Only two bodies in the Solar System are known to have persistent surface liquid: Earth and Saturn's moon Titan. Unlike Earth, Titan's liquid is methane rather than water, and most of the methane is in the moon's thick atmosphere instead of on the surface. Therefore, instead of large oceans, Titan possesses smaller, shallower seas in the polar regions. The equatorial region is marked by sand dunes, similar to fields found on Earth. Studies of Titan's atmosphere show the tropics to be free of precipitation, making it drier than the driest desert on Earth.