A mountain on Mars that is almost as tall as Denali in Alaska appears to be surprisingly light, scientists reported on Thursday.
For more than four years, NASA’s Curiosity rover has been exploring Mount Sharp, located within an ancient meteor impact crater known as Gale and rising more than three miles high. Now measurements of tiny changes in gravity, recorded by the rover as it climbed in elevation, could help solve the question of how the mountain formed.
The official name of the mountain is Aeolis Mons, but mission scientists have nicknamed it after Robert P. Sharp, a Mars expert who died in 2004. It consists of layer upon layer of sedimentary rock, which offer an easy-to-read history book of Martian geology. That was one of the attractions that led NASA to choose the site for the Curiosity mission, which landed on Mars in 2012.
But sedimentary rocks typically form at the bottoms of lakes and oceans, not at the tops of mountains.
Some scientists surmised that the 96-mile-wide Gale crater was once a lake that slowly filled to the brim with sediment, which was then blown away by winds, leaving Mount Sharp. Others suggested that particles blew in from other parts of the planet and piled up at the center of the crater in the shape of a mountain.
In the new research, published on Thursday in the journal Science, the gravity measurements indicated that the rocks beneath Curiosity were not solid but porous, lowering their density. This finding suggested that the crater was never completely filled with sediment, because the porous rocks would not have been strong enough to support all of the weight without being compressed.
That conclusion fits in with the notion that both hypotheses of Mount Sharp’s formation are true in part, said Ashwin R. Vasavada, the mission’s project scientist and an author of the Science paper. Its bottom may consist of carved-out lake sediments, while the top part may have built up from windblown particles.
“That’s an idea that’s been gaining strength,” Dr. Vasavada said.
The findings were the result of a clever technical improvisation, using sensors that were intended to provide engineers with information on the status of the rover. Doing so effectively added a new scientific instrument to Curiosity, years after the rover landed on Mars.
The brainstorm came from Kevin W. Lewis, a professor of earth and planetary sciences at Johns Hopkins University and a member of the mission team. He realized that modern devices such as smartphones have chips that measure the forces of acceleration. That’s how an iPhone knows to rotate the screen depending on whether you are holding it vertically or horizontally.
This kind of sensor, known as an accelerometer, can measure changes in the pull of gravity. On Earth, geologists use variations in gravity to probe for underground features such as earthquake faults and ore deposits. “Wouldn’t it be wild if we had something like that on Mars?” said Dr. Lewis, the lead author of the Science paper.
Then he realized that there were indeed accelerometers on Mars. The Curiosity rover was using such devices to track the tilt of the vehicle as it moved across the surface.
Those measurements provided a record of the force of gravity on Curiosity. “Luckily, the engineers had already been taking a perfect data set, basically since day one,” he said.
Because the sensors were not meant for Dr. Lewis’s purpose, the data were “quite noisy,” he said. “The data would jump around pretty severely from day to day.”
After calibrating the measurements and averaging out the variations, the researchers found that gravity indeed waned slightly as Curiosity climbed about 1,000 feet in elevation. But it was less than what would have been measured if Curiosity had been hoisted 1,000 feet into the air. That is because of the additional gravitational attraction from the mass of the mountain.
The change in the force was small. For a person who weighs 150 pounds on Earth (and just 57 pounds on Mars because of the weaker gravity), climbing 1,000 feet up Mount Sharp would shed about one-tenth of an ounce.
The researchers calculated that the bedrock under Curiosity had a density of about 100 pounds per cubic foot. However, the minerals that make up the rocks were about 70 percent more dense than that. This led Dr. Lewis and his colleagues to conclude that the rocks must be porous.
John P. Grotzinger, a geology professor at the California Institute of Technology who served as the project scientist during Curiosity’s first three years on Mars, said Dr. Lewis’s idea to tap into the engineering data was “a great example of real scientific creativity.”
But he was less certain of the conclusions.
Curiosity’s examination of the rocks at the surface suggest that sediment grains are tightly cemented together, not porous. Dr. Grotzinger also noted sizable swings in the gravity measurements, even when Curiosity was moving on roughly level ground. That means that the material beneath Mount Sharp might not be as uniform as assumed in the calculations by Dr. Lewis and his colleagues. The gravity data might reflect low-density geological structures deeper underground, not the properties of the Mount Sharp rocks.
“You just have to somehow invent a geologically plausible scenario,” Dr. Grotzinger said. “To me, it leaves some wiggle room.”