Astronomy Lecture Number 10

Mercury is so dense that
its origin is hard to explain. One solution is to assume a massive
impact tore most of its crust away and left mostly the core. We
believe that the Earth had a similar history, which accounts for the low-density
Moon.
Mercury has scarps, or fault
lines, that may indicate that Mercury's crust has shrunk as it cooled.
Mercury has a huge impact
basin, the Caloris Basin--a monster impact.
One can learn something about
the surface composition by looked at spectral colors.
Venus
Venus appears completely cloud
covered. Swirling wind patterns can be seen in the clouds, as shown
in the photos from Galileo, below:
Images from Galileo 1 day apart,
except the bottom two images were taken 2 hours apart.
Note that the clouds are lagging behind at the equator,
and the patterns move right to left in the figure.
The temperate zones have the equivalent of Jet streams.
The clouds trap allow some visible
light through, but trap infra-red light due to the presences of greenhouse
gases in the atmosphere. This makes the surface
temperature a whopping 700 K! Russia landed several landers
on the surface, as shown in the accompanying images. The temperature
and pressure conditions were so high that the landers survived only a few
hours, despite their diving bell construction.
Venera 13 Lander images
Venera 14 Lander images
Images
of Venus' surface features were obtained with the Magellan spacecraft, using
a radar mapper to see through the clouds.
Craters
are rare, shallow and ejecta stay close to crater due to dense atmosphere.
Surface is "new" -- only 1 billion years old.
We hardly need to show you pictures
of the Earth, but in a moment we will contrast Earth with the other terrestrial
planets. Features of Earth are that it has water(!), clouds in the atmosphere,
very few craters, many volcanoes due to plate tectonics.
Mars
Mars has been visited
by more spacecraft than any other planet (other than Earth). The
latest orbiter is Mars
Global Surveyor. Here are some
images from that spacecraft. Mars has polar caps, a (thin) atmosphere,
clouds, snow, water (very little above the surface) -- so it has many characteristics
like Earth. Three new spacecraft (all landers) arrived in Dec. 2003 and Jan. 2004. The ESA Beagle has not been heard from. However, the NASA Spirit and Opportunity rovers are working fine, and looking for signs of water-related geology.
One of the important considerations
for a planet, especially for whether it can support life, is its surface temperature.
On Earth, we are fortunate to have temperatures that allow the existence of
liquid water. It is likely that liquid water is essential to the development
of life. We can estimate how hot a planet should be just from its distance
from the Sun. The light and heat from the Sun decreases with distance
according to an inverse square law (1/r2
just like gravity). The temperature depends on two things--how close the
planet is to the Sun, and how much energy it absorbs (how dark or light it is).
Mercury is so close to the Sun (about 0.4 AU), that its average temperature
should be about 440 K. In fact, it rotates slowly (its day is 176 Earth-days
long--exactly 2 Mercury years!), so the side facing the Sun gets up to 700 K
while the night side, facing away from the Sun, gets down to a chilly 100 K
(-170 C)! This is an example of tidal locking, which is strong for Mercury
due to its closeness to the Sun. Venus, at 0.7 AU, should be cooler, only about
230 K. As we mentioned, though, the clouds of Venus, and the greenhouse
effect, causes its temperature to be much higher--740 K. Earth and the
Moon are in the "habitable zone" where liquid water can exist. However,
the Moon has no water because its gravity is too small to retain an atmosphere.
Finally, Mars, at 1.5 AU, is predicted to have an average temperature of about
218 K, close to the observed average of about 223 K. This is below the
freezing point of water (273 K), but Mars does show evidence of once having
had water. It is becoming more accepted that water still exists in the
form of ice below the surface.
From the figure
below, you can see why Mercury has the highest density--its metallic core
is very large compared to its size. Earth and Venus are about the
same, while Mars, which has the lowest density, also has the smallest relative
core size. The Moon has little or no metallic core.
We know that Earth has a
molten core of metal, and as the Earth spins there are motions set up in
this molten core that generates a magnetic field. It is this magnetic
field that makes compasses work to show the direction of North. Venus
also probably has a molten core, but because it spins very slowly (once
every 243 Earth days) it does not generate a magnetic field. Mercury,
Mars, and the Moon have solid cores, so they also do not have much of a
magnetic field.
