It turns out that the grooves on Phobos are being caused by tidal forces — in other words, by the pull of Mars’s gravity. Phobos isn’t a tightly packed ball of rock, either; it’s more like a loose agglomeration of dust and rubble. It’s slowly falling apart, and at some point in the next 30 to 50 million years it’ll be gone.
This is amazing to imagine. I hope humans are living on Mars at the time to witness this.