For a long time, data from space probes and rock analyzes have indicated that there is water ice on the moon and water bound in the rock. Now two new studies show that the lunar surface has significantly more and more widely distributed occurrences of water ice and molecular water than previously assumed. According to this, water ice does not only exist in a few large craters, but in shadow zones on a total area of over 40,000 square kilometers. In addition, some of the water bound in the lunar regolith does not appear to be a hydroxyl molecule (OH), but rather as molecular water (H2O).
Because the moon has no atmosphere, water in liquid or gaseous form does not remain on its surface – it escapes immediately into space or is photochemically decomposed by the hard radiation. But for around ten years we have known that there are some refuges for the life molecule. According to data from space probes, the permanently shaded, very cold areas of large polar craters could even have layers of water ice meters thick. However, it was not yet clear whether such cold traps also existed in smaller shadow areas. Spectroscopic measurements also indicate that bound water is also present in the lunar rocks at higher latitudes. “However, it is unknown whether this water is molecular water in the form of H2O or hydroxyl (OH),” explain Casey Honniball from the University of Hawaii in Manoa and her colleagues.
H2O instead of OH in the moon rocks
Two studies now provide answers to these questions. In the first, Honniball and her team evaluated observational data from the SOFIA (Stratospheric Observatory for Infrared Astronomy) aircraft telescope. They used this to scan the lunar surface for infrared radiation in the range of six micrometers wavelength in August 2018. “A fundamental vibration of the water molecule creates a spectral signature at six micrometers that is not shared by any other hydroxyl compound,” the researchers explain. This allows this measurement to differentiate whether the water, which has long been known and bound in regolith, is present as an H2O molecule or as an OH molecule. For their study, they analyzed SOFIA data for a crater region in the southern polar region and a comparison area on the lunar equator.
It was shown that the moon’s surface emits infrared radiation in the range of six micrometers in wavelength. “On the basis of our comparisons, we attribute this signal to the presence of molecular water,” report Honniball and her colleagues. According to their calculations, the lunar regolith could contain around 100 to 400 micrograms of H2O per gram of rock material at high latitudes. Most of this molecular water is not present as a water cover or water layer on the regolith surface, but is hidden in the rock material. “The water detected by SOFIA sits inside the lunar rock granules or is trapped between granules that are shielded from the harsh lunar environment,” the scientists explain. It is also not yet clear how widespread this molecular water is on the moon.
Water ice even in small shadow areas
Paul Hayne from the University of Colorado in Boulder and his team have investigated another water source – the water ice in lunar “cold traps”. “If you imagined standing on the lunar surface near one of the poles, you would see shadows everywhere,” explains Hayne. Because the light, which only falls at an angle in the polar regions, never reaches many crater edges, but also never reaches small depressions and gullies. They are therefore permanently in the shade. So far, however, it has not been clear whether such smaller shadow areas will remain cold enough for water ice to remain there permanently or at least for a long time. To clarify this, Hayne and his team used data from NASA’s Lunar Reconnaissance Orbiter (LRO) space probe to create a detailed model of the lunar topography and the temperatures occurring there. From this they determined how many potentially icy cold traps there are in the size range from one kilometer to one centimeter on the moon.
The evaluation showed that 0.15 percent of the moon’s surface is permanently in shadow. Around two thirds of these permanent shadow zones could be cold enough to hold ice deposits. Most of them are located in the lunar polar regions above 80 degrees latitude. Contrary to previous assumptions, however, these cold traps are not only found in large craters, but can also be located in the smallest, a few centimeters or meters in shadow zones, as the researchers report. A total of around 40,000 square kilometers of the lunar surface could be covered by such cold traps – that’s a good twice as much as previously assumed. “If we’re right, then water for future lunar missions could be far more accessible than we thought,” says Hayne. “Astronauts would then not have to descend into the dark, deep shadows of the crater, but could simply walk around looking for shadow zones that are one meter in size – ice could be just as likely there.”
Source: Nature Astronomy; Casey Honniball (University of Hawaii, Manoa) et al, doi: 10.1038 / s41550-020-01222-x; Paul Hayne (University of Colorado, Boulder) et al., doi: 10.1038 / s41550-020-1198-9