Joly is an impact crater on Mars, located at 74.7°S latitude and 42.7°W longitude in the Mare Australe quadrangle.
It measures 79.9 kilometres (49.6 mi) in diameter and was named after Irish physicist John Joly (1857–1933).
The name was approved in 1973, by the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN).
It freezes out directly onto the surface of the permanent polar cap, which is made of water ice covered with layers of dust and sand.
By the time temperatures rise in the spring, the frost layer has become a slab of semi-transparent ice about 0.91 metres (3.0 ft) thick, lying on a substrate of dark sand and dust.