November 6, 2010
Permafrost, Snow Cones and Fairy Castles
Comet Hartley 2 as seen from the EPOXI spacecraft this week. Comets are one source of water for lunar polar ice.
Although the discovery of ice on the Moon comes from a wide variety of different measurements, they are all “remote sensing.” We have not yet landed near these deposits and examined them up close. Thus, we do not know the physical nature of lunar polar ice. Having spent the last couple of weeks at several meetings in which this became an issue, I’ve been thinking about the nature of lunar ice. What is lunar polar ice? Is it a smooth pond of solid ice? Perhaps it is broken-up blocks and slabs of tough, compacted ice chunks. Maybe it’s a porous, void-filled snow-like aggregation of microscopic ice pieces. The question of the nature of lunar ice is not academic. If we plan to go to the Moon to harvest this ice to support human presence and space transportation, we must understand the physical nature of the deposits.
Although the details are probably complex, the concept of lunar ice deposition is simple. Water ice is stable in the cold, dark areas near the lunar poles. Any water that is made or deposited on the Moon is not stable in sunlit areas, and so will migrate across the surface. If it gets into one of these polar cold traps, it is there forever – no known process exists to remove it. Thus, even though the addition of water is extremely slow, over very long periods of time, a substantial amount of water may accumulate there.
But what is the physical nature of these deposits? Our expectations derived from experience here on Earth are probably misleading. We deal daily with water in liquid form and ice on Earth is usually made by the freezing of liquid water. This results in crystalline ice, as water molecules in solid form assume an ordered, tightly bonded lattice structure. As everyone knows, this material is both hard and tough and greatly resists attempts to break or gather it using normal digging tools. Water also can crystallize directly from vapor form into a solid as frost, usually found as an extremely thin coating that is very soft and easily scraped and removed from the object on which it forms.
Water that freezes within soil forms a tough, indurated deposit that can be quite difficult to dig or excavate. In the polar regions of the Earth, this material is frozen solid year-round and is called permafrost. Permafrost is extremely hard and difficult to excavate. Buildings in arctic regions require heavy equipment to dig and move the permafrost, including the use of explosives to break up the rock-hard frozen soil. If lunar ice is like this stuff, it will be extremely difficult to dig up and mine.
In contrast, snow is soft and easy to excavate. Snow is created when precipitation droplets (rain) freeze before they land on the ground. Typically, airborne dust particles will nucleate these droplets. Small drops have time to crystallize into magnificent ice crystals which famously, are each unique and individual. Sometimes larger water drops freeze quickly in flight and form ice blobs which may land on the ground as hail. In any event, if this material accumulates on the ground, we have a porous, weakly bonded deposit that is easily scooped up, usually by cursing inhabitants wielding large flat shovels.
Neither of these two accumulation scenarios occur on the Moon. We don’t know whether lunar ice is deposited (e.g., by comets hitting the Moon) or made (e.g., by solar wind hydrogen reacting with mineral surfaces). But however it is deposited, the water exists as individual molecules in gaseous form. Although this water is found all over the Moon, it is not stable everywhere. The molecules hop around the surface randomly, not slowing down until they land at a cooler locality and don’t stop until they reach a cold trap. The Moon loses most of these water molecules by a variety of mechanisms, including escape, disassociation and combination with minerals. The lucky few that reach a polar cold trap are there forever.
So what form do lunar ice deposits take? They are not now and never have been in liquid form, so crystallization into dense, “ponds” of ice is not likely. This lack of history as a liquid also means that “permafrost” (at least as we understand that term from terrestrial experience) is not likely either. Both of these ice forms ultimately require a freeze-thaw cycle, even if the time frame for such a cycle is hundreds of years. The lunar cold traps are cold now and have been for billions of years. And for this length of time, they have been gathering water molecules, sometimes at very high rates of accumulation (as when a comet strikes the Moon nearby) but usually at very slow, steady rates of accumulation.
Lunar ice probably is very porous, or at least “solid” but weakly bound together. The tight bonding of crystalline ice is made during the transition from liquid to solid during freezing. This doesn’t happen on the Moon; the water is added to the surface through direct ballistic deposition as individual molecules. In addition to the accumulation of water in the form of extremely tenuous vapor, dust and soil particles may interact with the water, creating a deposit with variable strength and water content. Even this material is likely to be loosely bound, as this mixing occurs at low temperatures and the water does not have a chance to re-crystallize, the usual reason for the steel-like hardness of permafrost. In astrophysics, a fine-grained, loosely bound structure is referred to as “fairy castle structure.”
Do we have any evidence that this guess may be correct? We have only a few indirect clues at present. The ejecta plume observed during the impact of the LCROSS upper stage was unusually narrow. The science team suggested that this was a result of impact into an unusually low density soil; the term they used to describe it was “fluffy.” In addition to the high CPR fill of anomalous craters seen in the Mini-RF radar images of the poles (which we interpret as ice), we also observe anomalously low CPR in the areas surrounding the anomalous craters. Extremely low CPR implies fine-grained, lower than average density deposits with few rocks. Yet because polar ice is geologically young (less than a couple of billion years), if there were rock-hard, crystalline ice in abundance, we might expect a higher than average radar CPR, caused by abundant angular blocks excavated by impacts. Such a signal is not observed.
