Jets on Mars
Nature, August 2006. CO2 Jets on Mars: Scientists have now found a better explanation for the dark spots and fans found on the south polar region of Mars. These spots, which can cover tens of square kilometers, are visible in the cryptic region. Originally, it was suggested that the spots were soil, exposed after early defrosting of the polar ice. However, an intensive new study with the Mars Odyssey Thermal Emission Imaging System (THEMIS) shows that the temperatures of the spots are inconsistent with expected temperatures of soil. Instead, the dark spots are within a few degrees of solid CO2 (~ -130°C). Soil, even when bordered by CO2, is greater than ~ -50°C. This suggests that the dark spots exists either on top, underneath, or inside of a layer of CO2. The previous suggestion that the spots are defrosted, CO2 ice free regions, no longer seems plausible.
This is an artist's rendition of what the Cryptic region might look like if one was standing on the surface of the polar cap. The cryptic region is believed to be an impermeable, translucent layer that allows sunlight to shine all the way through to the underlying soil. The sunlight heats the soil under the ice, leading to sublimation of the dry ice from the base. The sublimation that occurs under the ice results in pressures at the base of the impermeable ice slab that lifts the ice. Eventually the ice slab cracks and allows the built up gas to escape as a jet. The gas coming out of these jets can reach speeds 30 feet per second (20mph). The jets carry dust and dirt from underneath the ice slab up into the atmosphere. The dirt and heavier dust fall back to the ground forming a large dark circle around the jet while the lighter dust is carried down wind.
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MRO THEMIS web site
Artistic credit to Arizona State University/Ron Miller
Both the north and south polar caps of Mars exhibit great change throughout the year. The southern cap though is privy to a spotting anomaly. These spots, which are often associated with fans and blotches, are dark regions that form in the southern spring and summer. Radial channels, called fans, which are much like the legs of a spider, are often collocated with the spots after the CO2 ice disappears. Oddly though, the fans remain at near CO2 ice temperatures into the spring. This suggests that the fans must actually be thinly deposited soil on top of the ice. Based on the thermal conduction of Martian soil, the deposits must be ~1mm thick. Any thicker and the soil would warm from sunlight faster than it can dissipate the heat into the underlying ice. These deposits are consistent with local wind patterns, suggesting that the soil is erupted from CO2 jets and is carried by the wind to form the fan pattern.
CO2 jets likely form in the cryptic region from sublimation of annealed CO2 ice on the border between the ice and the soil. The sublimed CO2 gas then builds in pressure, lifting the ice above. At some point, it manages to crack the ice, escaping in the form of a jet, while also taking along some of the soil for the boundary where it sublimed. The depression left from the excavated soil, works as a primer for the following year. This means, that because of the previous erosion, spots will typically form again in the same location.
More information is available at the following web sites:
These temperatures are from the TES thermal bolometer which measures radiant energy from 6µm to 100µm. This spectral region includes the strong 15µm atmospheric CO2 band and the 9µm atmospheric dust band, so atmospheric temperatures can have a small effect on the measured temperature.
Temperature Comparisons at the MPL Landing Site:
The first two images show the temperatures in the Martian south polar region just prior to the arrival of the Mars polar lander. The color transition from blue to green shows the current edge of the south polar cap. Circles are spaced at 10 degrees of latitude. 0° longitude is straight up. The third figure shows the current albedo of the polar cap. The region that was made up of dark CO2, or Cryptic CO2, has sublimed away much quicker than other areas of the seasonal polar cap. The fourth image shows the sublimation rate of CO2 in kg/day/m2. The green outline marks the area considered for landing. Click on the image to see an enlarged view.
The first two images show the temperature data for the Martian Polar Landing region just prior to its arrival. The left image is 2 am data and the right image is 2 pm data. The third image is the Lambert albedo and the fourth image is the estimated sublimation rate in kg per square meter per day. There is no dry ice remaining in these zoomed-in images. The plus sign is the targeted landing site. Click on the image to see an enlarged view.
What did the Mars South Polar Cap look like in years past?
Here are some side-by-side comparisons of TES data from 1998 and Viking IRTM from 1997, taken for approximately the current season on Mars. A comparison of past data shows a polar cap receding in much the same manner year after year.
October 1st-3rd, 1999:
The left image is a 20 µm image of the south pole reconstructed from Viking 2 IRTM data (Jan 1977). The arrows point in the direction of midnight. The next image is TES data taken November 20th-21st, 1997, at about the same season. The last image is more current TES bolometric data from Oct 1st-3rd,1999.
November 27th-29th, 1999:
The left image is a 20µm image of the south pole reconstructed from Viking 2 IRTM data (Mar 1977). The next image is TES data taken January 13th, 1998, at about the same season. The last image is the most current TES bolometric data from Nov 27th-29th, 1999. The arrows point in the direction of midnight.
October 7th-9th, 1999
The left image is the albedo of the south pole reconstructed from Viking 2 IRTM data (Jan 1977). The arrows point in the direction of midnight. The next image is TES data taken November 20th-21st, 1997, at about the same season. The last image is the cap's more recent Albedo data. All three Martians years show the Cryptic Region in approximately the same location.
November 27th-29th, 1999
The left image is the albedo of the south pole reconstructed from Viking 2 IRTM data (Mar 1977). The next image is TES data taken January 13th, 1998, at about the same season. The last image is the cap's most recent Albedo data. The arrows point in the direction of midnight.
August 2006, Letters to Nature: The OMEGA team suggests an alternate and competing model for the presence of CO2 ice in the south polar cryptic region in this Nature submission.
May 12, 2005: "Scientists think they have an answer to the long-standing mystery of why the permanent icecap on Mars' South Pole is offset from the pole itself. Simply put, it's colder and stormier in that hemisphere.
But that is only part of the equation, they say, and new understanding about Mars' climate and its polar regions may suggest clues to finding water in the planet's equatorial zone - where it would be easier to land a spacecraft - and opening the door to future exploration." Read more at the Oregon State University website or read the journal article at LPI.