Water Ice on the Innermost Planet

In the early 1990s, scientists noticed features on Mercury that appeared bright on radar. It was suggested that the features are water ice. The reflectivity and other properties are similar to water ice at the Martian south pole, and icy satellites. Mercury has a very slight axial tilt (0.034 degrees, compared to the approximately 23.4 degrees of Earth). This means that some areas, such as the interiors of craters near the poles, are permanently in shadow, never receiving direct sunlight.
For a while, this idea sat without much more being able to be done with this. Not all of the surface was mapped by images, so the theory about the radar-bright regions could not be confirmed. Mariner 10 flybys in 1974 and 1975 provided images for under half of the poles.
The MESSENGER (MErcury Surface, Space ENvironment GEochemistry, and Ranging) spacecraft became the first spacecraft to orbit the innermost planet in 2011. With a better resolution in its data, MESSENGER found more radar-bright spots, some indeed in the interior of craters. Using this data, the author’s of today’s articles inspected the radar-bright features, their properties, and whether or not they are water ice. Published in the American Geophysics Union’s publication Journal of Geophysical Research: Planets in 2013, Nancy L. Chabot, Carolyn M. Ernst, John K. Harmon, Scott L. Murchie, Sean C. Solomon, David T. Blewett, and Brett W. Denevi are the authors of “Craters hosting radar-bright deposits in Mercury’s north polar region: Areas of persistent shadow determined from MESSENGER images.”
The improved resolution of the MESSENGER data led to seeing features over ten degrees from the poles, associated with small craters (diameters less than 10 kilometers (6.2 miles)). In its yearlong primary mission, starting in March 2011, MESSENGER imaged all of the south pole repeatedly, and took images of the majority of the north pole as well (the extensive imaging seen in the south pole could not be repeated, due to the orbit of the spacecraft). However, nearly all of Mercury was imaged. With a variety of sunlight angles, this gave a good indication of where permanently shadowed areas are, and also showed images taken around local noon, where the shadows will the the smallest, constraining the area that is in permanent shadow.
An artist's impression of Mercury, with enhanced color for the planet. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
An artist’s impression of Mercury, with enhanced color for the planet. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Around seventy percent of the radar-bright features are found in persistently shadowed areas, as anticipated. For the remaining features, they are within 4 kilometers (2.5 miles) of the permanently shadowed areas, or in the interiors of small craters. This is within the inherent uncertainty in the data. Examining 305 craters with diameters equal to or less than 10 kilometers (6.2 miles) revealed that 92 of them had radar-bright features.
Almost all craters within ten degrees of the north pole contained radar bright features. The majority of the exceptions had a diameter of over twenty kilometers (12.4 miles). The south pole was nearly identical, but with exceptions near 0 degrees east longitude.
Below 80 degrees north, features preferred to stay near 90 degrees east, and (less so) 270 degrees east, with much fewer of the radar-bright features appearing near 0 or 180 degrees east. This is because Mercury’s non-circular orbit leads to “cold poles” (90 and 270 degrees east), which occur when farthest from the sun, and “hot poles” at 0 and 180 degrees. The cold poles receive much less sunlight than the hot poles, and show a difference of 130 degrees in temperature (Celsius/Kelvin scale; a 234 degree difference in Fahrenheit).
Maps created by MESSENGER's scientific equipment. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Maps created by MESSENGER’s scientific equipment. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
The temperature of permanently shadowed areas depends on the size of the crater and the latitude. Poleward of 10 degrees, water ice would be theoretically cold enough to be stable on the surface for the timescale of a billion years. Towards the equator from 82 degrees, in diameters less than 40 kilometers (24.5 miles) the ice would not be stable on these timescales, as it would have slightly warmer temperatures. A slight covering of rocky debris could act as insulation, however, and keep it stable. Additionally, the rocky covering could explain slight differences in sets of radar data.
There is, however, one more question that must be addressed: who said it was water ice in the first place? One suggestion was elemental sulfur. This can deal with higher temperatures, but if the radar bright deposits were elemental sulfur, a sulfuric ice cap would be expected–and they are not present. Elemental sulfur would also not explain the reflectivity and other properties similar to ice. Another suggestion is that the features are silicates. However, the moon’s poles do not have similar radar signatures, and while the silica compositions of the moon and Mercury differ, this still is illogical. So the features are not likely to be sulfur or silicates; back to water ice. We are not sure where it came from or when it arrived, but the planet closest to the sun indeed seems to have a form of water on its surface.

 

Bibliography
Artist’s Impressions: MESSENGER” N.p., n.d. Web.
The image of MESSENGER at the beginning of the article, from the spacecraft’s website done by NASA and collaborators.
Chabot, N. L., C. M. Ernst, J. K. Harmon, S. L. Murchie, S. C. Solomon, D. T. Blewett, and B. W. Denevi (2013),Craters hosting radar-bright deposits in Mercury’s north polar region: Areas of persistent shadow determined from MESSENGER images, J. Geophys. Res. Planets, 118, 2636, doi:10.1029/2012JE004172.
Today’s main article.
National Aeronautics and Space Administration. “Unmasking the Secrets of Mercury.” N.p., 2015 May 1. Web.
NASA page from which I took the final image in the post.
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