Life on Earth ...and elsewhere?

Hands-on activities for grades 5–10 EDUCATOR RESOURCE GUIDE

Astrobiology in your classroom
Students come to class filled with questions about life on other

worlds. They have seen fantastic scenarios presented on television

and in films. They have read and heard about exploring and colonizing space. They have followed the drama of manned and robotic

space missions. The answers to the questions that arise out of

these experiences are often complex and multidimensional. How can

teachers meaningfully address such questions?

The hands-on activities in this Educator Resource Guide lay the conceptual groundwork for understanding questions fundamental to the

field of astrobiology. They enable students to examine the nature of

life, what it requires, its limits, and where it might be found. Through

these experiences, students learn important ideas related to the

search for extraterrestrial life. 

After completing the activities in this guide, students will understand

more about the nature of life, the habitability of planets, how astrobiologists gain insight into the possibility of extraterrestrial life, the

interdisciplinary nature of astrobiology, and the difficulties and subtleties of the search for life. Not only will students have a better

grasp of the concepts, but they will also deepen their appreciation

for the many things that are necessary in order for life to arise and

persist. Ultimately, the ambition of the guide is to inspire students to

stay abreast of developments — and perhaps even to become

involved — in the search for life on other worlds.

This Educator Resource Guide was written by Chris Randall (TERC) with

help from TERC’s Astrobiology: The Search For Life curriculum project staff

(NSF #ESI-9730728). It is in the Public Domain and is not protected by

copyright. Permission is not required for duplication. Illustration and design

by Michelle Hlubinka. 

Cover images courtesy of NASA and the Hubble Heritage Team:

heritage.stsci.edu/public/gallery/galindex.html#gallery

Life on Earth ...and elsewhere? Life on Earth ...and elsewhere?

2 INTRODUCTION
Activity Overview

2 What does life need to live? ........................................................................................11

Understanding that Life Needs Water, Nutrients, and Energy

• Grow organisms in one of 12 classroom environments and identify

common requirements (e.g., water, nutrients, and energy)

• Design a mission to identify habitable places by searching for water,

nutrients, and energy

• The Math Connection — Measuring Calories

3 What makes a world habitable? ........................................................................23

Assessing Planets As Candidates for Life

• Examine Habitability Cards, which discuss each planet and the six

large moons in terms of water temperature, atmosphere, energy, and

nutrients, and identify the top candidates for life in the solar system

• The Math Connection – Inverse Square Law

4 What can life tolerate?............................................................................................................37

Identifying the Range of Terrestrial Life

• Meet some extremophiles. What conditions do they tolerate?

• Play a card game similar to Rummy. Students create a set by matching an

extremophile, an extreme habitat on Earth, and an extraterrestrial habitat

that may be similar to an Earth habitat.

• Assemble a crew of extremophiles and target them to specific locations 

on a planet or moon

• Debate the ethics of sending Earth life to other worlds

5 Is there life on other worlds?....................................................................................49

Determining the Chances of Extraterrestrial Life

• Discuss the question, “What is the chance that we are the only life 

in the universe?”

• Determine what we need to know to predict the chances of extraterrestrial life

• Make estimates for each Drake Equation term and discuss the range 

of predictions in the class

• The Math Connection – Estimation

* For time estimates,

one day equals a 

50-minute class

period 

1 What is life?......................................................................................................................................................5

Defining Life

• Compare real and fake or live and dead objects and brainstorm ideas

about what life is

• Refine the definition by playing 20 Questions to identify an object or organism

• Test the definition by comparing “mystery” samples

2–3 DAYS *

2 DAYS *

1–2 DAYS *

2–4 DAYS *

1–2 DAYS *

ACTIVITY

ACTIVITY

ACTIVITY

ACTIVITY

ACTIVITY

ACTIVIT Y OVERVIEW 3
A Background Reading

starts the investigation

and informs teachers

about the topic.

The Recommended

Procedure details how to

implement the activity.

The Activity Guide

leads students through

an activity step-by-step.

ACTIVIT Y 5 53 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Do you think there is intelligent life in our galaxy with which we can

communicate? In 1961, Dr. Frank Drake identified eight terms to

help people think about what would have to take place for such

communication to be possible.

See what you think the chances are by making your own estimate

for each of the terms below. The conservative and optimistic values indicate the range of opinion among scientists with regard to

each term. You can use the conservative or optimistic estimates

or use another value, depending on your own intuition.

To estimate the number of worlds in the Milky Way

galaxy that have intelligent life that we can detect

using radio technology, follow these steps:

1 Make estimates for each of the eight terms listed

on the Using the Drake Equation worksheet.

2 Convert percentages to decimals before multiplying.

3 Multiply the estimates you made for the eight terms.

Is there life on other worlds? Activity Guide

Introduction

What to Do

= X X X The number of worlds within

our Galaxy that have intelligent life whose radio emissions should be detectable

Number of stars

in the Milky Way

galaxy Percent of these

stars that are

appropriate

Percent of appropriate stars that

have planetary

systems

To get a sense of what your students think

when they hear the term “extraterrestrial,”

ask them:

Have the class define the term, extraterrestrial. Try to get them to broaden the meaning to include anything from microbes to

plants to intelligent creatures to any living

thing.

Mention that astrobiology involves thinking

about whether or not there is extraterrestrial life, where it might be, and how we can

learn more about it.

To have students identify the factors related

to the existence of extraterrestrial life, ask

them what information they would need to

determine the probability of extraterrestrial

life. You might begin by saying, “Let’s see if

we can estimate how many worlds out

there have life (or intelligent life with which

we can communicate). What would we

need to know?” Try to keep your prompting

Students typically think

of “extraterrestrials” in

terms of science fiction. Prompting them

to consider other

forms of life often

changes their answers,

or at least the reasons

they provide. 

In 1961, Dr. Frank

Drake developed an

equation to estimate

the number of other

civilizations that exist in

our Milky Way galaxy

that we can detect

using radio technology.

It does this by making

estimates of eight component factors. Since

we are unsure whether

there is life out there,

let alone intelligent life

with which we can

communicate, there is

no correct solution to

the equation — the

value of each factor is

open to interpretation. 

The Drake Equation

Recommended Procedure

2

3

STEP

STEP

4

STEP

1

STEP

Are there 

such things as 

extraterrestrials? 

What do 

you mean by 

extraterrestrials?

What is the chance 

that we are the only

life in the universe?

Do you think that 

there are any forms 

of life elsewhere in

the universe? Why?

Would you be 

interested in a planet

inhabited by microbes,

plants, and insects? Why

or why not?

• The chances for life 

elsewhere in the galaxy

could be either high 

or low, depending on

whether one’s assumptions and estimates are

conservative or optimistic.

• Finding even simple life

forms will be a breakthrough discovery.

• The large number of 

stars significantly 

increases the possibility

of extraterrestrial life.

• Understand that our

galaxy contains a large

number of stars 

• Understand and 

Estimate the terms of the

Drake Equation

• Calculate how many civilizations with radio technology we can 

communicate with

• one Activity Guide 

(pages 53 and 54) 

for each student

• blank overhead transparencies

• transparency markers

• calculators

M ATERIALS

SKILLS

CO NCEPTS

ACTIVIT Y 3 23 50 ACTIVIT Y 5

If life is playing a game of planetary hide and seek

with us, then our job is to find it. But where in this

immense solar system should we begin to look,

and what should we be looking for? One way

astrobiologists narrow the number of possible

“hiding places” is to understand what makes a

planet or moon habitable. They then look closely at these habitable places.

No life beyond Earth has ever been found. Does

this mean that life is a rare accident that happened on Earth due to an extraordinary set of circumstances and is unlikely to happen elsewhere?

Currently, all other planets and moons seem to lack

at least one major requirement for life. Despite this

fact, Europa, Mars, and possibly Titan seem to have or

have had conditions conducive to life. Most astrobiologists

feel that, if extraterrestrial life were found, it would be bacteria-like, living beneath a planet’s or moon’s surface, and using

chemical energy for its needs.

Finding any kind of life beyond Earth would

be a profound discovery. It would help us

understand more about how planets and

moons can generate the chemistry that

leads to life and about the conditions that

life can tolerate. Furthermore, it would help

provide some important clues to the question

of whether life is a rare or common occurrence

in the universe.

What makes a world habitable? 3 ACTIVITY

To assess the possibility of life in the

solar system

Activity 3 builds on

the work the students have done

defining and examining life. Students

apply this work to

the question of

whether there is life

on other planets and

moons, a question

that people have

long pondered and

that drives much of

the space program.

This activity also

familiarizes students

with the planetary

bodies in the solar

system.

1 to 2 class periods

TIME

CONTEXT

PURPOSE

The Think About It

questions help students

reflect on what they have

learned. They can be

used as the basis for a

class discussion, as

homework, or as an

assessment.

Blackline Masters of

any required printed

materials are placed at

the end of an activity.

The Math Extension

explores a mathematical

concept related to the

activity.

Distance

Dis

1

tance 2 is twice that of Distance 1

Distance 3 is three times that of Distance 1

original

square at

Distance 1

new square

at Distance 2

Teachers: Cut up one or two sets of these

cards so that students may draw their

environments at random.

What does life need to live? Environment Cards

Sprout seeds such 

as radish, grass, bean,

squash, or alfalfa

seeds in damp soil 

or on a damp paper

towel in a plastic 

bag. Place in an 

illuminated area 

once germinated.

Place pond

water in an

open container.

Include some

sediment from

the bottom and

material such

as sticks or

plants. 

Place half an apple 

in an open bowl. 

Keep moist.

Cover slightly crushed

grapes with water.

Place brine shrimp eggs 

in salty water. Follow 

the instructions on the 

container.

Add sugar to

a graduated

cylinder until

it reaches the

5 mL mark. Do

the same with

the yeast. Add

the yeast and sugar to 

50 mL of warm water.

Moisten a slice

of bread but do

not soak it.

After leaving it

in the open air

for a day (it may need to be

moistened once or twice

during this interval), seal it

in a plastic bag. Place a few dried beans in

200 mL water.

Spread cottage cheese,

yogurt, or cream cheese in

the bottom of a container.

Cover several lettuce 

leaves with water.

Mix 5 g of

cornstarch

with 95 g

of soil. 

Add

enough

water to

give the mixture a doughy

consistency. Keep moist. Cover a handful of hay 

with water.

ACTIVIT Y 2 15 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE ACTIVIT Y 4 43 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

1 How can life and conditions on Earth be used as a model for life on other worlds?

2 If you were able to send a test tube of one kind of extremophile to Mars, which

extremophile would you choose? Why? 

3 If you were able to send a test tube of one kind of extremophile to Europa, which

extremophile would you choose? Why? 

4 If you could genetically engineer a new extremophile so that it had the traits of two

different kinds of extremophiles, which two traits would you merge if your

extremophile were to live on Mars? Europa? Explain.

(continues)

What can life tolerate? Think About It

Name _

Date _

Using This Guide

4 USING THIS GUIDE
What is life?

To develop a working definition of life

Activity 1 takes a

close look at what

characterizes life

and sets the stage

for subsequent activities that examine

what life requires,

what physical limits

it can tolerate, and

where it might be

found in the solar

system.

2 to 3 class periods

TIME

CONTEXT

PURPOSE

1 ACTIVITY

We think that we can recognize something as being alive or not

alive. But when scientists study very small samples or very old fossilized materials, the signs of life or past life are not easy to recognize. Furthermore, dormant and slow-growing organisms appear to

be non-living unless observed under the right conditions or over long

periods of time.

Surprisingly, there is no firm scientific definition of life. There is no

single test that can establish the presence or absence of life nor single characteristic that applies to all living things. However, one can

begin to define life by listing the characteristics that almost all living

creatures share. For example, Earth’s life forms all:

• have carbon-based chemistry

• have a membrane or wall that creates an internal

environment`

• use energy to maintain an internal state

• require liquid water

• are able to extract energy from the environment

• carry out metabolic processes resulting in 

the exchange of gases and solid materials (i.e.,

consuming raw materials and producing wastes)

• exhibit some type of growth, cell division, reproduction, or replication

• are able to have a population evolve and adapt to

the environment.

Some non-living objects, such as fire, possess many of these

characteristics and some arguably living organisms, such

as viruses, possess only a few. Are some characteristics

more fundamental than others?

There are two characteristics that are particularly useful

in helping distinguish living from non-living entities — the

ability to reproduce and the ability to produce and perpetuate genetic variation among offspring. Put another way, one could say that life is a self-contained chemiACTIVIT Y 1 5
6 ACTIVIT Y 1

• There is no firm scientific

definition of life, no single

test that can establish the

presence or absence of life,

and no single characteristic

that applies to all living

things.

• There are characteristics

that almost all living creatures share.

• Two characteristics useful

in helping distinguish life

from non-life are the ability

to reproduce and the ability

to produce and perpetuate

genetic variation among offspring.

• Observe carefully and

Record data

• Define life operationally

• Apply definitions and Draw

conclusions 

• Contribute thoughtfully to

group and class discussions

• one Activity Guide (page 9)

for each student

• pairs of objects (see Step 1)

• for each group:

• two hand lenses

• three jars

• hot tap water in a 

container

• 150 mg sand

• 15 mg sugar

• half a packet of active 

dry yeast

• one fizzing-style 

antacid tablet

M ATERIALS

SKILLS

CO NCEPTS

cal system capable of undergoing Darwinian evolution. This largescale, long-term view of life acknowledges that individual organisms

must still carry out many of the small-scale, short-term functions listed above. In fact, many of the tests that scientists design to detect

life on other worlds look for byproducts related to these short-term

functions. So, while detecting life depends on many immediately recognizable characteristics, for life to persist in the grander scheme of

things, it must evolve and adapt to changing conditions.

Few of the characteristics on this list lend themselves to quick, onetime tests, and many require multiple observations over a period of

time. Some characteristics leave traces after an organism has died.

