Early in the Curiosity rover’s trek across Gale Crater on Mars, team member and Los Almos National Laboratory planetary scientist Nina Lanza reported finding surprisingly high concentrations of the mineral manganese oxide. It was showing up as a blackish-purple fill to cracks in rocks, and possibly as a surface covering to others.
Lanza, who had some experience with the common and much-debated mineral– found in the American Southwest and other arid climes — initially proposed that it just might be related to terrestrial rock varnishes. This was a bold proposal because manganese-oxide rock varnishes on Earth are almost always associated with microbes, which are known to concentrate the mineral. So was this a biosignature coming from Mars?
Two years later, Lanza and others on the Curiosity team have published a paper describing in detail the regular detection of Martian manganese oxide, sometimes in concentrations higher than what is found on Earth. Based on the surrounding geology and geochemistry, the team then concluded that when the mineral was formed, the Mars atmosphere had levels of oxygen much higher than previously imagined.
This conclusion flows from the fact that the mineral is only formed, on Earth at least, when plentiful oxygen and plentiful water are present. Indeed, manganese oxides (and many other minerals) began forming in earnest here only after the so-called “great oxygenation event” that, through bacterial photosynthesis, delivered vastly more oxygen to Earth’s atmosphere.
On Mars, the manganese oxide was found largely in sedimentary rock cracks, and to geologists that means it was distributed via flowing water after the rocks had solidified.
Finding substantial water and oxygen together on a planet — in our solar system or beyond — has often been described as providing a strong case for a habitable, and perhaps inhabited, planet.
This all sounds suggestive of life on what Lanza calls “middle-aged” Mars. But here’s where things get tricky.
The paper also pretty much dismisses the possibility that the manganese oxide was a rock varnish formed by microbes. Instead, it argues that the oxygen almost certainly was formed, in massive amounts, by non-biological processes — possibly including the widespread breakdown of H2O after the protective Martian magnetic field began to fall apart, letting in destructive radiation. The lighter hydrogen would, in this scenario, sail away from the planet, leaving high oxygen concentrations for a time.
“Based on some of the manganese oxide abundances, we think there had to be a lot of oxygen in the air when they formed,” she said. “We can only hypothesize how the oxygen got there, but it does suggest a Martian atmosphere different than what we expected, at least for a period of time.”
I read the Lanza paper, in the journal Geophysical Research Letters, while thinking about an upcoming gathering about exoplanet biosignatures — a NASA-sponsored workshop to take place in late July in Seattle.
The field of “remote biosignatures” is just now becoming a hot discipline as the hunt for possible life on exoplanets speeds up.
It will still be years before the launch of a space mission advanced enough to analyze an exoplanet with the power and range needed to detect signs of life. But it is nonetheless considered time now to dive deep into the the question of what might constitute a biosignature — something identified in an exoplanet atmosphere and even on the surface that can only be produced by life. Clearly, it’s essential for NASA to know what are the most compelling extrasolar signatures of possible life before it begins looking in earnest. And so a new scientific community is being formed.
“We’re at a juncture now where we’ve become a science of biosignatures, remotely observed,” said Nancy Kiang of NASA Goddard Institute for Space Studies in New York City.
She is one of the science leads of the Seattle workshop, which is organized by the NASA Astrobiology Program and NASA’s Nexus for Exoplanet System science (NExSS) program. The workshop will be a review of what has been learned about remote biosignatures over the past decade or so, and how the field should go forward.
It’s findings and recommendations will be forwarded on to two NASA science and technology definition teams studying what an exoplanet life-detection space mission for the 2030s could and should be able to do.
From the beginning of scientific thinking about exoplanet biosignatures, oxygen and ozone were the prime targets because they are abundant in our atmosphere only because of photosynthesis. Oxygen bonds quickly with other molecules, and so to keep an atmosphere filled with oxygen requires constant replenishment. Especially early on, scientists described the detection of oxygen or ozone in an exoplanetary atmosphere has a very promising signal that the planet was home to life.
That is still the case, but with a substantial and growing number of caveats.
While oxygen, and its byproduct ozone, remain central to the biosignature search, their exoplanetary context has become increasingly important as well.
That’s because scientists have found or theorized various pathways for putting abundant oxygen into an atmosphere without any biology or photosynthesis whatsoever. The seemingly once oxygen-rich Martian atmosphere is a potential example relatively close to home.
The result of this has been the growing awareness of a “false positive” problem when it comes to exoplanets, oxygen and life. How will scientists know whether an oxygen or ozone rich atmosphere is being sustained through biology or non-biological means?
This is an issue that has many researchers working in labs and with their computer models to get a firmer handle on how they can distinguish the false positives from the real positives. And it will be a much discussed subject in the workshop ahead.
With the false positive issue in mind, Kiang said that the remote biosignature effort nonetheless remain focused on oxygen. But the effort and understanding, she said “it’s becoming a lot more nuanced.”
“We have to look at the full environmental context of the planetary system– many different factors. We can find oxygen and water on a planet and that still does not mean life. ”
Biosignature science is definitely science at the very edge. It is not only essential to understand exoplanets, but Kiang said new ways of thinking are making researchers ask questions about why things are as they are on Earth, too.
Her own specialty involves photosynthesis and plant pigments, and so she looks at how different kinds of light from different kinds of stars could effect the search for extrasolar biosignatures. We reflectively think of plants here as green, but Kiang says that exoplanet science raises the complicated question of ‘Why?”
David Catling, a planetary scientist and astrobiologist at the University of Washington, has studied the origins and dynamics of atmospheres of the early Earth, Mars and now exoplanets, with a focus on oxygen as the key element.
But he sees the same problems with oxygen as a singular biosignature for exoplanet life, and says that field should be looking for a “cocktail of biogenic gases” in their atmospheres. The presence of oxygen together with methane in an atmosphere, for instance, would be “hard to explain without a biosphere.”
“There is no single silver bullet,” he said, “but oxygen comes closest.”
He also said that other compounds that are produced by life on Earth — such as nitrous oxide and hydrogen sulfide — are potential exoplanet biosignatures, but they exist in only small amounts in Earth’s atmosphere and at similar levels would be impossible to detect on distant exoplanets.
(This abundance issue has not stopped MIT professor Sara Seager, a pioneer in studying exoplanet atmospheres, from collecting spectroscopic samples of scores of other gases produced by living things. Working with colleague William Bains, Seager has been cataloguing the chemical signatures of specialized gases from the likes of plankton. These often sulfur and chlorine based gases exist in tiny amounts on Earth, but Seager argues they could be much more widespread on other planets.)
Catling said that there is one sure way to know that an exoplanet is home to life: if compounds made by intelligent beings, rather than nature, are detected.
Short of that, he says, it is doubtful that we will ever make a 100 percent convincing detection of life on an exoplanet from afar. Or even if we are on another exoplanet. There are just too many conflating issues regarding biosignatures.
Take, for example, the manganese oxide finding on Mars. Catling says that while the oxygen needed to form the mineral could come from the atmosphere, it could also come from compounds on the surface of Mars known to contain large amounts of bound, but potentially unbindable, oxygen. Gases, he said, can be made available for further chemistry in many ways.
While an exoplanet biosignature that comes with a 100 percent may be unattainable, the process of learning how to get to a daunting 90 or 95 percent certainty is indeed pushing ahead at the edges of space and biological science. Expect difficulties, and expect surprises.