Like Earth, Mars spins on an axis tilted about 25 degrees from its orbital plane. Mars has no large satellite like the Moon, just its two small moons Phobos and Deimos. As a result, the tug of gravity from the Sun and the large planets causes a slow wobble in the tilt, or obliquity, of its axis. During periods of higher obliquity, the atmosphere is thicker, dust storms are more intense, and water now trapped at the poles moves to the equatorial region to form mountain glaciers. Many glacial landforms from the last time this occurred can still be seen on Mars.