Authors: Vladimir S. Airapetian, Charles H. Jackman, Martin Mlynczak, William Danchi, Linda Hunt
First author’s institution: Goddard Space Flight Center and American University
Status: Published in Nature Scientific Reports, open access
Once upon a time, the detection of abundant molecular oxygen in an exoplanet’s atmosphere was considered to be a “slam-dunk” indication of a biosphere. O2 is an exhaust gas from the production of glucose, life’s molecular Clif Bar. The heavy, disequilibrium presence of oxygen in the Earth’s atmosphere only exists because lots of biomass keeps pumping it in.
However, as observing capabilities have advanced to the point where we can actually start studying exoplanet atmospheres, the widening theoretical picture has become much more interesting. Would molecular oxygen indicate the presence of life? Well, maybe… and maybe not. It could be generated without life when UV insolation breaks up H2O or CO2 molecules. This is not significant in the Earth’s atmosphere, but today’s paper notes that it could be significant in dry, CO2-dominated atmospheres of planets around M-dwarfs.
Detection and verification of biosignatures will be hard, and not only because false positives will try to fool us. The signals are tiny, and will require long integration times with next-generation space telescopes and extremely large telescopes (ELTs). But those telescopes will be heavily oversubscribed, and observation times will have to be kept down to the absolute minimum.
Therefore, it is to our benefit to be clever and find complementary and easier ways of inferring the presence of a biosphere.
The authors of today’s paper consider chemistry that is “downstream” from the most direct biomarkers. They are interested in molecules with wide rovibrational absorption or emission bands (molecules can twist, turn, and stretch in ways that correspond to a wide range of energies) because observations will forseeably be limited to low-resolution spectroscopy.
The authors consider a nitrogen-dominated atmosphere, like present-day Earth’s. Nitrogen is handy for life because it can reassemble itself with hydrogen to help form stringy organic molecules. But before that happens, if an active star showers planets with electrons and UV and X-ray photons (XUV), N2 and H2O molecules will be broken up and reassemble into others containing sulfur or deuterium, which in turn join up with O2 to produce nitric oxide (NO).
For years, the Earth’s atmospheric NO emission levels have been under the watchful eye of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. When solar activity increases, atmospheric NO lights up and emits at 5.3 microns at the level of hundreds of gigawatts. As solar activity goes down again, NO emission falls with it. A similar correlation is exhibited by hydroxyls (OH) (Fig. 1), and, yes, O2.
And yet, aside from the occasional baby burp, our own Sun is fairly magnetically calm. By comparison, K and M dwarf stars erratically blast extreme UV and X-rays (XUV) at anything within reach. This can actually make a planet uninhabitable, as is probably the case for the nearest known exoplanet, Proxima Centauri b. But if the XUV flux is not too bad, it can provide a possible beacon that rises and falls with stellar activity.
The authors of today’s paper set up a grid of atmospheric models to trace the levels of ‘beacon’ molecules in an Earth-like atmosphere under different levels of coronal mass ejections (CMEs). They find that NO levels do indeed have a roughly linear correlation with the intensity of the CME (Fig. 2). In fact, NO levels can increase hundreds or thousands of times, leading to emission that rapidly outpaces that expected from collisions with atomic oxygen in an abiotic atmosphere. Ergo, if a strong NO emission at 5.3 microns were measured in an exoplanet atmosphere after a stellar event, there may exist biotic oxygen.
Fortuitously, emission signals from NO, OH, and N2 would be strong enough that they would be observable with low-resolution spectroscopy of R~10-40. The authors find that James Webb Space Telescope (JWST) MIRI detector could make a S/N=10 detection in an hour and a half. (Compare this to the hundreds or thousands of hours of integration time to discern other kinds of biosignatures.)
In recent years, theories of potential biosignatures have moved away from the geocentric assumption that a biotic atmosphere should look like our own. But today’s paper reminds us that we shouldn’t forget to learn from our own atmosphere to make additional predictions of what we might find, especially if the predicted signals are easy to detect.
Today’s paper wraps up by calling for more modeling, so as to determine the timescales of NO-emission events. The physics of the atmosphere under violent solar events is still not fully understood, and the Decadal Survey–no, not that Decadal Survey, but the one for heliophysics–regards this as an area of high priority. Detection of NO emission on a different planet altogether would bring us full circle.