WRITTEN ON THE MOON’S WEARY FACE are signs of the damage it has endured for the past 4.5 billion years. From impact craters to the dark plains or ‘maria’ left behind by volcanic eruptions, the scars are all that remain to tell the tale of the past.
But these features only hint at the processes that once acted—and act today—to shape the surface.
To get more insight, Meg Rosenburg and her colleagues at the California Institute of Technology have put together the first comprehensive set of maps revealing the slopes and roughness of the Moon’s surface, based on detailed data collected by the Lunar Orbiter Laser Altimeter (LOLA) on NASA’s Lunar Reconnaissance Orbiter.
Like wrinkles on skin, the roughness of craters and other features on the Moon’s surface can reveal their age.
“The key is to look at the roughness at both long and short scales,” says Rosenburg, who is the first author on the paper describing the results, published in the Journal of Geophysical Research earlier this year.
The roughness depends on the subtle ups and downs of the landscape, a quality that the researchers get at by measuring the slope at locations all over the surface.
To put together a complete picture, the researchers looked at roughness at a range of different scales—the distances between two points—from 17 metres to as much as 2.7 kilometres.
“Old and young craters have different roughness properties—they are rougher on some scales and smoother on others,” says Rosenburg. That’s because the older craters have been pummelled for eons by meteorites that pit and mar the site of the original crater, changing its shape.
“Because this softening of the terrain hasn’t happened at the new impact sites, the youngest craters immediately stand out,” says Gregory Neumann, a co-investigator on LOLA at NASA’s Goddard Space Flight Centre.
By looking at where and how the roughness changes, the researchers can get important clues about the processes that shaped the Moon.
A roughness map of the material surrounding Orientale basin, for example, reveals subtle differences in the ejecta, or debris, that was thrown out when the crater was formed by a giant object slamming into the Moon.
That information can be combined with a contour map that shows where the high and low points are.
“By looking at both together, we can say that one part of Orientale is not just higher or lower, it’s also differently rough,” Rosenburg says. “That gives us some clues about the impact process that launched the ejecta and also about the surface processes that later acted to modify it.”
The smooth plains of the maria, which were created by volcanic activity, have a different roughness “signature” from the Moon’s highlands, reflecting the different origins of the two terrains. Maria is Latin for “seas,” and they got that name from early astronomers who mistook them for actual seas.
Adapted from information issued by Elizabeth Zubritsky, NASA’s Goddard Space Flight Centre.
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