Mars Revealed

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Module 12 Mars - the Red Planet
Activity 2 Mars Revealed
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Learning Outcomes
In this Activity, we will investigate (a) the
atmosphere of Mars, and (b) the surface of Mars
- cratering, ice caps, volcanism, Martian
tectonics
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• With the renewed interest in sending space
  missions to Mars, the amount of information we
  have about it is increasing markedly - faster
  than the rate at which we can organize that
  information into a coherent understanding of the
  red planet!

So, like many areas of astronomy at present, our
knowledge and understanding of Mars is undergoing
an exciting revolution, and some of the concepts
we put forward here are likely to be refined,
modified or even thrown out in the near future...
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(a) The Atmosphere of Mars
The thin, carbon dioxide atmosphere of Mars is
very dry. It contains 30 times less water vapour
than does Earths atmosphere.
Pathfinder image of pre-dawn ice clouds in the
eastern sky
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Why is the Martian atmosphere so thin and dry?
Mars has a low escape velocity (5 km/s, compared
to 11.2 km/s for Earth) and so gas atoms can
escape relatively easily. Also, no ozone layer
means no protection for water vapour molecules.
Ultraviolet radiation from the Sun can break up
the molecules via photodissociation - hence the
dry atmosphere.
The Martian atmosphere may once have been much
denser and contained water - see the next
Activity.
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Although very thin, the atmosphere supports
weather patterns, including huge dust storms.
The following two Hubble Space Telescope images,
taken about a month apart in 1996, show a dust
storm near the edge of the Martian north polar
cap
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north polar cap
1000 km longdust storm atedge of polarcap
Storm has almost disappeared,leaving behind
acomma-shapedcold front typefeature in the
atmosphere
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The dust storms become global, lasting for a
month ortwo, when Mars is closest to the Sun.
Dust in the atmosphere affects the colour of the
sky, changing with the seasons. In winter, the
skies become clearer as the dust particles tend
to adhere to carbon dioxide ice particles and
precipitate out.
Mars Pathfinder image of the Martian sunset
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• The Mars Pathfinder mission found evidence of
  frequent dust devils (or willy willies),
  which may be another way in which dust is mixed
  into the atmosphere.

Dust devil seen by MOC on the Mars Global
Surveyor, April 2001
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(b) The Surface of Mars
The surface of Mars is reddish in colour, due to
rusted iron minerals in the soil. At the Viking
landing spot, the soil was composed of 19 ferric
oxide (rust) and 44 silica minerals.
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Martian rocks have been found to be basaltic
(i.e. volcanic in origin), typically containing
small holes indicating that they have been formed
from frothy gas-filled lava.
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• Cratering on Mars

Craters are shallower on Mars than on the Moon
because the effect of gravity is twice as strong
(resulting in less material being completely
thrown out in an impact).
Martian craters typically are strongly eroded by
dust storms, having lost the surrounding rays
and ejecta thatwe see associated with craters on
the Moon and Mercury.
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• According to someplanetary scientists,the Gusev
  crater may have been the site of an ancient
  lakebed.
• Well look at thedebate about water on Marsin
  the next Activity.

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By looking at the crater count, we can get an
idea of the relative ages of the Martian
hemispheres.
Northern hemisphere younger surface, fewer
craters, repeated lava flows.
Southern hemisphere old, heavily cratered
surface.
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• The Martian Polar Caps

The Martian north pole is mostly water ice, with
carbon dioxide ice in its outer reaches. The
carbon dioxide iceforms on top of the water ice.
At the low atmospheric pressures on Mars, water
ice forms when the temperature drops to about
190K, but carbon dioxide ice wont form until
the temperature drops to about 150K.
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The ice caps exhibit a layered structure with
alternating layers of ice with varying
concentrations of dark dust.
The orbit of Mars around the Sun is rather
eccentric (e 0.093), and the southern winter
occurs near aphelion, making it much more severe
than northern winter. Therefore the carbon
dioxide ice caps extend further fromthe south
pole in the southern winter, than they do from
the north pole in the northern winter.
Click here to see a movie of seasonal changesin
north polar ice cap of Mars
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During the Spring and Summer of 1998, the Mars
Orbiter Laser Altimeter flashed laser pulses
toward the Martian surface from the Global
Surveyor spacecraft and recorded the time it took
to detect the reflection.
This timing data has now been translated into the
following detailed topographic map of Mars
north polar terrain. According to the NASA press
releaseThe map indicates that the ice cap is
about 1,200km across, a maximum of 3km thick, and
cut by canyons and troughs up to 1km deep. The
measurements also indicate that the cap is
composed primarily of water ice with a total
volume of only about four percent of planet
Earths Antarctic ice sheet.
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• Volcanism on Mars

