Like the faces of veterans comparing war wounds, the surface of our moon is scarred by a lifetime of damage—impact craters pockmarked with even more craters, sprayed ejecta, discolored regions laid down by volcanic flows. Studying these characteristics can reveal much about the processes that formed them, say Caltech graduate student Meg Rosenburg and her advisor Oded Aharonson, who have created the first comprehensive sets of maps revealing the roughness of the moon's surface.
The maps are based on detailed data collected by the Lunar Orbiter Laser Altimeter (LOLA) on NASA's Lunar Reconnaissance Orbiter. The LOLA instrument is the first multibeam laser altimeter to fly on an orbiter; every laser pulse the instrument sends toward the moon contains five beams that strike and bounce off the moon, revealing information about surface features. "That's very special because it allows us to measure the true gradient of the surface at a particular scale," says Rosenburg, the first author on a paper published in the Journal of Geophysical Research (JGR) describing the maps and what they mean.
The physical appearance of a landscape—and, in particular, its "roughness"—represents the "culmination of all the processes that have worked to create and modify it," Rosenburg says. "The landforms that make up different terrains on a planetary surface are created and modified by landscape processes that act at different scales, sometimes competing with each other. This suite of processes, whatever they are for a particular body, can change over time and as a result the terrain evolves."
For example, the moon's maria (the Latin word for "seas," because they looked like oceans to early observers), which were formed by vast flows of dark basaltic lava, are relatively smooth compared to the moon's highland regions. The moon's countless craters may be young or old, which also affects their roughness; the old will have battered floors and jagged sides, because they've been struck repeatedly over the eons by additional meteorites, but may also have been shaken smooth on different scales by moonquakes. "Often we can quantify signatures of various surface processes in the topography itself," says Rosenburg, who hopes to "sort out which processes are responsible for which features, at large and small scales, and how that varies with the age of the surface, or the type of terrain," Rosenburg says.
In the JGR paper, the researchers looked at roughness at several scales (where "scale" is the distance between two successive measurement points), ranging from 17 meters (around 57 feet) up to 2.7 kilometers (approximately 1.6 miles). "The small laser spot spacing of about 57 meters is a vast improvement over previous instruments," Rosenburg says. "We can explore scales on the moon now that have never been accessible before for this kind of analysis."
The new study is preliminary in the sense that it mainly quantifies the moon's roughness features without a detailed analysis of the processes that created them (that's Rosenburg's next project). However, Aharonson says, "one thing we can say is that the mare-covered plains change in their roughness properties according to age, as is also the case for lava flows on Earth. Crater ejecta rays that are sometimes difficult to identify in images also have a distinct roughness signature."
Rosenburg is now conducting a theoretical study to determine the relationship between the roughness properties and the cratering flux, "as well as intensity of the smoothing processes," Aharonson says.
"I want to know which factors are the major players in determining the statistical properties of a cratered surface," Rosenburg says. "So I start with a flat plane and add craters to it until it reaches saturation—meaning that any new crater that comes in on average erases another crater of that size. Two big factors are the size-frequency distribution of craters that are added—that is, how many small craters vs. large craters there are—and the shape of the craters. We also have to consider downhill diffusion of material," she explains, "either because everything is shaken up a bit when a new crater is emplaced or due to the myriad small impacts below the resolution of the model itself."
The idea, Rosenburg says, is that "we take our model, which is still much simpler than the moon itself, and try to find out what's important in determining what we ultimately observe. We think this will help us to understand current surface processes at relatively small scales on the moon, and it will definitely clarify discussions of cratered surfaces on bodies for which we don't have a lot of data, much less altimetry."