One way astrobiologists search for extraterrestrial life is to search

for biosignatures, large-scale, telltale signs of life such as oxygen

and methane in the atmosphere (these reactive gases are quickly

removed by natural processes).

Recommended Procedure

Distribute a hand lens and one pair of objects to each student. Have

students examine the objects and create a list of characteristics

associated with life. What tells them that something is alive? After

five minutes, have students trade objects and examine a second pair.

Have them complete Questions 1 and 2 on the Activity Guide. With

the following pairs, students will immediately know which one is

alive, yet articulating how they know this can be very challenging.

• live ant in a container and a plastic ant (available at toy and joke shops)

• live flower and a similar kind of silk, paper, or plastic flower

• live leaf and a similar kind of silk, paper, or plastic leaf 

• live tree leaf and a dead tree leaf of the same kind

• live grass and dead grass of the same kind

• a dead house fly and a plastic fly (available at toy and joke shops)

• live cricket (from a pet shop) and a plastic cricket

• live earthworm (from the yard or a bait shop) and a gummy worm candy

Form groups of four students and have them develop a common set

of characteristics that can be used to identify life. Have each student

record this list under Question 3 on the Activity Guide.

In Steps 2, 4, and 11, the class creates working definitions for life.

However, to avoid too much repetition, groups should proceed on

their own in Steps 2 and 4, without reviewing and processing the

lists of characteristics as a class. By Step 11 all groups should have

refined their thinking and be able to contribute meaningfully to a

class discussion.

1STEP

2STEP
ACTIVIT Y 1 7

The intention of Step 3

is to hone in on the key

attributes of life. Poorly

phrased questions can

be answered truthfully

and still be misleading.

Encourage students to

word their questions

carefully and try to

identify characteristics

that are fundamental

to life.

Other ambiguous 

mystery objects include

mules (which are 

sterile), clouds, lights,

streams, crystals, televisions, viruses, and

cars. Only go through 

a second example if

absolutely necessary.

Mentioning these

examples will reduce

the number of ambiguous mystery objects

students will have 

available.

Steps 5 to 10 are 

adapted with permission from Lesson 5 

of Johnson Space

Center’s Destination

Mars activity packet.

sn-io.jsc.nasa.gov/sn/

outreach/activities/

destmars/destmars.htm

3STEP

4STEP

Have the students test their working definitions of life by playing an

abbreviated version of the familiar game, 20 Questions. Have each

group think of something either living or non-living. When it is their

turn, the class can ask the group ten (or five, depending on time)

yes-no questions to determine whether the mystery object is alive or

not. The class may not ask direct questions such as “Is it alive?” but,

instead, should ask about what the mystery object looks like, does

(or does not do), and uses or produces.

Before beginning, model an example that uses an ambiguous mystery object to demonstrate the need for clear questioning. Have the

class ask you five to ten questions to determine whether your object

is living on non-living. Any time a student asks you a poorly phrased

question, challenge the student to clarify his or her actual meaning.

An icicle is a good choice because it shows many of the attributes of

life. For instance, it grows, requires water, consumes raw materials,

has a complex internal structure made up of ice crystals, divides in

two when it breaks, responds to its environment, and produces

waste products when it drips and when the water evaporates or sublimates. Fire is another good example because it takes in nutrients

(fuel and oxygen), gives off energy (light and heat), grows, expands

its geographical coverage, moves, reproduces, and produces waste

products (heat, carbon dioxide, and smoke). So, how can scientists

justify that icicles and fire are not alive? Icicles and fire do not have

cell membranes or walls, do not use energy to maintain an internal

state, and do not have heritable traits that can be transferred.

Consequently, they may meet a limited definition for life, yet they do

not meet certain fundamental criteria.

After each group has challenged the class to identify its mystery

object, have each group answer Questions 4 to 6 on the Activity

Guide and create a list of some fundamental characteristics of life.

For a characteristic to qualify for the list, there can be no exceptions.

For example, if “moving independently” were suggested, one would

counter that plants are unable to move independently. Therefore,

moving independently could not be considered a fundamental characteristic. This step can be begun in class and completed as homework.

For each group, prepare the set of three jars described below.

5 Distribute one set to each group.

STEP

Jar Sand Sugar Other

1 50 mg sand 

(3 tablespoons)

5 mg sugar 

(1 teaspoon)

2 50 mg sand 

(3 tablespoons)

5 mg sugar 

(1 teaspoon)

5 mg active dry yeast (heaping teaspoon or half of a 1/4 oz packet) 

3 50 mg sand 

(3 tablespoons) 

5 mg sugar 

(1 teaspoon) 1 fizzing antacid tablet, crushed
8 ACTIVIT Y 1

Explain that it is difficult to tell whether or not there is anything alive

in the jars. Ask students to observe all three samples and write their

observations under Question 7 on the Activity Guide. They can smell

and touch the samples but not taste them. Encourage them to take

some material out and examine it with the hand lens, returning the

material to the correct jar each time. 

Give each group a small cup of hot tap water (50–55°C). Ask students to pour the water so that each sample is just covered. Have

students record their observations under Question 8 on the Activity

Guide. Students should look for and record differences caused by

adding water. 

Jar 1 will show no activity. Jar 2 will begin to show activity after

about 5 minutes. Jar 3 will immediately fizz vigorously.

Ask groups to look for signs of life. Have them use their lists of fundamental characteristics to determine whether there is something

alive in any of the jars. Have them write their conclusions and reasoning under Questions 9 and 10 on the Activity Guide. 

Reassemble as a class and create a class chart showing each group’s

conclusions for whether there is something alive in the three jars.

Probe their thinking by asking:

Which criteria were most important in helping you draw your conclusions?

What conclusions did you draw about the rapid fizzing of Jar 3?

What kinds of follow-up tests might provide you pertinent information?

Does no change mean that a sample contains nothing that is alive?

After students have engaged with the questions sufficiently, tell

them the contents of the three jars. Ask them how one might distinguish between a living and a non-living chemical change. What tests

can they suggest to show that the yeast is indeed alive — what can

yeast do that the fizzing antacid cannot do?

Have the class create a working definition for life, drawing on their

lists of fundamental criteria, experimental work, and discussion.

Have them write this definition under Question 11 on the Activity

Guide.

Use the Think About It questions as a basis for a class discussion, as

homework, or as an assessment.

6STEP

7STEP

8STEP

9STEP

10 STEP

11 STEP

12 STEP

Our senses are of limited use in detecting

microscopic life.

However, we can cause

changes that help us

understand what is

contained in a sample.

For example, we can

encourage microbes to

grow and become visible by culturing them,

or we can cause chemical changes to occur

and infer what is in the

sample based on the

byproducts.

Students can continue

to feed the yeast, and

the reaction should

continue for several

days. They can also

observe them under

the microscope and

see internal structures

and cells dividing in

two.
ACTIVIT Y 1 9 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What is life?

Activity Guide 

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 From your teacher, obtain a pair of objects. Examine them and create a list of characteristics associated with life. What tells you that something is alive?

2 Repeat Step 1 with a new pair of objects

3 In your group develop a common set of characteristics that can be used to identify life.

4 After you have finished playing 20 Questions, write down an example of a carefully

worded question that helped identify characteristics that are fundamental to life.

5 Write down an example of a poorly worded question that led to a misleading answer

which really did not help identify characteristics that are fundamental to life.

6 In your group develop a common set of characteristics that can be used to identify life.

This task is similar to what you did in Question 3, but since then, you have thought

more about what it means to be alive, and your list could be quite different.

7 Obtain three jars containing mystery samples from your teacher. Describe and compare

the contents of the three jars. If your teacher gave you a small sample of material from

one of the jars, how would you know in which jar it belongs? Look carefully.

8 Describe and compare the three samples after you added water.

9 Use your list of fundamental characteristics to determine whether there is something

alive in any of the jars. Write your conclusions and reasoning below.

10 Which of your criteria were most important in helping you draw your conclusions?

11 In the space below, write the working definition for life you developed with your class.
10 ACTIVIT Y 1 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What is life?

Think About It

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 Why is defining life difficult?

2 When you compared the pair of objects, which characteristics were most useful to you

in distinguishing the real one from the manufactured one or the live one from the dead

one?

3 When playing 20 Questions, what was the most challenging mystery object or organism? What made it so difficult to determine whether it was living or non-living?

4 Since our senses are of limited use in terms of helping determine what is in each sample jar, what are some other ways that we can use to figure out what is in each jar? 

5 What would you tell someone to help them distinguish between a one-time chemical

reaction and a chemical reaction associated with life?

6 Describe some follow-up tests that could provide additional information and help you

decide whether or not the jars contained something alive.

7 Does no change in a jar mean that a sample contains nothing that is alive? Explain.
Astrobiologists begin their assessment of whether a planet or

moon is a promising candidate for life by seeing if any liquid

water, energy sources, and nutrients are present. 

Many astrobiologists use the availability of liquid

water as the primary criterion for judging whether a

planet is a candidate for life. While other liquids do

exist (e.g., ammonia, methane, or ethane), they exist

at temperatures far below the level conducive for life.

The life discovered to date seems to be limited to a temperature range of about minus 15°C to 115°C. Under the

right conditions, water can be liquid over this entire range.

Furthermore, water is an important vehicle for transporting and

delivering dissolved chemicals as well as being an important chemical reactant in its own right. Therefore, planets and moons with a

way to cycle liquid water such as geothermal or atmospheric cycles

have a mechanism for delivering the chemicals required by living

organisms.

All life requires energy. Organisms use either light energy or chemical

energy to run their life processes. Light energy is available only to

organisms that live on or close to a planet or moon’s surface. For

life to exist there, the surface must be within a temperature range

conducive for life and be protected from harmful ultraviolet radiation

and charged solar particles. Strong magnetic fields and thick atmospheres around some planets and moons provide just this kind of protection. At some point, light energy from the sun becomes too dim

to be a viable energy source. Thus, on planets that are unprotected

or too distant from the sun, the only option for organisms is to live

beneath the ground and to depend on chemical energy for their

needs. Some microbes on Earth obtain energy from organic (carbon

containing) compounds. Others rely on inorganic (without carbon)

compounds such as sulfide and manganese compounds. There are

even microbes that can use both organic and inorganic compounds

as a source of energy. In each case, microbes break complex compounds into simpler ones to obtain a small amount of energy from

What does life need to live?

To establish the fact

that all life requires

water, nutrients, and

energy to live

After developing a

working definition

for life in Activity 1,

students now take a

broader look at life

by considering the

kinds of things all

living organisms

require. Because

organisms need to

obtain certain essentials from their environment, this activity places life within

a planetary context

and lays the foundation for discussing

habitability in

Activity 3.

2 class periods and

brief observation

times over the following week.

TIME

CONTEXT

PURPOSE

2 ACTIVITY

ACTIVIT Y 2 11
12 ACTIVIT Y 2

this chemical change. This energy is sufficient to power microbial

life. For life to arise and persist for billions of years, organisms need

a constant supply of these compounds. Processes such as volcanic

activity, weathering, lightning, and even the activity of life itself make

these energy-supplying compounds available.

Nutrients are the raw materials organisms need to construct and

maintain their bodies. While the solid planets and moons in our

solar system all have the same general composition, local conditions and processes have led to variations in the concentrations and

availability of different chemical compounds. As a result, nutrients

required by life are more available on some planets and moons than

on others. Atmospheres can also serve as a source of nutrients.

For example, nitrogen from nitrogen gas can be used to make proteins and carbon from carbon dioxide or methane can be used to

make carbohydrates and fats.

Once students understand life’s common requirements, they are

ready to think about how a planet might provide those essentials.

Habitable planets are ones that are able to provide organisms a

dependable supply of liquid water, nutrients, and energy. Since

chemistry works by the same rules throughout the universe, most

scientists think that extraterrestrial life will use the same things for

their life processes as life on Earth does. Consequently, when

searching for extraterrestrial life, astrobiologists search both for

direct evidence of life and for habitable conditions.

Recommended Procedure

Distribute the Activity Guide. Ask each student to answer Question 1 about what one

must provide any organism in order to have

it live a long time. Lead a class discussion

and create a list of students’ ideas.

Cut up one or two sets of Environment

Cards have students pick one from a hat. 

On the Activity Guide, have them answer

Questions 3 to 5.

To make sure their setup is safe and has the

necessary elements (see Teacher notes on

the 12 environments for growing organisms),

check the instructions that students wrote 

in Question 5 for maintaining conditions in

their environment. After approving their

approach, have them create the environment

according to their instructions.

The list should include

requirements such as

water, an energy source

(e.g., light, food, carbohydrates, fats, or sugar),

nutrients (e.g., chemicals, substances in soil

and fertilizers, and vitamins and minerals),

and a suitable habitat

(e.g., suitable temperature range, protection).

You may want to tell

students to loosely

cover their containers

with plastic wrap with 

a few holes poked in it,

and to keep Environments 3 to 12 out of

direct sunlight.

1STEP

2STEP

3STEP

• All known life requires

liquid water, nutrients,

and energy to live.

• Organisms use either light

energy or chemical energy

to run their life processes.

• Habitable planets are able

to provide organisms a

dependable supply of liquid water, nutrients, and

energy.

• Plan how to maintain a

setup to promote the

growth of organisms

• Create and Maintain their

environments

• Make Predictions based on

a model (the environment)

• Generalize and Extrapolate

from a model

• Design an instrument

package that can detect

conditions favorable to life

• Contribute thoughtfully to

group and class discussions

• one Activity Guide (pages

16 and 17) for each student

• How to tell what’s growing

in your environment (page

19) key for each group

• 1 to 2 sets of environment

cards (page 15)

• materials for the twelve

environments (see page 14,

Teacher notes on the 

12 environments for 

growing organisms)

M ATERIALS

SKILLS

CO NCEPTS
ACTIVIT Y 2 13

Have students complete Activity Guide Questions 7 to 10. This could

be done in class or as homework. Over the next week or two, ask

students to observe their environments and identify what is growing

by using the How to Tell What Is Growing in Your Environment key.

