Latitude constant latitude and constant longitude, which are constructed with reference to the rotation axis of the Earth. The primary reference points are the poles where the axis of rotation of the Earth intersects the reference surface. Planes which contain the rotation axis intersect the surface at the meridians; and the angle between any one meridian plane and that through Greenwich (the Prime Meridian) defines the longitude: meridians are lines of constant longitude. The plane through the centre of the Earth and perpendicular to the rotation axis intersects the surface at a great circle called the Equator. Planes parallel to the
Latitude dependent mantle depth over the whole planet for the water locked up in the north and south polar caps. Mantle forms when the Martian climate is different than the present climate. The tilt or obliquity of the axis of the planet changes a great deal. The Earth’s tilt changes little because our rather large moon stabilizes the Earth. Mars only has two very small moons that do not possess enough gravity to stabilize its tilt. When the tilt of Mars exceeds around 40 degrees (from today's 25 degrees), ice is deposited in certain latitude bands where much mantle exists today. Latitude dependent