Michael PostonApr 03, 2013

LPSC 2013: Seeing in Permanent Shadow

While I was at the Lunar and Planetary Science Conference I noticed a sudden spike in tweets Wednesday afternoon about a talk given by Nancy Chabot. I was chagrined to have missed such a popular talk, so am very pleased to have found (through the Young Scientists for Planetary Exploration Facebook group) someone who was there and who was willing to write about it: Michael Poston. Thanks very much to Michael for this contribution! Unfortunately, the thing that made people most excited about Nancy's talk -- her images, which Michael will describe to you below -- are not yet available on the Web, not until they get released through a peer-reviewed publication. I will be sure to post them when they come out! --ESL

The case for water ice hidden in permanently shadowed regions at the north pole of the planet Mercury received another boost recently. On Wednesday March 20, 2013 at the Lunar and Planetary Science Conference, Nancy Chabot presented the very first visible-light images of what is in the shadows of these polar craters.
As Emily reported previously, radar return from Arecibo shows signals consistent with, but not unique to, large deposits of water ice inside the permanently shadowed regions at Mercury's poles. Numerical models suggest temperatures plenty cold to maintain water ice for millions of years, exactly where the radar signals show potential ice. Measurements of neutron depletion are consistent with hydrogen-bearing molecules, such as water, at the poles, but cannot resolve their exact location in the same way as radar can.  The final piece of the pre-existing story is the strength of the reflection from the MESSENGER Laser Altimeter instrument, which measures the strongest reflection from its 1064 nm infrared laser in the coldest parts of permanent shadow.

Permanently shadowed, radar-bright regions on Mercury
Permanently shadowed, radar-bright regions on Mercury Shown in red are areas of Mercury’s north polar region that are in shadow in all images acquired by MESSENGER to date. The polar deposits imaged by Earth-based radar are in yellow. This comparison indicates that all of the polar deposits imaged by Earth-based radar are located in areas of persistent shadow as documented by MESSENGER images.Image: NASA / JHUAPL / CIW / NAIC, Arecibo Observatory

Any one of the above pieces of data would not be very convincing on its own, but with all these items pieced together the case for water ice is actually quite strong. However, for us humans, even scientists, it is often said that 'seeing is believing'. The new piece of data that Dr. Chabot presented gives us that first chance at seeing into the shadows. After several months of building up statistics and tweaking the settings on the MESSENGER spacecraft's cameras to collect the light bouncing off of nearby sunlit crater walls into the shadows, they came out with their first two satisfactory images in time for presentation at LPSC.

Dr. Chabot wasted little time prepping the room full of hundreds of eager scientists for what we were about to see and jumped quickly to the images (which should be considered preliminary). The first image was of Prokofiev crater, the largest crater near Mercury's North pole. Prokofiev shows a large region on the sunward side that is cold, permanently shadowed, reflective to the laser, and radar bright. Dr. Chabot first showed the normal visible image with the shadow, then blinked to the shadow-piercing image. The room burst into 'oohs and aahs' at what we saw! There was clearly an area of higher reflection where the shadows had just been, consistent with a high-albedo material such as water-ice.

Prokofiev's permanent shadow
Prokofiev's permanent shadow Prokofiev, named in August 2012 for the Russian composer, is the largest crater in Mercury's north polar region to host radar-bright material. MESSENGER has found evidence that within the cold, dark, permanently shadowed regions of Prokofiev, water ice is exposed on the surface.Image: NASA / JHUAPL / CIW

The next thing Dr. Chabot showed us was perhaps more impressive. She blinked the radar-brightness map of Prokofiev on top of the image and showed the excellent agreement of the reflective areas with the radar signal. One potentially confusing element of the image is that it showed a distribution of craters even on the putative ice. However, ice can be cratered, so this in itself is not a major problem.

An example of one of Dr. Chabot's early exploratory images from her conference abstract, showing Chesterton crater near Mercury's south pole, is below. The new images that Chabot presented at the meeting were much more impressive than this; the correlation between radar bright zones and higher visible light reflection was much more obvious.

In Chesterton's permanent shadow
In Chesterton's permanent shadow Average MDIS mosaic of Chesterton crater (37 km diameter) with: (a) Arecibo radar image in yellow [1]; (b) WAC broadband-filter, 10-ms-exposure image overlain, showing details of the permanently shadowed surface within the crater; (c) both radar (yellow) and WAC broadband filter images.Image: NASA / JHUAPL / CIW / NAIC, Arecibo Observatory / Nancy Chabot

Dr. Chabot's second image was of Kandinsky crater, a central-peak crater with a large area of shadow covering nearly its entire floor. Consistent with the shadows, temperature predictions, laser reflectance, and radar brightness data, the shadow-piercing image showed a uniformly bright floor. However, in contrast to Prokofiev's deposits, the floor of Kandinsky crater shows very few craters, suggesting Kandinsky to be a much younger crater. The age of the crater is important for determining when and where the ice came from, be it Mercury's interior, delivery by comets, or some means of producing the water slowly over time from the solar wind, but Dr. Chabot did not touch on this topic.

Kandinsky crater, Mercury
Kandinsky crater, Mercury Kandinsky crater is located very close to Mercury's north pole, and its floor is permanently shadowed.Image: NASA / JHUAPL / CIW

Let me close by reminding the reader that data shown at conferences is often preliminary and until the paper comes out (often even after the paper comes out!) the story can still develop quite a lot with additional data. For example, one major issue with this new data is that is appears to contradict the laser altimiter albedo data showing weaker reflection near the edge of the ice patches, which was hypothesized to be due to a dark material. Clearly, more observations are needed. However, as of the time of Dr. Chabot's presentation, the MESSENGER spacecraft had just completed its extended mission and was up for review for another extension. Considering the recent accomplishments of the MESSENGER team and how much can still be learned from this spacecraft, I certainly hope it gets another year.

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