A team of researchers led by Dr. Sander Goossens of NASA’s Goddard Space Flight Center has found evidence that the crust of the Red Planet is not as dense as previously thought. The research is published in the journal Geophysical Research Letters.
A new map of the thickness of the Martian crust shows less variation between thicker regions (red) and thinner regions (blue), compared to earlier mapping. This view is centered on Valles Marineris, with the Tharsis Montes near the terminator to its west. The map is based on modeling of Mars’ gravity field by Goossens et al. Image credit: NASA / Goddard / UMBC / MIT / E. Mazarico.
“The crust is the end-result of everything that happened during a planet’s history, so a lower density could have important implications about Mars’ formation and evolution,” Dr. Goossens said.
He and his colleagues from NASA’s Goddard Space Flight Center mapped the density of Mars’ crust, estimating the average density is 2,582 kg/m3 (about 161 pounds per cubic foot). That’s comparable to the average density of the lunar crust.
Typically, the Martian crust has been considered at least as dense as Earth’s oceanic crust, which is about 2,900 kg/m3 (about 181 pounds per cubic foot).
The new value is derived from Mars’ gravity field, a global model that can be extracted from satellite tracking data using sophisticated mathematical tools.
The gravity field for Earth is extremely detailed, because the data sets have very high resolution. Recent studies of the Moon by NASA’s GRAIL mission also yielded a precise gravity map.
The data sets for Mars don’t have as much resolution, so it’s more difficult to pin down the density of the crust from current gravity maps. As a result, previous estimates relied more heavily on studies of the composition of the Martian soil and rocks.
“As this story comes together, we’re coming to the conclusion that it’s not enough just to know the composition of the rocks. We also need to know how the rocks have been reworked over time,” said co-author Dr. Greg Neumann.
The team started with the same data used for an existing gravity model but put a new twist on it by coming up with a different constraint and applying it to obtain the new solution.
A constraint compensates for the fact that even the best data sets can’t capture every last detail. Instead of taking the standard approach, known to those in the field as the Kaula constraint, the authors created a constraint that considers the accurate measurements of Mars’ topography.
“With this approach, we were able to squeeze out more information about the gravity field from the existing data sets,” said co-author Dr. Terence Sabaka.
Before taking on Mars, the scientists tested their approach by applying it to the gravity field that was in use before the GRAIL mission.
The resulting estimate for the density of the lunar crust essentially matched the GRAIL result of 2,550 kg/m3 (about 159 pounds per cubic foot).
From the new model, they generated global maps of the crust’s density and thickness. These maps show the kinds of variations they expect, such as denser crust beneath Mars’ giant volcanoes.
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Sander Goossens et al. 2017. Evidence for a low bulk crustal density for Mars from gravity and topography. Geophysical Research Letters 44 (15): 7686-7694; doi: 10.1002/2017GL074172
