What Curiosity has yet to tell us about Mars

After five years on Mars, the Curiosity rover is an old pro at doing science on the Red Planet. Since sticking its landing on August 5, 2012, NASA’s Little Robot That Could has learned a lot about its environs.

Its charge was simple: Look for signs that Gale crater, a huge impact basin with a mountain at its center, might once have been habitable (for microbes, not Matt Damon). Turning over rocks across the crater, the rover has compiled evidence of ancient water — a lake fed by rivers once occupied the crater itself — and organic compounds and other chemicals essential for life.
NASA has extended the mission through October 2018. And there’s still plenty of interesting chemistry and geology to be done. As the robot continues to climb Mount Sharp at the center of the crater, Curiosity will explore three new rock layers: one dominated by the iron mineral hematite, one dominated by clay and one with lots of sulfate salts.

So, here are four Martian mysteries that Curiosity could solve (or at least dig up some dirt on).

Does Mars harbor remnants of ancient life?
Curiosity’s Mars Hand Lens Imager can take microscopic images, but preserved cells or microfossils would still have to be pretty big for the camera to see them. What the rover can do is detect the building blocks for those cells with its portable chemistry lab, Sample Analysis at Mars. The lab has already picked up chlorobenzene, a small organic molecule with a carbon ring, in ancient mud rock. Chains of such molecules go into making things like cell walls and other structures.
“We’ve only found simple organic molecules so far,” says Ashwin Vasavada, a planetary scientist at NASA’s Jet Propulsion Laboratory who leads Curiosity’s science team. Detective work in chemistry labs here on Earth could shed light on whether bigger organic molecules on Mars’ surface might degrade into smaller ones like chlorobenzene.

Curiosity could still turn up intact, heavier-duty carbon chains. The rover carries two sets of cups to do chemistry experiments, one dry and one wet. The latter contains chemical agents designed to draw out hard-to-find organic compounds. None of the wet chemistry cups have yet been used. A problem with Curiosity’s drill in December 2016 has held up the search for organics, but possible solutions are in the works.
How did Mars go from warm and wet to cold and dry?
That’s one of the million-dollar questions about the Red Planet. Curiosity has piled on evidence that Mars was once a much more hospitable place. Around 3.5 billion years ago, things changed.

The going theory is that particles from the sun stripped away much of Mars’ atmosphere (and continues to do so) when the planet lost most of its protective magnetic field. “That caused the climate to change from one that could support water at the surface to the dry planet it is today,” Vasavada explains. Curiosity found a higher ratio of heavy elements in the current atmosphere, adding credence to this argument — presumably the lighter elements were the first to go.

There’s also a chance that as the rover hikes up Mount Sharp it could capture regional evidence of the wet-to-dry transition. So far, Curiosity has investigated rocks from the tail end of the wet period. The new geologic layers it will encounter are younger.

“Hopefully we’ll be able to get some insight by looking at these rocks into some of the global changes happening that maybe no longer permitted a lake to be present on the surface,” says Abigail Fraeman, a research scientist at NASA’s Jet Propulsion Lab.
Does Mars really have flowing water today?
Some mineralized salts absorb water and release it as liquid when they break down at certain temperatures. The Curiosity team looked for the bursts of water that might result from such a process in Gale crater and came up empty.

But in 2015, the Mars Reconnaissance Orbiter snapped images of shifting salt streaks indicative of actively flowing water. The images are the best evidence yet that liquid water might not be a thing of the past.

Mount Sharp has similar dark streaks, and Curiosity periodically takes pictures of them. “It’s something we keep an eye on,” Vasavada says. If the streaks change in a way that might indicate that they’re moving, the rover could corroborate evidence of modern-day water on Mars. But so far, the streaks have stayed stagnant.

Where does the methane in Mars’ atmosphere come from?
On Earth, microbes are big methane producers, but on Mars, methane’s origins are still unclear. Early on Curiosity detected extremely low levels of the gas in Mars’ atmosphere. This baseline appears to subtly fluctuate annually — perhaps driven by temperature or pressure. Curiosity continues to monitor methane levels, and more data and modeling could help pinpoint what’s behind the annual ups and downs.

At the end of 2014, researchers noticed a spike 10 times the baseline level. Scientists suspect that methane sticks around in the air on Mars for only about 300 years. So, the methane spike must be relatively new to the atmosphere. “That doesn’t necessarily mean it’s being actively created,” Vasavada says. “It could be old methane being released from underground.” Minerals interacting with subterranean water sometimes make methane gas.

Mars’ methane could also be the product of planetary dust particles broken down on the surface. And yet another possible explanation is biological activity. “We have zero information to know whether that’s happening on Mars, but we shouldn’t exclude it as an idea,” says Vasavada. So, Martian life is unlikely but can’t be completely ruled out.