AST 248 HW for Chapter 8
Ryan Richards
Review Questions
Briefly summarize the evidence, both real and imagined, that led to widespread belief in a
martian civilization by the end of the nineteenth century.
During the nineteenth century, it was noted that Mars was similar to the Earth. A day on Mars
was about 24 hours and had polar ice caps. It was also speculated that some thought they saw
surface canals on Mars.
Why isn’t liquid water stable at the martian surface today? What happens to water ice that
melts on Mars?
There is no liquid water on the surface of Mars due to a few reasons. Mars is very cold most of
the time (thin atmosphere). As a result most of the liquid water on the surface would freeze into
ice. Even as the temperature increases, Mars still has very low atmospheric pressure which
would cause the water to evaporate.
How do martian seasons differ from Earth seasons? Describe the major seasonal changes
that occur on Mars?
Let us first note that both Mars and Earth have seasons due to their tilt away from their axis
(Mars has a lower tilt than the Earth). There are two main reasons why the martian seasons are
different for Earth seasons.
1) The Martian year is twice as long as the Earth’s so their seasons are (nearly) twice as
long.
2) Martian seasons are affected by their elliptical orbits. This differs from Earth as the
Earth’s tilt (tilt refers to angle different between the Earth and its rotation axis) is the only
reason for seasons.
Mars is closer to the Sun during its southern hemisphere summer and farther during its
southern hemisphere winter. Planets move quicker when they are closer to the Sun (as we
expect from energy conservation, the book says Kepler’s second law but it’s easier to
understand from the former), Mars has more extreme seasons. It has a shorter and warmer
summer while it has a longer and colder winter than its northern hemisphere.
More specifics are given on page 266.
How do we know that different regions of the martian surface date to different eras in the
past? What have we learned about changes in martian volcanism during the past eras?
We know that different regions of the martian surface date to different eras in the past because
the crater densities are different. Regions with more craters mean that there was less geological
activity to change the surface and remove the craters. From this analysis, we know that volcanic
activity on Mars has died down over the past few billion years.
Summarize the evidence suggesting that Mars must have been warm and wet, possibly
rainfall, in its distant past.
There are orbital images that suggest the existence of terrain features which may be due to
flowing water, such as dried up riverbeds and erosion. Rovers have found mineral traces that
normally form in water.
What evidence suggests that water might still flow at or beneath the martian surface
today? Why do we think that Mars might still have subsurface liquid water today?
The first evidence suggesting that water might still flow is the existence of gullies on the crater
walls. Signs of recent flowing water may suggest water springing up from underground before it
evaporates.
Why do we conclude that Mars must once have been warmer with a thicker atmosphere,
and what gases could have made such an atmosphere possible?
We conclude that Mars must have once been warmer with a thicker atmosphere due to the
volcanic activity. Volcanic activity would have released carbon dioxide thus making a denser
atmosphere.
What is the leading hypothesis concerning how Mars lost its once-thick atmosphere? What
role does Mars’s size play in this hypothesis?
The lading hypothesis supporting how Mars lost its thick atmosphere has to do with its magnetic
field. The book mentions that early on, Mars most likely had molten, convecting metals in its
core. Both convection and rotation should have produced a magnetic field which would have
protected the atmosphere. However, the magnetic field would weaken as Mars cooled down and
convection stopped. As we know cooling down is due to the lack of greenhouse gases.
Mars’s size played a role because it was too small to retain its internal heat. As a result, over
time mars would have eventually lost most of its atmosphere. Volcanic activity would have
ceased as the interior cooled down and the magnetic field strength would lessen allowing solar
winds to strip the gases from the martian atmosphere.
Briefly summarize the Viking experiments and their results. Do the results constitute
evidence of life? Explain.
The Viking experiments took martian soil and analyzed it to see if it contained any microbes.
They exposed the soil to different conditions and looked for a biological response. Initially some
of their experiments seemed to support that there was life, however the possibilities for life were
eventually ruled out. Scientists deduced that this may have been caused by some chemical
reactions instead of biological ones.
More specifics are given on pages 281-282.
What is the potential significance of atmospheric methane to the search for life on Mars?
The significance and the intrigue arise because of the origin of the methane gas. Scientists
believed that there are three reasons why methane may be present in the martian atmosphere.
They eliminated the possibility of comet impacts since these are such rare events. However, the
other two possibilities are still very much possible. These are both volcanism and life of Mars.
Scientists believe that the presence of methane has some biological origin since they have
observed that methane vary regionally across Mars and seasonally. They aren’t able to
completely rule out volcanic activity. Volcanism is still important since the heat necessary for
methane release may be enough to have underground liquid water.
How do we know that ALH84001 really came from Mars, and how have we learned its
history?
Let’s first note that the ALH84001 was a meteorite that was scooped up from Antarctica (Allan
Hills region) in 1984.
We know that it came from Mars because the gas trapped inside of it appeared to be very similar
in both its chemical and isotopic composition to the atmosphere of Mars. We’ve learned about its
history through radiometric dating and its studying its structure and composition.
More specifics can be found on pages 287-288. In particular a very nice timeline is given in
Table 8.2 on page 287.
Test Your Understanding
The first human explorers on Mars discover that the surface is littered with the ruins of an
ancient civilization, including remnants of tall buildings and temples.
This is surprising. There would have been evidence supporting this as there have been extensive
observations made on the Mars’s surfaces.
We discover a string of active volcanoes in the heavily crated southern highlands.
This is somewhat surprising. We know that volcanic activity has decreased over time. Otherwise,
Mars have a thicker atmosphere than it does today.
We find underground pools of water on the slopes of one of the Tharsis volcanoes.
This is believable. Mars has retained some internal heat which makes it possible for liquid water
to be underground. This heat may be volcanic heat.
We discover Mars was subjected to global, heavy rainfall less than 1 billion years ago.
This is surprising. The book states that there was stable rainfall during some periods between 2
and 3 billion years ago.
Photos from future orbiters show that new gullies have formed alongside some of the ones
already seen in crater walls from orbiting spacecraft.
This is believable (see the discussion above).
We find a lake of liquid water filling a small crater close to one of the dry river channels.
This is not believable since surface flowing water would either be evaporated or frozen.
The first fossils discovered on Mars come from the canyon walls of the Valles Marineris.
This is believable. The book mentions that the Valles Marineris may have been formed due to
tectonic activity. It also mentions that the valley network may have been shaped by flowing
water and evidence (spectroscopy) has shown minerals that may have formed in water.
A sample return mission finds fossil evidence not only of martian microbes, but also of
photosynthetic plants that lived on the exposed surfaces of martian rocks.
This is believable if Mars was warm enough and there was an abundance of liquid water.
We discover that the martian polar caps have in the past extended more than twice as far
forward the equator as they do now.
This is believable (see discussion on Mars’s axis tilt).
We find rocks on Mars showing clearly that planet once had a global magnetic field nearly
as strong as Earth’s magnetic field.
This is believable since there was high volcanic activity in early Mars which would be sufficient
in creating a strong magnetic field.
Investigate Further
Martian Fossil Hunting. On Earth, we cannot find fossil evidence of life dating to times
prior to about 3.8 billion years ago. If life ever existed on Mars, is it possible that we would
find older fossils than we find on Earth? Explain.
The most important thing to consider here is the fact that geological activity on Mars has slowed
down significant over time. It is possible that Mars did have significant geological activity about
3.8 billion years ago. If there are fossils older than this, then it is possible that they may be some
today if they survived this activity. If this is indeed the case, we can conclude that there may be
some fossils since geological activity has slowed down on Mars.