Have them write their observations under Question 11 of the Activity

Guide.

To help students understand that all life has certain basic requirements in common, conduct a class discussion based on their answers

to Questions 9 and 10, which examine what all life requires. Using

the categories identified in Step 1 such as energy, raw materials (i.e.,

nutrients), and water, have students identify how each environment

provides these essentials. The class list might look like the following.

Assuming that all life requires energy, raw materials, and water, ask

students how they would go about looking for life in the solar system. Instead of searching for actual life forms, could one make a

plausible argument for searching for extraterrestrial life by searching

for water, nutrients, and energy? Have them list the tools and instruments they would include on a robotic spacecraft that was sent to

look for life on another planet. Have them articulate their reasons for

including each tool or instrument.

Use the Think About It questions as a basis for a class discussion, as

homework, or as an assessment.

While students may be comfortable with the idea that chemical compounds such as sugar can be an energy source for organisms, this

may well be the first time that they have heard that inorganic compounds also contain energy. When microbes absorb such compounds

and break them down into simpler compounds, they obtain enough

energy to satisfy their energy needs. One vivid way to reveal the

energy contained in the inorganic substance iron is to ignite some

fine steel wool. Hold a small wad with some tongs and ignite it with

a match. It will spark and flare. As the Fe0 oxidizes to Fe+3, light and

heat energy are given off. Iron-oxidizing bacteria oxidize iron to satisfy their energy needs. Since only a small amount of energy is available, they need to oxidize a lot of iron to satisfy their needs. Other

everyday examples of the energy contained in inorganic compounds

include match heads, which contain sulfur and phosphorous, and

many explosives, which contain nitrogen compounds. While

microbes do not absorb match heads and explosives, these substances demonstrate the energy available from sulfur, phosphorous,

iron, and nitrogen-based compounds.

Energy Source

• Sunlight

• Carbohydrates 

(e.g., sugar, starch, 

and cellulose

• Other organisms

Source of Raw Materials

• Soil

• Nitrogen and carbon

dioxide in the air

• Chemicals contained in

food

• Other organisms

Source for Water

• Rain

• Groundwater

• Moisture in food

• Other organisms

Astrobiologists look for

planets and moons

that are or were habitable. A critical aspect

of being habitable

means having a

dependable supply of

energy, raw materials,

and water. In Step 6,

students can design

instrument packages

that either look for habitable conditions or

that look for direct evidence of life.

4

STEP

5STEP

6STEP

7STEP

Extension
14 ACTIVIT Y 2

1

Seeds

Sprout seeds such as radish, grass, bean,

squash, or alfalfa seeds in damp soil or on

a damp towel in a plastic bag. Place in an

illuminated area once germinated.

Plants Watch for the development of a root, stem, and leaves.

2

Pond water 

Place in an open container. Include

some sediment from the bottom and

material such as sticks or plants. Notice

how the numbers and kinds of organisms change as the water ages.

Protists Single-celled organisms visible under the microscope.

Insects May be attached to floating materials and in bottom

sediments.

Algae Gives water a greenish color.

Plants May be rooted in the sediment. 

3

Apple

Place half an apple in an open bowl.

Moisten occasionally.

Molds Many molds look cottony. As the molds age, look for

sporangia, spore-filled spheres on top of a stalk.

Bacteria Bacterial colonies are masses of millions of bacteria

and appear as shiny discs.

4

Grapes

Crush slightly and cover with water.

Molds See description in #3.

Bacteria Water becomes cloudy. Bacterial colonies may appear

on pieces of grape that are above the waterline.

5

Brine shrimp

Brine shrimp eggs are available at pet

stores. Follow the instructions on the

container.

Crustacean Brine shrimp will emerge from dormancy and 

swim about the container, visible to the naked eye.

Technically, the “eggs” are cysts, produced when 

conditions become unfavorable to adult brine shrimp.

6

Yeast

Place 5 g of yeast in 50 mL of warm

water. Add 5 g of sugar. Replenish the

sugar every other day. Eventually, alcohol, a waste product, builds up to toxic

levels. At this point yeast form thick

walls and become dormant. 

Fungi Yeast is a single-celled fungus. It will convert the 

sugar to carbon dioxide and alcohol, producing 

bubbles and a sweet, bread-like odor. It can be seen 

to bud into two cells under a microscope’s low power.

7

Bread

Moisten a slice but do not soak it. Leave

it in the open air for a day without letting

it dry out. Then seal it in a plastic bag.

Mold White molds (Rhizopus nigricans), pinkish molds

(Neurospora), and green molds (Aspergillus) are common bread molds.

Bacteria See description in #3.

8

Dried beans

Place a few dried beans in 200 mL

water. After they soften, crush them

slightly.

Mold See description in #3. Green molds such as 

Aspergillus and Penicillium are common.

Bacteria Water becomes cloudy. Bacterial colonies may appear

on any bean parts that are above the waterline.

9

Cottage

cheese,

yogurt or

cream

cheese

Spread cottage cheese, yogurt, or

cream cheese in the bottom of a container. Keep the surface moist.

Mold See description in #3. Green and white molds are 

common. 

Bacteria Colonies may appear as shiny discs. Lactobacillus acidophilus breaks down the lactose sugar in milk and

gives yogurt its consistency. Certain molds are utilized

to age cheeses.

10

Lettuce

Cover several lettuce leaves with water.

Keep very moist.

Bacteria Bacteria quickly break down lettuce leaves, turning

them to runny mush.

11

Cornstarch

and rich soil

Mix 5g of cornstarch with 95g of soil.

Add enough water to give the mixture a

doughy consistence. Many soil organisms will feed on the starch. Keep moist.

Mold See description in #3.

Bacteria Colonies may appear as shiny discs.

12

Hay

Cover a handful of hay with water. Keep

covered with water.

Bacteria The water becomes cloudy.

Protists Single-celled organisms visible under the microscope.

Mold Mold may appear on any hay above the waterline.

ENVIRO NMENTS CULTURING TIPS H OW TO TELL WHAT’S GROWING

Teacher notes on the 12 environments for growing organisms
Teachers: Cut up one or two sets of these

cards so that students may draw their

environments at random.

What does life need to live?

Environment Cards

Sprout seeds such 

as radish, grass, bean,

squash, or alfalfa

seeds in damp soil 

or on a damp paper

towel in a plastic 

bag. Place in an 

illuminated area 

once germinated.

Place pond

water in an

open container.

Include some

sediment from

the bottom and

material such

as sticks or

plants. 

Place half an apple 

in an open bowl. 

Keep moist.

Cover slightly crushed

grapes with water.

Place brine shrimp eggs 

in salty water. Follow 

the instructions on the 

container.

Add sugar to

a graduated

cylinder until

it reaches the

5 mL mark. Do

the same with

the yeast. Add

the yeast and sugar to 

50 mL of warm water.

Moisten a slice

of bread but do

not soak it.

After leaving it

in the open air

for a day (it may need to be

moistened once or twice

during this interval), seal it

in a plastic bag. Place a few dried beans in

200 mL water.

Spread cottage cheese,

yogurt, or cream cheese in

the bottom of a container.

Cover several lettuce 

leaves with water.

Mix 5 g of

cornstarch

with 95 g

of soil. 

Add

enough

water to

give the mixture a doughy

consistency. Keep moist.

Cover a handful of hay 

with water.

ACTIVIT Y 2 15 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE
16 ACTIVIT Y 2 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What does life need to live?

Activity Guide 

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 List some things one must provide an organism in order to have it live a long time.

2 From your teacher, obtain a description of an environment for growing organisms.

3 What essential materials does the initial environment provide to organisms living in it?

4 Which of these essential materials may run out in less than ten days?

5 What must you provide in order for the organisms living in your environment to live a

long time? Below, write a brief set of instructions (five to ten steps long) for how to

maintain your environment over the next ten days so that organisms will have what

they need to live.

6 After your teacher approves your plan, create the growing environment. 

(continues)
ACTIVIT Y 2 17 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What does life need to live?

Activity Guide (continued)

7 Predict what will be growing in your environment in ten days.

8 How will you tell that the organisms inhabiting your environment are alive?

9 What are some things each environment in the class has in common with the others? 

10 Based on the class’s environments, list the things you think are essential for life to exist.

11 Observe your environment for ten days. Use the How to tell what’s growing in your

environment key to identify what is growing in it 
18 ACTIVIT Y 2 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What does life need to live?

Think About It

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 In general, what does all life require in order to live?

2 Even though all life on Earth requires energy, raw materials, and water, is that sufficient evidence for saying that all life on Earth is related? Why or why not?

3 What are the strengths and weaknesses of the argument that if all life on Earth

requires energy, raw materials, and water, then extraterrestrial life must require 

these things too?

4 Describe what a planet or moon must have in order to be habitable.
ACTIVIT Y 2 19 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

What does life need to live?

How to tell what’s growing in your environment 

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

Algae 

Look for grass-green patches, green water, floating clumps, or green films on the surface of

the water, sediments, or container. Algae can range in size from microscopic to many meters

in length. All algae lack roots, stems, and leaves.

Plants

Look for roots, stems, and leaves. The green color of chlorophyll is a common indicator of plants.

Insects

Look for organisms with six legs. They can be found in soils, in bottom sediments, and

attached to floating materials.

Bacteria

Bacterial colonies are masses of millions of bacteria. Look for shiny discs up to 50 mm in

diameter growing on moist (but not submerged) solid surfaces. Bacteria often make water

look cloudy. Bacteria quickly soften and break down soft materials such as lettuce leaves.

Crustaceans

Look for organisms that have a hard outer shell, segmented body parts like a crab, and more

than six legs (such as a pill bug).

Molds (a type of fungus)

Look for fuzzy growth on moist, solid surfaces. Many molds look cottony. As the molds age, look

for sporangia, black, spore-filled spheres on top of a stalk. White molds (Rhizopus nigricans), pinkish molds (Neurospora), and green molds (Aspergillus and Penicillium) are common bread molds.

Fungi

Look for a spreading growth that sends thread-like strands throughout the material on which

it is growing. Yeast is a microscopic single-celled fungus. It converts sugar to carbon dioxide

and alcohol, producing bubbles and a sweet, bread-like odor. Yeast colonies can appear yellow

or pink. Molds (see below) are also fungi.

Protists

Look for microscopic, single-celled organisms that move about, usually in an aquatic or very

moist environment. There are three general shapes: those having a long tail (flagellates),

those covered with short hairs (ciliates), and those that have a blobby appearance and move

by changing the shape of their body (amoebae). Make microscope slides and view them

under the microscope.
20 ACTIVIT Y 2

One domain of mathematics, and one especially important in science, is measurement. While certain kinds of measurement are

logically obvious, other kinds, such as measuring the energy content

of food, are not. Food energy is measured in calories, a standard

unit of heat energy. Calculating calories is a two-step process that

involves transforming food energy into heat, something that is more

easily measured.

Scientists determine the number of calories in a food sample by

burning it and measuring the heat energy that is given off. One 

calorie is the amount of energy needed to raise the temperature 

of one gram of water one degree Celsius. (See Step 10 for a discussion of the difference between a nutritionist’s Calorie and a physicist’s or biologist’s calorie.)

The accuracy and validity of an experiment or measurement depend

on one’s ability to eliminate potential sources of error. In fact, an

important research skill is developing sensitivity to sources of error.

The setup suggested below is intentionally skeletal. Keeping it basic

helps students see clearly how one measures the energy content of

food. The spare setup also invites a great deal of error and provides

opportunities to identify sources of error and reduce them by refining the setup and by controlling variables.

Write the definition for calorie on the board. Ask the class how many

calories are required to raise the temperature of 1 gram of water 10

degrees Celsius and 100 grams of water 5 degrees Celsius (10 and 500

calories, respectively). Then, ask how they can determine the number of calories in a sample of food using this information. Challenge

them to express it in an equation to measure calories in food.

As a class, develop a procedure to measure calories in food similar to

Steps 4 to 7. Make sure to instruct students to wear safety goggles

at all times and not to eat the food or touch the fire or hot can.

Give each group a sample of one of the foods, and instruct them to

use the class procedure to measure the calories in their sample. To

ensure having multiple data points, make sure at least two groups

test each kind of food.

• can (soup or soda 

can size)

• ring stand with clamp

• thermometer

• matches 

• food sample holder

(stick a pin into a cork)

• sample of food 

(marshmallows, 

peanuts, taco chips)

• 50 mL of water at room

temperature

• glove or forceps to

grasp hot materials 

M ATERIALS

The joule is the SI unit

of energy, replacing the

calorie as a unit of heat

energy. A joule is

defined as (a) the

amount of work done

by moving one newton

one meter, or (b) the

work done by one

ampere flowing

through the resistance

of one ohm for one

second. To convert

calories to joules, multiply the calories by 4.19.

Math Extension

What does life need to live?

Recommended Procedure

1STEP

2STEP

3STEP
ACTIVIT Y 2 21

Set up a calorie-measuring device

similar to the figure at the right.

Pour 50 ml of water into the can

and measure its initial temperature.

Make sure the thermometer is not

touching the bottom of the can.

Ignite the food sample with a

match. When the food sample has

burned completely, record the final

temperature of the water.

Calculate the calories in the food

sample. 

Create a data table on the board showing the calories measured by

each group. Based on the table, determine which food contains the

most energy.

Ask if this experiment is a fair way to compare the energy content of

foods. Ask how one could fairly compare the energy content of different foods.

