In the 1970s, NASA’s Viking 1 and 2 spacecraft touched down on Mars and conducted three experiments designed to detect signs of life. One experiment came back with positive results, two came back negative, scientists argued over the ambivalence, and astrobiology stagnated.
“We didn’t make progress for decades,” NASA’s chief scientist James Green tells Inverse.
Green doesn’t want to see that happen again, which is why he is one of the co-authors of a new paper published Wednesday in the journal Nature that proposes a new framework and scale for determining just when evidence for alien life rises to the level of discovery.
The paper details the Confidence of Life Detection, or CoLD scale, designed not only to help scientists determine how close they are to discovering alien life and to maintain comity during debates, but to clearly communicate findings leading up to and including such a historic discovery as, well, aliens.
“We're at a point where we need to be able to communicate not only scientist to scientist, but scientists to the public,” Green says. “Science isn't done until it's communicated.”
What the paper says— The paper introduces CoLD as a progressive, seven-level scale for orienting scientists as they search for and assess evidence of extraterrestrial life. Each level builds on the last to provide scientists with confidence that by the time they reach level seven, they’ve truly discovered something remarkable.
- Level 1: Detection of a signal or biosignature that suggests biological activity.
- Level 2: Rules out contamination and determines the signal makes sense for the environment where its found.
- Level 3: Makes predictions for how biology might produce the signal in the environment.
- Level 4: Rules out all non-biological means of producing the signal.
- Level 5: Adds observation of independent biological signals to the initial biosignature.
- Level 6: Includes future observation ruling out alternative ideas proposed after level 1 discoveries.
- Level 7: Independent follow-up observations of biology in the environment confirms the presence of life.
There have been quite a few Level 1 observations of biosignatures with regards to potential life on Mars. There was the alleged detection of bacterial metabolism during the labeled release experiments on Viking 1 in 1976, and the potential signs of micro-organisms on a Martian meteorite — Alan Hills 84001 — found in Antarctica, and the detection of methane gas on Mars by ground-based telescopes.
But a biosignature isn’t enough. CoLD then asks scientists to take the next step and verify whether that biosignature makes sense in the environment in which it’s found and if there could be anything contaminating the signal. Detecting methane on Mars is a good biosignature, Green says, because “life here on Earth generates methane like crazy,” but it wouldn’t make sense if Mars were volcanically active, providing an abiotic alternative for the presence of methane.
And methane in Earth’s own atmosphere may have contaminated observations of methane on Mars by terrestrial telescopes.
NASA has already taken steps to expand that initial observation to what would be considered Level 2 of the CoLD scale by sending a spacecraft, Curiosity, which landed on Mars in 2012, to detect methane on the Martian surface. That eliminated the possibility of Earthly methane contaminating the telescope signal.
Level 3 of the scale requires a demonstration or prediction of the biosignature being produced in the environment. NASA scientists are already there when it comes to the search for life on Mars: They know that if life exists on Mars, it probably exists under the surface, where it’s shielded from radiation and there are possible aquifers of water to support metabolism.
If Martian bacteria were living beneath the surface in underground aquifers and producing methane, it would likely seep up through the surface in the way NASA missions have detected it.
“So Level 4 would be all known non-biological sources of the signal have been eliminated,” Green says, ruling out any geological or other sources of methane. “Are we there yet with methane? Not quite yet.”
That’s where future missions could come in. A future mission could collect Martian methane and measure which isotopes of carbon — carbon-12, -13, or -14 — occur in that Methane.
“Life loves carbon-12,” Green says, but doesn’t use carbon-13 or -14. “If we got a slug of methane and a lot of it, and we looked at the isotope, and it was all (carbon-12), we'd be through [Level] four.”
A future mission carrying a larger version of the Ingenuity helicopter might be the perfect instrument for such a measurement, Green says. Even better, a 40-pound, methane detecting helicopter could fly a grid pattern over the Martian surface to see if methane is seeping out in patterns that correlate with underground aquifers.
That would take measurements up to Level 5.
“Then (Level) 6 is really all about, let's go down and in and taste it,” Green says. “Have a mission that crawls around in these underground aquifers looking for the microbes.”
At that point, the presence of biology has been confirmed, and Level 7 just expands on it.
Why is it important?— Using the CoLD scale would be useful for several reasons, beginning with redirecting the sometimes heated scientific debates around the detection of biosignatures toward obtaining evidence that moves the discussion up the CoLD scale.
“What we find is that once somebody writes a paper on a biosignature, everyone wants to attack the individual rather than attack the next step,” Green says. “It's sort of like policing ourselves, putting rigor in what we do in a way that makes progress, and stopping the floundering around.”
And that could translate directly into what proposed missions NASA decides to support, and those it doesn’t. Before taking the role of chief scientist in 2018, Green was director of the Planetary Science Division at NASA, where he made funding decisions for the Mars program, among other research. He would be much more comfortable funding a project that is trying to move research forward from Level 1 to Level 2 than another project that is focusing on the same Level 1 biosignature detection.
“It allows us to put our money in the right direction to make progress,” Green says. “We would like to have researchers use a scale every time they write a paper,” and self-evaluate where their work lies on the CoLD scale, he says.
What’s next?— The paper opens with, “Our generation could realistically be the one to discover evidence of life beyond Earth. With this privileged potential comes responsibility.”
If it were even 10 years ago, Green says, he would not have felt comfortable writing that sentence. But advances in technology and the conceptual approach to detecting extraterrestrial life have led him to believe in the importance of the scientific community accepting a framework like the CoLD scale for analyzing a possible discovery he sees as ever more likely.
Beyond the ongoing Mars missions seeking more evidence, the next step is for the scientific community to weigh in and decide what else should be on the CoLD scale, so that the future debates in astrobiology, perhaps the most important debates, can be conducted and communicated effectively.
“If we don't put some rigor in this, we're going to really delay this fabulous opportunity to announce, and clearly delineate with solid science backing it up all the way, the most important claim of finding extraterrestrial life,” Green says. “I'd like to do it in my lifetime.”
Abstract: Our generation could realistically be the one to discover evidence of life beyond Earth. With this privileged potential comes responsibility. The magnitude of the question of whether we are alone in the Universe, and the public interest therein, opens the possibility that results may be taken to imply more than the observations support, or than the observers intend. As life-detection objectives become increasingly prominent in space sciences, it is essential to open a community dialogue about how to convey information in a subject matter that is diverse, complicated and has a high potential to be sensationalized. Establishing best practices for communicating about life detection can serve to set reasonable expectations on the early stages of a hugely challenging endeavour, attach value to incremental steps along the path, and build public trust by making clear that false starts and dead ends are an expected and potentially productive part of the scientific process. Here we endeavour to motivate and seed the discussion with basic considerations and offer an example of how such considerations might be incorporated and applied in a proof-of-concept-level framework. Everything mentioned herein, including the name of the confidence scale, is intended not as a prescription, but simply as the beginning of an important dialogue.