1

National Aeronautics and Space Administration

www.nasa.gov

Produced by the NASA 

Astrobiology Program to 

commemorate 50 years of 

Exobiology and Astrobiology 

at NASA.

#4

Issue

2nd EDITION
Astrobiology

A History of Exobiology and Astrobiology at NASA

This is the story of life in the Universe—or at least the story as we know it so far. As 

scientists, we strive to understand the environment in which we live and how life relates to this environment. As astrobiologists, we study an environment that includes 

not just the Earth, but the entire Universe.

The year 2010 marked 50 years of Exobiology and Astrobiology research at the 

National Aeronautics and Space Administration (NASA). To celebrate, the Astrobiology Program commissioned this graphic history. It tells the story of some of the 

most important people and events that have shaped the science of Exobiology and 

Astrobiology. At only 50 years old, this field is relatively young. However, as you will 

see, the questions that astrobiologists are trying to answer are as old as humankind.

Concept & Story

Mary Voytek

Linda Billings

Aaron L. Gronstal

Artwork

Aaron L. Gronstal

Script

Aaron L. Gronstal

Editor

Linda Billings

Layout

Jenny Mottar

Special thanks to Leslie Mullen and Daniella Scalice

Copyright 2017, NASA Astrobiology Program. 

First Edition printed in 2013
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Issue #4

Missions to the Outer Solar System

The year 2010 marked the 50th anniversary of NASA’s Exobiology Program, established in 1960 and expanded into a broader Astrobiology Program in the 1990s. To 

commemorate the past half century of research, we are telling the story of how this 

field developed and how the search for life elsewhere became a key component 

of NASA’s science strategy for exploring space. This issue is the fourth in what 

we intend to be a series of graphic history books. Though not comprehensive, the 

series has been conceived to highlight key moments and key people in the field as 

it explains how Astrobiology came to be.

-Linda Billings, Editor
Earth’s orbit sits at a spot 

that is ‘just right’ for life, and 

has helped astronomers define 

what they call the ‘habitable’ or 

‘Goldilocks’ zone around stars.

But farther away, beyond the 

main asteroid belt, lies a cold, 

dark region where the Sun’s 

warming light grows dim.

Adrift in the darkness are 

gas giant planets, icy moons, 

and frigid dwarf worlds.

In this region of space, there 

are planets so big that their 

atmospheric storms are larger 

than the entire Earth...

Scientists once thought that

this region of space was no

place for life as we know it.

It took some truly spectacular 

journeys to show us just how 

important the outer Solar 

System is for astrobiology.

...and their gravitational fields

could crush human and robotic

explorers in an instant.

2

Issue 4—Missions to the 

Outer Solar System.
3

The giant planets of the outer

Solar System have fascinated

humans for a long time. (1)

They are very far away, but Jupiter 

and Saturn are so giant that they 

shine brightly in the night sky.

When Galileo Galilei spotted Jupiter’s 

moons,* it occurred to him that the 

Universe might not be 

centered on the Earth. (2)

To truly understand the outer Solar System,

astrobiologists needed close-up views that

only space missions could provide.

By the time the space age 

dawned, scientists like Harold

Urey* began to wonder what 

planetary moons were made of.

In 1965, NASA scientist Gary 

Flandro realized that the 

planets were in a rare position 

relative to one another—an 

event that only occurred 

every 175 years! (5,6)

NASA was ready 

for a game of interplanetary pinball!

*see Issue #1

Hmm...

“...the Sun remains 

fixed in the centre of 

the circle of heavenly 

bodies, without changing 

its place...” (3)

“The interiors 

of objects of similar mass 

[to the Earth’s moon]... must 

have risen above the melting 

point of ice in their 

interiors...”

“Hence the

water of the Jovian

moons must all be at or

near their surfaces!”

What if we 

used Jupiter’s 

powerful gravity 

as a slingshot?

I have 

and idea.

“In fact, water

flows, instead of

terrestrial lava flows, 

may occur from time

to time.” (4)

A spacecraft 

could use the energy to 

visit all the giant planets 

in one ‘Grand Tour!’
4

It took a lot of hard work

both scientifically and

politically to get the first

missions off the ground...

It took seven months for Pioneer 10 to make the 

435 million-kilometer trek through the asteroid 

belt, proving the journey was possible. (8)

Pioneer was a ‘flyby’ mission, meaning both

spacecraft flew past the giant planets and

their moons rather than orbiting them.

