LPSC: Lunar Reconnaissance Orbiter results
Posted by Emily Lakdawalla
08-03-2010 14:17 CST
A week later and I am finally getting to the mountains of notes I took on Moon-related talks I saw at the Lunar and Planetary Science Conference (LPSC) held in Houston last week. Unlike previous years at LPSC, the Moon was really the leader, with the most talks and posters, even more than Mars, which has never, to my knowledge, happened in the years (since 1998) that I have been attending the conference. On top of that, there was an especially international flavor to the lunar sessions, because a very large contingent of Chinese scientists had come to the meeting, as well as a smaller Japanese group and the usual European contingent. (There were only a couple of Indian scientists -- there would have been more, but according to Jitendra Goswami of Chandrayaan-1, their visas did not come in time. Grr.)
Today I'll attempt to summarize what I learned about Lunar Reconnaissance Orbiter, which, inevitably, is abbreviated LRO. The conference came just before LRO's first planned release of data to the world via the Planetary Data System, an event scheduled for a week from today, March 15; everything gathered during the period from their initial high orbit through their first three months of operations (through, that is, December 15) is going to be public then. It will be a fire hose of data!
I skipped the first few talks in Monday's session to catch up on the rovers, so came in for Mark Robinson's presentation on LROC's first results (LROC is the Lunar Reconnaissance Orbiter Camera). A few operational things I learned from his presentation: we haven't seen many images from the LROC wide-angle camera instrument yet, which may result in part from an early problem with that camera's flight software that was fixed from September 20 forward. The wide-angle camera has a 60-degree field of view and will eventually produce impressive maps of the Moon in color, but photometric calibration of those images is a huge challenge -- there is a large amount of variation in brightness across each wide-angle image because the Sun-Moon-spacecraft angle varies so much across the image. As a result, color variations across each WAC image are much larger in terms of photometric effects than they are in terms of the actual color variations from point to point across the Moon; so calibrating these images will require a great deal of care. The prime mission's campaign to map the entire surface of the Moon in color began on the day of Robinson's talk, March 1.
Following these updates he marched through numerous specatcular examples of LROC images; his favorites seemed to be ones showing impact melts within and near craters, and it's hard to disagree. He also showed three photos, not yet released to the public, of Russian landers Luna 20, 23, and 24. You can see in the image of Luna 20 a little extension to its shadow -- Robinson said that was the raised sampling arm, "like it's waving at us." Luna 24 was particularly interesting because it appears to have landed exactly at the rim of a small and very fresh bright-rayed crater. So now there is an explanation for a mystery I didn't know about, which is why Luna 24's samples didn't seem to match the composition derived from regional mapping -- it landed in fresh crater ejecta. "Geologic context is everything!" Robinson remarked. He said that if an extended mission is funded, LROC could map the entire Moon at a resolution of 1-2 meters per pixel, which would be astonishing.
Next up was Ben Bussey presenting on results from Mini-RF, a synthetic aperture radar instrument sister to Mini-SAR on Chandrayaan-1. Bussey began a trend in the LRO session, reporting on how the results from his instrument could be used to derive topography -- from that point on, it seemed every single other LRO instrument, no matter its main science goal, was being used to derive topography! In Mini-RF's case, the derived topography is being used to perform correction of the radar images, which can be quite distorted if topography is not taken into account. The main scientific goals of Mini-RF and Mini-SAR involve the mapping of the lunar poles, which (because they are poorly lit by the Sun) are best explored using radar; but Bussey reported that LRO has taken advantage of excess downlink capacity -- LRO has a dedicated ground station at Goddard space center -- to permit Mini-RF to map quite a lot of the Moon's lower latitudes, taking the data on the nighttime leg of the orbit, when the optical instruments can't be used. He showed some radar images of the Apollo 11 and 14 landing sites "just to prove, using a different part of the electromagnetic spectrum, that we did go to the Moon," and remarked that the Mini-RF data would be included in the March 15 data release.
