The [[2001 Mars Odyssey]] found much evidence for water on Mars in the form of images, and with its [[Neutron spectroscopy|neutron spectrometer]], it proved that much of the ground is loaded with water ice. Mars has enough ice just beneath the surface to fill [[Lake Michigan]] twice. In both hemispheres, from 55° latitude to the poles, Mars has a high density of ice just under the surface; one kilogram of soil contains about of water ice. But close to the equator, there is only 2% to 10% of water in the soil. Scientists think that much of this water is also locked up in the chemical structure of minerals, such as [[clay]] and [[sulfate]]s. Although the upper surface contains a few percent of chemically-bound water, ice lies just a few meters deeper, as it has been shown in [[Arabia Terra]], [[Amazonis quadrangle]], and [[Elysium quadrangle]] that contain large amounts of water ice. The orbiter also discovered vast deposits of bulk water ice near the surface of equatorial regions. Evidence for equatorial hydration is both morphological and compositional and is seen at both the [[Medusae Fossae]] formation and the [[Tharsis Montes]]. Analysis of the data suggests that the southern hemisphere may have a layered structure, suggestive of stratified deposits beneath a now extinct large water mass.