[ptha]

It's powered batteries, from the article:

At the back of the robot are two titanium spheres—each somewhere between the size of a yoga ball and a beach ball—filled with batteries that power a year of continuous operation.

It also has a sensor so it drives into the current.

The oversized treads make a mess of the seafloor. “Even though it is moving very slowly, it doesn't take much to create this huge dust storm,” says Sherman. “We always want to be driving into the current, so that it can push the sediment that is disturbed behind us.” So before the rover moves, it uses a sensor to get an idea of the current direction of the … er, current, then heads straight for it.