The powerful James Webb space telescope will be launched shortly. Once deployed and positioned at Earth-Sun Lagrange Point 2, it begins to work. One of its tasks is to examine the atmosphere of exoplanets and look for biosignatures. It should be easy right? Just scan the atmosphere until you find oxygen, then close your laptop and go to the pub: fanfare, confetti, Nobel Prize.
Of course, Universe Today readers know it’s more complicated. Much more complicated.
In fact, the presence of oxygen is not necessarily reliable. It is methane that can send a stronger signal that indicates the presence of life.
Oxygen seems obvious in a planet’s atmosphere when looking for signs of life, but it is not. Its presence or absence is not a reliable indicator. The history of the earth makes this clear.
The modern earth’s atmosphere contains about 21% oxygen, and we know that most of it comes from organisms in the planet’s oceans. But there is a problem: when cyanobacteria started producing oxygen as a by-product of photosynthesis on ancient Earth, it was a long time before the atmosphere was oxygenated, possibly a billion years.
What if we investigated an exoplanet, found no oxygen, and then moved on without realizing that there was life down there, at the beginning of this world’s oxygenation process? What if we were a billion years too early and life hadn’t yet oxygenated the exoplanet’s atmosphere? Rocky planets have many oxygen sinks, and biologically produced oxygen would not be found free in the atmosphere until those sinks became saturated.
Artistic idea of early Earth after several large asteroid impacts that brought magma to the surface. Photo credit: Simone Marchi / SwRI
This is what happened on Earth, and what we expect could happen on other rocky worlds. On Earth, geological activity throws magma from the mantle onto the crust. Much of the cladding material, such as iron, combines with atmospheric oxygen and pulls it out of the atmosphere.
This is one reason why planetary researchers are focusing on other things like methane (CH4). In a new article, the researchers examined the potential of methane to signal biological activity. They say that abundant methane in a planet’s atmosphere is unlikely to come from volcanoes and is most likely of biological origin.
The title of the paper is: “Abundant atmospheric methane from volcanism on terrestrial planets is unlikely and strengthens the case for methane as a biosignature.” The lead author is Nicholas Wogan of the Department of Earth and Space Sciences at the University of Washington and the Virtual Planetary Laboratory at the University of Washington. The paper is published in the Planetary Science Journal.
Detecting potential biosignatures such as methane in the atmosphere of distant exoplanets is difficult. But once something like methane is discovered, harder work awaits you. Its presence must be examined in the context of the planet itself.
NASA’s James Webb Space Telescope is the largest and most technically complex space telescope NASA has ever built. One day, hopefully soon, it will finally start. Photo credit: NASA / Chris Gunn
Biosignature researchers haven’t been idly waiting for the James Webb Space Telescope to launch. You have put a lot of thought into recognizing biosignatures with the telescope. Scientists have suggested that planetary atmospheres with abundant methane and carbon dioxide imbalance could be a powerful biosignature. In their work, the authors point out that “… only a few studies have examined the possibility of non-biological CH4 and CO2 and related contextual cues.” In this case, non-biological means volcanoes.
The trio of researchers investigated volcanic false alarms of the CH4 + CO2 biosignature on two types of planets: one with only volcanic submarine outgassing, a water world, and another like Earth, with both subaerial outgassing and subaerial outgassing. This figure from the study shows some of the parameters used in the models. Photo credit: Wogan et al., 2020.
Using a thermodynamic model, the authors wanted to investigate whether the outgassing of volcanic magma on earth-like planets can bring CH4 and CO2 into the atmosphere. In essence, what they found was that volcanoes are unlikely to produce the same amounts of methane as biological sources. It’s not impossible, just unlikely.
This is mainly because hydrogen likes to stay in magma. H2O is highly soluble in magma, which limits the amount of H that is degassed and consequently limits the amount of CH4 in a planet’s atmosphere. Another reason is that CH4 itself needs low temperature magma to exhaust, while most of the earth’s magma is at a higher temperature.
A figure from the study. (a) and (b) show normalized methane production for an ocean world and an earth-like world. (c) and (c) show methane production multiplied by the earth’s magma production rate. At the modern Earth’s magma production rate, volcanoes are likely to produce negligible CH4, reinforcing the case for methane as a biosignature. Photo credit: Wogan et al., 2020
In those unlikely cases where volcanism could produce large amounts of methane, the authors would also produce carbon dioxide. The ancient archaic earth was much more volcanically active than the modern earth. During the Archaic Age, the earth’s heat flux was up to three times as high as it is today. According to the study, it could have produced 25 times more magma than modern earth and a lot more methane. However, the same activity that produced all of this methane would also produce a lot more carbon dioxide. The authors point out that this is a demonstrably false positive result. However, if copious amounts of methane are detected with no accompanying amounts of CO2, that is a more reliable biosignature.
An artist’s impression of the early archaic earth, when the planet was much more volcanically active. Photo credit: By Tim Bertelink – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=57273984
The authors say it would be difficult to explain the evidence of methane and carbon dioxide without looking at biological sources, at least for planets similar to Earth. They also concluded that a small or negligible amount of carbon monoxide detected in an atmosphere enhances the CH4 + CO2 biosignature because “… life easily consumes atmospheric CO, while the reduction in volcanic gases is likely to lead to that CO accumulates in the atmosphere of a planet. “
The researchers conclude on a cautionary note, indicating that this work is all based on what we know about the Earth and other planets in our own solar system. To what extent this knowledge can be extended to thousands of different exoplanets is unclear.
“These conclusions should be made with caution as they are based on what is understood about processes on Earth and in our solar system. This can be a very sparse sample of what is possible,” they write.