Pioneer 10 visited Jupiter and then flew out

toward the edge of the Solar System.

With knowledge gained from Pioneer,

NASA was ready for the next step.

Technology from the Mariner*

Program was adapted... and on

came the Voyagers! (10)

Pioneer 11 flew past Jupiter and Saturn 

before it, too left for the stars—but 

more on that in later pages.

...but in 1972, the Pioneer 10

spacecraft launched. It was

followed by Pioneer 11.

9

Charles F. Hall, Pioneer

Project Manager

*see Issue #3

Colleen Hartman, Deputy Director 

for Science, Operations and Program 

Performance at NASA Goddard.

“We are really only twelve 

generations away from Galileo 

and his first crude look at [Jupiter]. 

Twelve generations later we are 

actually there, measuring many of 

the characteristics of the 

planet itself.” (2)

“After [Pioneer 10] 

passed Mars on its long 

journey into deep space, it was

venturing into places where

nothing built by humanity had 

ever gone before.” (7)
5

Like Pioneer, the twin Voyager 

spacecraft were designed for 

planetary flybys. But they were 

even more powerful than their 

predecessors.

Voyager 1 arrived at 

Jupiter on March 5, 1979. 

Voyager 2 followed later 

that year on July 9.

Voyager 1 visited Jupiter 

and Saturn, while Voyager 2 

collected data from Jupiter, 

Saturn, Uranus and Neptune!

As a planet, Jupiter was obviously 

no place for life as we know it...

...but watching phenomena like

clouds on the gas giant has helped

scientists better understand

physical processes here on Earth.

Thanks to the relative positions of the

planets, the flight time to Neptune was 

reduced from thirty years to just

twelve! (see page 3).

But for astrobiologists, the real

prizes of Pioneer and Voyager

observations were Jupiter’s moons!

Studying Jupiter also provided clues 

about the origin and evolution of giant 

planets around distant stars.

11
6

The Pioneers took mass and

density measurements of Io,

Callisto, Ganymede and Europa.

Astrobiologists were shocked that 

Jupiter’s large moons were more 

than balls of frozen ice and rock!

When the Voyagers

made even more detailed observations, 

astrobiologists were 

stunned.

Jupiter’s moons were active, diverse worlds 

where dynamic geological processes could 

be seen from a spacecraft.

Callisto’s surface was very old, and covered with

craters. Many of the craters were barely visible, 

and could only be seen because they were a 

different color than the surrounding surface.

Ganymede was actually bigger 

than the planet Mercury!

Ganymede also had two distinct types of

terrain—cratered and grooved. These

features might indicate that its crust

was once rocked by tectonic activity.

Scientists wondered if some of the

crater features had been erased by

geological activity on Callisto’s icy crust.

It was the first time 

active volcanoes had been 

spotted beyond Earth.

There was even more serious stuff

happening at Io, where Voyager 2

discovered stunning volcanic activity!

Finding such a hot and active world so far from

the Sun was a shock. After studying the data sent

back by Voyager, scientists determined that the

heat Io experiences is caused by ‘tidal forces.’

We now know that Io is the 

most volcanically active world 

in the entire Solar System.

12, 13
7

Io’s orbit is affected by the nearby moons 

of Europa and Ganymede. Their gravity 

fields tug on Io whenever they come close.

After the moons move apart, 

Jupiter’s massive gravity 

pulls Io back into line.

This gravitational ‘tug-of-war’ causes

‘tidal bulging’ of up to 100 meters

(330 feet) on Io’s surface. (14)

The ‘squishing’ and ‘releasing’ 

is what heats up the moon.

Tidal heating of Io raised questions 

about whether or not the same process could provide a source of heat 

and energy on other moons that lie 

far from the warmth of the Sun.

While photos of Io showed exciting

geology in action, images from Europa

were more puzzling. Europa was relatively 

smooth, but it had intersecting lines 

stretching all over its surface. (15)

In low resolution images from

Voyager 1, scientists thought

the lines could be evidence of

tectonic processes.

However, high-resolution photos from 

Voyager 2 left scientists scratching 

their heads. The grooves of Europa’s 

lines were not as deep, and the ridges 

not as high, as they had expected.

Europa was a mystery that called for 

more than just a passing visit... and that 

would come with Jupiter’s first orbital 

mission—the Galileo spacecraft.
8

Political pressures almost stopped

Galileo before it began. (8)

...and taking on these two major programs 

was a huge strain on the budget. In 1981, 

Galileo was close to being cancelled.

