The Moon: Water ice may be more common than previously thought

Guest “Ice, ice baby!” by David Middleton

NASA Study Highlights Importance of Surface Shadows in Moon Water Puzzle
Aug 02, 2021

The shadows cast by the roughness of the Moon’s surface create small cold spots for water ice to accumulate even during the harsh lunar daytime.

Scientists are confident that water ice can be found at the Moon’s poles inside permanently shadowed craters – in other words, craters that never receive sunlight. But observations show water ice is also present across much of the lunar surface, even during daytime. This is a puzzle: Previous computer models suggested any water ice that forms during the lunar night should quickly burn off as the Sun climbs overhead.

“Over a decade ago, spacecraft detected the possible presence of water on the dayside surface of the Moon, and this was confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy [SOFIA] in 2020,” said Björn Davidsson, a scientist at NASA’s Jet Propulsion Laboratory in Southern California. “These observations were, at first, counterintuitive: Water shouldn’t survive in that harsh environment. This challenges our understanding of the lunar surface and raises intriguing questions about how volatiles, like water ice, can survive on airless bodies.”

In a new study, Davidsson and co-author Sona Hosseini, a research and instrument scientist at JPL, suggest that shadows created by the “roughness” of the lunar surface provide refuge for water ice, enabling it to form as surface frost far from the Moon’s poles. They also explain how the Moon’s exosphere (the tenuous gases that act like a thin atmosphere) may have a significant role to play in this puzzle.

Water Traps and Frost Pockets

Many computer models simplify the lunar surface, rendering it flat and featureless. As a result, it’s often assumed that the surface far from the poles heats up uniformly during lunar daytime, which would make it impossible for water ice to remain on the sunlit surface for long.

So how is it that water is being detected on the Moon beyond permanently shadowed regions? One explanation for the detection is that water molecules may be trapped inside rock or the impact glass created by the incredible heat and pressure of meteorite strikes. Fused within these materials, as this hypothesis suggests, the water can remain on the surface even when heated by the Sun while creating the signal that was detected by SOFIA.

But one problem with this idea is that observations of the lunar surface show that the amount of water decreases before noon (when sunlight is at its peak) and increases in the afternoon. This indicates that the water may be moving from one location to another through the lunar day, which would be impossible if they are trapped inside lunar rock or impact glass.

[…]

The researchers point out that this new study could help us better understand the role shadows play in the accumulation of water ice and gas molecules beyond the Moon, such as on Mars or even on the particles in Saturn rings.

The study, titled “Implications of surface roughness in models of water desorption on the Moon”, was published in the Monthly Notices of the Royal Astronomical Society on August 2, 2021.

NASA/JPL

The Moon is covered with craters and rocks, creating a surface “roughness” that casts shadows, as seen in this photograph from the 1972 Apollo 17 mission. Image Credit: NASA

Water may have also been more abundant early on in the Moon’s geological history than had been previously thought. Most of the rocks returned by the J Missions (Apollo 15, 16 & 17) contained traces of hydroxyl. The most famous example is probably Rusty Rock, returned on Apollo 16.

LMP Let me whack this thing right here. It’s so good that I can’t pass it up. All right, there’s a good place to whack.

CDR Oh, that’s hard – you got it! Demolished it.

LMP That’s a great rock. Look at that! I’m sorry we didn’t get it documented before, but that’s a good sample. I think it’s a crystalline rock.

CC Okay, let’s go ahead and document it now – – so we get the location of the one that’s still in place. It didn’t look like it moved.

CDR No, he didn’t move anything there. I’m going to do an up-sun on this documentation (107-17523-25)

LMP Okay, I’ll get a cross-sun here. It’s a grayish bluish – rock, Tony, in the matrix with some white clast in it. The matrix is so fine-grained, I can’t tell, but it’s definitely got a blue cast to it and there are inclusions of a whitish – it looked like plag to me.

CDR And then, needle-like black crystals in it, too. I see one in there that’s a millimeter wide by 3 mm long, and some other needle-like crystals in it.

LMP Here’s another piece – came off the same rock.

CDR It has this white clast in it. It’s got to be a breccia, Charlie.

66095 “Rusty Rock” Impact Melt Rock

  • CDR = Commander John Young
  • LMP = Lunar Module Pilot Charlie Duke
  • CC= EVA CapCom Tony England
Figure 1: 66095 was chipped from a small boulder (0.5 m). AS16-108-17632. 66095 “Rusty Rock” Impact Melt Rock

Introduction
Lunar sample 66095 was collected from a boulder on the rim of a 10 meter crater at the base of Stone
Mountain (figure 1). During the original examination of 66095 by (M. Bass in Butler 1972), an unusual
amount of colored stain (figure 3) was reported on the surface and interior of 66095 (LSPET 1973).

66095 “Rusty Rock” Impact Melt Rock

Figure 3: Close-up photo of metallic salts or “rust” on surface of 66095 (location unknown). Note the
appearance of a crust under the colored salts. Field of view about 1 cm. NASA S72-48424.

Rusty Rock was the mother-of-all breccias.

Petrography
Most of 66095 (~80%) is composed of a fine-grained, subophitic to ophitic impact melt-rock (figures 5, 6 and 7), which also contains a wide variety of lithic clasts (from basalt to anorthosite) (Garrison and Taylor 1980,
Hunter and Taylor 1981). The suite of lithic clasts found in 66095 contains every highland rock type except norite.

66095 “Rusty Rock” Impact Melt Rock

This was the prevailing view of the presence of volatiles in lunar rocks at the time:

“One of the most striking features of the moon is its great depletions in volatiles, such as C, N, H2O, Pb, Bi and Tl. Apparently these elements were left behind in the solar nebula when the moon accreted. For an understanding of the moon’s chemistry, it would be of interest to know the magnitude of this depletion, relative to cosmic or terrestrial abundances. The only elements for which this can be estimated with some confidence are Tl and Pb ——.”
–Krahenbuhl et al. 1973

66095 “Rusty Rock” Impact Melt Rock

It was even suspected that Rusty Rock may have been contaminated on its return from the Moon.

It is possible that anhydrous metal salts (chlorides?) in 66095 combined with the moisture in the LM, CM,
tropical Pacific and/or individual terrestrial laboratory, yielding terrestrial-like hydrogen and oxygen isotopic
signatures (Friedman et al. 1974; Epstein and Taylor 1974). However, it is difficult to see how moisture penetrated into the sample to “rust” the interior metal grains.

66095 “Rusty Rock” Impact Melt Rock

Rusty Rock contained more water than any other rock sample returned from the Moon.

Epstein and Taylor (1974) and Friedman et al. (1974) carefully studied the temperature release and isotopic
composition of H2O released from 66095. Samples of 66095 were found to have far more H2O than any other
rock sample and somewhat more H2O than any lunar soil. However, isotopic analysis indicated that the ä2H and ä18O were similar to that of terrestrial water.

66095 “Rusty Rock” Impact Melt Rock

It’s interesting to note that the rocks returned on Apollo 11, 12 and 14 as well as the Soviet Luna return missions (16, 20 & 24) were as depleted in volatiles as expected… All of these missions landed in mare (basalt lava plains); whereas the three J Missions, which returned rocks less depleted in volatiles, landed in or adjacent to lunar highlands (Lunar Landing Sites).

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August 8, 2021 at 08:43PM

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