Admittedly, the evidence for this story is very weak. To determine the true physical and chemical nature of lunar polar ice, we must examine and study it in detail from a suitably equipped surface rover. Such a mission has been repeatedly proposed and I note that it is one of the proposed mission studies in the National Academy’s Planetary Exploration Decadal Survey. For a resource that may change the rules of spaceflight, determining its properties should be a high priority for exploration.
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Dr Spudis:
I have to agree 100% with your position that we should try to return to the Moon ASAP and employ ISRU to the maximum exent possible.
There is one nagging alarm in the back of my mind when I hear other people discuss turning all that ice into rocket fuel, which will presumably be needed for those thousands of wealthy tourists that Space Adventures hopes to send to all those lunar resorts that NewSpace advocates anticipate.
At some point in the next few centuries I expect that there will be large number of people working and living on the Moon, and although they will probably employ closed environmental systems that fully recycle air and water, I wonder if we will need to place some limits on the amount of H2O that we will permit to be sucked into the vacuum of outer space as propellant? There is currently a vast amount of H2O at the lunar poles, certainly much more than enought to satisfy the fuel requirements of a large number of planetary science missions. But like the huge herds of Buffalo that once roamed the great plains, will we want to take some small steps to make sure that that this resource does not ever entirely disappear?
I am sure that this has already crossed your mind, and I would love to hear your analysis of the situration.
Sincerely,
Nelson Bridwell
Comment by Nelson Bridwell — November 6, 2010 @ 11:57 pm
Nelson,
Although we are still in the process of estimating the total amount of water on the Moon, the lowest estimates are in excess of several hundred million tonnes. The actual values are likely several factors higher than this number. This is enough water to support a significant lunar city for several hundred years, by which time I am guessing that humanity will have spread widely across the planets of the Solar System. I think that a human outpost on the Moon bootstraps our permanent presence in space. By the time we start running out of lunar water, we’ll be in the position to replenish it from cometary and asteroidal sources in space.
Comment by Paul D. Spudis — November 7, 2010 @ 4:46 am
In comparison, the gross mass of an Ares V is something like 3 thousand metrics tons. If there is only 200 million metrics tons of ice in the lunar cold traps at the poles, that would roughly equate to the mass of 60,000 Ares Vs. If we use anything close to that then we will be doing very well!
However, at some time in the future I don’t think that it would be a bad idea to reserve perhaps 50% of the ice for colonization purposes. If we make a ballpark assumption of 1 metric ton of H2O/O2 per lunar resident, that much ice should support a lunar population of 100 million, which sounds enormous by Apollo standards, but is only about 1% of the Earth’s population for a total surface area of 50 million square miles, about the same as our continents.
Comment by Nelson Bridwell — November 7, 2010 @ 2:39 pm
If lunar regolith is a fairy castle structure, might this be a poor surface to drive prospecting rovers and excavators on? Might they be constantly getting stuck as their tracks whip through this fluffy material?
Also, could a clear tarp be placed over ice of any form, sealed somewhat at the edges, and mirrors from sunlit areas directing light & heat through the tarp to heat the icy regolith somewhat until steam comes out of the regolith. This sucked out by tubes into storage containers which would accumulate pure ice?
Comment by JohnHunt — November 7, 2010 @ 6:02 pm
If lunar regolith is a fairy castle structure, might this be a poor surface to drive prospecting rovers and excavators on?
Possibly. But apparently, the ice concentration varies both laterally and vertically, so “normal” lunar surface may be interwoven with fluffy surface. But we need a prospecting surface mission to know these properties for certain.
As for your proposal to extract water without moving the soil, that of course is a highly desirable thing to do — saves effort, time and money. A number of processes have been thought about, but we need to be on the Moon to experiment with them.
Comment by Paul D. Spudis — November 8, 2010 @ 4:31 am
I’ve always pictured lunar ice in my mind as being ice that is in crevices, I’ve never pictured it as being fields of it. I don’t know why – maybe ignorance as I cannot recall having seen fields of it on any of the images that we have been shown globally.
I am very very puzzled about why nobody has ever been back to the moon though.. in all these years. I just don’t understand that. It forever fuels the conspiracy theory on that.
Comment by Beryl Crystal — November 9, 2010 @ 9:50 am
Beryl,
In truth, no one really knows the physical nature and distribution of lunar polar ice. This post was my best guess — it is no doubt wrong in several respects. The Moon always and continually surprises us.
As far as the reason no one has gone back to the Moon, after Apollo, it was decided to focus efforts on establishing routine access to low Earth orbit before any step beyond. We’re still struggling with that step.
Comment by Paul D. Spudis — November 10, 2010 @ 8:41 am
Very interesting. I had never thought about the possibility of vaporized water migrating in the vacuum gravity of Luna to the cold-traps. Quite reasonable.
It all comes down to the condition of the water there. A rover should have been sent to Luna instead of Mars.
