Dry ice on Mars

Earth’s obliquity—the angle between its spin and orbital axes, currently 23.4°—doesn’t change much, due to the stabilizing influence of the Moon. Mars, in contrast, doesn’t have a large moon, so its obliquity is less constrained, wobbling by up to tens of degrees on time scales of 10⁵–10⁶ years. The resulting variation in solar heating at the Martian poles has a profound effect on the polar icecaps: Substantial amounts of material sublimate and refreeze, creating a complex structure of overlapping layered deposits. Most of those layers are primarily water ice. But now, radar data from the Mars Reconnaissance Orbiter reveal at least one large deposit, shown in the figure, of solid carbon dioxide. Because CO₂ ice and H₂O ice have different indices of refraction, a layer’s composition affects how the radar sees the layers under it. If the deposit in question were H₂O ice, it would yield unrealistic distortions in the underlying topography; the refractive index that gives the most realistic topography is an excellent match to CO₂ ice. If the deposit were vaporized—as it certainly was at some point in the past, possibly around 600 000 years ago when the obliquity was particularly large—it could nearly double the mean atmospheric pressure, from 6 mbar to 10.5 mbar. Such an increase would have important implications for the frequency and intensity of dust storms and the stability of liquid water on the Martian surface. (R. J. Phillips et al., Science, in press, doi:10.1126/science.1203091 .)—Johanna Miller