KAGUYA’s last images
Published on Jun 20, 2009 at 3:36 pm.
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Filed under moon, space exploration.
Japan’s KAGUYA (formerly SELENE) probe crashed into the surface of the Moon as planned this past week. Like many space probes to the Moon, the last action commanded from Earth was to crash into the Moon. This is in part a safety move to make sure that the dead spacecraft does not pose a hazard to any later lunar missions. However, it is also an opportunity to scrape one last bit of scientific data from the spacecraft. The spacecraft crashed into the Moon very near the Moon’s south pole. This is a very important and very poorly studied region of the Moon. The dynamics of orbits means that it is easiest to send a spacecraft from Earth to orbit the Moon nearer the Moon’s equator. Polar orbits are tougher. So, the Moon’s polar regions have been the least studied. Yet, there is reason to suspect that comet impacts on the Moon may have temporarily enshrouded the Moon in water vapor over the history of that body. The floors of deep craters in the Moon’s polar regions might never see sunlight. Thus, those crater floors would be cold — very cold. Any water vapor drifting into those craters would freeze to the surfaces there. In the lab, I’ve used devices called cold traps in vacuum systems. These traps are simply metal plates that are in thermal contact with a reservoir of liquid nitrogen (or some other cryogenic substance). Though the pressure in the vacuum system is near zero, there is always a little gas left. As the gas molecules in the system (water vapor, grease outgasing, residual atmosphere, etc) contact the metal plates, they tend to stick. The pressure in the system drops as more gas is removed by the cold traps. A very similar thing may happen in the craters in the Moon’s polar regions. At least, that is the theory.
I remember being at a conference when the radar data from the Clementine mission suggested that perhaps some of that water ice had been found in the lunar polar craters. Then, over a year later, it hit the public airwaves, with news reporters suddenly jumping on the news. There was a special Nightline segment on the discovery of lunar polar ice. My students were sort of dumbfounded, asking me what all the news was about. After all, didn’t I just tell them the same thing earlier in the semester? Hmm. But, the news media went a bit overboard, in my estimation. They made the ice deposits out to be giant lakes of ice. Sadly, some astronomers jumped on the same bandwagon. Then, there was talk of the giant lakes of ice being reservoirs of water that would support moonbases and colonies.
Now, I can’t say for certain that there are not giant lakes of ice in those craters. However, that is not how I envision any ice there. For one thing, more rocky meteors that icy comets likely hit the Moon. Even the comets are not composed 100% of ice! The Moon itself is a rocky body, and anything big hitting the lunar surface fast enough will pulverize the ground and send rocky debris everywhere. There will be a lot more dust and dirt tossed around than just water vapor. So, I’d imagine that the water vapor drifting into the craters might result in some sort of deposit like a thin frost at best. Now, if all that happened was that the frost was added to over the years, then you might wind up with a lake of ice. But, what about all of the other dust and rocky debris? That gets tossed into the craters, too. And smaller meteorites slam into the craters and churn up the surface, volatilizing some ice and burying nearby ice with dust. So, I always thought of the ice in those craters to be a mixture of ice and dust, more like permafrost than like a frozen lake. Granted, this is not my field of expertise, so I could be wrong on this.
Since the Clementine data, several spacecraft have slammed into the Moon. None have kicked up measurable levels of water. KAGUYA is the latest to run into the lunar surface, and we’ll have to wait to see the data collected by astronomers studying the ejecta of the impact. So far, though, the results are consistent with what I would expect. It would be exciting to find pools of frozen water on the Moon, but I really don’t expect that to happen.
But, as KAGUYA plunged to its end, it was busy sending back images of the lunar surface. I’ve reproduced a few of them here for you (one at the top of the post, and the rest below). These are very high resolution images — some of the best showing the lunar surface from a spacecraft. They show a rugged and foreboding terrain. The impact site was in the shadow, so the last few images are mostly black, with just the tips of crater ridges and mountains lit by the Sun. This is typical of the terrain that I have seen from this part of the Moon. Even if there are giant pools of frozen water in these craters, it would be very difficult to reach the ice. Landing a manned craft on the Moon is tough enough under the best of circumstances. Landing in this terrain would be very difficult. There are very few flat and level places to land. Even a lunar surface vehicle would have a tough time making it across this terrain. So, I am not so sure that a moonbase or lunar colony would really be able to get to any ice deposits here (at least not with our current level of technology).
Though these were the last images sent back by KAGUYA, they are very important. This part of the Moon seriously needs more research, so planetary geologists will be pouring over these images looking for clues to the nature of the Moon’s surface in this area. I am looking forward to hearing their findings.
-Astroprof
Images courtesy JAXA/NHK
