LPSC: Friday afternoon on Martian basins and planning for future lunar missions
This is Ted Stryk's last contribution from his trip to the Lunar and Planetary Science Conference, two weeks ago now. Apologies to him for posting so late, and many thanks for all the notes. --ESL
by Ted Stryk
Herb Frey reported on an effort to hunt for hidden impact basins in new maps of Mars' crustal thickness. About 30 percent of Mars is in a condition where impact basins might be observable in this gravity data. He finds 50% more basins than we originally thought were there. It has been assumed from crater-counting that most large basins on Mars formed at around the same time, during a spike in the impact rate near the end of the period of heavy bombardment. Using crustal thickness models to identify old basins, he located several in the cratered highlands, a few under lowlands. Eleven candidate impact basins that he found are larger than 1,000 kilometers in diameter. Right now, crater counting data indicates that these 11 basins formed within the cratering rate spike at the end of heavy bombardment. However, smaller basins seem to show no such spike and appear over a wide, even spread of ages. Some are highly magnetized, so they formed before the demise of the Martian dynamo, while some are not, which also indicates that they formed over a wide range of ages. Alternatively, the crater retention ages could indicate some global resurfacing event that altered the crater retention ages. The apparent wide range of ages of small basins present a challenge to all hypotheses except this resurfacing idea, or the notion that there was a spike only in large impactors but not small ones, something that is hard to justify.
Matt Seigler talked about ice at the lunar poles. Figuring out how long ice could persist at the poles requires you to look at the Moon's inclination, not just its obliquity. He showed how the Moon's obliquity [the angle its spin axis makes to its orbit around Earth] has varied throughout history to at times be 77 degrees! (too bad it's not like that now -- we could see the far side) for a few brief times; subsolar latitude could have been up to 70 degrees at times. This means that lunar polar ices, if they exist, could not be primordial remains from billions of years ago; their age would have to be in the millions, not billions. Alternatively, primordial lunar ice could exist but it must be at least 10 meters deep. Such ice could possibly be migrating upward now -- given the rate the Moon would lose ice, it must have a deep source underground to replenish it because of loss rate coupled with the fact that it couldn't be accumulating any more ice under current conditions. All of these conclusions depend on models of the rate and timing of the Moon's recession from Earth. The key point is that lunar volatiles cannot have been sitting near the surface since early in the Moon's history, even in currently shadowed regions near the poles. Because the history of lunar recession is uncertain, the conclusions of this paper were stated by the authors to be far from certain. Chandrayaan-1 Mini-SAR data doesn't go deep enough to test whether polar ice is present at 10 meters' depth, and Kayuga's resolution is too low. A powerful SHARAD-like radar might help test this paper's conclusions.
Charles Hibbitts argued that lunar polar volatiles may exist as adsorbed molecules, not water ice. Adsorption would allow very different distributions and affect how it would be detected and retrieved. Radar data over the poles is inconsistent with the presence of massive ice. Neutron detectors on Lunar Prospector were not sensitive enough to detect state of H+. In conclusion, current data on hydrogen at the lunar poles could be explained without ices; volatiles may be present but in more exotic forms. Therefore, future missions shouldn't count on finding these resources in easy to utilize forms.