Tuesday, October 28, 2014

Polar Ice Deposits on Mercury

NASA’s MESSENGER spacecraft has obtained the first ever optical images showing the presence of water-ice and other frozen volatiles within the permanently shadowed interiors of craters near Mercury’s north pole (Chabot et al., 2014). It may come as a surprise that water-ice is present on Mercury since Mercury is the closest planet to the Sun and surface temperatures at its equatorial regions can soar above 400°C. However, near Mercury’s poles, there are a number of craters whose interiors are permanently shadowed from the Sun. Since Mercury does not have an atmosphere to transport heat around the planet, the permanently shadowed interiors of these craters serve as cold traps where water-ice and other volatiles can remain frozen there.

Figure 1: Mercury.

 Figure 2: Locations of water-ice deposits in the shadowed interiors of craters on Mercury.

Over two decades ago, Earth-based radar observations provided the first indications that water-ice might be present on Mercury’s poles. MESSENGER entered orbit around Mercury on 18 March 2011 and has been observing the planet from orbit ever since. In late 2012, the presence of polar water-ice deposits on Mercury was confirmed by MESSENGER through a combination of observations involving neutron spectrometry (Lawrence et al., 2013), measurements of surface reflectance at the near-infrared wavelength of 1064 nm (Neumann et al., 2013) and thermal modelling (Paige et al., 2013).

The polar deposits of water-ice and other frozen volatiles on Mercury were imaged using the Wide-Angle Camera (WAC) on MESSENGER’s Mercury Dual Imaging System (MDIS). Although the polar deposits never receive direct sunlight, they could still be imaged by taking advantage of the very low levels of sunlight scattered off illuminated crater walls. Images of the permanently shadowed interior of the 112 km wide Prokofiev crater, the largest crater near Mercury’s north pole, show an area with widespread surface water-ice deposits. The area shows up in the WAC images as a region with higher reflectance compared to its surroundings.

Numerous smaller craters cover the floor of Prokofiev crater. The area with surface water-ice deposits within Prokofiev crater has a similar cratered terrain as the neighbouring sunlit surface. This indicates that the water-ice deposits were placed there after the formation of the underlying craters, suggesting that the water-ice deposits were placed there relatively recently instead of billion of years ago. Furthermore, the water-ice deposits appear to be uniform, again implying a recent emplacement. Because if the water-ice deposits were there before impacts excavated the craters, a patchy appearance would result since the craters and their ejecta would have buried parts of the water-ice deposits.

WAC images of other craters with permanently shadowed interiors show areas of lower reflectance believed to be water-ice deposits covered by a thin, overlying layer of dark, organic-rich volatile material. These lower reflectance deposits extend to the edges of the permanently shadowed regions and terminate sharply. The sharp boundaries indicate that the deposits are relatively young since the long process of lateral mixing by impacts has yet to smudge the boundaries. WAC images of surface volatile deposits in Mercury’s polar craters show that these deposits are relatively young. The deposits were either delivered to the planet recently or continuously restored at the surface through an ongoing process.

- Chabot et al., “Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft”, Geology 15 October 2014, v. 42, no. 10
- Lawrence et al., “Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements”, Science 18 January 2013: Vol. 339 no. 6117 pp. 292-296 DOI: 10.1126/science.1229953
- Neumann et al., “Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles”, Science 18 January 2013: Vol. 339 no. 6117 pp. 296-300 DOI: 10.1126/science.1229764
- Paige et al., “Thermal Stability of Volatiles in the North Polar Region of Mercury”, Science 18 January 2013: Vol. 339 no. 6117 pp. 300-303 DOI: 10.1126/science.1231106