March 1, 2010
Ice at the north pole of the Moon
Radar mosaic of the floor of the north polar crater Peary, showing many craters with elevated CPR inside, but not outside, their rims. This material is probably water ice.
Last year, India’s Chandrayaan-1 lunar orbiter spent eight months mapping the surface of the Moon. I had the honor of being the Principal Investigator of an experiment on that mission, the Mini-SAR imaging radar. The purpose of this experiment is to map and characterize the deposits within permanently dark areas of the poles. These dark areas are extremely cold and it has been hypothesized that volatile material, including water ice, may be present in quantity here. Our radar team has just finished the first round of analysis of data returned by the Mini-SAR for the north pole and results will soon be published in the technical journal, Geophysical Research Letters.
Mini-SAR is a lightweight, low power imaging radar. It uses the polarization properties of reflected radio waves to characterize the lunar surface composition and physical state. Mini-SAR transmits pulses of left-circularly polarized radar. Typically, reflection from planetary surfaces reverses the transmitted polarization, so that Mini-SAR radar echoes from the Moon are right circularly polarized. The ratio of received power in the same sense transmitted (left circular) to the opposite sense (right circular) is called the circular polarization ratio (CPR). Most of the Moon has low CPR (about 0.3), meaning that a reversal of polarization is the norm, but some specific areas have high CPR (greater than 1.0). These include very rough, rocky surfaces (such as a young, fresh crater) and ice, which is transparent to radio energy. In this latter case, the radar penetrates the ice and is scattered and reflected multiple times by inclusions and flaws in the ice, resulting in the reflection of many same sense polarization echoes, leading to higher CPR values than normal. High values of CPR are not uniquely diagnostic of either surface roughness or ice; we must take into account the geological setting of the high CPR signal to interpret its cause.
Many craters near the poles of the Moon have interiors that are in permanent shadow from the Sun. These areas are very cold and water ice is stable permanently there. Fresh craters show high degrees of surface roughness (high CPR) both inside and outside the crater rim, caused by sharp rocks and block fields that are distributed over the entire crater area. However, Mini-SAR found craters near the north pole that have high CPR values inside, but not outside their rims. This relation suggests that the high CPR is not caused by roughness, but by some material that is restricted within the interiors of these craters. It is not geologically reasonable to expect rough, fresh surfaces to be present inside a crater rim but absent outside of it. The craters that show this enhancement are all permanently cold and dark, where ice is stable. We thus interpret this high CPR to mean that water ice is present in these craters.
Over forty small (2-15 km diameter) craters near the north pole of the Moon are found to contain this elevated CPR material. The total mount of ice present at the pole depends on how thick it is; to see this elevated CPR effect, the ice must have a thickness on the order of tens of wavelengths of the radar used. Our radar wavelength is 12.6 cm, therefore we think that the ice must be at least two meters thick and relatively pure. At such a thickness, more than 600 million metric tones of water ice are present in this area. Such an amount is comparable to the quantity estimated from the 1998 Lunar Prospector (LP) mission’s neutron spectrometer data (several hundred million metric tones). The LP neutron spectrometer only sees to depths of about one-half meter, while we penetrate at least a couple of meters, so the neutron data would underestimate the total quantity of water ice present.
The emerging picture from many experiments on several different lunar missions indicates that the creation, migration, deposition and retention of vast amounts of water are occurring on the Moon. Such an astounding result was totally unexpected by most lunar scientists, including myself. The emerging picture is consistent with earlier studies from the 1994 Clementine mission and subsequent Lunar Prospector as well as the more recent reports of the presence of water-bearing minerals at high latitudes (Moon Mineralogy Mapper), the detection of water vapor in the LCROSS impact plume (a few percent water content at its target site), and a variety of new supporting measurements, such as the discovery of unexpectedly cold polar temperatures by the DIVINER experiment on NASA’s Lunar Reconnaissance Orbiter (as cold as 25 degrees above absolute zero, colder than the estimated surface temperature of Pluto). The Moon experiences complex geological processes that were wholly unexpected before the recent results.
