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Identifying Life from Varying Atmospheres

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Earth from space
Earth’s axial tilt — the cause of the seasons — interacts in interesting ways with signatures of life. Could observations of varying atmospheric gases give away the presence of life on planets beyond our solar system? [NASA]

There’s no hiding — changes in Earth’s atmosphere over the seasons are a dead giveaway to the fact that Earth hosts life. Now a new study explores whether we might use atmospheric seasonality like Earth’s to detect life on other planets.

Looking for Change

Most of the searches for life beyond our planet focus on identifying static biosignatures, like the presence of methane or large amounts of oxygen in an exoplanetary atmosphere. This approach suffers from many ambiguities, however — including a high likelihood of false positives (processes that chemically mimic life signatures but aren’t life) and false negatives (non-detections despite the presence of life).

CO2 and CH4 levels

Earth’s atmospheric carbon dioxide (top) and methane (bottom) levels vary seasonally, as seen in these data from NOAA’s Earth System Research Laboratory. [Olson et al. 2018]

In a new study led by Stephanie Olson (UC Riverside and NASA Astrobiology Institute Alternative Earths and Virtual Planetary Laboratory Teams), a team of scientists has proposed an alternative approach: to search for distinctive variability of exoplanet atmospheres that indicates the presence of life.

Seasons and Life

if you’re like me, you probably haven’t spent a lot of time thinking about interactions between the Earth’s biosphere and its axial tilt. Nonetheless, this interplay is responsible for detectable and distinctive seasonal changes in our planet’s atmosphere!

seasonal variations in gases

This schematic shows how oxygen and carbon dioxide levels in the atmosphere vary in opposing phase seasonally, with the increased sunlight in summer driving greater conversion of carbon dioxide into oxygen. [Olson et al. 2018]

Since so much of our globe is covered by photosynthesizing life, the seasonal availability of sunlight regulates the conversion of carbon dioxide to oxygen, providing a signature in our atmosphere that varies over the course of the year. And photosynthesis isn’t the only culprit! Other biological products evolve seasonally as well — as the surface temperature on our globe changes throughout the year, biological rates, gas solubility, precipitation patterns, and more all respond accordingly.

Olson and collaborators ask a simple question: if our atmosphere varies distinctively in a way that reveals the presence of life on Earth, can we search for similar variation on other planets?

Gaseous Signatures

To answer this question, Olson and collaborators examine the potential for seasonal variation of several atmospheric gases: carbon dioxide, methane, molecular oxygen, and ozone. For a weakly oxygenated planet (like early Earth), the authors find that a detectable indicator of life may be seasonal variations in the strength of ozone spectral bands at ultraviolet wavelengths. This variation serves as a tracer of the seasonality of molecular oxygen.

seasonal O3 spectral line

On a planet with the right conditions, seasonal oxygen oscillations could create an observable difference in the depth of the ozone spectral line, as shown here. [Adapted from Olson et al. 2018]

To discover such a signature in the atmospheres of distant planets, we’ll likely need extended direct imaging; transit spectroscopy, such as that expected from the James Webb Space Telescope, will probe planets at only one point in their orbits, precluding the detection of seasonal changes. Olson and collaborators therefore advocate that upcoming direct-imaging missions, like LUVOIR and HabEx, include ultraviolet observing capabilities.

What is the likelihood that we’ll actually be able to detect seasonal changes in the atmospheric gases of distant exoplanets? More detailed modeling will need to be performed to say for certain — but in the meantime, this study presents an interesting additional technique we can add to our arsenal and explore further in the future!

Citation

Stephanie L. Olson et al 2018 ApJL 858 L14. doi:10.3847/2041-8213/aac171