In 1972, the astronauts of Apollo 17 pushed the lunar mess across the moon's surface, measuring gravity with a special instrument. There are no astronauts on Mars, but a group of clever scholars have realized they only have tools for similar Martian buggy experiments that work.
In a new article in Science, researchers are specifying how they adjust the sensors used to propel the Curiosity Rover and turn them into gravimeters that measure changes in gravitational attraction. This gives them the opportunity to measure the subtle rock climbers on the lower Sharp Peak, which rises 15 km from the base of the Gale Crater, and which curiosity rises from 2014. The results? It turns out that the density of these rock layers is much lower than expected.
Just like a smartphone, Curiosity brings accelerometers and gyros. Moving your smartphone allows these sensors to determine its location and the way it is turned. Sensors of curiosity do the same, but with much greater precision, playing a decisive role in navigating the Martian surface of each device. Knowing the orientation of the ribbon also allows engineers to accurately point their instruments and a multi-directional antenna with high gain.
By happy coincidence, the accelerometers of the burrow can be used as the gravity of Apollo 17. Accelerometers detect gravity on the planet every time the rover is stationary. Using engineering data from the first five years of the mission, the authors of the paper measured the gravity tug of Mars on the Rover. As curiosity climbs up the Sharp Mountain, the mountain adds extra weight – but not as much as the scientists expected.
"Lower Sharp Mountains are surprisingly porous," says lead author Kevin Lewis of Johns Hopkins University. "We know the lower layers of the mountain have been buried over time. This seals them, making them more dense. But this finding suggests that they have not been buried by as much material as we thought.
The Science of Marg Baggie
Astronauts from Apollo 17 drove a buggy through the Taurus-Litrus valley on the moon, intermittently stopping 25 measurements. Lewis studied the Martian gravitational fields using data collected from NASA's orbit and was familiar with Apollo 17's gravimeter.
The paper uses over 700 measurements of the Curiosity accelerometers made between October 2012 and June 2017. These data are calibrated to "noise" filtering, such as the effects of temperature and slope of the ridge during its climb. The calculations were compared to Mars gravity patterns to ensure accuracy.
The results are also compared with the mineral density estimates of the Curiosity Chemistry and Mineralogy tool, which characterizes the crystalline minerals in rock samples using X-rays. These data helped to know how many portions are the rocks.
The Mountain of Mystery
There are many mountains in craters or canyons on Mars, but few approach the scale of Mount Sharpe. Scientists are still not sure how the mountain has grown inside the Geyl Crater. One idea is that the crater was filled with sediment. How much of it is complete remains a source of debate, but the thought is that many millions of years of wind and erosion eventually excavate the mountain.
If the crater had been filled to the edge, all this material had to be pressed or compacted into the multiple layers of fine-grained sediments beneath it. The new paper shows that the lower layers of the Sharpe Mountain are sealed only half a mile to a mile (1 to 2 km) – much less than if the crater was fully filled.
"There are still many questions about how Sharpe Mountain has evolved, but this article adds an important part of the puzzle," says study co-researcher Ashwin Vassavada, a Curiosity scientist at NASA's Jet Propulsion Laboratory. JPL runs the mission of the Mars Science Laboratory in Curiosity.
"I am excited that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the Rover," he adds.
Lewis says Mars has a lot of mystery behind the Sharpe Mountain. Its landscape is similar to Earth, but it is sculpted more than wind and sand than water. They are planetary brothers and sisters, both known and quite different.
"For me, Mars is the mysterious valley of the Earth," says Luis. "It is similar, but it is shaped by different processes. It feels so unnatural for our earthly experience.