Ice on Mercury

The Pre-MESSENGER Picture

Mercury would seem to be one of the least likely places in the solar system to find ice. The closest planet to the Sun has temperatures which can reach over 700 K. The local day on the surface of Mercury is 176 earth-days, so the surface is slowly rotating under a relentless assault from the Sun. Nonetheless, Earth-based radar imaging of Mercury has revealed areas of high radar reflectivity near the north and south poles, which could be indicative of the presence of ice in these regions (1-3). There appear to be dozens of these areas with generally circular shapes. Presumably, the ice is located within permanently shadowed craters near the poles, where it may be cold enough for ice to exist over long periods of time. The discovery of ice on the Earth's moon can only serve to strengthen the arguments for ice on Mercury.

How was the evidence for ice found?

Why are these radar-bright areas thought to be ice?

Ice is highly radar reflective and the radar reflections off ice tend to be highly depolarized, unlike typical silicate rock which comprises the bulk of Mercury's surface. While not as highly reflective as other icy solar system objects, such as Europa, Ganymede, and Callisto, these areas are still significantly more reflective than silicate material. Moreover, the depolarized nature of the reflections is also an indicator of water ice. The Arecibo results show that the radar reflective areas are concentrated in crater-sized spots. At the south pole, the location of the largest area appears coincident with the large crater Chao Meng-Fu (shown at left) and the smaller areas with other identified craters. At the north pole, much of the area containing the radar bright spots was not imaged, and so cannot be correlated with known craters. However, for the imaged areas at both poles most of the areas have been loosely correlated with known craters (3). Craters near the poles could provide the permanent, or near-permanent (see 5), shading required for ice to exist on Mercury. The radar results indicate the reflective areas are probably relatively uncontaminated ice. However, the lower reflectivity compared to pure ice features indicates the ice may be covered by a thin layer of dust or soil or else does not completely cover the crater floor (6). Note that no direct unequivocal detection of ice has been made. The coincidence of the radar bright areas with large, possibly permanently shadowed, polar craters is strong circumstantial evidence for ice. However, the radar reflections could be explained by an enhancement of some other radar reflective material, such as metal sulphides or other metallic condensates, or precipitated sodium ions.



How can ice survive on Mercury?

How did the ice get there originally?

How can this discovery be tested?

Direct observations of Mercury from Earth are difficult because Mercury is so close to the Sun. The only effective way to study the polar regions beyond radar observations is to send a space probe equipped with an imager and spectrometry instruments. Missions to Mercury are difficult because the planet is deep in the Sun's gravitational well. The only mission to visit Mercury was Mariner 10 (shown at right) which had three flybys in 1974 and 1975. Each of these flybys occurred when the same portion of the planet was lit by the Sun, so only about half the planet was imaged. By their very nature, the interiors of shadowed craters are too dark to image, so these pictures do not shed any light on whether or not ice exists inside these craters. A mission to Mercury called MESSENGER launched in 2004 went into orbit around Mercury in 2011.



References

Author/Curator:

Dr. David R. Williams, dave.williams@nasa.gov

NSSDCA, Mail Code 690.1

NASA Goddard Space Flight Center

Greenbelt, MD 20771

+1-301-286-1258



NASA Official: Dr. David R. Williams, david.r.williams@nasa.gov

Last Updated: 27 November 2012, DRW