But the planetary science community made its voice 

heard, and, in the end, both missions were approved!

NASA had also approved the powerful 

Hubble Space Telescope...

Galileo started work early with two Earth flybys.

It spotted a huge impact basin on the far side of

the Moon and signs of ancient lunar volcanism.

Galileo left Earth in 

October 1989 aboard 

the newly minted 

Space Shuttle.*

The spacecraft performed 

flybys of Venus, Earth and 

the asteroids Gaspra and Ida 

(with its tiny moon Dactyl).*

Almost six years after

launch, Galileo arrived

at Juptier and released

an atmospheric probe.

The probe 

cut through 

Jupiter’s 

turbulent 

atmosphere...

...gathering data about the planet’s 

composition and weather.

After 58 minutes, 

it was vaporized!

Dick Malow, former staff

director of the House

HUD–Independent Agencies

Appropriations Subcommittee

*see Issue #3

“Rarely have 

I ever seen such a 

successful lobbying 

campaign... It was 

masterfully 

done.” (8)
9

Galileo circled through the Jovian system and

collected much more data than the Pioneers

or Voyagers did on their passing visits.

For instance, Galileo measured Io’s

temperature and found volcanoes

hotter than those on Earth!

But Galileo’s real prize for astrobiologists 

was Europa. Images from the Voyagers 

were nothing compared to the spectacular 

world Galileo revealed.

Europa had few craters on 

its surface. This means that 

the ice is constantly changing, 

wiping away the evidence of 

impacts after they happen.

Although frozen solid, the surface of Europa 

had large ‘ice rafts’ the size of cities that 

appeared to have broken off and floated 

apart. Galileo also showed that Europa had a 

thin oxygen atmosphere and ionosphere. (18)

Astrobiologists traced the web of intersecting lines and 

fissures that covered Europa. They noted the nuances in 

their colors and the ways in which they reflected light.

Were the cracks fresh ice that was 

pushed to the surface from below?

In July of 1994, 

Galileo witnessed 

something spectacular 

(and unexpected)...

Jim Erickson,

former Galileo

project manager.

17

The comet

Shoemaker-Levy

crashing into

Jupiter!

“Galileo was positioned

to see the actual impact on

the backside and provide

images and data concerning

exactly the magnitude of

those impacts.” (16)
10

Careful examination of the patterns and

shapes led scientists to theorize that

the icy surface was just a shell resting

above a liquid layer deeper down. (19)

Then, Galileo’s magnetometer 

spotted strange directional 

changes in Europa’s magnetic 

field...

But any ocean on Callisto would have to 

be extremely deep, because there are 

large craters all over Callisto’s surface.

Galileo did discover a magnetic field at Ganymede.

...raising questions about what lay beneath

the large moon’s solid surface. (22)

Similarly, Galileo found a 

magnetic field at Callisto...

Margaret Kivelson, 

principal investigator for 

Galileo’s magnetometer.

“We have good reason 

to believe the surface 

layers of Europa are made 

up of water that is either 

frozen or liquid.”

“The direction that 

a magnetic compass on Europa 

would point to flips around in a 

way that’s best explained by the 

presence of a layer of electrically 

conducting liquid, such as 

saltwater, beneath the ice.”

“But ice is not a good

conductor, and therefore we

infer that the conductor may

be a liquid ocean.” (20)

“It will be

interesting to see

whether this same type of

phenomenon occurs at

Jupiter’s moon

Ganymede.” (21)
11

While the Voyagers were busy putting Europa 

on the astrobiological map, scientists made a 

fantastic discovery back on Earth.

Researchers Tjeerd van Andel

and Jack Corliss spent six hours

exploring the seafloor at 9,000

feet below the Pacific near the

Galapagos Islands. (23)

They returned to the

surface with samples that

stank like rotten eggs.

These findings raised many 

questions that changed our 

view of habitability and the 

origin and evolution of life.

The smell of hydrogen sulfide led scientists to discover that the depths of 

Earth’s oceans support more life than 

anyone expected—and some organisms 

thrived without light from the Sun.

We learned of new energy

sources, like undersea

volcanoes, that supported

entire communities of life.*

Scientists began to explore 

environments on Earth that 

could be similar to a Europan 

ocean—from arctic islands to 

subsurface Antarctic lakes.

