Guest “Eis, Eisbaby!” By David Middleton
NASA study highlights the importance of surface shadows in the moonwater puzzle
August 02, 2021
The shadows cast by the roughness of the lunar surface create small cold spots where water ice can accumulate, even during the harsh lunar day.
Scientists are confident that water ice can be found in permanently shaded craters at the poles of the moon – in other words, in craters that never get sunlight. However, observations show that water ice is also present on much of the lunar surface during the day. This is a mystery: previous computer models suggested that any water ice that forms during the moonlit night should burn off quickly when the sun rises over them.
“Over a decade ago, the spacecraft discovered the possible presence of water on the daytime surface of the moon, and this has been confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy [SOFIA] in 2020, ”said Björn Davidsson, scientist at NASA’s Jet Propulsion Laboratory in Southern California. “These observations were initially counterintuitive: water should not survive in this harsh environment. This calls into question our understanding of the lunar surface and raises fascinating questions about how volatiles like water ice can survive on airless bodies. “
In a new study, Davidsson and co-author Sona Hosseini, a researcher and instrument scientist at JPL, suggest that shadows created by the “roughness” of the lunar surface provide refuge for water ice and allow it to stand as surface frost away from the die Pole of the moon. They also explain how the moon’s exosphere (the thin gases that act like a thin atmosphere) can play a significant role in this puzzle.
Water traps and pockets of frost
Many computer models simplify the lunar surface, making it flat and structureless. It is therefore often assumed that the surface away from the poles heats up evenly during the lunar day, which would make it impossible for water ice to remain on the sunlit surface for a long time.
So how is it that water is discovered on the moon beyond permanently shadowed regions? One explanation for the discovery is that water molecules can be trapped in the rock or in the glass that is created by the incredible heat and pressure of meteorite impacts. Melted in these materials, as this hypothesis suggests, the water can remain on the surface even when heated by the sun while generating the signal detected by SOFIA.
One problem with this idea, however, is that observations of the lunar surface show that the amount of water decreases before noon (when the sun is at its peak) and increases in the afternoon. This suggests that the water can move from one place to another during the lunar day, which would be impossible if they were trapped in lunar rocks or glass.
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The researchers suggest that this new study could help us better understand the role of shadows in the accumulation of water ice and gas molecules beyond the moon, such as on Mars or even on the particles in Saturn’s rings.
The study, entitled “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 in craters and rocks, creating a surface roughness that casts shadows, as seen in this photo from the 1972 Apollo 17 mission. Image source: NASA
Water could also have been more abundant at the beginning of the Earth’s history than previously assumed. Most of the rocks returned from the J missions (Apollo 15, 16 & 17) contained traces of hydroxyl. Probably the most famous example is Rusty Rock, which returned at Apollo 16.
LMP Let me beat this thing up. It’s so good that I can’t help it. All right, there is a good place to hit.
CDR Oh, that’s difficult – you got it! Got it demolished.
LMP That’s a great rock. Look at this! I’m sorry we didn’t document it beforehand, but that’s a good example. I think it’s a crystalline rock.
CC Okay, let’s document it now – this is how we get the location of what is still there. It didn’t look like it moved.
CDR No, he didn’t move anything there. I’m going to do an Up-Sun in this documentation (107-17523-25)
LMP Okay, I’m getting 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 definitely has a blue cast and there are whitish inclusions – it looked like a nuisance to me.
CDR And then there are needle-shaped black crystals in it. I see there a millimeter wide and 3 mm long and some other needle-shaped crystals in it.
LMP Here’s another piece – came from the same rock.
CDR It has that white clast in there. It has to be a breccia, Charlie.
66095 “Rusty Rock” Impact Melt Rock
Apollo 16 crew information
- CDR = Commander John Young
- LMP = pilot of the lunar module 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
The lunar sample 66095 was collected from a boulder on the edge of a 10 meter high crater at the base of the rock
Mountain (Figure 1). The original investigation of 66095 by (M. Bass in Butler 1972) revealed an unusual one
Amount of color stains (Figure 3) was reported on the surface and inside of 66095 (LSPET 1973).
66095 “Rusty Rock” Impact Melt Rock Figure 3: Close-up of metal salts or “rust” on the surface of 66095 (location unknown). Notice that
Appearance of a crust under the colored salts. Field of view approx. 1 cm. NASA S72-48424.
Rusty Rock was the mother of all breccias.
Petrography
The largest part of 66095 (~ 80%) consists of a fine-grained, subophitic to ophitic impact melt rock (Figures 5, 6 and 7), which also contains a large number of lithic clasts (from basalt to anorthosite) (Garrison and Taylor 1980,
Hunter and Taylor 1981). The sequence of lithic clasts found in 66095 contains all highland rock types 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 noticeable features of the moon is its great lack of volatiles like C, N, H2O, Pb, Bi and Tl. Apparently these elements were left behind in the solar nebula as the moon grew. In order to understand the chemistry of the moon, it would be of interest to know the extent of this depletion in relation to cosmic or terrestrial abundances. The only elements for which this can be estimated with any degree of certainty are Tl and Pb – “
– Krahenbühl et al. 1973
66095 “Rusty Rock” Impact Melt Rock
It has even been suggested that Rusty Rock may have been contaminated from the moon on its return.
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 supplying terrestrial hydrogen and oxygen isotopes
Signatures (Friedman et al. 1974; Epstein and Taylor 1974). However, it is difficult to see how moisture penetrated the sample to “rust” the internal 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 temperature release and isotopes
Composition of the H2O released from 66095. Samples of 66095 were found to contain far more H2O than any other
Rock sample and a little more H2O than any lunar soil. The isotope analysis showed, however, that 2H and 18O were similar to those of terrestrial water.
66095 “Rusty Rock” Impact Melt Rock
It is interesting to note that the rocks returned by Apollo 11, 12 and 14 as well as the Soviet Luna return missions (16, 20 and 24) were depleted of volatiles as expected … All of these missions ended up in Mare (basaltic lava plains) ; whereas the three J missions, which brought back less volatile-poor rock, landed in or next to the lunar highlands (lunar landing sites).
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