Evidence of water in the shadows of craters or enclosed in glass beads like microscopic snowflakes has recently revealed that the Moon’s surface is much less desiccated than we ever imagined.
One mystery that astronomers are trying to solve is precisely where this layer of icy water came from. One surprising possibility that appears is an elementary rain from our own atmosphere, emitted by the Earth’s magnetic field.
Water is not exactly a rare substance in space. Given the right places to hide, it can pass inside asteroids, cover up comets and even cling precariously to the darkness of Mercury’s craters.
It makes sense that, from time to time, some of this splashes on the Moon. But with the scorching heat of the Sun and the lack of protection against the vacuum of space, it is not expected to last long.
To explain the surprising amount of moisture found on the lunar surface, researchers have proposed a more dynamic form of production: a constant “rain” of protons driven by the solar wind. These hydrogen ions are transformed into mineral oxides from the Moon’s dust and rocks, tearing off chemical bonds and forming a loose, temporary alliance with oxygen.
It is a solid hypothesis, which would be given a boost thanks to the observations of the most exposed (and most bound) water molecules that quickly succumbed to the vacuum of space as long as the Moon is protected from the solar wind.
Our own planet is pretty well protected from the constant breeze of ions blown by the Sun, thanks to a magnetism bubble that surrounds it. This force field not only surrounds us, but is dragged in the form of a tear by the solar attack.
For a few days each month, the Moon traverses this magnetosphere, receiving a brief respite from the water of the Sun’s proton.
An international team of researchers has recently used plasma and magnetic field instruments in the Japanese Kaguya orbiter to pinpoint this precise moment in the Moon’s orbit. Spectral data from Chandrayaan-1’s Moon Mineralogy Mapper (M3) were used to map the distribution of water on the Moon’s surface at its highest latitudes.
The results were not exactly what anyone expected.
In short, nothing happened. The time series of the Moon’s watery signature revealed no appreciable difference in the three to five days it spent hiding from the Sun’s wind.
These results could mean a few things. One is that the entire solar wind hypothesis is a bust, and other reservoirs are responsible for replenishing the Moon’s surface water.
But another intriguing possibility that doesn’t require us to abandon the idea of the solar wind is that the Earth’s magnetic field simply picks up where the Sun leaves it.
Previous research has suggested that the plasma sheet associated with our planet’s magnetosphere could produce the same amount of hydrogen ions as the solar wind, especially toward the lunar poles.
Certainly not all are offered the same amount of punch, but researchers hypothesize that even some heavy-weight hydrogen ion can create more than its fair share of water. And lower-energy protons may be more easily held in place, so they are less likely to fall apart in the moments after their formation.
There is also every possibility that oxygen from the upper atmosphere over our poles is transmitted through the extensive stretch of vacuum to collide with the Moon, especially during periods of enhanced geomagnetic activity.
If all this sounds pretty speculative, so be it. Right now, we just have a pretty amazing water map that just doesn’t fit your favorite models.
But it points in some exciting directions to the emerging field of lunar hydrodynamics. Since the researchers only mapped the distribution of water at higher latitudes, it would be worth looking closer to the equator of projected losses in the future.
On a practical level, perhaps we should rely heavily on a supply of lunar frost replenishment as fuel and life support, if the Moon were to become a springboard for space exploration.
If nothing else, we are slowly gathering an understanding of the water cycle in space to help us better understand the connections between our planet and its only natural satellite.
This research was published in Letters from astrophysical journals.