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Tanya Harrison • Jul 24, 2015

Help map Mars' south polar region!

The science team of NASA's Mars Reconnaissance Orbiter (MRO) wants your help in mapping out the weird and wonderful features of Mars' south polar region! In partnership with Zooniverse, they've launched a new website called Planet Four: Terrains. The site asks users to browse through MRO Context Camera (CTX) images and identify key features in order to aid in finding targets for the High Resolution Imaging Science Experiment (HiRISE) camera, also aboard MRO.

One of the main uses of CTX is to provide—as the name suggests—context for HiRISE images. This is because CTX provides moderately high resolution (6 m/pixel) coverage over large areas, while HiRISE provides ultra-high resolution coverage (up to 25 cm/pixel) with a much smaller footprint. The two cameras work in concert to help paint a picture of Mars' surface.

Comparison of CTX, HiRISE, and MOC NA image footprints
Comparison of CTX, HiRISE, and MOC NA image footprints CTX, HiRISE, and MOC are three cameras that have provided high-resolution images of Mars. This diagram compares their typical footprint sizes across Ius and Tithonium Chasmata, Mars. Each camera's design makes different trades among resolution and areal extent. MOC on Mars Global Surveyor acquired long, skinny strips at about 6 meters per pixel. CTX on Mars Reconnaissance Orbiter provides images of the same resolution, but covering a much larger area. HiRISE, also on Mars Reconnaissance Orbiter, can take much higher resolution images at about 0.3 meters per pixel, but covers a proportionally smaller area. The base map to this image is the Viking Orbiter MDIM or global mosaic.Image: NASA / JPL / MSSS / Tanya Harrison

Planet Four: Terrains lets users research the different types of features they will encounter around the south pole of Mars, including what these features look like at CTX scale and how scientists think they formed. These features include the descriptively named "spiders" and "Swiss cheese terrain". (There are even "baby spiders"—how cute is that?)

Spiders are thought to form from the release of CO2 gas. There is no known Earth analogue—so far they appear to be an uniquely martian feature. In late fall into winter in Mars' south polar region, CO2 ice deposits onto the surface. Sunlight then penetrates through the translucent layer of ice, warming the underlying ground. This causes the ice to sublimate at the ice-ground boundary, carving channels (the "spiders") into the ground beneath the seasonal layer of ice. The spiders are then exposed in spring and summer as the seasonal ice layer sublimates away entirely.

Spiders at CTX scale
Spiders at CTX scale Ice-free view of spiders in Mars' south polar region, taken in southern summer. This image is shown at ~10 m/pixel, downsampled slightly from CTX's native 6 m/pixel resolution.Image: NASA / JPL-Caltech / MSSS / ASU / Tanya Harrison
Spiders at HiRISE scale
Spiders at HiRISE scale Zoomed in view of spiders visible in the previous CTX image, taken at a different time of year (southern autumn). The spider channels are filled with bright CO2 ice.Image: NASA / JPL-Caletch / University of Arizona

Swiss cheese terrain only occurs in the southern polar ice cap of Mars. The terrain consists of pits and scarps in the CO2 ice, which are actually actively eroding on Mars today as discovered with the narrow-angle Mars Orbiter Camera (MOC NA) aboard Mars Global Surveyor back in 2001. Their erosion arises from differing rates of CO2 ice deposition and accumulation over multiple seasons. The pits eventually form in places where the net rate of CO2 ice sublimation is higher than the net accumulation.

Example of Swiss cheese terrain at CTX scale
Example of Swiss cheese terrain at CTX scale Swiss cheese terrain is characteristic of Mars' south polar cap, thought to form due to sublimation of the cap's CO2 ice.Image: NASA / JPL-Caltech / MSSS / Tanya Harrison

In addition to image guides to teach users what to look for, the website includes a forum where users can interact, ask questions, and discuss their observations. From looking at the forum, it looks like the site has already attracted some enthusiastic citizen scientists!

You can read more about the work behind the Planet Four: Terrains project on the team's blog.

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