An annotated 360-degree view from the Curiosity mast camera. Dust remaining from an enormous recent storm can be seen on the platform and in the sky. And holes in the tires speak of the rough terrain Curiosity has traveled, but now avoids whenever possible. Make the screen bigger for best results and enjoy the show. (NASA/JPL-Caltech/MSSS)
When it comes to the search for life beyond Earth, I think it would be hard to point to a body more captivating, and certainly more studied, than Mars.
The Curiosity rover team concluded fairly early in its six-year mission on the planet that “habitable” conditions existed on early Mars. That finding came from the indisputable presence of substantial amounts of liquid water three-billion-plus years ago, of oxidizing and reducing molecules that could provide energy for simple life, of organic compounds and of an atmosphere that was thick enough to block some of the most harmful incoming cosmic rays.
Last year, Curiosity scientists estimated that the window for a habitable Mars was some 700 million years, from 3.8 to 3.1 billion years ago. Is it a coincidence that the earliest confirmed life on Earth appeared about 3.8 billion years ago?
Today’s frigid Mars, which has an atmosphere much thinner than in the planet’s early days, hardly looks inviting, although some scientists do see a possibility that primitive life survives below the surface.
But because it doesn’t look inviting now doesn’t mean the signs of a very different planet aren’t visible and detectable through instruments. The Curiosity mission has proven this once and for all.
The just released and compelling 360-degree look (above) at the area including Vera Rubin Ridge brings the message home.
Those fractured, flat rocks are mudstone, formed when Gale Crater was home to Gale Lake. Mudstone and other sedimentary formations have been visible (and sometimes drilled) along a fair amount of the 12.26-mile path that Curiosity has traveled since touchdown.
The area the rover is now exploring contains enough hematite — iron oxide — that its signal was detectable from far above the planet, making this area a prized destination since well before the Mars Science Laboratory and Curiosity were launched.
Like Martian clays and sulfates that have been identified and explored, the hematite is of great interest because of its origins in water. Without H2O present many eons ago, there would be no hematite, no clay, no sulfates. But as Mars researchers have found, there is a lot of all three.
I like to return to Mars and especially Curiosity because it provides something unique in the cosmos: an environment where scientists today have ground-truthed the hypothesis that early Mars was once habitable, and found unambiguous results that it was.
That doesn’t mean that the planet necessarily ever gave rise to, or supported, living organisms. But it’s a lot more than can be said for other targets for life beyond Earth.
NASA’s Europa Clipper may determine some day that beneath the ice crust of that moon of Jupiter is an ocean that is, or was, habitable. But that determination is still years away. Same with Saturn’s moon Enceladus, which some see as habitable beneath its ice, but no mission is currently approved to determine that.
And when it comes to exoplanets and possible life on them, it is both a logical and alluring conclusion that some support living organisms — there are, after all, billions and billions of exoplanets, and the cosmos is filled with the elements and compounds we find on Earth.
But we remain quite far away from consensus on what an exoplanet biosignature might be, and much further away from being able to confidently detect the probable biosignature elements and compounds on distant exoplanets.
And so for now we have Mars as our most plausible target for life beyond Earth.
It wasn’t that long ago that the NASA exploration mantra for Mars was “follow the water,” under the assumption that life needed water to survive.
But Curiosity and satellites orbiting Mars have found abundant proof that water did play a major role in the planet’s early times. Not only has Curiosity found that a lake existed on and off for hundreds of millions of years at Gale Crater, but researchers recently announced the presence of a large reservoir of liquid water beneath the southern polar region.
What’s more, evidence of briny surface streams on steep Martian cliffs in their warm season has grown stronger, though it remains a much-debated finding.
But with the water story well established, researchers are focused more on organics, minerals and what can be found beneath the radiation-baked surface.
Curiosity has been working for months around Vera Rubin Ridge, though for much of that time with a big handicap — the rover’s long-armed drill wasn’t working. Important internal mechanisms stopped performing in late 2016, and it wasn’t until late spring of 2018 that a workaround was ready.
After one successful drilling, the next two failed. But there was no drill problem with those two; the rock on the ridge was just too hard to penetrate. It makes sense that the rock would be very hard because it has withstood millions of years of powerful winds blowing across Gale Crater, while other nearby rock and sediments were carried away.
The best way to discover why these rocks are so hard is to drill them into a powder for the rover’s two internal laboratories. Analyzing them might reveal what’s acting as “cement” in the ridge, enabling it to stand despite wind erosion.
Most likely, said Curiosity project scientist Ashin Vasavada, groundwater flowing through the ridge in the ancient past had a role in strengthening it, perhaps acting as plumbing to distribute this wind-proofing “cement.” In this case, it would be some variation of hematite, which in crystal form can be pretty hard on its own.
On its third attempt — and after a prolonged search for a “soft” spot in the ridge — the Curiosity drill did succeed in digging a hole and bringing back some precious powdered contents for study in the two onboard labs.
After the exploration of Vera Rubin Ridge and its hematite will come explorations of large deposits of sulfates and phyllosilicates (clays) — both formed in water as well — further up Mt. Sharp.
I find the landscape of Mars that Curiosity shows us to be captivating, but also sobering when it comes to the search for life beyond Earth.
Here is the planet closest to Earth (during some orbits, at least), one that has been determined to be habitable 3 to 4 billion years ago, one that can be studied with rovers on the ground and orbiting satellites — and still we can’t determine if it ever actually supported life, and probably won’t be able to for decades to come.
The big confounding factor on Mars really is time. Life could have come and gone billions of years ago, and intense surface radiation could have erased that history and made it appear as if life was never there. (This is one reason why Mars scientists want to dig deeper below the surface, where the effects of radiation would be much reduced.)
Time may be a powerful obstacle when it comes finding signs of life on exoplanets as well. If life exists elsewhere in the cosmos, it surely comes and goes, too. The odds of us catching it when it’s present may be low, despite all those billions and billions of planets. (Given the way that exoplanet biosignatures work, the life needs to be present at the time of observation.)
Or maybe the time for life in the cosmos has really just begun.
Harvard-Smithsonian astrophysicist Avi Loeb argued several years ago that life on Earth may be a premature flowering, compared with what may well happen later and elsewhere. (Column on his intriguing ideas is here.)
A majority of stars in the cosmos are red dwarfs, or M stars. They take eons to stabilize and then generally continue in a steady state for much longer than a G star like our sun. So, he argued, life in the cosmos around red dwarfs may not become widespread for some time, and then could last for a very long time if and when it did arise.
But enough about time — other than to perhaps take a little more time to enjoy the 360-degree view of Mars and Curiosity that brings thoughts like these to mind.