Shadows have always been known to be excellent hiding spots. They can even hide unexpected things from the earth. According to a new NASA study, there could be water moving from shadow to shadow on the moon – even in daylight.
Scientists have long accepted the fact that there is water on the moon – especially in the permanently shadowed craters at the poles, which is why they recently funded a “hopper” mission to study them. But there are parts of the moon that are sometimes exposed to the sun and sometimes in shadow. Previously, scientists thought it would be difficult for water ice to exist in these environments, but it turns out that they may have been wrong.
UT video discussing part of the lunar environment.
Data from SOFIA, one of NASA’s commercial aircraft-based observatories, confirmed that water exists on the lunar surface exposed to daylight. But models suggested that any water that might have been there should have been scorched by the sun. The data contained a critical clue which then led to a hypothesis that takes advantage of two other factors in the lunar environment.
This critical indication was that the amount of water measured by SOFIA decreases on the lunar “morning” and then increases on the lunar “afternoon”. If it were just burned away, the amount would have steadily decreased over time. It also ruled out that the water was trapped in rock formations by an earlier meteorite impact. However, one explanation that fits the data is that the water migrates to different parts of the moon over the course of a lunar day. To find out if this was possible, the scientists at the Jet Propulsion Laboratory began studying the environmental conditions of the moon.
Map of some traces of water ice on the moon, mostly in shady craters.
Credit – Shuai Li, University of Hawaii SOEST / HIGP
One focus of her study was the “jagged” shadows that lay over the surface of the moon and move with the position of the sun in the sky. These shadows are mainly formed by rocks or cliffs rather than crater walls and are usually not very large. As the sun irradiates these areas, it increases its temperature dramatically, in some cases up to 120 ° C. After the sun has moved on and an area is re-eclipsed, the temperature can drop as low as -210 ° C. The heat is not effectively transferred between these two areas, even though they are literally touching, as there is little to do with no atmosphere that provides the thermal conductivity necessary to equilibrate temperatures as there is on Earth.
With these cold temperatures in the shade, pockets of water ice are certainly possible, but what about the change in quantity over the course of a lunar day? While there isn’t enough atmosphere to perform heat transfer on the moon, it could be enough to perform hydrotransfer.
Graphic showing the new mechanism (right) and what scientists may have previously kept the water on the lunar surface (left).
Credit – NASA / JPL-Caltech
Although the moon is generally perceived as a completely vacuumed desert, it has an “exosphere” made up of traces of gases that are gravitationally bound to the moon’s surface before being blown away by solar winds. Water ice frozen in shaded areas that are then exposed to the sun would likely join this exosphere as vapor and transition to a colder, shadowy area and then freeze again as part of the lunar day cycle.
That all sounds good in theory, but so far that’s just theory. So the JPL team decided to develop a sensor that could help collect data to prove or disprove this theory. This tiny sensor, known as the Heterodyne OH Lunar Miniaturized Spectrometer (HOLMS), would actually look for hydroxyl, which is similar to water, and can likely shed some light on how much water is in the exosphere. Hydroxyl is much more reactive than water, making it an easier target for quantitation by sensors like HOLMS.
Unfortunately, the time for deploying such sensors could be running out. Efforts are already being made to establish permanent and commercial operation on the Moon, and such efforts run the risk of adding artificial water vapor or other contaminants to the otherwise pristine lunar surface. Fortunately, the sensors are already equipped for use on some lunar landers such as the Autonomous Pop-Up Flat Folding Explorer Robot (A-BUFFER), which could serve as the vanguard of all human missions to the moon.
Until such a sensor is launched, however, we only have remote observations and theories. But if there’s one lesson to be learned from all of this, it’s to stay away from shadows on the moon – it’s really, really cold there and you could slip on ice.
Learn more:
JPL – NASA study highlights the importance of surface shadows in the moonwater puzzle
RAS – Effects of surface roughness in models of water desorption on the moon
UC Boulder – Tiny moon shadows can harbor hidden stores of ice
SciTech Daily – NASA says cold shadows on the lunar surface can explain the lunar water mystery
Mission statement:
Apollo era image of an astronaut on the moon.
Credit – NASA
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