Astronomers have discovered that sulfur may be a key to helping us narrow down our search for life on other planets. It’s not that sulfur is a great indication that a planet is inhabited. Instead, it’s the opposite: Significant amounts of sulfur dioxide in a planet’s atmosphere is a good sign that the world is uninhabitable and we can safely cross it off the list of candidates.
One of the holy grails of modern astronomy is finding life on an alien planet. But that is an extremely daunting task. The James Webb Space Telescope is unlikely to be able to identify biosignatures — the atmospheric gases produced by life — in any nearby worlds. And the upcoming Habitable Worlds Observatory will be able to scan only a few dozen potentially habitable exoplanets.
One of the big hurdles is that biosignature spectra are usually very weak. So one way to narrow down the list of potential candidates is to focus on the ability of a planet to host life, mainly in the form of water vapor in its atmosphere. If a planet has a lot of water vapor, it might have a good chance of hosting life as well.
This requirement is the basis of the habitable zone, the region around a star where the radiation onto a planet isn’t too little that all the water freezes out and isn’t too much that the water boils away. In our solar system, Venus is near the inner edge of the habitable zone, and its surface reaches temperatures of over 800 degrees Fahrenheit (427 degrees Celsius) underneath a thick, choking atmosphere. On the opposite end, Mars is essentially frozen out, with all of its water locked up in polar ice caps and under the surface.
But even a search for water has difficulties. For example, from great distances, it’s very difficult to tell Earth (inhabited) apart from Venus (uninhabited and outright hostile to life). Their atmospheric spectra are just too similar when you’re trying to hunt for water vapor.
In a recent preprint paper, astronomers note that they’ve found a different signature gas that might be a useful tool for separating uninhabitable worlds from potentially habitable ones: sulfur dioxide.
Warm, wet worlds like Earth have very little sulfur dioxide in their atmospheres. That’s because rain can pick up atmospheric sulfur dioxide and wash it down into the oceans or into the soil, essentially cleansing it out of the atmosphere.
And, ironically, planets like Venus also have very little…
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