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As we continue our search for life out in the universe, it's important that we leave no stone, or moon, unturned.

Hosted By: Reid Reimers
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This episode is sponsored by Wren, a website with a monthly subscription that helps fund projects to combat the climate crisis.

Click the link in the description to learn more about how you can make a monthly contribution to support projects like rainforest protection programs. [ INTRO ] As we search for life in this big, beautiful, universe, it’s essential to leave no stone … or moon… unturned. There are plenty of planets out there that could conceivably harbor life, but also tons of moons.

As of 2022, we've found about 5000 planets outside our solar system, and many of these are gas giants. Now, we don’t expect these exoplanets themselves to be habitable. It’s unlikely that much would be able to live in the conditions of a gas giant.

But they could have exomoons made of ice or rock with conditions more friendly to life. Within our own solar system, Jupiter and Saturn each have dozens of moons, and a few of them are seen as the best candidates for life in our Solar System. Outside of Earth, of course.

So let’s take a look at a couple of Saturn’s moons and see how what we’ve learned from them has allowed us to expand our search for life in the universe. The first thing to look for is liquid water, since it’s a vital component of life here on Earth. The habitable or “Goldilocks Zone” around a star is where the right conditions for liquid water exist.

T his means a planet is not so close to a star that it’s too hot and all the water evaporates, but also not so far that it’s too cold and all the water freezes. If life is possible without liquid water, we don’t really have a clue what that would be. So this liquid water zone gives us a way to narrow down the search.

However, moons complicate this a bit. Having a big planet nearby can both heat a moon up and cool it down, depending on the circumstances. Planets can reflect a star’s light onto a moon, and maybe even radiate some heat of their own.

On the other hand, a small moon orbiting a big planet could have lots of long eclipses, which would block the star’s light. Another source of heat for moons is tidal heating. This is when the gravitational force from the planet deforms the moon as it orbits.

How much tidal heat a moon gets depends on how far away it is from its planet and the eccentricity of its orbit. That refers to how stretched out the ellipse of its orbital path is. The further from a perfect circle, the greater the deformation the moon experiences between the closest and furthest points of the orbit, and the more the moon will heat up.

The combination and balance of all these forces is complex. For example, the closer the moon is to a planet, the more likely eclipses are, which would cool it down. But that would be counteracted by tidal heating, which could easily make it too hot.

If the balance is just right, though, it could effectively make moons outside the habitable zone… habitable. Saturn’s moon Enceladus might be right on the money as far as nailing this balance. Saturn is well outside our Sun’s Goldilocks zone, which only includes Earth and Mars.

And the surface of Enceladus is covered in ice. However, in 2005 the Cassini spacecraft spotted strange geysers of liquid shooting up from its surface. This points to the possibility of an underground liquid water ocean on the moon, which could be a prime environment for finding microbial life.

We know tidal heating plays a significant role in keeping this ocean from freezing, Tides and other factors we know about only explain a small fraction of the heat observed. One possibility is that the moon used to have a more eccentric orbit that provided more tidal heating. Maybe what we see today is a slowly cooling remnant of this.

This could mean that liquid water isn’t stable on Enceladus in the long term – which would be a point against the moon having enough time to evolve and support life. A second key feature on a habitable planet is an atmosphere. Much of life on Earth and many of the chemical reactions that life depends on rely on our atmosphere.

We expect the same could be true elsewhere. But keeping an atmosphere on a moon is easier said than done. Here on Earth, our planet’s magnetic field protects our atmosphere from the solar wind which could otherwise strip it away.

But most moons don’t have their own magnetic fields, leaving any atmosphere vulnerable to erosion from the solar wind. In fact, the only moon we know of with a significant atmosphere is Saturn’s moon Titan. Titan’s atmosphere is made of mostly nitrogen and methane, and the methane undergoes reactions that are particularly exciting.

They produce many of the chemical components that are considered the building blocks of life, making the moon prime potential real estate for extraterrestrial microbes. Despite not having a magnetic field, Titan has somehow managed to keep its atmosphere around. Even just chemical reactions from sunlight would destroy all the methane in less than 100 million years.

So the best guess is Titan’s atmosphere gets replenished from underground sources. This seems to be relatively stable, too. Models suggest the replenishment has been going on for the last billion years.

There is still a lot we don’t understand about both of these moons. NASA has plans to send a lander to Titan in 2026 that will arrive in 2034. There are no solid plans for missions to Enceladus yet, but NASA and others are studying multiple proposals.

And in terms of the search for habitable moons elsewhere, while we have never fully confirmed the existence of specific exomoons, two promising candidates were discovered in 2018 and 2022. Which means learning from the moons of our own solar system will become increasingly important as we find more moons outside of it. And it will help us shine light into all the narrow, unexpected corners that life might be hiding.

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