Engineers began developing 

instruments that could send 

future missions through the 

ice of Europa to explore its 

hidden depths.

*More on this in later issues
12

Decades after Galileo’s launch, astrobiologists 

are still studying data from the mission and 

making new discoveries about Europa.

For instance, Earth scientists 

compared bumpy features on Europa 

(called chaos terrains) with similar 

features at Earth’s poles. (24)

Based on what we know from 

Earth, these chaos terrains may 

indicate that lakes of water exist 

within the ice of Europa’s shell.

Britney Schmidt, 

Georgia Tech

Don Blankenship, 

University of 

Texas at Austin.

“One opinion in the

scientific community has been if

the ice shell is thick, that’s bad for

biology. That might mean the surface

isn’t communicating with the

underlying ocean.”

“Now, we see evidence 

that it’s a thick ice shell that can 

mix vigorously and new evidence for 

giant shallow lakes. That could make 

Europa and its ocean more 

habitable.” (25)

“This new understanding 

of processes on Europa would 

not have been possible without the 

foundation of the last 20 years of 

observations over Earth’s ice 

sheets and floating ice

shelves.” (25)
13

If material from Europa’s surface is mixed through the ice and 

into the subsurface ocean, as the presence of chaos terrains 

suggests, Europa could be a much more suitaible place for life. (24)

With the discovery that Europa might

support habitable environments for life,

NASA decided it was too dangerous to

leave the Galileo spacecraft stranded in

Jupiter orbit when the mission ended.

To ensure Galileo would not accidentally

crash into Europa and contaminate the

moon, the spacecraft was deliberately

destroyed by plunging it into Jupiter’s

crushing atmosphere in 2003. (18)

Galileo’s fiery end completed

humankind’s explorations at

Jupiter for a time.

But there were more surprises

waiting at humankind’s next stop

in the Solar System... Saturn.
14

Pioneer 11 and the two Voyager

spacecraft made passing visits

of the splendid, ringed giant.

These flybys provided data

about the planet, its rings and

many of its bizarre moons.

Tiny Enceladus showed

evidence of tectonic

activity. It was covered

in faults and valleys.

And then there

was Titan...

Although bigger than 

Mercury, Titan wasn’t as big 

as astronomers expected.

It turned out that the moon’s 

thick atmosphere made it look 

bigger than it actually was.

Pioneer and Voyager couldn’t

see through the Titanic haze,

but they still sent back data

that stunned astrobiolgists.

Scientists began to develop theories about 

how organic molecules like ethane could be 

formed in Titan’s complex atmosphere.

Mimas had a crater so huge

that scientists thought the

impact must have almost

broken the moon apart!

26

27

28

29

30

31
15

Some scientists even thought that

these molecules could form lakes or

oceans on the moon’s hidden surface.

Chemical activity on Titan 

might help explain what was 

happening in Earth’s ancient 

atmosphere when life first 

arose on our planet.

As with Jupiter’s moon Europa, 

astrobiologists had to wait for an 

orbital mission at Saturn in order 

to get a good look at Titan.

That chance came with the

international Cassini-Huygens 

mission. Cassini launched in 1997 

and arrived at Saturn in 2004.*

Cassini’s first order of business 

was to release the European 

Space Agency’s (ESA) Huygens 

probe toward the mysterious, 

gas-shrouded moon, Titan.

The mission was named for two 

17th century scientists that made 

important observations of Saturn 

in the early days of astronomy.**

*see Issue #3 for some 

important observations 

Cassini made en route. 

** see Issue #1

32
As Huygens warmed 

the soil, it detected 

bursts of methane 

gas. (38)

Nobody knew 

what Huygens 

would see on 

its dramatic 

descent!

There were unambiguous

signs of features carved

by flowing liquid. (33)

As the probe transmitted data

from its six instruments, people

on Earth held their breath.

When Huygens became the first

explorer to land on a world of the

outer Solar System, it touched down

on a soft, but solid surface. (36)

Titan is so cold that 

the landing site had 

chunks of water ice 

instead of rocks. (37)

In fact, Huygens images 

gave us a birds-eye view 

of what appeared to be a 

dark, hydrocarbon lake.

Huygens captured spectacular images, showing 

that Titan had some remarkably Earth-like 

features, such as lakes and drainage canals.

Scientists had no idea what would 

happen when Huygens reached the 

surface. Some theories suggested it 

34 would splash into an ocean of methane!

