December 23, 2008
Moon water – again
Model of the distribution of ice at the lunar south pole
The question, “Is there water on the Moon?” is still with us. Although water is not stable on the lunar surface in vacuum, the poles of the Moon contain deep craters whose floors are in permanent shadow. These dark areas are extremely cold – only about 50º above absolute zero. If a water molecule gets into one, no known physical process can remove it.
Where would such water come from? The Moon is constantly pelted by meteorites and comets, many containing water, either as ice or bound into mineral structures. This water is mostly lost to space during an impact, but some molecules may hop around on the surface for an extended time (minutes to hours). If by chance a water molecule fell into a polar dark area, it would be trapped there forever. Of course, this would be an extremely slow process, but the Moon is old (over 4,500 million years) and has plenty of time.
We will soon be obtaining new information on the poles of the Moon from the ongoing Chandrayaan-1 orbital mission. On that spacecraft, the Mini-RF SAR experiment will use radar to map the poles, including all of the dark regions. Unlike neutron spectroscopy, radar probes a couple of meters below the surface and is sensitive to the presence of ice, not hydrogen.
The first hint that there may be ice in these polar cold traps came from a radio experiment on the Clementine mission in 1994. Four years later, a small satellite called Lunar Prospector (LP) carried an instrument designed to measure the amount of neutrons given off the Moon’s surface. Hydrogen absorbs neutrons, so when the LP investigators saw a decrease in neutron flux near the lunar poles, they concluded that excess amounts of hydrogen are present there.
A problem with the Lunar Prospector data is that its maps of hydrogen concentration are low in spatial resolution; we cannot identify any structure in the data smaller than about 40 km. Thus, in the LP neutron data, we see a large, smeared out area of enhanced hydrogen. We cannot tell if this excess is confined to the dark floors of permanently shadowed craters (consistent with the presence of water ice) or just an overall enrichment of hydrogen near the poles (consistent with implanted solar wind protons.)
Last week, new models of the distribution of water ice near the lunar poles were published. These maps indicate that if the polar hydrogen is present as water ice, ice concentrations may exceed 1 weight percent in some areas. This sounds like a small amount, but when added over a large area, it could constitute hundreds of millions of tons of water ice on the Moon. Moreover, because the neutron instrument only senses the outer 30 cm of the surface, total concentrations could be up to ten times greater than these results.
Of course, a model is not new data, but merely an attempt to envision how hydrogen might be distributed over the lunar poles. In concert, both neutron and radar data sets will provide an abundance of information that may allow us to finally resolve this vexing question: Is there water on the Moon?
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Ultimately, we still want ground truth, dont we ? One thing i dont get is how we are sending multibillion dollar science laboratories to Mars, but we havent sent something relatively as simple as CMU IceBreaker rover to lunar craters to get an answer to that crucial question.
Comment by kert — December 24, 2008 @ 2:58 am
Hi Kert,
You are absolutely correct — we will have to go to the surface and get ground truth at some point. No remote measurement is definitive.
The problem is that a surface rover mission is very challenging. The permanently dark regions are extremely cold and of course, solar power doesn’t work. You need a nuclear power source for long-lived operations. Also, you cannot see the dark cold traps from Earth, so you need a relay satellite in lunar orbit for communication with the Earth. Both of these requirements increase mission cost and complexity.
Comment by Dr. Paul D. Spudis — December 24, 2008 @ 5:58 am
Perhaps an antenna could be left at a higher point, on the rim of a crater, as a relay much like the broad band antenna I use for my internet connection. That might cost a bit less than a satellite in lunar orbit. I have read that power can be beamed by microwave which might replace a nuclear power source. Of course that might be a little more complicated to set up than a nuclear power source.
Comment by James West — December 29, 2008 @ 8:35 pm
Wouldn’t it be great if it was possible to put a large array of solar panels in synchronous orbit around the moon. We would then be able to transmit power to numerous places including some dark ones. That would enable us to conduct numerous experiments over a long period of time using the panels many times instead of having to send new power sources each time. It could even be combined with a relay station for transmission of information.
Comment by James West — December 30, 2008 @ 12:53 am
James,
A permanent ground-based radio relay is feasible, but only after you already have significant infrastructure on the Moon. My comments above were in regard to a near-term, robotic rover mission to characterize the polar volatiles.
For power, you just can’t beat simple solar arrays, placed on a spot which receives (near-)permanent sunlight. Such places exist near both poles. But again, this is only for power after you have a lunar installation. For near-term robotic missions, you need a portable power source and a small radioisotope generator is the simplest (if not the cheapest) solution.
Comment by Dr. Paul D. Spudis — December 30, 2008 @ 6:16 am
Dr. Paul, thank you for replying. The idea for a relay antenna on the rim of a crater came from a recent article I read refering to a flat area on the rim of a crater that was possibly suitable for landings. I was not aware that a relay antenna could not be attached to a spacecraft transporting a robotic rover. I didn’t know it had to be a permanent ground based antenna to relay information. So much for trying to simplify things.
The link I have included sort of answered my question about a reusable solar array in orbit around the moon, particularly the paragraph that refers to lunar orbits. It does look to me like Nasa would be on the right track if they were to pursue this, apparently, more practical approach to space exploration. The article is by Henry Brandhorst, Jr. at the Space Research Institute, Auburn University, AL.
Thanks again for answering.
http://pdf.aiaa.org/preview/CDReadyMIECEC08_1836/PV2008_5601.pdf
Comment by James West — December 30, 2008 @ 8:11 pm
I was not aware that a relay antenna could not be attached to a spacecraft transporting a robotic rover. I didn’t know it had to be a permanent ground based antenna to relay information.
I’m sorry — I misunderstood your proposal. Yes, you could do this, although the rim of Shackleton crater is visible from Earth only about half the time, so you would be out of communication 50% of the month. But it is a feasible strategy. You would also have to operate within the crater such that it is always visible to the spacecraft on the rim, which means the spacecraft has to be right on the very edge of the crater. The easiest way to do that would be to land and then move the lander to the edge of the crater, possibly with wheels as landing footpads.
Thanks for responding!
Comment by Dr. Paul D. Spudis — December 31, 2008 @ 6:07 am
Then, at the crater’s edge, the antenna could telescope or unfold up for better range.
What fun!
I sure hope the economy improves so the space program doesn’t get pushed aside.
Also, I hear a lot of grumbling from people who think the whole program is useless, even though they talk on cell phones every day.
Happy New Year!!!
Comment by James West — January 1, 2009 @ 1:06 am
Wow! Amazing no idea how new the discovery of Moonwater is currently. October 9th,2009 was very recent. I am so flabbergasted at the new technology that was created by Mr.Spudis, it interesting that this equipment can identify ice meters below surface.I am so glad my teachers assigned me a project to look at recent events in the space race. These are concepts that reach out side of the box my only ? is what will this lead too. What does this mean for the future in ten years that water molecules exist on the moon? Is there going to be space shuttles and permanent bases? That I have not been able to find a clear answer. Im also open for answers.
Kenny
Millennium
Tracylc.net
Comment by Kenny — March 23, 2010 @ 11:25 pm
Kenny,
Yes, it turns out that there is an enormous amount of water on the Moon, enough to support a small colony for many years. Whether we take advantage of that fact to establish a foothold there remains to be seen.
Comment by Paul D. Spudis — March 24, 2010 @ 4:30 am