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Thanks to over half a century of sending robots to Mars, astronomers know that the Red Planet used to have huge amounts of liquid water on its surface. But billions of years ago, it dried up.

The story usually goes that, as Mars lost its atmosphere, that water evaporated and escaped into space, except for the little bit that ended up as ice at the poles or beneath the surface. But new research challenges that picture, and paints a new one. According to a paper published last week in the journal Science, as much as 99 percent of Mars’s water wasn’t lost.

Instead, it’s hiding in the crust. One way scientists figure out how much water Mars used to have involves looking for a specific type of hydrogen. It’s called deuterium, and it has an extra neutron in its nucleus.

When deuterium becomes part of a water molecule, it makes the water a bit heavier, so it’s less likely to escape the planet’s gravitational pull. That means that as Mars dried up and lost more of the lighter water to space, the relative amount of deuterium water in the atmosphere increased. So, to figure out how much water Mars used to have, scientists look at the ratio of deuterium to regular hydrogen in Mars’s current atmosphere.

Then, they compare it to the ratio in meteorites that broke off the planet billions of years ago. From there, they can back out how much hydrogen was lost over time — and how much water Mars used to have. The trouble is, this team looked at how much water we think Mars used to have, and the rate at which the planet is losing water today.

And they argue that those numbers don’t match up. According to them, the current rate at which Mars is losing water isn’t enough to have dried the entire planet in even a few billion years. Something else had to be happening.

As to what that “something else” is — well, they offered an explanation inspired by our own planet. On Earth, water regularly gets trapped in our planet’s crust to make clays and other hydrated minerals. Then, as tectonic plates move around and the crust gets melted and recycled, that water eventually makes its way back into the water cycle.

So, this team proposes that something similar happened on Mars — but with a twist. See, hydrated minerals have also been found on the Red Planet. But because Mars doesn’t have tectonic activity like Earth’s, the water molecules have just been sitting there, trapped for billions of years.

So, according to this paper, that’s where Mars’s “lost” water is hiding: in ancient, hydrated minerals. In fact, based on their data, the team estimated that over Mars’s lifetime, anywhere from 30 to 99% of its water could have been trapped inside its crust. That range is so big because we don’t actually know if the rate Mars is currently losing water is the same as it’s always been.

To figure that out, we’ll need to analyze more Mars rocks — which future missions should help with. But either way, studies like this are a reminder that the hunt for Mars’s water is far from complete. And no matter how much we think we know, there are still plenty of questions to answer.

Meanwhile, last week in the Journal of Geophysical Research: Planets, two astronomers gave us an update on the interstellar visitor ‘Oumuamua. And while it’s still not aliens, these authors propose it is evidence of something we haven’t seen before: an extrasolar Pluto. ‘Oumuamua was spotted in 2017. And based on how its position in the sky changed over time, astronomers were able to figure out its trajectory — and that it came from somewhere beyond our solar system.

After that, the big questions were where it came from, and also what it was, because that wasn’t obvious. For instance, the rock kind of acted like a comet. Or at least, it got a slight push from the Sun, as the sunlight converted the rock’s ice into gas — which is something we see with comets.

But unlike most comets, ‘Oumuamua didn’t have any kind of noticeable, gassy tail. It was also shaped like a pancake, and was flatter than any known body in our solar system. But this new paper offers an explanation that explains almost all of that.

Based on how much the Sun pushed this rock around, and the known reflectivity of different ices, the team figured out ‘Oumuamua’s composition. And a key component turned out to be nitrogen. We don’t see a lot of nitrogen ice on comets in our solar system, but you know what does have a lot of it?

Pluto. So, according to these scientists, ‘Oumuamua is probably a chunk of a Pluto-like planet, maybe from the Perseus arm of the Milky Way. They think the rock got knocked off its parent in a collision about half a billion years ago.

And that collision was so violent, that rock escaped the gravitational pull of its star and ended up visiting us. The authors didn’t say anything about how this relates to ‘Oumuamua’s lack of a tail, but they did point out that being made of nitrogen also helps explain its pancake shape. According to the authors, that would have been caused by the outer layers of ice getting worn away by energetic particles flying through the cosmos.

So, it’s been four years since ‘Oumuamua came on our radar. And although it’s still in our solar system, it will escape the Sun’s influence in a couple of decades. But while it’s with us, this object may ultimately give us brand-new knowledge of exoplanets.

It’s the first evidence of Pluto-like worlds beyond our solar system. And with new telescopes getting brought online in the next few years, astronomers hope to be able to find more of these nitrogen rocks to keep learning even more. If you’re in the market for some incredible space merch, or if you’ve been following along with our Pin of the Month series, we wanted to mention that this is the last week to get our March pin!

This month, the pin is our take on a dark matter star — a kind of hypothetical star made entirely of invisible dark matter. And yes, it absolutely glows in the dark. If you want one, they’ll only be available until the end of March, and we’ll have a new design in April.

You can learn more at DFTBA.com/SciShow. [ outro ].