This experiment is not a fair way to compare the energy content of

foods because the food samples are of different sizes. Use the table

from Step 8 to show students that, for each food type, there is a

range in the number of calories they measured. Note that this setup

has many sources of error. Have students research bomb calorimeters to see how scientists try to reduce experimental error when

measuring the energy content of foods.

Have students compare the caloric values they measured for each

type of food with the nutrition labels on the food containers. Ask

them why their values differ from those on the labels. Ask how fats,

carbohydrates, and proteins compare in energy value.

One reason for the difference is considerable experimental error. 

For example, the apparatus is inefficient and uninsulated, so heat

that should warm the water is lost to the room. Another difference

stems from the fact that nutritionists measure food energy in

Calories (capital C), not calories. The difference between a 

nutritionist’s Calorie and a physicist’s or biologist’s calorie is that 

a Calorie equals 1000 calories, or one kilocalorie. To calculate the

Calories in the above experiment, multiply calories by 1000. 

4STEP

5STEP

6STEP

7STEP

8STEP

9STEP

10 STEP

To be confident that all

temperature changes

are due to the burning

food, use room temperature water. If hot or

cold water is used, the

water will lose or gain

heat independently of

the heat contributed by

the burning food.

Since a gram of water

equals one milliliter of

water, one can calculate the number of

calories by multiplying

the change in temperature in degrees Celsius

by either the water’s

mass in grams or its

volume in milliliters. 

= x ( – ) Starting

Temperature

°C

Final

Temperature

°C

Mass of

Water

(grams)

calories

To be fair, one must

determine the calories

on a per-gram basis.

Only when one compares the caloric value

of equal amounts of

food can one determine which kind of food

contains more energy.

If you have the inclination, consider having

students determine

the per-gram caloric

value by massing the

food sample before

and after burning and

divide the total number

of calories by this difference in weight. 
What does life need to live?

Math Extension Activity Guide

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 Write the name of your food sample below.

2 What was the initial temperature of the water?

3 What was the final temperature of the water?

4 How many calories did your food sample contain?

calories = (mass of water) X (change in temperature)

5 What happened to the heat energy that did not

go into raising the temperature of the water?

6 Suggest some improvements to the setup to reduce experimental error and to

increase the accuracy of your energy measurement.

7 Is this experiment a fair way to compare the energy content of foods? Explain.

8 How can you fairly compare the energy content of different foods?

9 Why were the calorie values on the nutrition labels so different from the calorie values you measured in each type of food?

10 How do fats, carbohydrates, and proteins compare in energy value?

22 ACTIVIT Y 2 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE
ACTIVIT Y 3 23

If life is playing a game of planetary hide and seek

with us, then our job is to find it. But where in this

immense solar system should we begin to look,

and what should we be looking for? One way

astrobiologists narrow the number of possible

“hiding places” is to understand what makes a

planet or moon habitable. They then look closely at these habitable places.

No life beyond Earth has ever been found. Does

this mean that life is a rare accident that happened on Earth due to an extraordinary set of circumstances and is unlikely to happen elsewhere?

Currently, all other planets and moons seem to lack

at least one major requirement for life. Despite this

fact, Europa, Mars, and possibly Titan seem to have or

have had conditions conducive to life. Most astrobiologists

feel that, if extraterrestrial life were found, it would be bacteria-like, living beneath a planet’s or moon’s surface, and using

chemical energy for its needs.

Finding any kind of life beyond Earth would

be a profound discovery. It would help us

understand more about how planets and

moons can generate the chemistry that

leads to life and about the conditions that

life can tolerate. Furthermore, it would help

provide some important clues to the question

of whether life is a rare or common occurrence

in the universe.

What makes a world habitable?

3 ACTIVITY

To assess the possibility of life in the

solar system

Activity 3 builds on

the work the students have done

defining and examining life. Students

apply this work to

the question of

whether there is life

on other planets and

moons, a question

that people have

long pondered and

that drives much of

the space program.

This activity also

familiarizes students

with the planetary

bodies in the solar

system.

1 to 2 class periods

TIME

CONTEXT

PURPOSE
24 ACTIVIT Y 3

• Except for Earth, each

planet or moon currently

has major limitations for

life as we know it.

• Looking for habitable conditions is easier than looking for actual organisms.

• If extraterrestrial organisms exist in our solar

system, they probably

live underground and,

thus, are very small.

• Europa, Mars, and Titan

may have or have had

habitable conditions.

• Sunlight intensity influences surface temperatures and whether organisms can use light as an

energy source.

• Sunlight intensity on a

planet or moon diminishes as the square of its

distance from the sun.

• Interpret images and

data and Compare, Sort,

and Categorize that data

• Generalize and Infer from

observations and Draw

conclusions

• Summarize information,

Synthesize understanding, and Present it clearly

• Contribute thoughtfully

to group and class discussions

• three to four sets of Habitability Cards (pages 29 to

34; laminate, if possible)

• one Activity Guide 

(page 36) and Key to

Habitability Factors

(page 35) for each 

student (copy back-toback, if possible)

M ATERIALS

SKILLS

CO NCEPTS

1

Recommended Procedure

STEP

2STEP

3STEP

4

STEP

To get a sense of student understanding

about habitability, ask students, “What

makes a planet or moon a good home for

living things?” Have each student write

down an answer.

If you feel that students need a quick

review of the key habitability factors, conduct a class discussion to develop a set of

basic criteria. You might ask:

In general terms what does life need?

Answer: Life needs food, water, and conducive

habitats (e.g., protection from radiation and suitable temperatures). Food is both a source of

energy and raw materials for construction.

Students will be able to make better use of the

Habitability Cards if they are aware of food’s

dual roles.

What kinds of things might limit life?

Answer: Extreme temperatures, high levels of

radiation such as ultraviolet radiation, and lack

of food and water can limit life.

To investigate the possibility of life in our

solar system, have students use the

Habitability Cards and the accompanying

key to assess the habitability of each planet

and moon in our solar system. [To make 

a set of cards, photocopy pages 29 to 34, 

double-sided if possible. Cut them into individual cards. To make them last longer, copy

them onto card stock or laminate them.] On

the Activity Guide, have each student rank

each planet or moon as a likely, unlikely, or

possible candidate for life and articulate the

reasoning behind his or her determination.

If students have been working on their own,

organize them in groups. Have each group

select its top three candidates for life.

Conduct a class discussion and record each

group’s analyses on a class chart. Have them

state the reasoning behind their choices.

Students should come

away from this discussion with a clearer (it

does not have to be

perfect!) sense of what

makes a planet conducive for life. What

they need before going

on is a rudimentary set

of criteria for judging

the possibility of life on

a planet.

Since students typically

take several minutes

with each card, there

is no need to prepare 

a large number of sets.

Students can share

cards, exchanging ones

they have used for new

ones. Alternatively, if

you feel that trading

will cause too much

disruption in your class,

organize the class in

groups of six students

and provide each group

a complete set of

Habitability Cards.

Photocopied versions

of the Habitability

Cards are usually lackluster and uninspiring.

You can download a

full-color version of the

Habitability Cards at

astrobio.terc.edu or

astrobiology.arc.nasa

.gov.
ACTIVIT Y 3 25

In the Habitability Cards, students read that, at certain distances

from the sun, sunlight is too dim to be a viable energy source. The

intensity of light diminishes as one moves further from the source.

Students know this intuitively from observing how the brightness of

a flashlight changes as one moves toward or away from a surface.

Interestingly, the light changes in a predictable, measurable way

that can be described by a mathematical formula called the

Inverse Square Law. Any student comfortable with fractions and

multiplying and dividing can calculate differences in light intensities. 

The Inverse Square Law applies to fields that radiate evenly in all

directions (e.g., light and magnetic and gravitational fields). Objects

that generate such fields are called point sources. The diagram

below shows the sun serving as a point source of light because it is

in the center of a sphere of light traveling evenly in all directions.

As the light spreads out, its intensity per unit area decreases. To

calculate the light intensity, one needs to know the light intensity at

some reference point. For our solar system, astronomers use

Earth as a reference point because they have measured the intensity of light reaching Earth from the sun to be 1370 watts per

square meter (W/m2). 

To calculate the light intensity at any position in the solar system,

one needs to have a reference intensity and to know the distance

from the light source. A convenient unit for talking about distances

in the solar system is the astronomical unit (AU.) One

AU equals 149,597,870 kilometers (about 93 million miles), the distance from Earth to the sun.

A sphere’s surface area grows as the

square of its radius. Therefore, light intensity depends on the square of the distance from the center of the sphere.

Mathematically:

difference in light intensity =

1/distance2

For example, Mars is 1.5 AU from

the sun. We can use the Inverse

Math Extension

What makes a world habitable?

Sun

1 unit 2 units 3 units
26 ACTIVIT Y 3

• overhead projector

M ATERIALS

Square Law to see how the light intensity on Mars compares with

the light intensity on Earth.

1/distance2 = 1 / 1.52 = 1/ 2.25 = 0.44

The light intensity on Mars is 44% that of Earth’s. Since the intensity on Earth is 1370 W/m2, then the intensity on Mars is 

0.44 X 1370 W/m2 = 603 W/m2

By increasing the distance from the sun by just one-half an AU, the

light intensity drops by more than half, relative to Earth.

Unlike the sun or a light bulb, both of which are point sources of light,

overhead and slide projectors use mirrors and lenses to focus light.

Consequently, the light from these projectors does not radiate evenly

in all directions, and they are not point sources of light. Nonetheless,

the model described in this Math Extension successfully approximates the Inverse Square Law, and light intensity does diminish with

distance in close accordance with the 1/distance2 relationship. 

As with most models, this one has its limitations — it illustrates certain aspects of the Inverse Square Law well while misrepresenting

others. With any model, students must be made aware of its

strengths and limitations. So, even though the Inverse Square Law

applies only to point sources, this model with its focused beam still

demonstrates the inverse square relationship fairly well. For a truly

accurate inverse square demonstration, use a light meter to measure the drop off in intensity around a 40 Watt (or less) light bulb. 

Recommended Procedure

Set a slide or overhead projector either ten centimeters or one meter

from a wall or board. Either of these distances will make the inverse

square relationship readily apparent. (This distance can vary. It is

really a function of how much space you have and the size of your

screen.) Note the distance between the board and the bulb or lens.

Project light onto the board and outline the illuminated square. 

Use a three-column table to keep track of the distance from the

board, the length of the illuminated square’s edge, and the area of

the illuminated square. 

Have students predict what will happen to the illuminated square

when you move the projector further from the board. (It increases in

size.) Ask them how the light intensity changes when you move the

projector further from the board. (It decreases.)

1

STEP

2

STEP

3

STEP
ACTIVIT Y 3 27

Move the projector twice

the original distance from the

board, explaining to your students

exactly what you are doing. Outline the

illuminated square. The edge of this illuminated square should be twice as long as the edge of

Step 2’s illuminated square, and its area should be four

times the area of Step 2’s square. Ask students how the light

intensity has changed, now that the same amount of light has to

cover four times the area. They should say that the light intensity

within the square is one fourth of the original light intensity.

Ask students to predict what the intensity will be if you move the 

projector three times the original distance from the wall. When you

move the projector this distance, the edge of this illuminated square

should be three times as long as the edge of Step 2’s illuminated

square, and its area should be nine times the area of Step 2’s square.

Consequently, the amount of light hitting each unit area in Step 5 is

about one ninth of the amount hitting each unit area in Step 2.

Have students create a graph comparing the projector’s distance

from the board with the area of the illuminated square. Then, introduce your students to the math of the Inverse Square Law. Refer to

the distance, length, and area data from the table you created: 

1/(Distance 1)2 = 1/12 = 1 Define the intensity at Distance 1 as the reference

intensity

1/(Distance 2)2 = 1/22 = 1/4 A value of 1/4 means that the intensity per unit

area at Distance 2 is 1/4 of the reference intensity

1/(Distance 3)2 = 1/32 = 1/9 A value of 1/9 means that the intensity per unit

area at Distance 3 is 1/9 of the reference intensity 

4

STEP

5

STEP

6

STEP

Distance

Dis

1

tance 2 is twice that of Distance 1

Distance 3 is three times that of Distance 1

original

square at

Distance 1

new square

at Distance 2

The square of light

at Distance 2 is four

times the area of the

original square at

Distance 1.
28 ACTIVIT Y 3

Using the graph or applying the Inverse Square formula, ask students to predict the size of the illuminated area if the projector were

moved four times the original distance. Try it, if possible. (Compared

with the illuminated square in Step 2, the edge of Step 6’s square

will be four times as long and its area will be 16 times as large.)

The Inverse Square Law enables you to calculate light intensities relative to a known light intensity. Step 8 uses the light intensity of

Earth (1370 W/m2

) as the known light intensity. Have your students

calculate the light intensity for planets and moons in our solar system relative to Earth. Then have them create a graph of their calculated values against their distances from the sun. 

Begin by using Mercury as an example. On average, Mercury is 0.39

A.U. from the sun. According to the Inverse Square Law, the amount

of light reaching Mercury is inversely proportional to Mercury’s distance from the sun, so: 

1/(0.39)2 = 6.575

Thus, the amount of light reaching Mercury is more than six and 

a half times (657.5%) of the amount of light reaching Earth. To determine the light intensity on Mercury, use Earth’s intensity, 

1370 W/m2

, as a reference intensity 

6.575 X 1370 W/m2 = 9,008 W/m2

Briefly discuss the pattern students found and the idea of a

Habitable Zone — the zone between the point where there is too

much and too little sunlight. Beyond the inner edge of the Habitable

Zone, it is too hot for life and surface water boils away. Beyond the

outer edge of the Habitable Zone, liquid water freezes. Also, at some

point beyond the outer edge of the Habitable Zone, sunlight

becomes too dim to be a viable energy source for organisms living

on or near the surface. Within the Habitable Zone, water can exist as

a liquid, given the right atmospheric conditions.

