The lack of an atmosphere exposes the Moon and other airless bodies, like asteroids, to space weathering. In this process, some combination of micrometeoroids and a stream of charged particles known as the solar wind alters celestial surfaces, making them darker and redder through the production of nanometer-scale iron particles.
The relative contribution of micrometeoroids and the solar wind to space weathering on the Moon has been unclear, with lunar soil samples providing scant insight. In a new study, Sim et al. took a different approach, using the optical properties of craters to show that the solar wind likely plays a dominant role in lunar space weathering.
The research team used data from a Japanese lunar orbiter known as Kaguya (or SELENE) to investigate light reflected off the walls of 1,872 lunar craters with diameters ranging between 5 and 120 kilometers. They divided each crater wall into four parts: equator facing, pole facing, east, and west. Then, they analyzed differences in optical properties between the walls of craters at different latitudes and longitudes.
The researchers found that at higher latitudes, pole-facing walls were less weathered—in other words, brighter and less red—than equator-facing walls. This finding is consistent with the geometry of the solar wind, which strikes the Moon mostly in a direction parallel to the equator. Pole-facing walls of craters located closer to the Moon’s poles angle more steeply away from the solar wind and are therefore more protected from it.
Meanwhile, differences between east- and west-facing walls varied with longitude in a way that suggests they are shielded from the solar wind whenever the Moon passes through the elongated tail of Earth’s magnetic field.
Together, these directional differences suggest a dominant role for the solar wind in lunar space weathering. Micrometeoroids are unaffected by Earth’s magnetic field, so if they played a bigger role, one would expect a more uniform weathering of lunar craters.
The mathematical relationships between crater wall reflectance and solar wind exposure seen in this study should hold true for other airless bodies and for other topographical features, such as hills, valleys, and volcanoes. Accounting for these effects could improve the accuracy of other measurements making use of optical data from airless bodies, such as surface age, mineralogy, and water abundance. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075338, 2017)
—Sarah Stanley, Freelance Writer