Defrosting Mars

Mars Global Surveyor
Kunowsky Crater MGS MOC Release No. MOC2-753, 10 June 2004
Image Credit: Mars Global Surveyor, Malin Space Systems

By launching the Mars Global Surveyor (MGS) spacecraft in November 1996, NASA and the Jet Propulsion Laboratory began America’s return to Mars after a 20-year absence. Tody MGS serves as an important relay satellite for communicating between Earth and the surface rovers now exploring two regions on opposite sides of Mars. When combined with the Mars Odyssey and Mars Express satellites, the communication network is the first constellation built for interconnecting Earth with the surface of another planet.

The Global Surveyor travelled nearly 750 million kilometers (466 million miles) over the course of a 300-day cruise to reach Mars on September 11, 1997. During mapping operations, the spacecraft circles Mars once every 118 minutes at an average altitude of 378 kilometers (235 miles). One fascinating features that these maps have revealed is seasonal changes on a global and local scale. While the surface rovers have primary missions to look for water, the orbiter views have provided pictures of how Mars changes as the north and south hemispheres move into summer and winter. Notable among these changes is the dramatic appearance of martian frost.

A sampler of MGS images is highlighted below. This frosty Mars offers complex chemistry as water-ice and frozen carbon dioxide (so-called, ‘dry ice’) deposit and sublime at different temperatures and different seasonal peaks.

The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image, acquired this image (right) in March 2004, shows Kunowsky Crater ringed by seasonal frost.

Kunowsky is about 67 km (~42 mi) in diameter. Wavy clouds form to the east (right) of the crater in early spring as winds circulate from west to east.

Mars Global Surveyor
Dunes with Frost MGS MOC Release No. MOC2-743, 31 May 2004
Image Credit: Mars Global Surveyor, Malin Space Systems

Springtime for the martian northern hemisphere brings defrosting spots and patterns to the north polar dune fields.

This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image (left) shows an example located near 76.7 degrees N, 250.4 degrees W.

In summer, these dunes would be darker than their surroundings. However, while they are still covered by frost, they are not any darker than the substrate across which the sand is slowly traveling.

Dune movement in this case is dominated by winds that blow from the southwest (lower left of the image) toward the northeast (upper right). The picure covers an area about 3 km (1.9 mi) across and is illuminated by sunlight from the lower left.

Seasonal frost can also enhance the view from orbit of polar polygonal patterns on the surface of Mars.

Sometimes these patterns look something like a city map, or the view from above of a city lit-up at night. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image (lower right) shows an example from the south polar region near 80.7 degrees S, 70.6 degrees W.

Mars Global Surveyor
Martian City Map MGS MOC Release No. MOC2-742, 30 May 2004
Image Credit: Mars Global Surveyor, Malin Space Systems

Polar polygons on Mars are generally believed, though not proven, to be the result of freeze/thaw cycles of ice occurring within the upper few meters (several yards) of the martian subsurface. The image shown here covers an area about 3 km (1.9 mi) across; sunlight illuminates the scene from the upper left.

The image (lower left) shows frost-covered sand dunes in the early northern spring of 2004 in the north polar region.

Sunlight illuminates the dunes from the bottom/lower left, but frost on slopes facing the lower right create the illusion of sunlight from that direction.

This dune field, which would appear quite dark in summertime, is located near 80.3 degrees N, 148.7 degrees W. The picture also covers an area about 3 km (1.9 mi) across.

Mars Global Surveyor
Frosty Dune Field MGS MOC Release No. MOC2-713, 1 May 2004
Image Credit: Mars Global Surveyor, Malin Space Systems

Astronomers have known for years that Mars possessed polar ice caps, but early attempts at chemical analysis suggested only that the northern cap could be composed of water ice, and the southern cap was thought to be carbon dioxide ice.

Recent space missions then suggested that the southern ice cap, existing all year round, could be a mixture of water and carbon dioxide.

Permafrost is water ice, mixed into the soil of Mars, and frozen to the hardness of solid rock by the low Martian temperatures. This is one reason why water ice has eluded detection in many previous surveys- because the soil with which it is mixed cannot reflect light easily and so it appears dark.

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The wide angle view of the martian north polar cap was acquired on March 13, 1999, during early northern summer. The light-toned surfaces are residual water ice that remains through the summer season. The nearly circular band of dark material surrounding the cap consists mainly of sand dunes formed and shaped by wind. The north polar cap is roughly 1100 kilometers (680 miles) across.Credit: NASA/JPL/Malin Space Science Systems

Using data collected by the recent Mars Express satellite, planetary scientists now know that the south polar region of Mars can be split into three separate parts. Part one is the bright polar cap itself, a mixture of 85% highly reflective carbon dioxide ice and 15% water ice.

The second part comprises steep slopes known as scarps’, made almost entirely of water ice, that fall away from the polar cap to the surrounding plains.

The third part was unexpected and encompasses the vast permafrost fields that stretch for tens of kilometers away from the scarps.

During the winter months in the northern hemisphere, scientists expect that carbon dioxide from the atmosphere will freeze onto the poles, making them much larger and covering some of the water ice from view.





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