- Phobos orbits Mars in an almost perfect circle, right along Mars's equator.

And this makes us question whether Phobos is really the asteroid it appears to be because captured asteroids usually orbit planets on very elliptical orbits that sometimes take them very far from their planet and sometimes being much closer.

- [Narrator] So if it's not an asteroid, where did this funny little moon come from?

- So in planetary science, the joke is that you just add an impact event, and an impact event can explain everything.

That might actually be true in this case, though, it's this one-off event, and it might have been that an object hit Mars and Phobos is the result of that impact collision between those objects, and that's what we're seeing.

- [Narrator] Phobos may not be a captured asteroid at all, but formed from debris thrown into orbit after a massive asteroid impact.

However, this still doesn't explain why it's covered in grooves.

Phobos is about 3,700 miles from Mars's surface, and every 100 years, moves closer by about six feet.

And as it does, the tidal forces caused by Mars's gravity becomes stronger.

- So just like the moon pulls on the Earth, dragging the oceans and causing the tide, Mars pulls on Phobos, causing tides as well.

So as you can see behind me right now, the tide is coming in as the moon is pulling on the water on the Earth.

The same process happens on Phobos due to Mars, except on Phobos, it's sand and rock that's moving instead of water.

- [Narrator] Those tidal forces are pulling Phobos apart, and with every orbit, the grooves widen and deepen.

Eventually, Phobos will drift so close, Mars's gravity will destroy it.

As Phobos breaks apart, most of the debris will fall to the surface of Mars, but the rest will remain in close orbit, spreading out to encircle the entire planet.

Giving Mars a wispy ring.

(soft atmospheric music)