We know that the Earth has
a molten core by studying earthquakes. There are two kinds of sound
waves set up in an earthquake--compressional or pressure (p) waves, and
translational or secondary (s) waves. It turns out that liquids cannot
transmit s waves, and we see that s waves do not travel through the Earth's
core, so we know there is liquid. How did the Earth get a liquid
core? Probably all of the planets started out in an undifferentiated
state, with rock and metal all mixed up together. But at some point
the temperature of each of the planets got high enough to start to become
liquid, just from the bombardment of planetesimals during the formation
of the planets. Once that happened, the dense metals could sink to
the core while the lighter crust floated to the outer surface. We
say that the planets are now differentiated.
Mercury, Mars, and the Moon are so small that they cooled off and have
become solid all the way through. Earth, and probably Venus, still
have liquid interiors and so have active volcanoes.
Plate Tectonics and the Moon
Because of the molten interior,
it is possible for the crust of the Earth to move due to motions in the interior--updrafts
of heat and energy that occur in the ocean basins. Venus
and Mars do not appear to have plate tectonics, although both show evidence
of trying. There are shield volcanoes that grow very large, because the
crust over "hot spots" stay in one place. There are huge rift zones where
the crust has been pulled apart, and split, but it does not cause the crust
to move--plate tectonics just cannot get started. Why is the Earth different
in this way? The answer may lie in the origin of the Moon.
The Moon appears to be made entirely
of the crustal material of the same sort as Earth's surface. It is strange
that it does not have metals in its core. It is thought that late in
the formation of the Earth, a giant impactor (planetesimal) struck the Earth.
Such a collision would have destroyed the impactor, the metals of the impactor
would sink to the center of the Earth, and much of the outer crust of the
Earth could have been torn off to later come together to form the Moon.
This may explain the unusually thin crust of the Earth's oceans. Without
the impact, the Earth might not have plate tectonics.
When Earth first formed 4.5 billion
years ago, it had a primary atmosphere made of light gases such as hydrogen
and helium, methane, ammonia, and water vapor. Very early in its lifetime,
after only a few hundred million years, nearly all of this atmosphere would
have been lost, either by escape into space or by combining with the rocky
crust. Sometime later, outgassing from the interior (due to volcanic activity)
added a new atmosphere, possibly supplemented by bombardment of comets, which
would have brought lots of water. Initially, the atmosphere would have been
mostly carbon dioxide, sulfer dioxide, and nitrogen.
The appearance of life (which initially
was anaerobic and did not use oxygen) actually changed the atmosphere by splitting
carbon dioxide into carbon and free oxygen. Over time, the oxygen levels built
up to the level we have today. At first, life would have remained in the oceans,
because without oxygen (and ozone) the UV radiation at the surface would have
been too harmful to life. After sufficient oxygen appeared in the atmosphere,
life could venture out onto land, around 500 million years ago.
Venus
Venus would have had a very similar
early history as Earth, with an early primary atmosphere, followed by a secondary
one of mostly carbon dioxide and sulfer dioxide. Unfortunately for Venus,
it did not develop oceans and life, so the carbon dioxide (greenhouse gas)
never left the atmosphere. As a result, Venus' temperature continued to rise
until the atmosphere itself got so hot that it can radiate away the energy
it receives from the Sun. If we were to place Earth at the distance of Venus,
it would likely also develop a runaway greenhouse
effect. The temperature would nearly double, and the oceans would
begin to evaporate. Water vapor is also a greenhouse gas, and the increased
greenhouse effect would make Earth hotter still, causing more evaporation,
etc. The process would run away, and give
Earth a thick atmosphere with a similar 700 degree temperature.
Mars
Mars appears to once have had a relatively
thick atmosphere, and a greenhouse effect that was sufficient to warm the
planet, despite its distance from the Sun, to an average temperature near
0 C (32 F). This apparently allowed liquid water to exist, perhaps in the
form of an ocean, and it must have rained occasionally to form the river beds
that we see today. But by about 3.5 billion years ago, the carbon dioxide
was lost in the form of rocks, possibly by dissolving in the oceans. This
reduced the greenhouse effect, cooling the planet and ultimately the water
was also lost by combining with rocks (which is why Mars appears red--it is
rusty!). Some water may remain in the form of ice, as we said.