This SciShow Space video is all about moons because we are launching a new moon-themed calendar!

We are so excited about this! You can find it for a limited time at ComplexlyCalendars.com. [♪ INTRO] The rocky worlds in our solar system often present astronomers with a cosmic whodunnit mystery.

Each is covered in craters, cracks, and crevices all begging for an answer to the question: What did this, and how? And nowhere is a confusing crime scene more evident than Mars’s moon Phobos, whose surface is covered in a dramatic series of groove-like structures. While the mystery remains after decades of investigation, one possible answer might say just as much about the future of Phobos as it does about its past.

American astronomer Asaph Hall discovered Phobos and Deimos in 1877 and named them after the god of war’s twin sons, fear and panic. And we would have to wait another century to get the first close-up views of each, thanks to NASA’s Mariner 9 orbiter in 1971. These views revealed both moons were dramatically different from our own Moon.

Each is less than 30 kilometers across, with an irregular shape and an orbit that is far closer to the Red Planet than the Moon’s orbit around Earth. Phobos, for example, gets as close as 6,000 kilometers away from Mars’s surface. That’s closer than any other moon gets to its planet in our solar system!

Like their Ancient Greek namesakes, Phobos and Deimos started off looking kind of like twins. But a few years later, NASA’s Viking orbiters revealed one major difference a series of long, deep grooves that stretch from nearly one end of Phobos to the other. And that’s not all.

Some of the grooves seem to criss-cross over the others, and that could suggest that Phobos suffered from multiple traumatic events in the past. Lucky for us, humanity’s cosmic detectives have spent decades on this cold case to catch, or at least identify, the perpetrator. The first suspect was fairly obvious: the astronomical body that smashed into the end of Phobos and created the 9 kilometer wide crater named after Hall’s wife, Angeline Stickney.

It makes sense that such a violent blow could cause cracks to radiate outwards from the impact site. And at first glance, that appears to be the case for the mysterious grooves on Phobos. But when scientists actually traced the grooves back to their point of origin, they found that they didn’t actually start inside Stickney crater.

Just near it. And what about the grooves that crossed over other grooves? How could only one impact explain that piece of evidence?

Was this all pointing to one big cosmic coincidence? Maybe not. To form that enormous crater, a bunch of rocks would have been thrown up and out of the impact site.

A lot of that debris would have been lost to space forever, but some surely rained back down onto Phobos. Coming in at a steep angle, some boulders could have bounced and skipped across the surface, gouging out the grooves as they went. And according to a paper published in 2018, once you take Phobos’s extremely weak gravity into account, these bouncing boulders could have traveled all the way around the moon, and then some.

That would explain the criss-cross effect. Boulders were bounding over the paths left by their destructive siblings during their first go-around. While this hypothesis paints a pretty complete picture, it has not satisfied every astronomer out there.

Some argue that the grooves have nothing to do with Stickney Crater. After all, there are other small moons in the solar system that have big craters, but no grooves. So maybe there’s another culprit.

An even bigger culprit. Like Mars itself. A paper published in 2016 suggests that Phobos’s grooves may actually be stretch marks caused by the tidal forces exchanged between a planet and its moon.

Just like Earth and the Moon deform each other a little bit with their gravitational tugs, the gravity of Mars and Phobos does, too. But Phobos is way closer to Mars, so the effect is much stronger. And unlike our Moon, it may not be made of strong enough stuff to withstand that tug.

Given its size, shape, and composition, some scientists think Phobos was a wayward asteroid that wandered a bit too close to Mars and got captured a long time ago. And also it seems that small asteroids are more often loosely-bound piles of rubble, rather than actual, solid objects. So Mars could be in the middle of pulling Phobos apart.

If that’s the case, these grooves are evidence not of a past assault, but a future murder. Due to the exchange of tidal forces, the orbit of Phobos is actually shrinking at a rate of about two meters per century. And the closer it gets to Mars, the stronger those tidal forces will be.

In 30 to 50 million years, the stress will be great enough to tear the moon apart. Which will be a problem for any people living on Mars 30 to 50 million years from now. This might sound like a pretty downer ending, but Phobos’s dramatic story helps illustrate just how dynamic the worlds beyond Earth can be.

And if you think Phobos is as cool as we do, you may enjoy the first ever SciShow Space Calendar because the month of May is devoted to Phobos in all of its splendor. That’s my birth month. It may be that I’m the one who did that.

And this composition. This is an actual photograph. I put a little Mars over here so you can get some context.

We are bringing you an entire calendar filled with our favorite moons in the universe. Now you have the opportunity to look at them every day while also supporting the people who brought you this video. You can head over to ComplexlyCalendars.com or click the link in the description to get your calendar.

And while you’re there, you can check out Complexly’s other beautiful calendars, from Bizarre Beasts to Journey to the Microcosmos! We’ve got moons, and microbes, and more. And we will ship it anywhere in the world.

Thank you so much for being excited about the universe with us. [♪ OUTRO]