{♫Intro♫}.

The Curiosity rover has been hard at work on Mars—both finding mysteries and trying to solve them. This is a detective story!

Last year it discovered that levels of methane in the atmosphere were mysteriously changing. And now our trusty rover has thrown us for a loop again: changing levels of oxygen gas. Scientists published their results last week in the Journal of Geophysical Research: Planets, and so far, they don't have a clear explanation for it.

Like the changing levels of methane, changing oxygen levels are likely caused by geologic processes—but they could also potentially be a sign of life. Either way, they'll probably tell us something we didn't know about the Red Planet. Curiosity has spent the past three Martian years—that's five years on Earth—in.

Gale Crater, sniffing the Martian air. It has a portable chemistry lab, called SAM, short for Sample Analysis at Mars, which measures the types and amounts of gases in the atmosphere. [

Credit: NASA]. It also keeps track of how they change over time. Unlike Earth, where the content of our atmosphere stays pretty constant, Mars's atmosphere changes a lot. Roughly 95% of Mars's super-thin atmosphere is carbon dioxide, and during the winter months, some of the CO2 in the air deposits into dry ice at the poles. [

Credit: ESA/DLR/FU Berlin/J. Cowart] As the air pressure drops in that region, other atmospheric gases rush into the area, mixing everything up. Come spring, that CO2 starts sublimating back into a gas. The cycle repeats every half-year, on alternating poles.

But that cycle only affects the amount of CO2 in the atmosphere. After CO2, nitrogen and argon are the next-most common elements in Martian air, and according to SAM, the number of those particles stays even across the seasons. Weirdly enough, that's not the case for oxygen.

By the end of each spring, the amount of oxygen SAM measured in the atmosphere increased. By fall, it fell to predicted levels, and in winter, the amount decreased. This suggests that something on Mars is producing this oxygen gas, and then several months later, something is removing it.

Scientists aren't yet sure how to explain this. They've ruled out the possibility that CO2 might be splitting apart to create extra oxygen gas—it doesn't decompose fast enough. And although solar radiation can sometimes break up molecules, they don't think that's what's happening to the oxygen to explain its decrease.

So it's still a mystery! And sadly, researchers think it's probably not aliens, and more likely the result of some chemical reaction in the Martian soil. For example, one experiment from the Viking landers in the late 1970s did show that soil could release oxygen gas if it had enough heat and humidity. [credit: NASA/JPL].

But it's not a perfect answer—that experiment was done in a completely different environment than the Gale Crater, and it still can't explain where the oxygen goes after summertime. The answer could be related to whatever is causing the rise and fall of methane levels, but the two cycles don't always line up. So they might be two completely different puzzles.

The good news is Curiosity is on the case! [

Credit: NASA/JPL-Caltech/MSSS] It's still collecting data, and maybe after a few more rounds of seasons, it'll have our answer. TOC Switch. In the meantime, other researchers have been hard at work trying to get to the bottom of a mystery a little farther out in the solar system. Specifically, they're trying to figure out how a tiny moon of Saturn got its unusual stripes.

The moon, Enceladus, has a set of tiger stripes—yes, that's actually what astronomers nicknamed them—around its south pole. [

Credit: ]. These four stripes are about 130 KM long, and they run parallel to each other with about 35 KM in between. They're basically fissures: long cracks in Enceladus's surface that connect the moon's subsurface ocean with the void of space. And that's cool for us, because water and other molecules shoot out of the fissures into space, which means spacecraft can study them to find out what's going on down there.[

Credit: NASA/JPL-Caltech].

Alt:https://www.jpl.nasa.gov/video/details.php?id=1458

Alt:https://www.jpl.nasa.gov/video/details.php?id=1404 But as useful as they are, we still don't really understand them. We don't know how they formed, why they're so parallel, or why they're only on one part of the moon. And only on one moon that we know of, for that matter. [NASA/JPL-Caltech]. But earlier this month, researchers presented one hypothesis.

They suggested that the stripes actually formed because of the moon's relationship with. Saturn. We already knew that as Enceladus orbits Saturn, the gas giant's gravitational tug heats up the moon's insides.

That's how this frigid moon manages to maintain a liquid ocean under its frozen surface. NASA/JPL-Caltech/Space Science Institute. As this new hypothesis goes, the moon slowly cooled over a very long period of time.

As it cooled, the total amount of ice slowly increased, and pressure built up on the frozen shell. When it couldn't take it anymore, it cracked. Since the ice is thinnest over the moon's south pole, this weak spot was the first to give way under the weight of the shell.

That first crack relieved the pressure across the whole planet, keeping cracks from forming anywhere else. For now. [

Credit: NASA/JPL-Caltech]. Once that fissure formed, ocean water would spew out of it into space. For the most part, it wouldn't be traveling fast enough to escape the moon's gravity, so it'd snow back down on either side of the stripe. After enough time—anywhere from a hundred thousand to a million years—-the weight from that new frozen water would cause more cracks to form, parallel to the original.

If it's right, this hypothesis would also explain why Enceladus is the only moon we know of that has this stripey feature: it's one of the smallest moons with an underground ocean. On a larger moon, any stripes that formed wouldn't last; gravity would cause the fractured ice to collapse and crush itself back into one piece. It's a promising hypothesis, but it hasn't gone through peer review yet, so it'll be a while before this hypothesis gets put to the test.

It's still possible that other geologic processes were involved. But so far, it's done the best job of explaining this tiny moon's tiger stripes. TOC OFF.

It can take a while to get to the bottom of puzzles like these, but exploring space is as much about finding the mysteries as it is about finding the answers. A lot of times these puzzles—the things that don't make sense—are the things that give us the most room to learn. Thanks for watching this episode of SciShow Space!

This episode is brought to you by this week's President of

Space: Matthew Brant. Matthew is one of our patrons on Patreon, which is a group of amazing people helping make science education free on the internet. We couldn't make these videos without our patrons like Matthew Brant, so thanks to all of you. If you're not a patron yet but you're interested in learning more about how to support us, you can go to patreon.com/scishow. {♫Outro♫}.