* The terrain in the southern hemisphere of Mars is ancient and heavily cratered. The pattern of ejected material from many Martian craters shows that the ejecta were very fluid and may have contained a high proportion of water.
* Widespread volcanic flows have eradicated the ancient, cratered
terrain in the northern hemisphere of Mars.
* Mars has many volcanos. The largest Martian volcanos are located
in the Tharsis region, a continent-size bulge. The great sizes
of the Tharsis volcanos imply that the Tharsis region has remained
over its magma source for billions of years. The heights of the
Tharsis volcanos show that the magma that formed them rose from
great depths, where pressure was large enough to lift the magma
to the summits of the volcanos. This shows that the lithosphere
of Mars is thicker than the lithosphere of the Earth.
* We can estimate the ages of Martian volcanos and lava flows
by measuring crater densities. Volcanic activity occurred all
over Mars about 2.5 billion years ago but has gradually become
restricted to a smaller and smaller region of the planet.
* The many faults and chasms on Mars show that there has been
considerable crustal motion. The most spectacular example of crustal
motion is the Valles Marineris canyon system. Much of the crustal
motion seems to have occurred as a consequence of the weight of
the Tharsis bulge.
* Two kinds of channels on Mars appear to have been cut by running
water. Runoff channels, resembling terrestrial river systems,
were formed when underground water or rainfall was collected from
a large region. Outflow channels were cut by great floods that
took place when large pools of water suddenly were produced in
a local region.
* Both the north and south poles of Mars have permanent polar
caps. The larger northern cap is made primarily of water while
the southern cap also contains a large percentage of carbon dioxide
(dry ice). A seasonal cap of dry ice forms about each permanent
polar cap in winter.
* The polar regions of Mars are covered with thick layers of sediment
formed from dust blown poleward by strong winds. It takes tens
of thousands of years to accumulate a single layer tens of meters
thick. The layering probably is caused by the precession of Mars's
polar axis.
* Pictures taken by the Viking landers show rolling countryside
littered with rocks ejected from nearby impact craters. The regions
around the landers resemble rocky deserts on the Earth.
* The Pathfinder lander and the Sojourner rover explored Ares
Vallis for about three months in 1997. Pathfinder images showed
abundant evidence of ancient floods while Sojourner measured the
chemical composition of rocks and soil.
* The atmosphere of Mars consists primarily of carbon dioxide.
The atmosphere is too thin to insulate the surface, where daily
temperature fluctuations are as much as 60 K. Atmospheric pressure
is lowest in the summer and winter when carbon dioxide is trapped
in one of the polar caps. Pressure is highest in the fall and
spring when carbon dioxide is released into the atmosphere.
* Frequent dust storms keep dust suspended in Mars's atmosphere
at all times. Great dust storms sometimes spread a thick layer
of dust over the entire planet. The seasonal changes in dark markings
are caused by deposition and removal of dust by seasonal winds.
Large dust particles are skipped along the surface by winds and
are very effective at eroding the Martian surface.
* Although Mars contains a smaller percentage of iron than the
Earth does, it has a core made of iron or iron mixed with sulfur.
The crust of Mars is rich in iron, giving Mars its reddish color.
* After Mars formed, radioactive decays heated its interior until
iron melted and accumulated in its core. Eventually the rocks
in the mantle melted partially to produce magma, which flooded
the surface and produced large volcanos. Mars expanded as it grew
hotter, perhaps causing its crust to pull apart and produce Valles
Marineris. Mars has cooled and become less active for the last
several billion years.
* Little water (in solid, liquid, or gaseous form) can be found
on Mars today. However, surface features such as its channels
show that Mars once had much more water than we have been able
to measure. It is likely that most of Mars's water is trapped
as permanently frozen, subsurface ice.
* Early in its history, Mars may have had an atmosphere thick
enough for a greenhouse effect to occur. The atmosphere may have
been warm and thick enough for liquid water to exist. As on the
Earth, carbon dioxide was incorporated into surface rocks. Unlike
the Earth, however, the lack of plate tectonics prevented carbon
dioxide from being recycled into the atmosphere. The atmosphere
dwindled, water froze, and Mars eventually reached its present
condition.
* The Viking biology experiments showed that the Martian soil
is chemically active, but probably does not harbor life.
* The evolution of the terrestrial planets can be pieced together
using information from each of them. Impacts and radioactive decays
heated each planet until much of its iron sank to form a core,
and light rocky material floated to the surface to become the
crust. The extent to which each planet has cooled and become volcanically
inactive depends on size. The smaller planets quickly became inactive
while the larger remain tectonically and volcanically active.
outflow channel, organic molecule, patera runoff, channel
1. Why is the daily temperature variation on Mars so much larger than we experience on Earth? Mars' atmosphere is too thin to insulate the surface
2. In what respect are the atmospheres of Venus and Mars most
alike? chemical composition
3. During some seasons the barometric pressure on Mars drops steadily
for months at a time. Why? the atmospheric gas is solidifying
on one of the polar caps
4. Which of the following is most likely to have been responsible
for the formation of Valles Marineris? surface cracking when Mars
expanded
5. How do we know that the volcanoes of Mars are billions of years
old? there are impact craters on their slopes
6. If there is a lot of water on Mars, where is it most likely
to be found? in ice layers beneath the visible surface
7. In which of the following forms can water NOT exist on Mars
today? lakes
8. Some of the Martian channels originate in chaotic (jumbled)
terrain. How were the chaotic terrain and the channels emerging
from them probably produced? melting of underground ice followed
by the flow of water
9. What was the result of the life-detection experiments that
were carried out on the Viking missions to Mars? no clear-cut
evidence of life was detected
10. Which of the following observations led astronomers in the
early 1900's to suspect that there might be life on Mars? seasonal
variations in dark surface markings