35

39

Carolyn Porco, Cassini

Imaging Team Lead.

“I am so shocked!”

“I really didn’t

expect the images

to be so easily

interpretable.”

“I thought 

we’d see patterns...

and I thought they’d

still be mysterious

to us.”

“But the images that 

we’ve seen... one of them 

is clearly a drainage 

pattern.” (40)

16
17

Every time Cassini passes Titan on its journey around the 

Saturn system, new details about the moon are revealed.

Instead of water, Titan’s lakes and 

seas are filled with a liquid mixture 

of ‘hydrocarbons’—carbon-based 

molecules like ethane and methane!

On Earth, methane is usually found as 

a gas—but it’s so cold on Titan that 

methane rains down as a liquid.

...but the moon could also 

have other habitats for life.

By watching how Titan is squeezed by the gravity of 

Saturn, Cassini may have found evidence that Titan 

harbors a subsurface ocean of liquid water. (42)

Astrobiologists are still studying what 

liquid hydrocarbons on Titan could mean 

for life’s prospects on the surface...

Cassini has spotted features 

like lakes, dunes, mountains 

and dramatic storms that make 

Titan seem almost ‘Earth-like.’

As with Venus, Titan’s environment can be

used to study our own planet, Earth, through

the science of comparative planetology.*

Sami Asmar, Cassini team 

member, NASA JPL

Luciano Iess, Cassini team

member, University of Rome *see Issue 3

41

“The tides on Titan

pulled up by Saturn aren’t 

huge compared to the pull 

the biggest planet, 

Jupiter, has on some of

its moons.”

“But, short of being

able to drill on Titan’s

surface, the gravity

measurements provide the 

best data we have of Titan’s 

internal structure.” (43)

“Cassini’s

detection of large

tides on Titan leads to

the almost inescapable

conclusion that there is 

a hidden ocean at

depth.” (43)
18

Cassini has returned spectacular 

data from other locations as 

well. Saturn’s rings have never 

been seen in such detail.

And dozens of Saturn’s 

bizarre and numerous 

moons had their moment 

in front of Cassini’s lens.

Tiny Enceladus captured the attention 

of astrobiologistswhen Cassini witnessed 

icy plumes erupting from the moon.

It was definitive 

evidence that Enceladus 

was geologically active.

Cassini has continued to study Enceladus’ 

plumes and their effects as they spray 

material all over the Saturn system.

Cassini found evidence that the 

plumes contain saltwater and organic 

chemicals. Could Enceladus also be 

hiding a subsurface ocean? And could 

that ocean be a habitat for life? (45)

Carolyn Porco, Cassini

Imaging Team Lead.

“For planetary

explorers like us, there is

little that can compare to the

sighting of activity on another

solar system body. This has

been a heart-stopper, and

surely one of our most 

thrilling results.” (44)
19

The Cassini mission continued to yield 

incredible discoveries about Saturn, and 

the astrobiological potential of its many 

moons, until its completion in 2017.

Like its sister spacecraft, Pioneer 11 is now at 

the distant edge of the Solar System, but it is 

silently headed toward the star Sagittarius.

They continued to collect data for many

years as they blasted out toward the

farthest reaches of the Solar System.

On January 23, 2003, a final, weak signal was recieved 

from Pioneer 10. The silent craft is now flying into 

interstellar space toward the red star Aldebaran. (46)

Voyager 1’s planetary mission was also 

complete with its flyby of Saturn.

It left the ringed planet

behind and blasted outward 

toward interstellar space.

As for the Pioneer

spacecraft, Jupiter and

Saturn were the only

planetary destinations

on their schedule.

47

Robert Kraemer,

NASA HQ

Larry Lasher, Pioneer 

Project Manager

“We can

say that we sent

Pioneer 10 off to

tweak a dragon’s tail,

and it did that and

more. It gave it a

really good yank

and... it survived.”

(2)

“Originally designed 

for a 21- month mission, 

Pioneer 10 lasted more 

than 30 years... I guess 

you could say we got our 

money’s worth.” (7)
20

Voyager 2, however,

had more work to do.

Uranus and Neptune can be seen by 

telescopes on Earth, but scientists 

didn’t know what to expect when 

Voyager 2 arrived.

Uranus itself appeared 

to be fairly inactive, 

with little variation in 

its layers of clouds.

But Voyager 2 discovered 10

new moons and 2 new rings

around the planet. (14)

Uranus may have

looked boring at

first, but it actually

has some pretty

strange features...