However Luna has its own unique challenges which Mars does not. Extremes of temperature. Day-night cycle length.
Sometimes you have to play God
Create the chicken before you get the egg. As expensive or inexpensive as it might be, building industry on Luna could very well be the way to achieve routine access to LEO.
Comment by Rhyshaelkan — November 10, 2010 @ 8:54 pm
However Luna has its own unique challenges which Mars does not. Extremes of temperature. Day-night cycle length.
Every space destination — not matter where it is — has its unique environment and design challenges. In the case of the Moon, water deposits are concentrated near the poles, where we have both permanent darkness and near-permanent sunlight. Although challenging, this lighting is also enabling: you can operate nearly constantly in the lunar polar regions through the use of solar generated power from arrays in the quasi-permanent sunlight.
Comment by Paul D. Spudis — November 11, 2010 @ 4:50 am
The environmentalist in me hopes that the exploitation of lunar ice will be limited to less than 5% of its total resource over the next 200 years.
Hopefully, by the 22nd century, water and hydrogen resources derived from the asteroids and possibly even from Jupiter’s moon, Callisto, will put the export of lunar water and hydrogen out of business so that the lunar ice can be preserved as a ‘natural wonder’ for future generations in the solar system.
Comment by Marcel F. Williams — November 11, 2010 @ 12:44 pm
Yes “peaks of eternal sunshine” are of great interest on our forum. So is possibility of keeping pace with the day/night terminator at such high latitudes.
Ben Bova mentioned in his science-faction book “Welcome to Moonbase” that it is possible to keep up with the day/night terminator at the equator with a brisk walk.
How much more easy would it be to do so at higher latitudes where the circumference is smaller? It might mean a couple/three course plotting people, to avoid pitfalls and to know where we are going next, and a few drivers zooming at best speed from location of interest to the next. However by doing so you could avoid the annoyance of the day/night cycle. This would require bouncing your transmissions off a relay or two.
I hope we move water harvesting to the NEOs as soon as we can to prolong the resources of Luna. In addition to other priceless elements we get from the NEOs, such as nitrogen or its compounds.
Comment by Rhyshaelkan — November 11, 2010 @ 3:46 pm
“Yes “peaks of eternal sunshine” are of great interest on our forum.”
Sadly, there are no peaks of eternal sunshine. Mentioned in Der Mond nach seinen kosmischen und individuellen Verhältissen oder allgemeine vergleichende Selenographie” (by Beer and Mädler in 1837… before Flammarion’s attribution in 1879 might I add) and stating with respect to the lunar polar mountains, “..many of these peaks have (with the exception of eclipses caused by the Earth) eternal sunshine”(pretty smart I thought… I found this reference by accident with Google Scholar, for those interested, in German) , however none seem to exist for either north or south pole.
“So is possibility of keeping pace with the day/night terminator at such high latitudes.
… that it is possible to keep up with the day/night terminator at the equator with a brisk walk. How much more easy would it be to do so at higher latitudes where the circumference is smaller?”
Not so easy to do because of the terrain (mountains/craters/etc in the way). Same for every latitude. The problem also exists that you must plan ANYWAY for not being able to stay in the Sun due to rover failures or bad terrain, so savings are not as great as thought.
“It might mean a couple/three course plotting people, to avoid pitfalls and to know where we are going next, and a few drivers zooming at best speed from location of interest to the next.”
Zooming is not likely due to risk to billion dollar (euro) equipment and astronaut lives. More likely for the rovers is the stately pace that would make a Model T seem fast…. the typical space-rated slow choreography that encourages us to hit the fast forward/play button when viewing video of these kind of real-time events.
It is best to have a robotic rover thoroughly map each potential route much before hand in order to determine minimum energy usage routes (least steep terrain, best surface properties) rather than risk more expensive equipment.
“However by doing so you could avoid the annoyance of the day/night cycle.”
It would be nice, but not possible at least for the south pole. The shadow characteristics (due to near and distant shadow casting terrain) are such that you can’t stay in the light (unless you teleport to the next location that has illumination… the sunlight patches do not stay next to each other). Still, some minimized time in the darkness (or energy usage) solution can probably be found. It depends on the characteristics of the rover and better topographic models (even LOLA is not enough right now to be definite).
Comment by James Fincannon — November 22, 2010 @ 3:54 pm
Sadly, there are no peaks of eternal sunshine.
No, but areas do exist that are sunlit for more than 70% of the daytime in winter and 100% of the day in local summer. Moreover, the eclipse periods are non-continuous, or reasonably short duration (less than 100 hours) and come at irregular intervals, as they are caused by local topography.
The latest lighting model results (based on Kaguya topography and validated with images from Clementine, SMART-1 and LROC) can be found here:
Bussey D.B.J., J.A. McGovern, P.D. Spudis, C.D. Neish, H. Noda, Y. Ishihara, S.-A. Sørensen (2010) Illumination conditions of the south pole of the Moon derived using Kaguya topography. Icarus 208, 2, 558-564.
Comment by Paul D. Spudis — November 22, 2010 @ 4:04 pm