Planet or Moon Distance from Light Intensity Watts per 

Sun (AU) (relative to Earth) Square Meter 

Mercury 0.39 6.575 9008 

Venus 0.72 1.929 2643 

Earth & Moon 1.0 1.000 1370 

Mars 1.5 0.444 603 

Jupiter & its moons 5.2 0.037 51 

Saturn & its moons 9.5 0.011 15 

Uranus 19.2 0.0027 3.7 

Neptune 30.1 0.0011 1.5 

Pluto 39.5 0.0006 0.8 

8STEP

9STEP

7STEP%
fast facts

Venus has a thick carbon dioxide atmosphere that traps

heat efficiently. The average surface temperature is

464°C. 

There is no surface water. The atmosphere has trace

amounts of water vapor (30 parts per million or

0.0000003%).

Venus’s atmosphere is 92 times that of Earth’s. It is 97%

carbon dioxide.

The thick clouds prevent much sunlight from reaching the

surface, so any life would have to depend on chemical

energy. Sulfuric acid clouds provide a potential source of

chemical energy.

In general, Venus and Earth have the same chemical composition, and Venus is volcanically active, giving it a way to

cycle chemicals important to life.

VENUS

fast facts

The average surface temperature is 15°C. Earth’s maximum temperature is 51°C (Libya) and its minimum is

–89°C (Antarctica). 

On Earth, water exists in all three states. The water cycle

delivers water to nearly every part of Earth. 

Earth’s atmosphere shields the surface from harmful

ultraviolet radiation and most meteorites, insulates the

Earth, and serves as a source of nutrients such as nitrogen and carbon. 

Plants capture sunlight and make possible the food chain.

High oxygen levels in the atmosphere enable life to use

high-energy, carbon-based energy sources (e.g., sugar).

Many microbes live off the chemical energy in inorganic

compounds such as iron and sulfur.

Everything organisms need to build and maintain their

bodies is already on Earth. Earth has processes such as

plate tectonics to cycle chemicals important to life.

EAR T H fast facts

Even though the surface temperature can reach room

temperature for a few minutes at mid latitudes, the average surface temperature is –63°C.

Though there is no surface water, features suggest that

Mars once had flowing surface water. There are also indications of thick layers of permafrost, soil locked in water

ice. The Northern and Southern ice caps contain water ice.

The Martian atmosphere is 95% carbon dioxide. The

atmospheric pressure is so low (one-thousandth that of

Earth’s) that surface water quickly boils away. The atmosphere is too thin to protect or insulate the surface of

Mars significantly. 

Mars is on the edge of the Habitable Zone, making sunlight a possible energy source. Chemicals made available

by volcanic activity early in Mars’s history may once have

been a possible energy source.

Mars and Earth have the same general chemical composition. Mars was volcanically active for its first two to three

billion years, giving it a way to cycle chemicals important

to life.

MARS

ACTIVIT Y 3 29

fast facts

The temperature at the cloud tops is –200°C while the

interior temperatures reach tens of thousands of

degrees. The churning of the atmosphere causes temperatures of the circulating gasses to change greatly over

short distances and periods of time.

The gas giants are made almost entirely of hydrogen and

helium, with very small amounts of water.

Gas giants release large amounts of their own energy,

keeping internal temperatures high and causing their

atmospheres to circulate constantly. The violent storms

created by this circulation would subject life to rapid and

extreme changes in temperature and pressure. 

Sunlight is dim but may be a viable energy source.

Obtaining sufficient amounts of chemicals in a gaseous

environment is difficult, making chemical energy an 

unlikely energy source.

A gas environment is too diffuse to concentrate nutrients

and make them available in a predictable, reliable way.

Having life arise or survive in such a constantly changing

environment is highly unlikely.

THE G AS GIANT S JUPITER

S A TURN

URANUS

NEPTUNE

LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE
VENUS

MARS

The surface is so hot, it melts in places,

causing depressions and lava channels.

High temperatures caused these depressions and lava channels (left).

volcano and

ash plume

North

Pole

Pathfinder landing site

Biggest volcano in the solar

system (800 km in diameter)

These two features indicate

flowing water.

Ophir Canyon

Frame width is 300 km

Middle East

South

Pole

Eroded

volcano

in Hawaii

Mangrove Delta in Bangladesh

Buenos Aires

(left side of bay)

Deforestation in Brazil

Frame width 10km

lava disks

EAR T H

Frame width 100km

G A S

GIANT S 

Saturn

Neptune

Jupiter

Uranus
G ANYMEDE 

EUR O P A

C ALLIS T O

I O

Surface close-up. Frame width is 50 km.

Computer-generated surface view.

Io’s

surface is

discolored by

sulfur compounds

from volcanic eruptions.

Callisto is the most heavily

cratered object in

the solar system.

The bright patches are 

a highly reflective 

material such as 

ice that oozed from

the interior.

Ganymede 

is Jupiter’s

largest

moon.
fast facts

At noon on the equator, the average surface temperature

is –150°C. In areas with volcanic activity, the lava flowing

across the surface can reach 1,250°C.

Io experiences almost constant volcanic activity, making it

the most active volcanic body in the solar system. This

activity and the hot interior drive out any water, and there

is no known liquid water or water ice on Io. 

There is essentially no atmosphere. A thin cloud of sulfur

compounds from Io’s constant volcanic activity surrounds Io.

Sunlight may be a viable energy source. Volcanic activity

has coated Io’s surface with compounds such as sulfur

and sulfur dioxide. On Earth, many microbes use such

compounds as an energy source.

Io is a solid body and the materials for life are likely to be

present. Volcanic activity could cycle chemicals important

to life.

IO JUPITER’S MOON

fast facts

At noon on the equator, the average surface temperature

is –145°C.

Europa is covered with a one- to ten-kilometer-thick crust

of water ice. There is strong evidence that this crust may

cover a 60–100-km deep ocean of water. An ocean of this

size would hold more water than there is on Earth!

There is no atmosphere. 

Sunlight may be a viable energy source. Scientists think

Europa’s core is hot enough to have volcanic activity

beneath its ocean. Such activity might make energy-rich

compounds such as sulfur compounds available. Europa’s

ice crust is also thickly dusted with another potential energy source, sulfur compounds from Io’s eruptions.

Europa is a solid body and the materials for life are likely

to be present. Possible volcanic activity and a large ocean

provide several ways to cycle chemicals important to life.

EUR O P A JUPITER’S MOON fast facts

At noon on the equator, the average surface temperature

is –121°C.

Ganymede’s surface and upper layers are an even mixture

of rock and water ice. There is no known source of heat to

melt the ice.

There is virtually no atmosphere.

Sunlight may be a viable energy source. There are no

known geologic processes to make chemicals available 

to organisms that rely on chemical energy.

Ganymede is a solid body and probably has the necessary

materials for life. However, Ganymede seems to lack any

processes that are necessary to cycle chemicals important to life.

G ANYMEDE JUPITER’S MOON

fast facts

At noon on the equator, the average surface temperature

is –108°C.

Callisto appears to be an ice-rock mix through and

through. Its low density suggests that it contains large

amounts of water ice. Some scientists think there is a

salt-water layer beneath the surface.

There is virtually no atmosphere.

Sunlight may be a viable energy source. If there is a saltwater layer beneath the surface, organisms may be able

to rely on chemical energy.

Callisto is a solid body and probably has the necessary

materials for life. However, Callisto seems to lack any

processes that are necessary to cycle chemicals important to life.

C ALLIS T O JUPITER’S MOON

32 ACTIVIT Y 3 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE
P L U T O

EAR TH’S MOON

TIT A N

A thick, hazy atmosphere

envelops Titan. The closest a

spacecraft has come to Titan is

about 400,000 km.

Because no spacecraft has ever visited Pluto, 

this computer-generated image based on 

telescopic observations is among the most 

accurate depictions we have of Pluto.

MER CUR Y

There are no

processes to 

remove the 

craters.

With no atmosphere,

meteors of all 

sizes hit the 

planet.
fast facts

The average surface temperature is –225°C.

All water is permanently frozen as ice.

There is essentially no atmosphere.

At this distance from the sun, sunlight is too dim to be a

viable energy source. Organisms would need to rely on

chemical energy.

Pluto and Earth have the same general chemical composition, but Pluto lacks any processes that are necessary to

cycle chemicals important to life.

P L U T O

fast facts

There is no atmosphere to moderate temperatures, and

temperature depends entirely on how much sunlight falls

on the surface. While the overall average surface temperature is –23°C, the daytime average is 107°C and the

nighttime average is –153°C. 

There is no known liquid water on the moon. In 1998,

NASA’s Lunar Prospector spacecraft detected water ice

at each of the moon’s poles.

There is no atmosphere. Without an atmosphere, the surface experiences large and rapid temperature swings,

which are hard for organisms to cope with.

The moon receives the same amount of sunlight as Earth,

making the sun a viable energy source. Chemicals made

available by volcanic activity early in the moon’s history

may once have been a possible energy source.

The moon and Earth have the same general chemical

composition, but the moon lacks any processes that are

necessary to cycle chemicals important to life.

f

f

ast

acts EAR TH’S MOON

The average surface temperature is –179°C.

Water-ice icebergs might float in an ocean of ethanemethane liquid or slush. There is virtually no water in the

atmosphere.

Titan has an atmospheric pressure 1.5 times that of

Earth. It is 90–97% nitrogen and 3–10% methane, a

composition more like Earth’s than the carbon dioxide

atmospheres of Mars and Venus. 

At this distance from the sun, sunlight is too dim to be a

viable energy source. Organisms would need to rely on

chemical energy.

Sunlight-driven reactions can turn methane into amino

acids, the building blocks of life. They could join into large,

complex molecules and rain down on the surface. There,

they could accumulate, covering the surface with thick,

gooey deposits of hydrocarbons. These conditions may be

similar to those on early Earth. 

TIT AN S A TURN’S MOON

34 ACTIVIT Y 3

fast facts

There is no surface water or water in the atmosphere.

There is essentially no atmosphere.

Living on or near the surface is impossible, so life would

have to live underground and depend on chemical energy. 

Mercury and Earth have the same general chemical composition, but Mercury lacks the processes that are necessary to cycle chemicals important to life.

MER CUR Y

The temperature on the side facing the sun is 252°C. On

the dark side, it is –183°C.

LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE
ACTIVIT Y 3 35 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Temperature

Water

Atmosphere

Energy

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

What makes a world habitable?

A key of habitability factors

At about 125°C, protein and carbohydrate molecules and genetic material (e.g., DNA

and RNA) start to break down. Cold temperatures cause chemicals in a living cell to

react too slowly to support the reactions necessary for life. Thus, life seems to be limited to a temperature range of about minus 15°C to 115°C.

Life as we know it requires liquid water. It can be available on an irregular basis with

organisms going dormant until it becomes available, but, eventually, it needs to be

available. On a cold planet or moon, there must be internal heat to melt ice or permafrost. On a hot planet or moon, the water will boil away or evaporate unless it is far

beneath the surface.

Atmospheres can insulate a planet or moon and protect life from harmful ultraviolet

radiation and small- and medium-sized meteorite impacts. In addition, atmospheres can

serve as an important source of biochemicals. For example, nitrogen from nitrogen gas

can be used for proteins, and carbon from carbon dioxide and methane can be used

for carbohydrates and fats. Atmospheres also moderate day-night and seasonal temperature swings. However, to serve as an effective shield or insulator, the atmosphere has

to be fairly substantial, as it is on Earth, Venus, and Titan. A planet or moon depends

on its gravity to hold an atmosphere. A small-sized body such as Pluto or Earth’s moon

has too little gravity to hold onto an atmosphere, making life on or near the surface

difficult.

Organisms use either light or chemical energy to run their life processes. At some point,

light energy from the sun becomes too dim to be a viable energy source. On Earth,

many microbes obtain energy from the sulfur, iron, and manganese compounds present

in the Earth’s crust and surface layers. When they absorb such compounds and break

them down, they obtain a small amount of energy from this chemical change. This energy is sufficient to power microbial life.

Nutrients

The solid planets and moons in our solar system have the same general chemical 

composition. As a result, the necessary raw materials to construct and maintain an

organism’s body are in place. However, a planet or moon needs to have processes such

as plate tectonics or volcanic activity to make these chemicals constantly available. In

addition, liquid water is a powerful solvent and is an important vehicle for transporting

and delivering dissolved chemicals. Therefore, planets or moons with volcanic activity,

plate tectonics, or a way to cycle liquid water have a way to supply the chemicals

required by living organisms.
36 ACTIVIT Y 3 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

Planet/Moon Rationale

MERCURY

VENUS

EARTH

Earth’s

MOON

MARS

JUPITER

Jupiter’s 

Moon IO

Jupiter’s

Moon EUROPA

Jupiter’s Moon

GANYMEDE

Jupiter’s Moon

CALLISTO

SATURN

Saturn’s Moon

TITAN

URANUS

NEPTUNE

PLUTO

LIFE ISLIKEL

Y

LIFE ISPOSSIBLELIFE ISUNLIKEL

Y

What makes a world habitable?

Searching for a habitable world
ACTIVIT Y 4 37

Our solar system has nine planets and

over 60 moons. Of all these worlds,

only Earth is known to have life.

Consequently, we must base any

search for extraterrestrial life on

what we know about life here.

Thanks to advancing technology, the past

decade has seen the discovery of organisms

living under conditions we consider extreme and

uninhabitable. These bacteria and bacteria-like

organisms are called extremophiles. They are

prokaryotes, a type of very small, single-celled

organism that lacks a nucleus. The fact that no

one expected to find organisms living under

extreme conditions underscores how much we still

have to learn about life!

Collecting acid-loving

extremophiles in

Carlsbad Caverns,

New Mexico. COURTESY OF 

VAL HILDRETH-WALKER

What can life tolerate?