Mars has many volcanoes, primarily in the
northern halfof the planet, including a dozen
which are very large indeed.
Three giant volcanoeson the Tharsis Bulge
Valles Marineris - more aboutthis canyon system
later
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• Most Martian volcanoes occur in the northern
  hemisphere, together with extensive lava flows.

Most cratering is seen in the relatively flat
southern hemisphere - which is therefore
presumed to be older.
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• The volcanic plains in the north lie at an
  average of several kilometres lower than older
  southern cratered uplands, very like lunar
  maria, and formed about same time - perhaps by
  a huge lava flow about three billion years ago.

The reason forthese differencesbetween the
north and south hemispheres isnot well
understood.
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The largest Martian volcano is Olympus Mons, a
shield volcano rising 25 km above the
surrounding terrain, with a diameter of more
than 500 km.
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• Mars Global Surveyor image of Olympus Mons

Collapsed volcanic cone, called a caldera
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3-dimensional image created from several images
of Olympus Mons
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• Many Martian volcanoes exhibit a number of
  craters, implying that they are quite old, but
  Olympus Mons has very few, implying that its
  surface cannot be more than hundred million years
  old and could be much younger.

It is possible that some of these great volcanoes
may remain intermittently active today, but we
have no evidence of this.
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• Olympus Mons rises 25 km above the surrounding
  terrain. Compare this to the highest volcano on
  Earth, Mauna Loa in Hawaii, which rises about 8
  km above the seabed, and Mount Everest, which
  rises 9 km above sea level.

The weight of Mauna Loa has depressed the
seafloor to form an underwater moat surrounding
it. As you have seen, Olympus Mons, roughly three
times higher than Mauna Loa, has not formed a
moat in the surrounding terrain the Martian
crust seems to be supporting it without
difficulty. Planetary scientists conclude that
the crust of Mars is significantly thicker than
that of Earth - perhaps twice as thick.
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• As we have seen, Mauna Loa and the other Hawaiian
  volcanoes are terrestrial examples of hot-spot
  volcanism, a form of volcanism where magma from a
  particularly hot region far below the surface
  rises and breaks through the crust as lava,
  forming a volcano. As the crustal plate moves
  due to tectonic forces (continental drift), the
  volcanic outlet and accumulated cooled lava
  shifts aside, and the next eruption of lava
  breaks through as a new volcano.

In this way, gradual tectonic plate movement
leads to a chain of volcanoes such as the
Hawaiian-Emperor Islandchain. This island chain
now extends 3800 km across the Pacific seabed.
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Here the Hawaiian island chain is superimposed on
animage of Olympus Mons.
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The giant Martian volcanoes appear to be formed
by hot-spot volcanism too, but in the case of a
volcano like Olympus Mons, the magma has kept
breaking throughthe one vent in the planets
crust for millions of years. So instead of a
chain of smaller volcanoes, singlegiant
volcanoes have been able to form.
Planetary scientists conclude that, unlike on
Earth, the crust of Mars is not separated into
moving tectonic plates.
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• Martian Tectonics?

The Tharsis bulge, a region approximately the
same sizeas North America, rises nearly 10 km
above the surroundingterrain, crowned by 4 great
volcanoes which rise another 15 km.
Located on the boundary between the cratered
uplands and the northern plains, the Tharsis
bulge is primarily tectonic. There is evidence
of extensive cracking in the crust surrounding
the Tharsis area.
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• False-colour image of part of the Tharsis region.

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• The Elysium region is a similarly uplifted
  volcanic plain, almost halfway around planet from
  the Tharsis Bulge. The cratering record
  indicates that the Elysium region is the slightly
  older of the two.

