The Objects > Lunar Science
Researchers Estimate Ice Content of Crater at Moon's South Pole
kodemunkey:
--- Quote ---NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft has returned data that indicate ice may make up as much as 22 percent of the surface material in a crater located on the moon's south pole.
The team of NASA and university scientists using laser light from LRO's laser altimeter examined the floor of Shackleton crater. They found the crater's floor is brighter than those of other nearby craters, which is consistent with the presence of small amounts of ice. This information will help researchers understand crater formation and study other uncharted areas of the moon. The findings are published in Thursday's edition of the journal Nature.
"The brightness measurements have been puzzling us since two summers ago," said Gregory Neumann of NASA's Goddard Space Flight Center in Greenbelt, Md., a co-author on the paper. "While the distribution of brightness was not exactly what we had expected, practically every measurement related to ice and other volatile compounds on the moon is surprising, given the cosmically cold temperatures inside its polar craters."
--- End quote ---
http://www.nasa.gov/mission_pages/LRO/news/crater-ice.html
JFincannon:
From the Nature article:
"The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility."
Note the 1 micrometer! Zuber states this means ~100 gallons in this crater! I guess there could be more but their instrument is only good enough for that first 1 micrometer.
The paper states “The 40 km by 40 km topographic model of Shackleton is derived from 5.358 million elevation measurements with an average of 0.34 altimeter measurements in each 10-m square area.” That means there is not even 1 laser beam bounce within a 100 square meter area! This is equivalent to 6% real data of the surface. Imagine your computer screen only showing 6% of the pixels. How do they get what the empty spaces are? Interpolation. So what is really in the empty spaces? Holes, rocks, ice crystals, snowmen or just smooth flatness?
Also, interestingly, the Zuber paper states that they used only reflectance data from between May 11, 2010 and May 30, 2010. This means the study used only a fraction of the 5.358 million data points (for the entire time period LOLA has operated) that were used to generate the laser topographic image of Shackleton Crater area. Using the LOLA public data site, it is possible to see that the fraction of used data points (in the short time range) is only _4%_(!) of the 5.358 million points. If you translate the number of used laser data shots, it turns out to be one, 5 meter diameter laser shot for every 83 meter by 83 meter area (the equivalent of a football field’s area) or 0.3% of the surface area. Try to recognize your computer screen with 0.3% of the pixels lit up! Hummmm. I am not going to assert that there are a lot of chunks of ice, ski slopes, snow men, etc in the vast space between these laser shots, but I do think they need to be a little more qualified in their statements. At least they emphasized the 1 micrometer.
One question unanswered is the quality of the laser energy measurements. Can they correlate reflectance with actual surface objects? Just looking at the data shows some high reflectances sometimes (up to 10%, but mostly between 3-6%). Is this just a bad reading? Or is it something real?
kodemunkey:
Maybe they used a small data sample as a proof of concept?
JFincannon:
No, I don't think so. LRO will not be able to operate long enough to get 100% of the data. If it has only got 6% since it started operating, the batteries on the spacecraft (or lasers) will have long since failed (or solar arrays degraded due to radiation hits by the Sun) before they even get 12%.
Scientists think differently than we do about this. They seem to think that statistically, on average, they are providing valid data. They did state in the paper that they "bin" data in these large area "bins", but they do not explain the ramifications of it, leaving this to the reader to determine. This annoys me. They should have a section of the paper stating specifically that 1) there may be alot more ice/water but since the laser cannot see below 1 micrometer, then it must wait for another instrument, 2) there may be ice between the 0.3% of the surface points that was painted by laser light.
A prior paper released a week or so before which had a different instrument (LEND) offered a depressing set of data about there being less water than first thought: the abstract states explicitly..."This means that all [permanently shadowed regions, PSRs], except those in Shoemaker, Cabeus and Rozhdestvensky U craters, do not contain any significant amount of hydrogen in comparison with sunlit areas around them at the same latitude." But after reading the actual paper, I see that their list of examined craters, a small fraction of ALL permanently shadowed regions, does not even include Shackleton Crater. The list has all the craters around Shackleton, but NOT Shackleton. Curious. And ones of the coauthors is Zuber. But in the paper they state "The full list of all PSRs both at the North and South poles consists of more than several thousand areas ranging from very small (<1 km2) up to quite large regions of more than 1000 km2. ....The set selected for analysis in this study all have areas >100 km2. The list of tested PSRs is presented in Table 1." This list is of 50 PSRs. So we went from a list of "several thousand" areas, down to 50 (minus Shackleton crater for some unknown reason but which is large enough to fit their arbitrary >100 km2 filter) which somehow means "ALL" PSRs. I imagine they chose large PSRs since the LEND data is of 1.5 km resolution and they can identify local landforms of 10 km by 10km.
Another limitation of this analysis was that they used the LOLA dataset to determine the state of "permanent" shadow. As I have shown, you can only get a certain coverage amount at the poles. The laser paints the surface densest at the poles, but still only paints ~6% of it. They are using a curvefit of these 6% of the surface to generate the map to examine illumination. So some rocks in a PSR could be sticking into the Sun (the Sun is at a shallow angle, near the horizon, at the poles), but are missed because they were not in the 6% of laser data. Conversely, the terrain casting shadows may be overestimated due to the sparse number of points.
kodemunkey:
How long is the LRO designed to last for?
Navigation
[0] Message Index
[#] Next page
Go to full version