'Atmospheric seasons could signal alien life'

The biological products in the atmospheres of potentially habitable planets outside our solar system may help detect alien life, scientists say.

These atmospheric fingerprints of life, called biosignatures, will be detected using next-generation telescopes that measure the composition of gases surrounding planets that are light years away, according to the research published in Astrophysical Journal Letters.

However, biosignatures based on single measurements of atmospheric gases could be misleading.

Now, scientists at the University of California, Riverside (UCR) in the US are developing the first quantitative framework for dynamic biosignatures based on seasonal changes in the Earth's atmosphere.

As Earth orbits the Sun, its tilted axis means different regions receive more rays at different times of the year.

The most visible signs of this phenomenon are changes in the weather and length of the days, but atmospheric composition is also impacted.

For example, in the Northern Hemisphere, which contains most of the world's vegetation, plant growth in summer results in noticeably lower levels of carbon dioxide in the atmosphere. The reverse is true for oxygen.

"Atmospheric seasonality is a promising biosignature because it is biologically modulated on Earth and is likely to occur on other inhabited worlds," said Stephanie Olson from the University of California, Riverside (UCR).

"Inferring life based on seasonality would not require a detailed understanding of alien biochemistry because it arises as a biological response to seasonal changes in the environment, rather than as a consequence of a specific biological activity that might be unique to the Earth," said Olson,.

Further, extremely elliptical orbits rather than axis tilt could yield seasonality on extrasolar planets, or exoplanets, expanding the range of possible targets.

The researchers identified the opportunities and pitfalls associated with characterising the seasonal formation and destruction of oxygen, carbon dioxide, methane, and their detection using an imaging technique called spectroscopy.

They also modelled fluctuations of atmospheric oxygen on a life-bearing planet with low oxygen content, like that of Earth billions of years ago.

They found that ozone (O3), which is produced in the atmosphere through reactions involving oxygen gas (O2) produced by life, would be a more easily measurable marker for the seasonal variability in oxygen than O2 itself on weakly oxygenated planets.

"It's really important that we accurately model these kinds of scenarios now, so the space and ground-based telescopes of the future can be designed to identify the most promising biosignatures," said Edward Schwieterman, a NASA Postdoctoral Program fellow at UCR.

"In the case of ozone, we would need telescopes to include ultraviolet capabilities to easily detect it," said Schwieterman.

He said the challenge in searching for life is the ambiguity of data collected from so far away.

False positives - nonbiological processes that masquerade as life - and false negatives - life on a planet that produces few or no biosignatures - are both major concerns.

"Both oxygen and methane are promising biosignatures, but there are ways they can be produced without life," Schwieterman said.

Olson said observing seasonal changes in oxygen or methane would be more informative.