4 ACTIVITY

To show that organisms living under

extreme conditions

on Earth can serve

as analogs for extraterrestrial life

Activity 4 explores

some of the reasons

to think that extraterrestrial life is possible. In doing so, it

draws on concepts

developed earlier in

the module such as

understanding what

life is, what it

requires, and what

makes a planet habitable.

2 to 4 class periods

TIME

CONTEXT

PURPOSE

Collecting heat-loving

extremophiles from a

hot spring in Yellowstone National Park.

COURTESY OF MICHAEL

MILSTEIN, BILLINGS

GAZETTE

Two views of 

heat-loving bacteria. COURTESY OF DR. 

HENRY ALDRICH,

UNIVERSITY OF

FLORIDA

Cold-loving bacteria COURTESY OF DR. HENRY

ALDRICH, UNIVERSITY OF

FLORIDA

Extremophiles adapted 

to cave conditions 

(e.g., dark, acidic, and

low levels of oxygen

and nutrients) COURTESY OF SPILDE,

NORTHUP, AND BOSTON

Collecting

cold-loving extremophiles by coring into the 

Antarctic ice sheet. 

COURTESY OF THE NATIONAL 

SCIENCE FOUNDATION
These tubeworms are about one meter

long. They lack a mouth or gut but are

able to absorb chemical-rich seawater

to feed the bacteria living 

in their bodies. The bacteria break down

the chemicals, providing 

energy for themselves and for 

their tubeworm hosts.

© WOODS HOLE OCEANOGRAPHIC

INSTITUTION

38 ACTIVIT Y 4

Along mid-oceanic ridges,

hydrothermal vents spew

clouds of chemicals and

minerals into the seawater.

Bacteria can metabolize

these chemicals, forming

the base of an entire

ecosystem that functions

in the absence of sunlight.

COURTESY OF P. RONA

OAR/NATIONAL UNDERSEA

RESEARCH PROGRAM

Extremophiles live at the limits of what life’s chemistry is able to tolerate. If organisms on Earth can thrive under such conditions, then

one might reasonably expect that similar conditions on other worlds

might support life, as well. In fact, as we explore our solar system,

we find mounting evidence for extraterrestrial conditions that may

support extremophiles.

The logic at work, both in the search for extraterrestrial life and in

this activity, is that Earth’s extremophiles can serve as models for

life elsewhere. It is plausible to think that we may find evidence of life

in any place that mirrors Earth’s life-sustaining environments.

In this activity, students play a card game that models the logic driving much of the search for extraterrestrial life. In the game, students start with an extreme habitat on Earth, find an extremophile

that might live under those conditions, and finally identify a similar

extraterrestrial habitat. By grouping these three elements, students realize that promising extraterrestrial habitats, and

maybe extraterrestrial life itself, do indeed exist.

Additionally, they realize that if extraterrestrial life is

found in our solar system, it will most likely resemble

one of Earth’s extremophilic microbes.

We are in the infancy of being able to detect extraterrestrial life. But daily, we learn more about the limits of

life and about extraterrestrial environments and are moving our

search forward.

• We must base any search

for extraterrestrial life on

what we know about life

on Earth.

• Extremophiles live at the

limits of what life’s chemistry is able to tolerate.

• Any place that mirrors

Earth’s life-sustaining environments may harbor life.

• As we explore the solar

system, we find evidence

for conditions that may

support life.

• If extraterrestrial life is

found in our solar system,

it will most likely be microbial and inhabit environments considered extreme

on Earth.

• Extract key information

from a reading

• Contribute thoughtfully to

discussions

• Draw conclusions and

Make inferences when creating sets

• Understand extremophiles

as analogs for extraterrestrial life

• Conceive of a plausible

mission

• Debate the ethics of sending Earth life to another

world

• one Activity Guide (pp.

41–42) for each student

• one set of Life on the Edge

cards (pp. 46–48) per four

students (laminate or copy

onto cardstock, if possible)

• poster materials

M ATERIALS

SKILLS

CO NCEPTS

These bacteria

extract energy

from inorganic

chemical sources.

They live inside

tubeworms by

the millions.

COURTESY OF DR.

COLLEEN CAVANAUGH,

HARVARD UNIVERSITY
ACTIVIT Y 4 39

Recommended Procedure

Read the Activity Guide (pages 41 to 42) so you are familiar with the

information that the students have available.

Have students read the information about extremophiles in the

Activity Guide (page 41).

In groups, have students discuss and answer the four questions that

follow the reading. Conduct a brief class discussion to make sure students understand:

• why extremophiles are either bacteria or bacteria-like organisms.

• that extremophiles require the extreme conditions to be ongoing and

continuous. With extremophiles, we are talking about long-term conditions, not short-term, one-time exposures.

• the range of conditions that life can tolerate.

Teach students the Life on the Edge card game. You can do this by

having students read the directions on the Activity Guide (page 45)

themselves, read and discuss the directions as a class, or demonstrate the game for the class. Make sure they understand:

• what constitutes a set.

• that there are duplicates of the organism cards. A table showing how

many of each card are available is printed on the Life on the Edge

rules sheet.

• that the three kinds of cards are linked by a particular environmental

condition. 

Have students play two or three rounds of the Life on the Edge card

game.

The table below shows the make up of the 48 cards in the deck. This

table is reproduced on the Life on the Edge rule sheet.

Emphasize that the

habitats described on

the Possible Extraterrestrial Habitat

cards are speculations

based on current data.

Confirming the existence of most of the

features described on

these cards requires

further exploration. 

To date, no extraterrestrial life has been found

in the solar system.

Linking specific Earth

organisms to extraterrestrial habitats is an

exercise in imagining

what type of extraterrestrial life might be

found based on adaptations that are successful in similar Earth environments.

1STEP

2STEP

3STEP

4STEP

5STEP

Environmental Number of Number of Number of Possible

Condition Organism Earth Habitat Extraterrestrial 

Cards Cards Habitat Cards

Hot 4 4 4 

Cold 4 4 4 

Acid 2 2 2 

Salt 3 3 3 

Radiation 3 3 3 
40 ACTIVIT Y 4

Base a class discussion on Think About It Questions 1 to 7. Students

can answer these questions on their own, in groups, as homework,

or as a class.

Challenge groups to select a “crew” of extremophiles to send to

Mars, Europa, or one of the other top candidates they identified in

Activity 3. To have the organisms survive once they reach their destination, groups must select a specific habitat on their target planet or

moon and describe a plausible way to deliver the extremophiles to

that habitat. Have groups create and then present a poster explaining their mission.

Discuss the ethical issues involved with sending Earth life to another

planet or moon and have students debate different positions. 

Some would say that sending Earth life to another world contaminates it and seriously compromises learning about how life arises,

evolves, and persists on other worlds. They also might say that Earth

life might out-compete extraterrestrial life forms and drive them into

extinction before we understand them or even know they are there.

Merely sending a spacecraft to another world might be enough to

damage a sensitive environment with rocket exhaust, physical

impact, and Earth materials. Even though NASA takes great pains to

sterilize its spacecraft, it is still possible for microbes to “hitchhike” a

ride to another world.

Others say that if humans are ever to inhabit other worlds, we need

to send microbes now to provide raw materials and change the environment in ways that will support serious colonization. This is called

terraforming. For example, microbes could help create an atmosphere, add oxygen to the environment, detoxify harmful compounds,

extract useful materials from the planet’s or moon’s crust, and establish a food supply.

7STEP

6STEP

8STEP

The Life on the Edge

game follows the rules

for Rummy. Students

can also use the cards

to play any card game

in which the cards are

grouped to make sets.

Concentration and Go

Fish are examples of

this kind of game.
ACTIVIT Y 4 41 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Our solar system has nine planets and over 60 moons. Of all these worlds, only

Earth is known to have life. Consequently, we must base any search for extraterrestrial life on what we know about life here. Over the past ten years, we have

discovered organisms living in places once considered extreme and uninhabitable. These bacteria and bacteria-like organisms are called extremophiles (philia

is Greek for “love”). No one expected to find organisms living under such extreme

conditions, which shows how much we still have to learn about life! 

If organisms on Earth can thrive under extreme conditions, then couldn’t organisms live under similar conditions on other worlds? As we explore the worlds in

our solar system, we find evidence for conditions that may support extremophiles. As a result, Earth’s extremophiles can serve as models for life elsewhere. 

Let’s meet some extremophiles. Extremophiles not only tolerate extreme conditions (extreme by human standards, anyway), but they require them! If you put

most of them in the kinds of conditions we like, they would die.

Hottest 113°C Pyrolobus fumarii (Vulcano Island, Italy).

Earth’s average surface temperature is 15°C

Coldest -15°C Crypotendoliths (Antarctica)

Deepest 3.2 km These bacteria live in the spaces between rock grains in the

underground under- Earth’s crust and are exposed to high levels of pressure, heat, 

ground and radiation.

Most acidic pH 0.0 These bacteria grow in caves. The acid-base scale is called the

pH scale where 0 is the most acidic and 14 is the most basic. 

Most life lives within a pH range of 5 to 8.

Most basic pH 11 Alkaliphilic bacteria are found in areas where large bodies of

water have evaporated and left behind layers of alkaline

(i.e., basic) minerals.

Highest 5 million Deinococcus radiodurans is a common soil organism. A dose

radiation dose rads of 1000 rads will kill a person. Less than 1 rad per year is 

normal, and zero rads is ideal.

Longest period 6 years Bacillus subtilis living in a NASA satellite that exposed test 

in space organisms to the extreme conditions of outer space.

Highest 1200 times This was a bacillus living at the bottom of the Marianas Trench, 

pressure atmospheric the deepest point beneath Earth’s oceans. Typically, atmospheric 

pressure pressure at sea level is 1013 millibar (14.7 pounds per sq. inch.)

Saltiest 30% salt Halophilic bacteria live in water with a 30% salt content. By 

comparison, seawater and human blood are about 3.5% salt.

Fresh water has very little salt.

What can life tolerate?

Activity Guide

Introduction

Meet the

Champions

This list is posted on 

NASA’s Astrobiology

Institute web site:

astrobiology.arc.nasa.gov

/overview.html

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________
42 ACTIVIT Y 4 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Why are all these extremophiles bacteria or bacteria-like organisms,

rather than being more like cockroaches, plants, or us? Mostly because

extremophiles are very simple organisms compared with multicellular

life. With fewer parts and fewer internal processes to coordinate, less

can go wrong. When something does go wrong, it is easier to repair the

damage and keep the organism living.

Some people will only be happy when we find a Hollywood-style alien.

However, astrobiologists would be ecstatic if we found evidence of even a

simple microbe anywhere beyond Earth. That would suggest that life

occurs whenever the conditions are right. Also, once microbes inhabit a

world, it opens up possibilities for the development of more advanced life.

The first life on Earth was microbial and look at us now!

Before starting today’s activity, answer the following questions.

1 What is an extremophile?

2 Why are extremophiles either bacteria or bacteria-like?

3 Could humans be considered extremophiles? Explain.

4 Complete this table with information on the conditions that life

can tolerate.

Environmental Maximum Level Minimum Level Typical Level

Condition Tolerated by Life Tolerated by Life for Humans

Temperature

Acid-Base Levels

Salt Levels

Radiation Levels 

What can life tolerate?

Activity Guide (continued)
ACTIVIT Y 4 43 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

1 How can life and conditions on Earth be used as a model for life on other worlds?

2 If you were able to send a test tube of one kind of extremophile to Mars, which

extremophile would you choose? Why? 

3 If you were able to send a test tube of one kind of extremophile to Europa, which

extremophile would you choose? Why? 

4 If you could genetically engineer a new extremophile so that it had the traits of two

different kinds of extremophiles, which two traits would you merge if your

extremophile were to live on Mars? Europa? Explain.

(continues)

What can life tolerate?

Think About It

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________
44 ACTIVIT Y 4 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

5 Describe the kind of extraterrestrial life that we are most likely to find in our solar

system. Why do you think it is the most likely kind?

6 What will finding evidence of microbial life on another world teach us about life in

general?

7 Draw two pictures: one of the kind of extraterrestrial life we might reasonably expect

to find in our solar system and the other of what you think most people think is out

there. If they are different, explain why you think they are different.

What can life tolerate?

Think About It (continued)
ACTIVIT Y 4 45 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Today you will play a card 

game that shows how life and 

conditions on Earth can be used 

as a model for life on other worlds.

The rules are similar to Rummy,

and the game is best played 

with two to four students. 

4 If the player has a valid set, he or she must lay

it down, read the three cards aloud, and explain

why it is reasonable to think that an organism

might exist on this other world. Other players

can challenge the set if they disagree.

5 The player discards a card and play continues in

a clockwise rotation.

6 The next player may take the discarded card or

select the top card of the draw deck.

7 Play continues until the first person goes out. If

all the cards from the draw deck have been

drawn, shuffle the discarded cards and use them

as a new draw deck.

1 Deal five cards to each player and place the remainder face down between the players. This will be the draw deck. Turn one card face up next to the draw deck.

2 The first player takes either the upturned card or the top card of the draw deck.

3 The player sees if he or she has a set (one card from each category). To be a valid set,

the organism must be able to live in the Earth and possible extraterrestrial habitats.

What can life tolerate?

Rules for Life on the Edge Card Game

Organism Card

describes a 

specific type of

extremophile

Earth Habitat Card

describes a specific

habitat on Earth

where this extremophile might be

found

Possible Extraterrestrial

Habitat Card

describes an 

extraterrestrial

habitat with 

similar characteristics to 

the Earth habitat that could

be home to extremophiles 

or extremophile-like life

LIFE

ON THE EDGE

The Makeup of the 

48 Cards in the Deck

Condition

Hot 4 4 4 

Cold 4 4 4 

Acid 2 2 2 

Salt 3 3 3 

Radiation 3 3 3
Organism Card 

Cold-Loving Bacteria

Little Known Fact:

Scientists have found

Cryotendoliths living at 

minus 15°C. Earth’s average

surface temperature is

15°C.

