Transcript Mars Revealed

Module 12: Mars - the Red Planet
Activity 2:
Mars Revealed
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
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...
(a) The Atmosphere of Mars
The thin, carbon
dioxide atmosphere
of Mars is very dry.
It contains 30 times
less water vapour
than does Earth’s
atmosphere.
Pathfinder image of pre-dawn ice clouds in the eastern sky
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.
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:
north polar cap
1000 km long
dust storm at
edge of polar
cap
Storm has almost
disappeared,
leaving behind a
comma-shaped
“cold front” type
feature in the
atmosphere
The dust storms become global, lasting for a month or
two, 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
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
(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.
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.
• 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 that
we see associated
with craters on the
Moon and Mercury.
According to some
planetary scientists,
the Gusev crater
may have been the
site of an ancient
lakebed.
We’ll look at the
debate about
water on Mars
in the next Activity.
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.
• The Martian Polar Caps
The Martian north pole is mostly water ice, with carbon
dioxide ice in its outer reaches. The carbon dioxide ice
forms on top of the water ice.
The south pole is mostly carbon dioxide ice.
At the low atmospheric pressures on Mars, water ice
forms when the temperature drops to about 190°K,
but carbon dioxide ice won’t form until the temperature
drops to about 150°K.
Because of this, the size of the
carbon dioxide ice component
of the polar caps changes
markedly from winter to summer
winter
spring
summer
at each pole.
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 from
the 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 changes
in north polar ice cap of Mars:
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 release:
“The 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 Earth’s Antarctic ice sheet.”
• Volcanism on Mars
Mars has many
volcanoes,
primarily in the
northern half
of the planet,
including a dozen
which are very
large indeed.
Three giant volcanoes
on the Tharsis Bulge
Valles Marineris - more about
this canyon system later
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.
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 for
these differences
between the north and
south hemispheres is
not well understood.
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.
Mars Global
Surveyor image
of Olympus
Mons
Collapsed
volcanic
cone,
called a
caldera
3-dimensional image created from several images of Olympus Mons
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.
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.
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 Island
chain. This island chain now extends 3800 km across the
Pacific seabed.
Here the Hawaiian island chain is superimposed on an
image of Olympus Mons.
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 through
the one vent in the planet’s crust for millions of years.
So instead of a chain of smaller volcanoes, single
giant volcanoes have been able to form.
Planetary scientists conclude that, unlike on Earth, the
crust of Mars is not separated into moving tectonic plates.
• Martian Tectonics?
The Tharsis bulge, a region approximately the same size
as North America, rises nearly 10 km above the surrounding
terrain, 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.
False-colour
image of part
of the Tharsis
region.
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 planet’s mantle, lifting up
regions of the crust rather than breaking through it as lava.
On Venus, a similar phenomenon causes oval ring patterns
called coronae,* but the thicker, less pliable crust on Mars
may prevent coronae forming.
* see the Activity on Observing the Surface of Venus
• 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).
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 on
Earth, caused by either volcanic
or sedimentary processes.
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
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.
Image Credits
NASA:
Pathfinder image of ice clouds in pre-dawn sky
http://nssdc.gsfc.nasa.gov/planetary/image/marspath_clouds_s39.jpg
Pathfinder image of Martian Sunset
http://nssdc.gsfc.nasa.gov/planetary/image/marspath_ss24_0.jpg
Hubble image of dust storm at Martian North Pole
http://nssdc.gsfc.nasa.gov/image/planetary/mars/hst_mars_dust_storm.jpg
Venus globe
http://nssdc.gsfc.nasa.gov/image/planetary/venus/venusglobe.jpg
Earth globe
http://pds.jpl.nasa.gov/planets/welcome/earth.htm
Mars globe
http://pds.jpl.nasa.gov/planets/welcome/thumb/marglobe.gif
A view of the Martian surface (Viking 1)
http://nssdc.gsfc.nasa.gov/image/planetary/mars/vikinglander1-1.jpg
Image Credits
NASA:
Mars - Twin Peaks (Pathfinder)
http://mpfwww.jpl.nasa.gov/MPF/parker/TwnPks_RkGdn_rite_sm.jpg
Gusev crater
http://ic-www.arc.nasa.gov:80/ic/projects/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 Pole
http://nssdc.gsfc.nasa.gov/image/planetary/mars/mars_so_pole.jpg
Valles Marineris
http://nssdc.gsfc.nasa.gov/image/planetary/mars/marsglobe1.jpg
Olympus Mons
http://nssdc.gsfc.nasa.gov/image/planetary/mars/olympus_mons.jpg
Image Credits
NASA:
Cratering on Mars
http://www.anu.edu.au/Physics/nineplanets/thumb/mar6cratICON.gif
East African Rift Valley, Kenya
http://images.jsc.nasa.gov/images/pao/STS32/10063457.gif
Layered outcrop, Valles Marineris
http://lunar.ksc.nasa.gov/mars/mgs/msss/camera/images/top102_Dec98_rel/layers/
Olympus Mons, Mars-Hawaii Comparison
http://cass.jsc.nasa.gov/images/shaw/shaw_S01TN.gif
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