CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
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Mars Geology

Mars' Atmosphere, Weather, and Frozen Volatiles

Mars' atmosphere today is a wisp compared to Earth's. At the lowest point on the planet, atmospheric pressure is only 1/80th of that on Earth. Over more than half the surface, it's less than 1/150th. At the low pressure on Mars' surface, water can't exist as a liquid. As soon as it melts from ice, the water evaporates. A glass of ice water poured onto the surface would boil away as violently as a glass of water on Earth thrown into a hot oven. However, extremely salty water - whose "vapor pressure", which can be thought of as tendency to boil, is reduced by the salt content - may survive for hours at a time in the warmest places.

Mars’ atmosphere is over 95% carbon dioxide, whereas Earth's air is 78% nitrogen and 21% oxygen. Most of the remaining few percent of Mars' atmosphere is nitrogen and argon, with traces of carbon monoxide and oxygen. The atmosphere is also nearly bone-dry. In the most humid regions of the planet, all of the water vapor would, if condensed out, form a layer thinner than a sheet of paper (3/1000th of an inch thick).

Despite the thinness of the atmosphere, the most dynamic aspect of Mars is its weather. Most obviously in the composite image below, taken from Mars Global Surveyor, is that Mars has clouds. Water ice clouds are most common:

  • around the volcanoes, where updrafts cool the thin Martian air and cause ice to condense ("orographic clouds")
  • high in the atmosphere in the equatorial region during northern summer ("aphelion" clouds, so named because during northern summer Mars is at the furthest point from the sun in its orbit)
  • surrounding each polar region during local autumn and winter ("seasonal" clouds)

An amazing aspect of Mars' clouds is how repeatable they are, Mars-year after Mars-year. The MGS/Mars Orbiter Camera, or MOC, watched the planet from 1997 to 2006, a task taken over since 2006 by the Mars Color Imager, or MARCI, on MRO. These cameras have assembled a fascinating record of repeatable cloud features. Year after year, similar clouds form in the same places. Without an ocean to drive chaotic meteorology, Mars' weather is more predictable than Earth's, year after year.

Water ice occurs on Mars' surface in the residual polar caps and as clouds.
(Image credit NASA/JPL/Malin Space Science Systems.)

Mars also has clouds of dust, raised from the surface by winds. They may occur at any season but are most common during southern spring and summer. At that time local dust storms are common in the southern hemisphere, and in some years they explode in intensity and become globe-encircling events. Famous "global dust events" occurred in 1971, the year Mariner 9 – the first Mars orbiter –arrived at the planet in 1971, in 2001 when Mars Global Surveyor was tracking Mars' weather, and during MRO’s mission in 2007. Smaller, more localized dust storms, like the one below, also occur around each pole in spring as the seasonal polar cap ablates.

The dust storm in this image is the light-colored, tongue-shaped feature
extending down and to the left of the northern seasonal polar cap.
(Image credit NASA/JPL/Malin Space Science Systems.)

This movie covers the period April 1999–August 2004, and shows
the growth and decay of a globe-encircling dust storm in 2001.
(Image credit NASA/JPL/Arizona State University.)

Watch the movie of a globe-encircling dust storm in 2001 and note the value
of "Ls," a short-hand notation for solar longitude. It is a measure of season,
with a value of 0 through 360. 0 is at the beginning of southern autumn.
The dusty season is Ls 180–240, southern spring. Watch in July 2001 when
a regional dust storm exploded to global scale.

Each polar region has a seasonal cap of frozen carbon dioxide that snows out of the atmosphere in autumn and early winter. On average the seasonal cap is a fews tens of centimeters – or about a foot – in thickness. It contains a trace of water ice, which lingers longer on the surface in spring after the frozen carbon dioxide evaporates. (On Mars, neither carbon dioxide ice nor water ice melts. It goes directly to gas because the atmospheric pressure is so low.)

Earth has a seasonal polar cap as well – winter snow, made of water ice. Earth's snow forms from a minor constituent of the atmosphere, water vapor. In contrast, Mars' carbon dioxide ice seasonal cap forms from the freezing of the main atmospheric gas. Such a large part of the atmosphere gets tied up in the seasonal cap that, in mid-winter, atmospheric pressure drops by one-fourth.

The polar deposits left after disappearance of the seasonal cap polar cap each year – the "residual cap" and underlying "polar layered deposits" – are made mostly of water ice. The northern cap is nearly all water ice mixed with dust and sand. The southern cap has a surface layer of carbon dioxide ice a few tens of meters (about one hundred feet) thick, underlain by a mixture of water ice, dust and sand.

This scarp in Chasma Boreale shows layering of the north polar residual cap and underlying
sedimentary deposits. The upper part of the sedimentary stack is the polar layered deposits
(red arrow). Parts of the layered deposits are underlain by a "basal layer" rich in sand that
predates the main layered deposits (blue arrow). Eroded sand from the basal layer forms
a giant field of dunes (white arrow) surrounding the north polar layered deposits.
(Image credit NASA/JPL/University of Arizona/Planetary Science Institute.)

One of the most important questions about Mars is, was its atmosphere ever thicker and perhaps temperatures warmer? Mars' channels suggest to some scientists that at one time Mars had a denser carbon dioxide atmosphere that warmed the planet by the greenhouse effect allowing liquid water to flow. Other scientists think that Mars was never globally warmer, and that valley networks formed by geothermal heating or from local rains after impact craters melted ground ice.

If there ever was a denser atmosphere, where did it go? There is not enough carbon dioxide in the polar caps to explain a warmer climate in the past, if that carbon dioxide was once in the atmosphere. Perhaps the ancient atmosphere was eroded away by the solar wind. An alternative idea is that carbon dioxide reacted with crustal rocks and was trapped as carbonate minerals. The search for these carbonates was a major motivation for CRISM, and constraining the ages and amounts of carbonates would help us to understand how Mars' channels formed.