The origin of these regions appears to be due to
rising surges of thick magma in the planets
mantle, lifting up regions of the crust rather
than breaking through it as lava. On Venus, a
similar phenomenon causes oval ring
patternscalled coronae, but the thicker, less
pliable crust on Mars may prevent coronae
forming.

see the Activity on Observing the Surface of
Venus
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Valles Marineris

• One of the most striking Martian features imaged
  by the Mariner spacecraft (and named after it) is
  Valles Marineris, a huge canyon system stretching
  at least 4000 km (nearly 1/4 way around Mars).

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• Valles Marineris is 600 km wide in places, and 6
  km deep - four times the depth of the Grand
  Canyon, which would fit easily into one of its
  side canyons.

Layered outcrop in part of the Valles
Marineris. Layering of this sort is seen
onEarth, caused by either volcanicor
sedimentary processes.
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Valles Marineris has features in common with the
East African Rift, part of a vast terrestrial
plate fracture which extends from southern
Turkey, through the Red Sea, East Africa into
Mozambique.

• Planetary scientists conclude that Valles
  Marineris is probably an ancient fracture caused
  by limited plate tectonics (due to the uplifting
  of the Tharsis Bulge and its volcanoes) which,
  however, failed to develop further on Mars.

Part of the East African Rift Valley
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• In the next Activity we will look at the search
  for evidence that liquid water, and perhaps life,
  once existed on Mars.

In the meantime, when you have finished this
Activity, use the CD-ROM which accompanies the
Universe textbook to view simulated fly-pasts of
the Martian surface, volcanoes and Valles
Marineris in the Animations Videos section.
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Image Credits
NASA Pathfinder image of ice clouds in pre-dawn
skyhttp//nssdc.gsfc.nasa.gov/planetary/image/mar
spath_clouds_s39.jpg Pathfinder image of Martian
Sunsethttp//nssdc.gsfc.nasa.gov/planetary/image/
marspath_ss24_0.jpg Hubble image of dust storm at
Martian North Polehttp//nssdc.gsfc.nasa.gov/imag
e/planetary/mars/hst_mars_dust_storm.jpg Venus
globehttp//nssdc.gsfc.nasa.gov/image/planetary/v
enus/venusglobe.jpg Earth globehttp//pds.jpl.nas
a.gov/planets/welcome/earth.htm Mars
globehttp//pds.jpl.nasa.gov/planets/welcome/thum
b/marglobe.gif A view of the Martian surface
(Viking 1)http//nssdc.gsfc.nasa.gov/image/planet
ary/mars/vikinglander1-1.jpg
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Image Credits
NASA Mars - Twin Peaks (Pathfinder)http//mpfww
w.jpl.nasa.gov/MPF/parker/TwnPks_RkGdn_rite_sm.jpg
Gusev craterhttp//ic-www.arc.nasa.gov80/ic/pro
jects/bayes-group/Atlas/Mars/special/Gusev/Syrtis
Region (Hubble)http//nssdc.gsfc.nasa.gov/image/
planetary/mars/marsglobe3.jpg 3D Mars North Pole
(Mars Global Surveyor)http//antwrp.gsfc.nasa.gov
/apod/ap981216.html Mars South Polehttp//nssdc.g
sfc.nasa.gov/image/planetary/mars/mars_so_pole.jpg
Valles Marinerishttp//nssdc.gsfc.nasa.gov/image
/planetary/mars/marsglobe1.jpg Olympus
Monshttp//nssdc.gsfc.nasa.gov/image/planetary/ma
rs/olympus_mons.jpg
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Image Credits
NASA Cratering on Marshttp//www.anu.edu.au/Phy
sics/nineplanets/thumb/mar6cratICON.gif East
African Rift Valley, Kenyahttp//images.jsc.nasa.
gov/images/pao/STS32/10063457.gif Layered
outcrop, Valles Marinerishttp//lunar.ksc.nasa.go
v/mars/mgs/msss/camera/images/top102_Dec98_rel/lay
ers/ Olympus Mons, Mars-Hawaii Comparisonhttp//c
ass.jsc.nasa.gov/images/shaw/shaw_S01TN.gif
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• Now return to the Module 12 home page, and read
  more about the surface of the Mars in the
  Textbook Readings.

Hit the Esc key (escape) to return to the Module
12 Home Page
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