Organism Card 

Cold-Loving Bacteria

Little Known Fact:

Scientists have found

Cryotendoliths living at 

minus 15°C. Earth’s average

surface temperature is

15°C.

Organism Card 

Cold-Loving Bacteria

Little Known Fact:

Scientists have found

Cryotendoliths living at 

minus 15°C. Earth’s average

surface temperature is

15°C.

Organism Card 

Cold-Loving Bacteria

Little Known Fact:

Scientists have found

Cryotendoliths living at 

minus 15°C. Earth’s average

surface temperature is

15°C.

Organism Card

Heat-Loving Bacteria

Little Known Fact: 

Scientists have found

Pyrolobus fumarii living in

113°C water.

Organism Card

Heat-Loving Bacteria

Little Known Fact: 

Scientists have found

Pyrolobus fumarii living in

113°C water.

Organism Card

Heat-Loving Bacteria

Little Known Fact: 

Scientists have found

Pyrolobus fumarii living in

113°C water.

Organism Card

Heat-Loving Bacteria

Little Known Fact: 

Scientists have found

Pyrolobus fumarii living in

113°C water.

Organism Card

Radiation-Tolerant Bacteria

Little Known Fact: Scientists have

found Deinococcus radiodurans

living after being exposed to radiation levels of five million rads. It

can tolerate high levels of both

ultraviolet radiation and radioactive decay. The lethal dose for

humans is 1000 rads. 

Organism Card

Radiation-Tolerant Bacteria

Little Known Fact: Scientists have

found Deinococcus radiodurans

living after being exposed to radiation levels of five million rads. It

can tolerate high levels of both

ultraviolet radiation and radioactive decay. The lethal dose for

humans is 1000 rads. 

Organism Card

Radiation-Tolerant Bacteria

Little Known Fact: Scientists have

found Deinococcus radiodurans

living after being exposed to radiation levels of five million rads. It

can tolerate high levels of both

ultraviolet radiation and radioactive decay. The lethal dose for

humans is 1000 rads. 

Organism Card 

Acid-loving Bacteria

Little Known Fact: 

Scientists have found 

bacteria growing on the

walls of caves living at 0.0

pH. Most organisms live

within a pH range of 5 to 8.

Organism Card 

Salt-Loving Bacteria

Little Known Fact: 

Scientists have found

halophilic bacteria living 

in water that is 30% salt. 

By comparison seawater

and human blood are about

3.5% salt.

Organism Card 

Salt-Loving Bacteria

Little Known Fact: 

Scientists have found

halophilic bacteria living 

in water that is 30% salt. 

By comparison seawater

and human blood are about

3.5% salt.

Organism Card 

Salt-Loving Bacteria

Little Known Fact: 

Scientists have found

halophilic bacteria living 

in water that is 30% salt. 

By comparison seawater

and human blood are about

3.5% salt.

Organism Card 

Acid-loving Bacteria

Little Known Fact: 

Scientists have found 

bacteria growing on the

walls of caves living at 0.0

pH. Most organisms live

within a pH range of 5 to 8.

46 ACTIVIT Y 4
Earth Habitat Card

Salt domes and brine are

often found in association

with petroleum deposits.

Earth Habitat Card

The Arctic ice cap is made

of water ice.

Earth Habitat Card

Water ice over two kilometers thick covers Antarctica.

The coldest temperature on

Earth, minus 89°C, was

recorded in Antarctica. 

Earth Habitat Card

Greenland is covered with a

two-kilometer-thick sheet of

water ice.

Earth Habitat Card

The evaporation of large

bodies of salt water has covered large areas of land with

thick layers of salt. 

Earth Habitat Card

Volcanic vents occur all

along the 17,000 miles of

Earth’s mid-oceanic ridges.

The water injected into the

ocean environment is

extremely hot.

Earth Habitat Card

Hot springs occur when

groundwater is heated and

rises to the surface. 

Earth Habitat Card

Processes in the Earth’s

crust produce extremely hot

groundwater.

Earth Habitat Card

Contact between volcanic

magma and underground

water produce pockets of

hot water.

Earth Habitat Card

The Arctic tundra has a

layer of permafrost beneath

it. Permafrost is soil locked

in water ice.

Earth Habitat Card

Natural deposits of uranium

can produce areas with high

levels of radiation.

Earth Habitat Card

When our atmosphere’s

ozone layer gets thin, Earth’s

surface can receive dangerous levels of harmful ultraviolet radiation.

Earth Habitat Card

Acidic groundwater is found

beneath much of the Earth’s

crust.

Earth Habitat Card

Acidic groundwater dissolves

certain kinds of rocks, forming caves and producing an

acidic environment for life. 

Earth Habitat Card

Salt occurs in Earth’s ocean

water. The salt concentration can rise dramatically as

water evaporates from

enclosed bodies of sea

water such as tide pools and

enclosed bays.

Earth Habitat Card

Radiation in the Earth’s

crust comes from the decay

of radioactive elements such

as uranium.

ACTIVIT Y 4 47
Possible Extraterrestrial

Habitat 

Just beneath Europa’s 

surface, there may be 

large pockets of salty brine.

Possible Extraterrestrial

Habitat 

During its first two to three

billion years, Mars had

water and volcanic activity.

This combination would likely

produce hot springs and

underground pockets of hot

water.

Possible Extraterrestrial

Habitat 

Europa’s ocean is probably

very salty.

Possible Extraterrestrial

Habitat 

The decay of radioactive elements such as uranium in

the Martian crust would create high levels of radiation.

Possible Extraterrestrial

Habitat 

Mars may have a layer of

water beneath its surface.

On Earth, such groundwater

is often acidic.

Possible Extraterrestrial

Habitat 

Acidic groundwater dissolves certain kinds of rocks,

forming caves. Mars may

have these kinds of rocks,

resulting in an acidic environment for life.

Possible Extraterrestrial

Habitat 

Ultraviolet radiation and

charged particles from the

sun bombard the surface of

Mars, which is completely

unprotected from these

kinds of harmful radiation.

Possible Extraterrestrial

Habitat 

Salt layers form when large

bodies of salty water evaporate. Mars may have had

large bodies of water that

have since evaporated, possibly leaving layers of salt.

Possible Extraterrestrial

Habitat 

Ultraviolet radiation bombards the surface of Europa,

which is completely unprotected from this kind of

harmful radiation. 

Possible Extraterrestrial

Habitat 

Processes in the Martian

crust may heat water below

the surface, producing pockets of hot groundwater.

Possible Extraterrestrial

Habitat 

The Northern Polar ice cap

on Mars is made of water

ice.

Possible Extraterrestrial

Habitat 

The core of the Southern

Polar ice cap on Mars

seems to be made of 

water ice.

Possible Extraterrestrial

Habitat 

Most of the Martian surface

has a layer of permafrost

beneath it. Permafrost is soil

locked in water ice.

Possible Extraterrestrial

Habitat 

Europa is completely covered by a one- to ten-kilometer-thick shell of water ice.

Possible Extraterrestrial

Habitat 

Evidence suggests that

Europa may have considerable volcanic activity beneath

its ocean. This volcanic activity would provide Europa’s

ocean with large amounts of

hot water.

Possible Extraterrestrial

Habitat 

The Martian surface has

deposits of a kind of iron

oxide called hematite.

Hematite is often associated

with organisms living in hot

springs.

48 ACTIVIT Y 4
Is there life on other worlds? People have pondered this question since ancient times. But now, for the first time in human

history, advances in the biological sciences, space exploration,

and space technology may finally make it possible to answer it. 

At first, “Is there life on other worlds?” seems a simple question to answer. However, it quickly becomes a complex web of

issues. What is life, anyway? How does it begin and evolve?

What conditions can life tolerate? What makes a planet or

moon habitable? How do we look for and identify extraterrestrial life? To attempt to resolve these kinds of questions, astrobiologists draw on many branches of science and employ many

research strategies such as fieldwork, laboratory research,

telescopic observation, and exploration with spacecraft.

Though we have not found any examples of extraterrestrial life,

comparisons with certain kinds of life on Earth suggest that

potential habitats for extraterrestrial life, and maybe life itself,

do indeed exist. However, contrary to popular notions, if extraterrestrial life is found in our solar system, it will most likely be

bacteria-like. 

In 1961, the astronomer Dr. Frank Drake suggested an organized framework for thinking about life in the galaxy. Known as

the Drake Equation, it provides a way to estimate the number

of worlds within our Milky Way galaxy that have intelligent life

and whose radio transmissions should be detectable. Drake

identified a sequence of eight terms to help people think about

what must occur before a world can be inhabited by a civilization with radio technology. This activity uses Dr. Drake’s framework to have students consider the implications of each term

and make their own estimates of life in the Milky Way galaxy. 

Is there life on other worlds?

To estimate the

number of worlds in

the Milky Way

galaxy that have life

By asking students

to consider the 

possibility of life —

simple to technological — in our galaxy,

Activity 5 builds on

the previous investigations of life and

habitability and gets

students to think

about the size and

composition of the

galaxy. 

1 to 2 class periods

TIME

CONTEXT

PURPOSE

5 ACTIVITY

ACTIVIT Y 5 49
= X X X

The number of worlds within

our Galaxy that have intelligent life whose radio emissions should be detectable

Number of stars

in the Milky Way

galaxy Percent of these

stars that are

appropriate

Percent of appropriate stars that

have planetary

systems

To get a sense of what your students think

when they hear the term “extraterrestrial,”

ask them:

Have the class define the term, extraterrestrial. Try to get them to broaden the meaning to include anything from microbes to

plants to intelligent creatures to any living

thing.

Mention that astrobiology involves thinking

about whether or not there is extraterrestrial life, where it might be, and how we can

learn more about it.

To have students identify the factors related

to the existence of extraterrestrial life, ask

them what information they would need to

determine the probability of extraterrestrial

life. You might begin by saying, “Let’s see if

we can estimate how many worlds out

there have life (or intelligent life with which

we can communicate). What would we

need to know?” Try to keep your prompting

Students typically think

of “extraterrestrials” in

terms of science fiction. Prompting them

to consider other

forms of life often

changes their answers,

or at least the reasons

they provide. 

In 1961, Dr. Frank

Drake developed an

equation to estimate

the number of other

civilizations that exist in

our Milky Way galaxy

that we can detect

using radio technology.

It does this by making

estimates of eight component factors. Since

we are unsure whether

there is life out there,

let alone intelligent life

with which we can

communicate, there is

no correct solution to

the equation — the

value of each factor is

open to interpretation. 

The Drake Equation

Recommended Procedure

2

3

STEP

STEP

4

STEP

1

STEP

Are there 

such things as 

extraterrestrials? 

What do 

you mean by 

extraterrestrials?

What is the chance 

that we are the only

life in the universe?

Do you think that 

there are any forms 

of life elsewhere in

the universe? Why?

Would you be 

interested in a planet

inhabited by microbes,

plants, and insects? Why

or why not?

• The chances for life 

elsewhere in the galaxy

could be either high 

or low, depending on

whether one’s assumptions and estimates are

conservative or optimistic.

• Finding even simple life

forms will be a breakthrough discovery.

• The large number of 

stars significantly 

increases the possibility

of extraterrestrial life.

• Understand that our

galaxy contains a large

number of stars 

• Understand and 

Estimate the terms of the

Drake Equation

• Calculate how many civilizations with radio technology we can 

communicate with

• one Activity Guide 

(pages 53 and 54) 

for each student

• blank overhead transparencies

• transparency markers

• calculators

M ATERIALS

SKILLS

CO NCEPTS

50 ACTIVIT Y 5
X X X X X

Average number

of habitable 

planets within 

a system

Percent of habitable planets that

develops life

Percent of 

planets with life

that develops

intelligent life

Percent of intelligent life that

develops radio

technology

Percent of radio

technologies that

are “current”

5STEP

6STEP

7STEP

8STEP

Students can succeed

in the next part of the

activity after naming

five or six factors in

this brainstorm.

Don’t worry about creating a usable equation.

The goal is to have students think about each

factor. Students should

be able to identify factors such as the number of stars, planets,

habitable planets, planets with life, planets

with intelligent life, and

planets with radio technology.

Explore an online 

version of the 

Drake Equation at

astrobio.terc.edu.

Since the computer

recalculates whenever

you change a value,

this online version may

be the best way for

students to manipulate

the terms easily and

obtain different results

quickly.

to a minimum. It is more important to have students think about the

terms than it is to get a complete set. Record their terms on the

board or overhead projector.

With minimal prompting, students can determine many of the factors

needed to estimate the number of worlds with intelligent, technology-using civilizations. Note that one can begin this list from any term

and work out from there to the rest of the terms. Some prompts you

might find helpful include:

• What does _____ depend on? 

• What might the next step be? 

• Are you saying we’ll find life on any _____ ?

Introduce the Drake Equation as one scientist’s effort to identify the

factors in the same way students did in Step 4. Have students complete the worksheet and estimate values for each term. Make sure

they understand that there are no right answers. The Drake Equation

simply helps us think about the factors involved in determining the

probability of communicating with civilizations that have radio technology.

After making estimates for each term, have students multiply the

eight terms and determine their estimate of the number of other civilizations that exist in our Milky Way galaxy that we can detect using

radio technology (Question 1 on the Activity Guide). The percentages

should be treated as decimals. (For example, 30% = 0.30; 5% = 0.05;

0.01% = 0.0001).

Have students report their estimates and discuss the range and their

implications. The Drake Equation can provide radically different

answers from 1 (we will never hear from intelligent extraterrestrials)

to billions (we will almost undoubtedly hear from them). 

Use the Think About It questions to guide a class discussion on the

implications of the numbers that the students estimated. There is a

Teacher Answer Guide to the Think About It questions on pages 57

and 58.