Uranus’ five largest

moons showed complex

surfaces that indicated

past geological activity.

...like the fact that

its rotational axis is

tipped on its side.

Telescopes on Earth have revealed clouds and huge 

wind storms, proving that Uranus is more dynamic 

than it first looked in Voyager images. (53)

Some studies even suggest that moons of Uranus, 

like Titania, might have subsurface oceans. (54)

While Voyager 1 rocketed away, Voyager 2

continued to Uranus and Neptune and

completed the origininal ‘Grand Tour’ of the

planets proposed in the 1960s (see page 3).

48

49

50

51

52
21

In the summer of 1989, Voyager 2

made it to Neptune. (14)

Neptune’s largest moon, 

Triton, was the last solid 

object Voyager 2 visited

before heading to the 

Solar System’s edge.

Uranus and Neptune provide important

information about the different types

of planets that exist in the Universe.

Scientists now know that they are

‘ice giants,’ a class of planet that

might be the most common around

stars other than our sun. (54)

Many scientists have called

for new missions to these

mysterious giants.

But with no new missions 

planned, Uranus and 

Neptune are keeping 

thier secrets for now.

Heidi Hammel, planetary 

astronomer, Association of 

Universities for Research 

in Astronomy.

55

“Uranus is a type of a

planet that we know very little

about. Thirty years ago we thought

Uranus and Neptune were just

smaller versions of Jupiter 

and Saturn.” (54)

“We’d like to study 

our local examples of 

this common type of 

planet.” (54)

Mark Hofstadter,

planetary scientist,

NASA JPL

“Well, that’s not what it’s 

like right now. It’s in a completely 

different season. The atmosphere’s 

turning on. There’s dark clouds and 

there’s bright spots and there’s 

all kinds of activity on 

this planet.” (56)

“When Voyager flew 

by Uranus in 1986, it 

was dead. There were 

maybe 10 clouds.”
With Voyager 2, all of the

major planets in the Solar

System have been visited.

Jupiter and Saturn have even 

been orbited by spacecraft.

On July 4, 2016, Jupiter welcomed a 

new, dedicated mission, named Juno.

Juno has given 

us a view of Jupiter 

that we’ve never seen 

before.

It’s the first 

craft to fly from 

pole to pole around 

an outer planet.

Juno is answering 

questions about Jupiter’s 

atmosphere and interior.

That can tell us 

how far from the Sun 

the planet formed.

For instance, how 

much water is in the 

atmosphere?

Candice Hansen, Planetary 

Science Institute (PSI), 

Juno Co- Investigator

We want to know 

how the Solar System 

formed, and Jupiter 

can help tell us.

After all, understanding 

formation is the first step in 

figuring out how our system 

became habitable.

In that sense, Juno 

could provide context 

for astrobiology studies 

in the Solar System 

and beyond. Scott Bolton, Southwest 

Research Institute (SwRI) 

Juno principal investigator.

Fran Bagenal, Univ. 

of Colorado, Juno 

Co-Investigator

Jonathan Lunine, 

Cornell University, 

Juno Co-Investigator

57

22
23

Beyond the major planets, many 

hidden corners of the outer 

Solar System remain unexplored...

...but NASA’s New Horizons 

mission is helping to change that.

In 2006, New Horizons launched 

toward small, distant Pluto, which 

is classified as a ‘dwarf planet.’

Before New Horizons, we 

knew little about Pluto 

and its five small moons.

Pluto does have an atmosphere 

that is escaping into space. (59)

58

Even the highest-resolution images from groundbased and orbital telescopes see these tiny, 

distant objects as little more than points of light.

However, the atmospheric escape 

was slower than scientists had 

expected before the mission. (60)

There are many theories about 

Pluto’s structure and evolution.

Some scientists even think that liquid, 

subsurface oceans could exist on Pluto 

and similar objects. (61, 62, 63, 64)

23
The surface of 

Sputnik Planitia is 

probably no more than 

10 million years old! 

(68)

The plains 

contain some ices 

that are nearly 

pure nitrogen.

There are also 

Methane- and waterrich areas on Pluto.

And reddishbrown areas 

that might be 

tholins! (71)

Pluto is truly 

unlike anywhere 

else in the Solar 

System.

The surface 

variations are a 

real suprise.

New Horizons returned spectacular 

images of the Pluto system. These firstever closeups held many suprises. (65)

The composition of Pluto’s surface 

is frozen, but surprisingly complex.