The quantity of water present at the lunar poles is significant; at the north pole alone, the 600 million metric tons of water there – turned into rocket fuel – is enough to launch the equivalent of one Space Shuttle (735 mT of propellant) per day for over 2000 years. The discoveries we are now making show that the Moon is an even more interesting and attractive scientific and operational destination than we had previously thought. The Moon is the key to sustainable human presence in space. Its resources enable us to create a reusable, sustainable transportation system, one that can routinely access not only the Moon, but all points of cislunar space. Once established, such a system can be used to go forward into the Solar System.
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“The emerging picture from many experiments on several different lunar missions indicates that the creation, migration, deposition and retention of vast amounts of water are occurring on the Moon.”
One question: Where is this water coming from?
Comment by Steve — March 1, 2010 @ 11:15 pm
The Moon is so close that you can almost touch it. We could live there, work there, and grow the American economy there for less than 0.5% of our total Federal budget. I wonder what future historians will say about a country that had a chance to own the rest of the solar system through its own technological know how for such a relatively tiny investment, but turned down the deal.
Its almost as if Thomas Jefferson decided not to purchase the Louisiana territories from France. What a mistake that would have been for the future of our country.
Napoleon Bonaparte, upon completion of the agreement, stated, “This accession of territory affirms forever the power of the United States, and I have given England a maritime rival who sooner or later will humble her pride.”
Comment by Marcel F. Williams — March 2, 2010 @ 12:45 am
Steve,
Where is this water coming from?
We’re not totally certain. There are a variety of possible external water sources, including the impact of comets and water-bearing asteroids, cosmic dust particles, and the reduction of surface oxides by solar wind protons (hydrogen). Apparently, some or all of these various processes are continually adding water to the Moon’s surface. Much of it is destroyed or lost to space by a variety of processes, but if it gets to the cold polar areas, it gets trapped and is stable there, essentially forever. If it sounds like a very slow process, it is, but over geological timescales of billions of years, appreciable amounts of water can accumulate.
Comment by Paul D. Spudis — March 2, 2010 @ 5:31 am
Paul – Any notion of what the carbon or nitrogen abundances in the cold traps might be? Is there a known/plausible lower bound?
Comment by Jay Manifold — March 2, 2010 @ 11:40 am
A good question to ask is how would we go about mining these resources at the lunar poles.
And how could you deliver these resources to lunar bases or colonies not located near the poles?
And how would you deliver these resources to a Lagrange point or lunar orbit?
Comment by Marcel F. Williams — March 2, 2010 @ 2:02 pm
Jay,
Any notion of what the carbon or nitrogen abundances in the cold traps might be? Is there a known/plausible lower bound?
The actual species present in the polar cold traps cannot be determined from radar data. But the spectral results from LCROSS suggest a variety of cometary volatiles are present, including ammonia, CO2 and simple organics. If they are present in cometary abundance, they would represent less than 5% of the total amount of water ice.
Comment by Paul D. Spudis — March 2, 2010 @ 4:18 pm
Marcel,
All very good questions indeed. I’m not sure that I have all the answers yet, but one thing I would say right now is that I see no reason NOT to locate a lunar outpost or base directly AT one of the lunar poles. Not only is water ice available there, but you also have near-permanent sunlight on certain mountain peaks, which both provides abundant, clean constant solar energy as well as a near-constant, benign thermal environment (about -50 C, +/- 10 C)
Whether the south or north pole is the optimum location remains to be determined.
Comment by Paul D. Spudis — March 2, 2010 @ 4:21 pm
Maybe hydrogen powered cannons like those proposed by the Quicklaunch company could be used to distribute solid polar materials to particular areas of the Moon during that area’s lunar night (just make sure your base is not located there:-)) while also being able to deliver payloads to lunar orbit and the Langrange points.
Infact, such as system might also be industrially useful on the surface of Mars, Mercury, Callisto, and the larger asteroids.
http://quicklaunchinc.com/
Comment by Marcel F. Williams — March 2, 2010 @ 7:56 pm
Paul!
Kudos on your latest results that confirm the previous findings. This is science at its best!
Comment by Warren Plattsw — March 5, 2010 @ 8:23 pm
Warren,
Many thanks! Getting the radar built and flown was an accomplishment by a lot of people from a variety of places around the country and in India, all of whom share in this discovery. It was and is my honor to be associated with them.
Comment by Paul D. Spudis — March 7, 2010 @ 11:44 am
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