It measures approximately 165 kilometers in diameter and was named after Claudius Ptolemaeus (Ptolemy), the Greco-Egyptian astronomer (c. AD 90-160).

[3] Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.

[4] This ice-rich mantle, a few yards thick, smooths the land, but in places it has a bumpy texture, resembling the surface of a basketball.

Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas.

The water comes back to ground at lower latitudes as deposits of frost or snow mixed generously with dust.