Crater counts showed that Pluto 

has been geologically active for 

the past 4 billion years!

66

67

A huge ice plain larger than Texas, 

informally dubbed Sputnik Planitia, 

has no craters. This means it is 

geologically young.

70

Alan Stern, SwRI, 

New Horizons Principal Investigator

Leslie Young, 

SwRI, Deputy 

Project Scientist

Cathy Olkin, 

SwRI, Deputy 

Project Scientist

69

24
25

Beautiful images from New Horizons 

showed that Pluto’s surface is covered 

with amazing geological features.

There were mountains, icy plains, 

glaciers, and possible dune fields... 

just to name a few. (65)

Even Pluto’s moons had 

things to teach us. (73) The brightness of Pluto’s small 

satellites was greater than more 

distant objects in the solar system 

called Kuiper Belt Objects (KBOs).

This difference supports the idea 

that Pluto’s largest moons are not 

objects captured from the Kuiper 

Belt by Pluto’s gravity.

Instead, they are likely 

remnants of a giant impact 

that shattered Pluto.

When the dwarf planet reformed from 

the debris, the leftovers became the 

satellites of the Pluto system.

74

75

72
26

After Pluto, New Horizons turned to 

look at the Kuiper Belt, where the craft 

made observations of a number of KBOs.

The Kuiper Belt is thought 

to be a ring of material 

inhabited by ‘ice dwarfs.’ 

These planetary embryos 

never became planets...

...but they can teach us 

about how our solar system 

formed and evolved. (76)

Out beyond the Kuiper Belt is thought 

to be a mysterious region of dust and 

debris known as the Oort Cloud.*

Objects in the Solar System’s outer 

reaches are not just balls of rock and 

ice. They actually appear in a range 

of colors - red, blue and white.

The dark colors are caused by

radiation blasting their surfaces.

...which can occassionally 

come crashing into the 

inner Solar System and 

impact worlds like Earth.

In 2019, New Horizons will reach 

the object 2014 MU69, another 

example of an early building 

block of the Solar System.

Pluto has shown us that 

some KBOs can be active.

For instance, white KBOs might 

have fresh material erupting from 

layers below the surface. (77)

This distant region may 

be a source of comets 

and other objects...

* Possibly 

10 to 10,000 

times as far!

26
27

As robotic explorers traversed the planets

of the outer Solar System, they also pointed

their cameras back toward home.

Voyager 1 was the first to photograph Earth as

a ‘pale blue dot,’ and showed astrobiologists

exactly what an inhabited planet looks like from

a distance of 6.4 billion kilometers (78).

...and now they are about 

to do the same for our 

entire solar system.

The Voyagers gave us an outsider’s perspective of Earth...

More than three decades after launch, 

the Voyagers are still active. Voyager 

1 is now the farthest-traveled object 

ever created by humankind.

In 2013, NASA announced that Voyager 1 had officially 

exited the Solar System and entered interstellar space. (80)

As they travel into the darkness 

beyond the influence of our sun... who 

knows what mysteries the Voyagers 

will uncover next!

Together, the Voyagers are providing information 

about how our solar system evolved into the only 

known system capable of supporting life. This 

information can help astrobiologists spot similar 

systems around distant stars.

“Look again at

that dot. That’s 

here. That’s home. 

That’s us.”

“On it everyone 

you love, everyone you 

know... ...every human being who

ever was... ...the history of our

species lived there-on a mote

of dust suspended in a

sunbeam.” (79)

Carl Sagan
28

Missions to the outer Solar System

have reshaped humankind’s knowledge

of our little corner of the Universe.

Moons of the giant planets are

more active than anyone thought

possible, and have raised questions

about the potential for life beyond

the traditional ‘habitable zone’

near the warmth of the Sun.

As astrobiologists continue to complete 

our view of the Solar System, we can begin 

to build a picture of what makes a system 

habitable... and where other Earth-like 

worlds might exist in the Universe.

By studying the gas giants 

and smaller icy bodies, 

we now understand how 

interconnected the Solar 

System truly is.

After all, that’s

what astrobiology is

about—finding other

worlds that life

could call home.

And the further we travel 

from our own home... the 

more we realize just how 

precious the Earth truly is.

Next issue...

Astrobiology

and the Earth!
29

Astrobiology

A History of Exobiology and Astrobiology at NASA

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