ACTIVIT Y 5 51
52 ACTIVIT Y 5

Calculating a number using the Drake Equation is an exercise in

estimation. When estimating, we provide definite numbers to

terms for which we have no actual values. However, we use whatever pertinent information is available to make an estimate as accurate as possible. An estimate is different from a sheer guess

because, with an estimate, one constrains the possibilities by

extrapolating from what one does know. 

Probabilities are different than estimates. Probabilities focus on

how often an event will occur out of a certain number of trials 

(e.g., one chance in a hundred or in a million). As a result, they are

expressed as percentages. Estimates are expressed as numbers.

Since the Drake Equation results in a number rather than in a percentage, it is an estimate. 

Enrico Fermi, a Nobel Prize-winning physicist, often challenged his

students with similar kinds of problems, such as estimating the

number of corn flakes in the United States. Known as Fermi

Problems, they rely on the same process — extrapolating from what

you know. Astrobiologists use the Earth in this fashion. By studying

our planet and its inhabitants, scientists have learned a great deal

about life and habitability. Using what we know enables us to make

predictions about life in distant places. 

Ask students how they would estimate the number of pens and pencils in the school. To give their estimate some basis, they might

count the actual number of pens and pencils in the classroom. This

number together with the number of classrooms in the school can

provide a rough estimate of the total number of pens and pencils.

As students think more deeply about the question, they will realize

that there are offices, teacher’s desks, and supply rooms that have

pens and pencils, too. Including these in their estimate makes it

increasingly accurate. 

One could expand this exercise by estimating the number of pens

and pencils nationwide.

Math Extension

Is there life on other worlds?

Recommended Procedure

1

STEP

2

STEP

3

STEP
ACTIVIT Y 5 53 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Do you think there is intelligent life in our galaxy with which we can

communicate? In 1961, Dr. Frank Drake identified eight terms to

help people think about what would have to take place for such

communication to be possible.

See what you think the chances are by making your own estimate

for each of the terms below. The conservative and optimistic values indicate the range of opinion among scientists with regard to

each term. You can use the conservative or optimistic estimates

or use another value, depending on your own intuition.

To estimate the number of worlds in the Milky Way

galaxy that have intelligent life that we can detect

using radio technology, follow these steps:

1 Make estimates for each of the eight terms listed

on the Using the Drake Equation worksheet.

2 Convert percentages to decimals before multiplying.

3 Multiply the estimates you made for the eight terms.

Is there life on other worlds?

Activity Guide

Introduction

What to Do
54 ACTIVIT Y 5 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Is there life on other worlds?

Using the Drake Equation

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 The total number

of stars in the Milky

Way galaxy

These numbers are based on observations of the stars

in our galaxy, the Milky Way galaxy, and of other galaxies

we believe to be like our own. Most scientists believe the

number of stars to be 400 billion.

100 billion 600 billion

TERM BACKG ROUND CONSERVATIVE

E STIMAT E

OPTIMISTIC

E STIMAT E

YOUR

E STIMAT E

2 The percent of

stars that are

appropriate

Many scientists believe that a star has to be like our

sun, which is a Main Sequence, G-type star. Only about

5% of the stars in our galaxy are G-type stars, though

about 10% are the closely related F- and K-type stars.

About 50% of stars exist in binary or multiple systems,

which many scientists feel make them inappropriate.

5% 15–45%

3 The percent of

these appropriate

stars that have 

planetary systems

Appropriate stars may not have planets circling them.

We have only just begun detecting extra-solar planets, 

so we don’t really know how common they are. 

5% 50–100%

4 The average number of habitable

planets or moons

within a solar 

system

Our only example of this term is our own solar system.

Could Earth be the only habitable place in our solar 

system? Is our system typical? Remember that if one

system has no habitable planets or moons and another

has four, the average would be two per system.

0.1

On average, there

is one habitable

planet in every

ten systems

4

On average,

there are four

habitable planets

in every system

5 The percent 

of habitable 

planets or moons

that develop life

Having a planet or moon that is appropriate for life 

doesn’t necessarily mean that life will arise. No real 

data are available to help us estimate this term. Earth is

the only planet on which we know there is life. However,

bacterial life existed on Earth shortly (geologically speaking) after its formation, possibly indicating that the development of life is easy. Many scientists believe that

whether or not life arises depends on many factors.

0.000001%

Life is a rare

accident that is

unlikely to happen elsewhere

100%

Life will arise if

conditions are

appropriate

6 The percent of 

planets with life

that develop intelligent life

On Earth, humans developed intelligence, apparently 

as an evolutionary advantage. However, this term

depends on how you define intelligence. Are dolphins,

gorillas, octopus, and ants intelligent? Furthermore, 

single-celled life existed on Earth very early, and multicellular life took 2.5 billion years to form (a very long

time, geologically speaking). Maybe the development 

of complex life, let alone intelligent life, is unusual.

0.0001%

or less 

Only one in a million planets with

life will develop

intelligent life

100%

Any planet with

life will develop

intelligent life

7 The percent of 

intelligent life that

develops radio

technology

Communication with intelligent extraterrestrials requires

that we hear from them. Given the vast distances of

space, they would probably send signals which travel 

at the speed of light, such as radio waves. On Earth,

humans have only just developed radio technology, so

possibly this term should have a low value. But, we did

eventually develop radio technology, so maybe this is 

true of all intelligent beings.

0.0001%

or less 

Only one in a million planets with

intelligent civilizations will develop

radio technology

100%

All intelligent life

will develop radio

technology

8 The percent 

of “current” 

civilizations 

having radio 

technologies

Will an extraterrestrial’s signals overlap with the lifespan

of the receiving civilization? Extraterrestrials that sent

signals a million years ago from a world a million light

years away would still overlap with us, even if they died

out long ago. So, how long do civilizations with radio

technology last? A high level of technological development could bring with it conditions that ultimately threaten

the species. Or maybe, once a society has radio technology, it may survive for a long time. Finally, radio signals

may give way to more advanced, less noisy technologies

such as optical fiber. No one would hear us then!

0.0001%

or less 

One in a million

civilizations with

radio technology

will develop it in

time to detect

signals from

another civilization

10%

One in a ten 

civilizations with

radio technology

will develop it in

time to detect

signals from

another civilization
ACTIVIT Y 5 55 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Is there life on other worlds?

Think About It

Name ________________________________________________________________________________________________________________________________________________________________________________

Date ______________________________________________________________________________________________________________________________________________________________________________________

1 To find out your estimate of the number of worlds in the Milky Way galaxy that

have intelligent life that we can detect using radio technology, fill out the Drake

Equation worksheet and multiply the eight terms together. Write your answer here:

2 Based on your estimates, how good are our chances of hearing from intelligent

extraterrestrials?

3 How does your answer to Question 2 compare to what you thought before you

began the activity?

4 Can your answer to Question 1 be less than one? Why or why not?

5 When making estimates, in which terms did you have the most confidence? The

least? Why?

6 Are you more optimistic or conservative when it comes to thinking about extraterrestrial life with radio technology in the Milky Way galaxy? Why?

7 How could you adjust the estimates in the equation to have it come out so that

Earth is the only planet in the Milky Way galaxy with radio technology?

(continues)
56 ACTIVIT Y 5 LIFE ON EARTH...AND ELSEWHERE?

NASA’S ASTROBIOLOGY INSTITUTE

Is there life on other worlds?

Think About It (continued)

8 If tomorrow’s newspaper headline read, “Message Received from Outer Space” what

would it mean to you?

9 What would your reaction be if we discovered microbes on another planet? Plants?

Insects? Mammals? Intelligent life?

10 If microbial life were discovered on another planet, what implications might such a

discovery have? 

11 How would you define extraterrestrial now? How does your current definition differ

from the one that the class developed earlier in the activity?

12 What do you think is the most abundant life form on Earth?

13 If life exists elsewhere, what do you think it will look like?
ACTIVIT Y 5 57

Is there life on other worlds?

Teacher Answer Guide to the Think About It Questions

These background notes provide answers to some of the questions on the Student 

Activity Guide and can be used to help guide a class discussion based on those questions.

Questions 2, 3, 7, 8, 9, and 11 ask students to reflect on their own feelings, perceptions, or

idea, and therefore have no background notes.

1 Based on the eight terms, what is your estimate?

While Terms 5 to 8 are percentages, the final number that students

obtain is a discrete number rather than a percentage or a probability.

This number represents a student’s estimate of the number of civilizations with radio technology that we can detect.

4 Can your answer to Question 1 be less than one?

While there is no right answer, there are wrong answers. Because

Earth exists, the final answer cannot be zero and probably should 

not be less than one. If students choose low probabilities for Terms 

5 to 8, they may need more than the 400 billion stars in the Milky

Way galaxy to obtain a final answer of one. This would require considering additional galaxies to account for the existence of Earth.

5 In which terms did you have the most or least confidence?

Estimating numbers for the eight terms becomes increasingly a matter of conjecture as one goes from Term 1 to Term 8. There is widespread agreement only for the first two terms.

6 Are you more optimistic or conservative?

An answer of one states that Earth is the only place with intelligent

life that has radio technology. There may still be life or even intelligent

extraterrestrials out there, but we cannot communicate with them

because they do not have radio technology. Any number larger than

one implies that we may receive signals from intelligent extraterrestrials someday. However, make the distinction between detection and

communication, which is a two-way exchange. With a small final number, actual communication is less likely. With a large final number,

actual communication becomes increasingly likely.

(continues)
58 ACTIVIT Y 5

10 What are the implications of discovering microbial life?

Many students express disinterest in discovering anything less than a

bona fide, Hollywood-style extraterrestrial. However, no life beyond

Earth has ever been found, which implies that life may be a rare accident that happened on Earth due to an extraordinary convergence of

circumstances and that it is unlikely to happen elsewhere. In this context, discovering microbial life would help us understand more about

how life arises and what conditions it can tolerate. Furthermore, it

would help answer the question of whether life is a common process

in the universe. In short, discovering microbial life beyond Earth would

be a profound discovery. By multiplying Terms 1 to 5, students can

make their own estimate of how many worlds in our galaxy have life of

any sort.

12 What is the most abundant life form on Earth?

We live in the age of bacteria. For the past three and a half billion

years, bacteria have been the dominant life form in terms of numbers

and biomass. They are key to many biological, geological, and chemical

processes, and many scientists think that multicellular organisms

became possible only after single-celled bacteria began living symbiotically within a cell membrane.

13 If life exists elsewhere, what do you think it will look like?

Astrobiologists feel that most extraterrestrial life will be bacteria-like,

living beneath a planet’s or moon’s surface and using chemical energy

for their needs. Animal life, and especially intelligent animal life, is

probably much rarer.

Is there life on other wDRAFT orlds?

Teacher Answer Guide to the Think About It Questions (continued)
Science Standards

Unifying Concepts and Processes 1 2 3 4 5

Systems, Order, and Organization • • • • Evidence, Models, and Explanation • • • • • Constancy, Change, and Measurement • • • Evolution and Equilibrium • • • Form and Function • •

Science as Inquiry

Abilities Necessary to do Scientific Inquiry • • • • • Understandings about Scientific Inquiry • • • • •

Physical Science

Structure & Properties of Matter, Objects, & Materials • • • Chemical Reactions • • Interactions of Energy and Matter •

Life Science

Biological Evolution • Matter, Energy, & Organization in Living Systems • • • Behavior of Organisms •

Earth and Space Science

Energy in the Earth System • • • Origin and Evolution of Planetary Systems • • Planetary Characteristics • Organization of the Solar System •

Science and Technology

Abilities of Technological Design • • • Understanding Science and Technology • • • •

Science in Personal and

Social Perspectives

Natural Resources • • • Risks and Benefits •

History and Nature of Science

Science as a Human Endeavor • •

Mathematics

Standards 1 2 3 4 5

Problem Solving • • •

Communication • • • • •

Reasoning • •

Connections • • •

Computation and Estimation • • •

Patterns and Functions •

Probability •

Measurement • •

Science Process 

Skills

Observing • •

Measuring •

Communicating • • • • •

Collecting Data • •

Inferring • • • •

Predicting • • • • •

Making Models • •

Hypothesizing • • • •

Interpreting Data • • •

Controlling Variables • •

Defining Operationally • •

Investigating • •

Extrapolating • •

Synthesizing • • • • •

National Standards What is life? What does life need to live? What makes a world habitable?

What can life tolerate?Is there life on other worlds?

What is life?What does life need to live?What makes a world habitable?What can life tolerate?Is there life on other worlds?

STANDARDS 59
“The search for life-sustaining worlds

could be a new unifying goal for

space research”

Daniel Goldin, NASA Administrator

Astrobiology is an interdisciplinary field. It provides a relevant, meaningful context for

students to explore key concepts in biology, chemistry, physics, mathematics, and Earth

and space science. This Educator Resource Guide provides opportunities for students in

grades 5–10 to master fundamental science concepts and develop inquiry skills. 

Visit nai.arc.nasa.gov for further information

on NASA’s Astrobiology Institute.

Visit astrobiology.arc.nasa.gov for further

information on the science of astrobiology.

Visit astrobio.terc.edu for more information

on TERC’s yearlong astrobiology curriculum.

This curriculum guide was sponsored by the 

NASA Astrobiology Institute (NAI), with funds 

from the NAI Lead Teams at NASA Ames Research

Center and NASA Jet Propulsion Laboratory, and

TERC’s Astrobiology Curriculum Project.

Version 1.2 Fall 2000

The Guide’s hands-on activities introduce 

students to core ideas in astrobiology by 

examining five key questions:

What is life?

What does life require?

Which planets and moons might 

be habitable?

How do Earth’s extremophiles support

the idea of extraterrestrial life?

What are the possibilities for life 

elsewhere in our solar system?

The Science and Mathematics Standards met 

by the activities are listed inside this cover.