After Bussey came Dave Smith on the Lunar Orbiter Laser Altimeter, or LOLA, which, as of February 25, had accumulated more than 1.2 billion measurements of lunar topography. Their coverage is dense enough that their global topographic digital elevation model has about 250-meter latitudinal resolution, but only 1-kilometer longitudinal resolution at the equator. At the poles, though, where the orbits are closely spaced (they actually cross), the longitudinal resolution is now about 40 meters. He showed a really cool animation of the varying lighting conditions at one of the poles, using a shaded relief map of the LOLA topography as a base. He said they also have produced a preliminary albedo map of the Moon at the laser wavelength of 1064 nanometers. Any photo of the Moon is an albedo map of course, and the LOLA albedo map will be lower resolution than that, but LOLA is an active instrument, so its albedo map is acquired with totally consistent "lighting" conditions (at zero phase), so it's a unique data set.
A really important activity for LOLA is co-registering the LOLA topographic profiles with the LROC high-resolution images; I can affirm from experience that having co-registered MOLA profiles with high-resolution photos of Mars really elevates the quality of geomorphic analysis using orbital data. But, Smith said (with my usual warning that these "quotes" are from my note-taking so I may have made transcription errors, caveat emptor), "Moving LOLA pixels into the right position on LROC images is a challenge. The timing isn't exact. This is a major current activity of the LROC and LOLA teams. Co-registration will be continuing task." Unlike the MOLA team, though, the LOLA team has a very handy tool to improve their understanding of the spacecraft's position in orbit around the Moon: laser ranging directly to the spacecraft. "We measure the range from Earth to LRO in order to position LRO very precisely, using laser ranging to spacecraft from Goddard Spaceflight Center [and elsewhere]. Eventually, improvements in the lunar gravity model will enable 50-meter horizontal positioning of LRO," that is, they'll know where LRO was at all times during the mission to a horizontal precision of 50 meters.
The way this laser ranging works is very cool. It happens when LRO's radio dish is pointed at Earth, while LOLA is active and also ranging to the lunar surface. Smith explained, "The receiver is inside [LRO's] dish, and the laser signal goes through a fiber optic cable -- no power is required -- into the back of the LOLA detector. The beaming is one-way," that is, LRO doesn't send the signal back for two-way ranging, "so it depends upon clocks on both Earth and spacecraft." The Earth clock will be a very stable instrument, the spacecraft one less so. "The LOLA detector alternates between detecting earth range signal and lunar surface signal continuously." There is work to be done before they can spit out their data on LRO's position. "We cannot reproduce LRO's [observed] orbit at 100-m [precision] with the pre-LRO lunar gravity model, not even when we include Kaguya gravity data. This even though Kaguya's gravity model is clearly superior to previous lunar gravity models for science purposes)." This, he explained, is because of LRO's unusually low, 50-kilometer science orbit, which "feels" a lot more small-scale variations in lunar gravity than previous spacecraft did. "Laser ranging has enabled us to understand the behavior of the clock on the spacecraft. It is behaving very well; we have a good clock up there. We will eventually be able to provide better than 1 millisecond timing for all instruments on LRO." That is, they will be able to correct the times spat out by the spacecraft's clock to produce time data precise to 1 millisecond. Still, given LRO's fast, low orbit, "That's 1.5 meters along the orbit -- still not good [detailed] enough for LROC!" LROC images have resolutions a third that length. He showed a chart that indicated the LRO clock is stable enough to notice the monthly effect of relativity, as the Moon speeds up and slows down in its elliptical orbit around Earth. Finally, Smith said, "So far, there has been no degradation in LOLA's lasers." There are two lasers, used alternately. "If on-orbit performance matches ground tests, there's every indication LOLA lasers will operate for three years."
