NASA's Curiosity rover and its suite of advanced tools have revealed all kinds of fascinating facts about Mars, but scientists here on Earth have now uncovered a surprising characteristic through one of its more rudimentary instruments. Using an accelerometer similar to what you'd find in a smartphone, the researchers have been able to gather more precise density measurements of its rocky layers, finding it to be much more porous than previously thought.

For more than six years now the Curiosity rover has been schlepping its way across Mars' Gale Crater, using its high-resolution cameras to image its more remarkable features, its drill to dig into the surface and its miniature onboard laboratories to break down samples and reveal organic molecules at least three billion years old.

There is a lot these kinds of instruments can and have told us about Mars, but scientists tracking five years of data from its onboard accelerometers have revealed some holes in the story. Just as they do in a smartphone, Curiosity's accelerometers can determine its orientation and motion, but with much greater precision and with the ability to measure the force of gravity at different points on the surface as well.

"Working from the rocks' mineral abundances as determined by the chemistry and mineralogy instrument, we estimated a grain density of 2,810 kilograms per cubic meter," said Travis Gabriel, a graduate student at the Arizona State University's School of Earth and Space Exploration. "However, the bulk density that came out of our study is a lot less — 1,680 kilograms per cubic meter."

The scientists retraced the steps of Curiosity over its first five years on Mars and logged the gravity force measurements at more than 700 points along the journey. The data as Curiosity began to ascend Mount Sharp, a 3-mile (4.8 km) tall mountain in the center of the Gale Crater, is where things started to get interesting. It was expected that as Curiosity made its way up the hill, the extra bulk beneath it would add further gravity to the equation. This proved to be somewhat true, but at much less than the scientists had predicted, suggesting the material underfoot is a lot less dense than suspected.

"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."

The discovery feeds intriguing new information into the ongoing debate over how Mount Sharp was formed. One line of thinking goes that crater was once brimming with sediment, with wind and erosion gently forming the giant mound over millions of years. All the while, the weight of all that material should have compressed the layers beneath it.

According to the new paper, however, the lower layers of Mount Sharp appear to have been compacted far less than what they should have been had the crater been completely filled – by only 0.5 to 1 miles (0.8 to 1.6 km) in total.

"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. "I'm thrilled that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the rover."

The research was published in the journal Science.

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