3. Modern Water

Discovery: vast near-surface ice deposits, and water in mid-latitude glaciers and both polar caps. Active water cycle includes snow and frost. Sheet ice and gullies in mid-latitude craters suggest recent episodes of liquid water formation.

Significance: completely shifted the paradigm of Mars today from a static, arid world to a planet still being shaped by water. If life evolved there, conceivably it may still survive.

Low neutron counts indicate hydrogen-enriched soil (H2O). Graphic: NASA/JPL/University of Arizona	Graphic: NASA/JPL/Los Alamos National Laboratory

The Martian water visible today is scanty and confined to the polar caps. But in a stunning discovery, Mars Odyssey’s gamma ray spectrometer showed that near-surface water deposits reach far into temperate latitudes – as much as 60º from the poles. The mass of water required to explain the data indicates ice or ice-cemented ground meters thick. Away from the poles exposed ice would sublimate, but ice was calculated to be stable in the pores of frozen soil particles. To verify the result, the Mars Phoenix lander was sent to a region with near-surface water, with the expectation that chemical tests would be needed to reveal it.

Boynton, W.V. et al., Science 297 (5578), 81 (2002).

Feldman, W. C. et al., Science 297 (5578), 75 (2002).

Mitrofanov, I. et al., Science 297 (5578), 78 (2002).

Mellon and Jakosky, J. Geophys. Res. 100, 11781 (1995).

Phoenix set down on a sheet of water ice beneath surface soil. (left) Soil blew away during touchdown, exposing ice beneath the lander. (This outcrop was dubbed “Holy Cow!”)

Segregated (pure) ice was unexpected. It indicates recent liquid water, but the site’s maximum summer temperature was –19ºC. This suggests that diffusion is important in the Martian water cycle: Equilibrium exists between the solid ice and atmospheric water vapor, with water molecules diffusing through the soil cover during daily and seasonal temperature changes. Under these conditions on Earth, interactions between water molecules and soil surfaces interfere with ice formation, so that liquid water films form on soil particles at temperatures below freezing (producing frost heaves, for example). During winter, when the films are subjected to deep cold from above, the result can be subsurface ice tables a few inches thick. The prospect of liquid Martian groundwater is of great interest: on Earth, segregated ice and its surroundings are an important bacterial habitat.

At its landing site 68ºN, the Phoenix lander witnessed snowfall (again unexpected) and frost; both interacted continually with the soil. (right) Frost on the ground around the lander. Color enhanced.

• Smith, P.H. et al. “H2O at the Phoenix Landing Site,” Science 325, pp. 58-61 (2009).
• Whiteway, J.A. et al. “Mars Water-Ice Clouds and Precipitation,” Science 325, pp. 68-70 (2009).
• Zent, A. “A Historical Search for Habitable Ice at the Phoenix Landing Site,” Icarus 196, 385 (2008).

Images: NASA/JPL-Caltech/University of Arizona/Max Planck Institute

Closeup of water ice protruding from the soil beneath the lander. Image: NASA/JPL-Caltech/University of Arizona/Max Planck Institute	In ‘Dodo’ trench, dug by Phoenix’s robot arm, exposed water ice sublimates over four sols. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University

From Plaut et al. The white track indicates MRO’s orbit. In the radar trace (upper), arrows mark the boundary between ice and the layer below; in the lower visual photo, arrows mark the edges of the glacier. SHARAD observation 214501. Lower photo: Mars Express High Resolution Stereo Camera. Annotations: Geophysical Research Letters	From Holt et al. The dotted line indicates MRO’s orbit. MRO Context Camera image P01_002294_1349. NASA/JPL/ Malin Space Science Systems

Temperate-zone glaciers: Common surface features resembling glacier traces (and glaciers) in temperate zones have been controversial, since surface ice should not form at temperate latitudes. But huge glaciers exist there, concealed (and protected) by surface debris and revealed by MRO’s ground-penetrating radar. They appear to be remnants from a recent ice age. (See: Climate Change) The examined ice masses extend for miles and are up to half a mile thick. If similar features planetwide are also glaciers, they contain a significant fraction of Mars’s water supply.

Holt, J. et al. “Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars,” Science 322 (5905), 1235-1238 (2008).

Plaut, J. J. et al. (2009), “Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars,” Geophys. Res. Lett., 36, L02203.

False-color HiRISE photo PSP_003583_1425: NASA/JPL/University of Arizona	Closeup (about 640m wide) showing braiding and convergent channels, with boulders scattered in a manner suggesting flow deposition.

Gullies exist in many crater walls. Dating them is challenging, but most appear geologically recent (they cut across other features), and a few are known to be new (See: Modern Processes). Many strongly resemble terrestrial gullies formed by water action. Liquid water is not stable on the Martian surface, so whether and how it was involved in gully formation is controversial. Suggestions include freezing point depression of groundwater by dissolved salts (eutectic brines can have melting points as low as -70ºC), and the gradual melting of emplaced snowpack left from a recent ice age. (See: Climate Change)

Malin, M.C. and Edgett, K.S. “Evidence for recent groundwater seepage and surface runoff on Mars.” Science, 288, 2330-2335 (2000).

Christensen, P.R. “Formation of recent Martian gullies through melting of extensive water-rich snow deposits.” Nature 422, 45-48 (2003).

Malin, M.C. et al. “Present-day impact cratering rate and contemporary gully activity on Mars.” Science 314, 1573 (2006).

View a gully “flyover” constructed from HiRISE images: http://hirise.lpl.arizona.edu/sim/movies/mro_flyover_640.mov