Next up was David Paige, reporting on Diviner results. Diviner looks almost identical to Mars Climate Sounder and is used to measure emission from the lunar surface in thermal infrared wavelengths; they'll get temperature maps of the Moon, which will allow them to determine where things are rockier and where they are dustier, just as TES did on Mars Global Surveyor. Paige said that "At night the Moon cools off to 100 Kelvin. But there are a few interesting thermal anomalies at lower latitudes," places where the surface is unusually warm at night, generally interpretable as fresher, less dusty rock. "Rocks can really affect temperatures; their high thermal inertia allows temperatures to remain higher than 200 Kelvin at night" in some places. Regardless of rockiness or dustiness, "Daytime temperatures are uniform," I read temperatures of around 375 to 390 Kelvin off of his graph. He showed a preliminary global rock abundance map and pointed out that -- younger craters show higher rock abundance values, which is a reality check indicating that their instrument is working. "There are fresh impact craters like Tycho that have fresh blocky material that have very high thermal inertia, which show up as bright red spots on the map. There are also cold spots, likely places covered with uniformly fine material."
That was it for Monday's session, but I picked up a couple more lunar talks on Wednesday afternoon, just before I left. Paul Lucey talked about using Diviner data to map the compositional properties of the lunar surface, essentially treating Diviner like a spectrometer. It is a spectrometer, but it doesn't have as high spectral or even spatial resolution as many orbiting spectrometers do now. "Diviner is not an imager, it is a pushbroom radiometer. It has 21-pixel noodles that can be hundreds or thousands of pixels long. We cover 10% of the equator each month." Still, Lucey showed that with only three wavelengths near 8 microns, you can discriminate among some important rock-forming minerals. In particular, you can notice whether the rocks contain silica-rich minerals like quartz and microcline, minerals that are very common on the highly geologically processed Earth but which are rare (hence, unusual and interesting) on the Moon.
Timothy Glotch picked up the thread of this discussion in the next presentation on highly silicic features on the Moon. Of five lunar features Glotch was focusing on in his talk, he said that four of them are so-called "lunar red spots, which were first seen in the 1970s using ultraviolet to near-infrared spectroscopy." He said that the Gruithuisen domes and Hansteen Alpha, which were previously proposed to be silicic volcanic features, show this high-silica feature in Diviner data. There are also craters like Aristarchus and Lassell that appear to have struck a silica-rich pluton (body of igneous rock that solidified from a melt below the surface without erupting onto the surface), so that the craters and ejecta have this high-silica signature. Then there was a weird spot on the far side of the Moon called Compton-Belkovich, previously noted for having an unusually high amount of thorium, which also showed up as being silica-rich in the Diviner data. Glotch said that other lunar red spots were not remarkable as seen by Diviner; interestingly, they also show unremarkable amounts of thorium. So thorium and silica seem to be linked on the Moon.
Somewhere in here I noted the challenge of the Q and A parts of these lunar sessions, where a lot of Chinese and Japanese scientists were standing up to ask questions. Their English wasn't necessarily any more strongly accented than the English spoken by European scientists; but because their accents are still unfamiliar to American ears, the speakers were having a terrible time understanding the Asian scientists' questions, and vice versa, leading to frustration on both sides. This is something that should improve with time and familiarity, now that there is more and more frequent interaction with scientists from China and Japan. There were also some interesting and amusing cultural divides. In the Monday session, I was told, the front row was filled with Chinese scientists who lifted cameras up and snapped photos of every single slide, something that you Just Don't Do at science conferences, at least at planetary science conferences in America! After Monday, session chairs opened every session with an announcement telling people not to do that. (By the way, it is kosher to ask presenters for a copy of their presentation or selected slides from their presentation, which they may or may not choose to provide to you.)
One Chinese questioner was asking either Bussey or Smith (I don't remember which, but believe it was Smith) about the accuracy of their tracking methods. Once the speaker managed to understand the question, he acknowledged the validity of the question but then became a bit testy, pointing out that even when Japan and China release their science data (Japan has done so for Kaguya, China has not for Chang'E, despite having announced that the data release would happen on March 1) that neither JAXA nor the Chinese space agency has any plans to release tracking data on the spacecraft, something that limits the data's usefulness for science. "At the moment only NASA releases this," he said.
There were many, many more presentations from LRO, but those were all I had time to sit in on as I flitted from Moon to Mars and other planets and moons. And that was just from half the week!
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