Some very clever people have figured out how to use MSL Curiosity’s navigation sensors to measure the gravity of a Martian mountain. What they’ve found contradicts previous thinking about Aeolis Mons, aka Mt. Sharp. Aeolis Mons is a mountain in the center of Gale Crater, Curiosity’s landing site in 2012.
Gale Crater is a huge impact crater that’s 154 km (96 mi) in diameter and about 3.5 billion years old. In the center is Aeolis Mons, a mountain about 5.5 km (18,000 ft) high. Over an approximately 2 billion year period, sediments were deposited either by water, wind, or both, creating the mountain. Subsequent erosion reduced the mountain to its current form.
Now a new paper published in Science, based on gravity measurements from Curiosity, shows that Aeolis Mons’ bedrock layers are not as dense as once thought.
Curiosity’s gravity measurements recall earlier days in Solar System exploration, when Apollo 16 astronauts used their Moon buggy, or Lunar Roving Vehicle, to measure the Moon’s gravity. That was way back in 1972. In our time, its robots instead of astronauts that are setting foot on distant worlds, but the spirit of exploration, and the science, is the same.
The new study is based on gravimetry, the measurement of very small changes in gravitational fields. It can only be done on the ground, versus large-scale gravimetry done from an orbiting spacecraft. To take these measurements, the research team re-purposed Curiosity’s accelerometers, instruments onboard the rover that are used for navigation.
When coupled with gyroscopes, accelerometers tell the rover where it is on Mars and which way it’s facing. Smart phones have them too, and they’re used by apps that allow you to point your phone at the sky and read the names of stars. Of course, Curiosity’s gyroscopes and accelerometers are far more accurate than anything inside a smart phone.
“I’m thrilled that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the rover.”
Study co-author Ashwin Vasavada, Curiosity’s project scientist, NASA’s Jet Propulsion Laboratory, Pasadena, California.
The team measured the change in the gravitational field of Mt. Sharp as the rover climbed it. Gravity weakens with altitude, and Curiosity’s instruments were re-calibrated to measure these tiny changes. From those changes, the density of the underlying rock was inferred.
The gravimetric measurements showed that the rock under the mountain is less dense than thought, meaning it is relatively porous. This goes against previous research showing that the crater floor used to be buried under several kilometers of rock.
“The lower levels of Mount Sharp are surprisingly porous,” said lead author Kevin Lewis of Johns Hopkins University. “We know the bottom layers of the mountain were buried over time. That compacts them, making them denser. But this finding suggests they weren’t buried by as much material as we thought.”
In their paper, the researchers show that their measurements include bedrock to a depth of several hundred meters, not mere surface rock. They measured an average density of 1680 ± 180 kg m -3. That’s much less dense than typical sedimentary rocks. Since sedimentary rocks gain density by being compacted underneath a greater accumulation of rock, their low density suggests they weren’t buried that deeply.
In a way, these findings only add to the mystery of Mt. Sharp’s formation, structure, and erosion. For instance, we still don’t know if Gale Crater was once completely filled with sediment, and that sediment was eroded to the modern shape of Mt. Sharp. It may be that only a portion of the crater was ever filled with sediment.
On the other hand, the summit of Mt. Sharp is higher than the rim of the crater. Based on that, other research has proposed Gale Crater was completely filled with sediment, and that Mt. Sharp is the remnant of a much taller mountain than we see now. But if that’s the case, then these new findings go counter to that. If these rocks at the lower reaches of Mt. Sharp were buried so deeply, their measured density would be much higher.
Another line of reasoning relies on Aeolian sedimentation. Aeolian means wind-driven. In this hypothesis, the wind carried sediment into the crater, depositing it onto Mt. Sharp and building it up into more or less the form it takes now. In that case, the rocks measured by Curiosity would never have been compacted. That would explain their low-density when compared to other buried, sedimentary rocks.
“There are still many questions about how Mount Sharp developed, but this paper adds an important piece to the puzzle,” said study co-author Ashwin Vasavada, Curiosity’s project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “I’m thrilled that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the rover,” he added.
This study won’t solve the debate over Gale Crater and Mt. Sharp, but it does add some clarity. It also shows the usefulness of rover-based gravimetric measurements in understanding the history of Mars.
Plus, it’s just really cool.
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