New Discovery on Mars: Ephemeral Polar Water Ice Mapped

After decades of studying the climate conditions of Mars, scientists have recently made a breakthrough observation. At the last conference of the American Geophysical Union (AGU) in San Francisco, Dr. Adrian Brown reported on the latest results of the instrument that led his team to find water on Mars. Brown, a research scientist at the SETI Institute who studies climate similarities between Mars and Earth, says that finding water on Mars makes the potential for past or even future life on Mars much better.

Brown's team found a substantial amount of ephemeral (short lived) water ice in the polar regions of Mars. Early models of the s easonal caps of Mars suggested the polar caps would be pure carbon dioxide (CO₂), and indeed, we now know that Mars' seasonal polar caps are 99% CO₂ ice. Later thinking on the subject predicted that enough water existed in the Martian atmosphere to form a band, or "annulus," of water ice around the polar caps as they receded during springtime. This was subsequently confirmed in the northern hemisphere, but water ice in the southern hemisphere remained elusive and difficult to find until now.

The Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, is the instrument that made possible the recent discovery of Brown's team. Johns Hopkins University in Maryland operates CRISM, which is carried onboard NASA's Mars Reconnaissance Orbiter. CRISM has twenty times the resolution of past Mars mineral mappers, allowing an unprecedented analysis of surface content, as well as the first compositional mineral maps of Mars. It collects a digital image made up of pixels; each pixel spans 15 to 20 meters on the surface of Mars. These pictures are then compiled or examined individually as surface footprints. "CRISM could take a picture of two Martian houses, just to give you an idea of how detailed these images are," Brown explained.

The results of the CRISM observations identify some striking similarities between the Earth and Mars. The recent study of the first CRISM data — reveals frozen H₂0, which appears in small patches within and on the edge of Mars' southern polar cap as it recedes in springtime. In addition to seeking this form of ice, Brown also studies ice composed of CO₂, with his search focused on the south polar region of Mars. The polar regions of both Earth and Mars have dynamic effects on the rest of the planet. The Sun's radiation on Earth and Mars has comparable effects, and mapping of the polar regions has lead some researchers to suggest the controversial idea that Mars is showing signs of global warming, much like on Earth.

A close examination of data from CRISM shows an image timeline of the seasonal south polar ice cap, as CO₂ ice makes the transition directly from a solid to a gaseous form in a process called sublimation. The sequence of images in this movie shows the strength of the 1.4 micron absorption band of CO₂ in the seasonal cap of the south pole of Mars. Red colors show deeper absorptions, typically due to larger grains or greater abundance of CO₂ ice. Blue shows little or no CO₂ ice, and green and yellow are in between. This allows scientists to track the receding edge of the CO₂ ice cap through the spring. Each frame of the movie is a combination of images acquired by CRISM over a 14-Earth-day period.

The south polar cap of Mars grows and retracts seasonally, similar to the polar caps on Earth. As Brown said, "Understanding that whole system as a function of time is really important and what CRISM allows us the opportunity to do. And that is the work that we were reporting on at the recent AGU conference, looking at the spring recession of CO₂ ice. We found some pretty interesting, unexpected effects that we haven't been able to find before with this CO₂ ice and water ice interaction."

Brown and the CRISM team intend to examine the so-called spring recession data more closely to determine whether the water ice is present in the form of clouds, or whether it is part of a surface deposit. The ephemeral nature of these deposits and their small extent have meant they have been overlooked in the past, but as CRISM and Mars Reconnaissance Orbiter carry out their continuing mission, the Mars community is gaining a greater understanding of what is happening in the most dynamic parts of the red planet.

For more information about Adrian Brown's research on Mars, visit: http://abrown.seti.org

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