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Evidence of water in the shadow of craters or encased in glassy pearls like microscopic snowballs recently revealed that the Moon’s surface is much less parched than we ever imagined.
The origin of this ice-water plating is a mystery that astronomers are currently trying to solve. One surprising possibility that emerges is elemental rain from our own atmosphere, delivered by the Earth’s magnetic field.
Water is not exactly a rare substance in space. Given the proper places to hide, it can slip inside asteroids, cover comets, and even cling precariously to the darkness of Mercury’s craters.
It makes sense that at least part of it splashes on the Moon every now and then. But with the scorching heat of the Sun and the lack of protection from the vacuum of space, this is not expected to last very long.
To account for the surprising amount of moisture found on the lunar surface, the researchers proposed a more dynamic form of production – a constant “rain” of protons driven by the solar wind. These hydrogen ions transform into mineral oxides in the dust and rocks of the Moon, tearing chemical bonds apart and forming a loose and temporary alliance with oxygen.
This is a solid hypothesis, which would be reinforced by observations of the most exposed (and more loosely bound) water molecules that quickly succumb to the vacuum of space whenever the Moon is sheltered from the solar wind.
Our own planet happens to be fairly well protected from the constant breeze of ions blown by the Sun, thanks to a bubble of magnetism surrounding it. This force field not only surrounds us, it is blown in the shape of a tear by the solar assault.
For a few days each month, the Moon traverses this magnetosphere, enjoying a brief respite from the downpour of protons from the Sun.
An international team of researchers recently used plasma and magnetic field instruments on the Japanese orbiter Kaguya to locate this precise moment in the orbit of the Moon. Spectral data from the Chandrayaan-1 Moon Mineralogy Mapper (M3) was then used to map the distribution of water on the Moon’s surface at its highest latitudes.
The results were not quite what everyone expected.
In short, nothing happened. The time series of the Moon’s watery signature revealed no appreciable difference between the three to five days spent sheltered from the solar wind.
These results could mean several things. The first is that the whole solar wind hypothesis has failed and other reservoirs are responsible for replenishing the surface water of the Moon.
But another intriguing possibility that doesn’t force us to abandon the idea of the solar wind is that the Earth’s magnetic field simply picks up where the Sun left off.
Previous research has suggested that the plasma sheet associated with our planet’s magnetosphere could provide roughly the same amount of hydrogen ions as the solar wind, especially towards the lunar poles.
Granted, not everything comes with the same amount of oomph, but the researchers speculate that even the occasional high-impact hydrogen ion could potentially create more than its fair share of water. And low-energy protons could be more easily held in place, so less likely to fall apart in the moments after they form.
There is also every possibility that oxygen from the upper parts of the atmosphere above our poles will be transported through the vast expanse of vacuum to collide with the Moon, especially during times of increased geomagnetic activity. .
If this all sounds rather speculative, that’s because it is. At the moment, we only have a pretty surprising water map that doesn’t quite match the favorite models.
But it points to exciting new directions for the emerging field of lunar hydrodynamics. Since the researchers only mapped the water distribution at higher latitudes, it will be worth looking closer to the equator for losses predicted in the future.
On a practical level, we might need to rely heavily on a supply of lunar frost for fuel and sustaining life one day, if the Moon becomes a springboard for space exploration.
If nothing else, we’re slowly assembling an understanding of a water cycle in space that helps us better understand the connections between our planet and its only natural satellite.
This research was published in the Letters from the Astrophysical Journal.
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