In October of last year, NASA revealed that their Stratospheric Observatory for Infrared Astronomy (SOFIA) had confirmed the presence of water on the moon’s sunlit surface. More specifically, the amount of water was about the same as what would fit in a 12-ounce bottle and it was discovered in a cubic meter of soil in the Clavius Crater.
While that was incredibly exciting news, scientists were still baffled as to how water could be present during the hot daytime temperatures on the lunar surface. The daytime temperature on the sunlit surface can reach temperatures as high as 260 degrees Fahrenheit (127 degrees Celsius). To put this into better perspective, water boils at an average temperature of 212 degrees Fahrenheit (100 degrees Celsius) depending on where you are located.
So, if the sunlit surface is hot enough to boil water, why is there still water ice in certain areas? According to a new study by NASA scientists, the “roughness” of the lunar surface in addition to its shadows could help to keep it from evaporating.
A previous study published in 2009 stated that the amount of water on the lunar surface changed throughout the day – there was less water before noon (the hottest part of the day) but when the temperatures cooled down in the afternoon, the amount increased. This suggested that some of the water was able to move around from being boiled to freezing over again. However, the thermophysical models used in that study were conducted on a mostly flat lunar surface. Another paper was published last year that described how water could have been held in small “cold traps” across the lunar surface.
In this most recent study, the researchers focused on desorption (a phenomenon where a substance is released through or from a surface) in addition to using a much more accurate model of the actual lunar surface. In fact, astrophysicist Björn Davidsson from the Jet Propulsion Laboratory (JPL) and JPL researcher Sona Hosseini referenced photos taken from the Apollo missions in order to update their model to a lot rougher lunar surface with craters and boulders.
In this new model, they realized that the roughness on the moon created shadows that kept the surface cool enough for the water to move around and not evaporate as Davidsson explained, “Frost is far more mobile than trapped water,” adding, “Therefore, this model provides a new mechanism that explains how water moves between the lunar surface and the thin lunar atmosphere.”
The researchers detailed their findings the Monthly Notices of the Royal Astronomical Society which read in part, “The model of the surface temperature of the Moon described in this paper has significant implications for understanding the presence and evolution of water on the lunar surface.” “It is of critical importance to take account of the surface roughness to get an accurate picture of the amount of water on the surface of the Moon.”
An illustration depicting the shadows on the rough lunar surface can be seen here.