Gullies on Mars Could Have Formed by Melting of Water Ice

Orbital image of martian gullies. Credit: NASA/JPL-Caltech/University of Arizona.

The surface of Mars is littered with geomorphological features that bear some resemblance to features on Earth. Of particular interest are channels or gullies, which have long fascinated the scientific community because of the possibility that they were created by water. The martian gullies are mainly found in the mid-latitudes where near-surface water ice is stable. Their distribution, however, includes elevated regions where the present-day atmospheric pressure on Mars is always below the point where liquid water is stable (the triple point of water). Thus, ice would sublime straight to a vapor phase rather than melt to form a liquid. The sublimation of carbon dioxide (CO2) has, therefore, been considered an alternate hypothesis for the formation of gullies.

J. L. Dickson of Caltech and colleagues recently conducted climate modeling to investigate conditions under which liquid water would be stable enough to create the martian gullies. As Mars does not have a large, close moon as Earth does, it is subject to considerably more axial tilt (obliquity) than Earth. The results of this modeling showed that when Mars is tilted to 35° (vs. its current 25°), the gully-forming regions of Mars experience pressures and temperatures conducive to the existence of liquid water (pressures > 612 Pa, temperatures > 273 K).

The authors then proposed a scenario for the formation of the gullies that involves both H2O and CO2. In this model, gullies first form during periods of high obliquity when liquid water can create the channels. As Mars returns to lower obliquity, melting ceases and sublimation of water dominates, leaving loose material collecting in the gully. Finally, the channel serves as a cold trap for CO2 and H2O; seasonal variations in temperature can then produce the occasional and localized modifications to the overall gully, which are observed today.

This scenario manages to combine both the H2O and CO2 hypotheses for gully formation and is consistent with the distribution of gullies observed on Mars, as well as gully morphology, the observed modern modifications to gullies, and the current lack of new gully formation on Mars. READ MORE