The basin is located in the southern highlands of Mars and is thought to have been formed about 3.9 billion years ago, during the Late Heavy Bombardment.

Its process of formation is still largely unknown, although it appears to be due to erosion of hard and soft sediment along with ductile deformation.

In addition, Mars orbiting laser altimeter (MOLA) data show that the contacts of these sedimentary units mark contours of constant elevation for thousands of km, and in one case all around the basin.

A lake in Hellas in today's Martian climate would form a thick ice at the top that would eventually sublimate away.

Recently, research with the Shallow Radar on the Mars Reconnaissance Orbiter has provided strong evidence that the LDAs are glaciers that are covered with a thin layer of rocks.

When the tilt (obliquity) of Mars increases the southern ice cap releases large amounts of water vapor.

The two tiny Martian moons do not stabilize their planet, so the rotational axis of Mars undergoes large variations.

Their major advantage over other sources of Martian water are that they can easily mapped from orbit and they are closer to the equator, where crewed missions are more likely to land.

Lineated floor deposits may be related to lobate debris aprons, which have been proven to contain large amounts of ice.

This ice-rich mantle, a few yards thick, smoothes the land, but in places it displays a bumpy texture, resembling the surface of a basketball.

The image at the right shows a good view of this smooth mantle around Niger Vallis, as observed with HiRISE.

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

[16] Remnants of a 50–100 meter thick mantling, called the upper plains unit, has been discovered in the mid-latitudes of Mars.

[19][20] Large areas of the Martian surface are loaded with ice that is protected by a meters thick layer of dust and other material.

Periods of high tilt will cause the ice in the polar caps to be redistributed and change the amount of dust in the atmosphere.

[32] Furthermore, at this high tilt, stores of solid carbon dioxide (dry ice) sublimate, thereby increasing the atmospheric pressure.

[31] When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust.

[35][36] The lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind.

Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hot magma melted huge amounts of ice in the frozen ground.

[2] The partially circular depressions on the left side of the channel in the adjacent image suggests that groundwater sapping also contributed water.

[43] A further analysis, published about a year later, revealed that the deposit could have been caused by gravity moving material down slope (a landslide).

[47] [48] Glaciers, loosely defined as patches of currently or recently flowing ice, are thought to be present across large but restricted areas of the modern Martian surface, and are inferred to have been more widely distributed at times in the past.

[51][53][54][55][56][57][58][59][60] Today, we consider lobate debris aprons (LDA's) and lineated valley fill (LVF) as about the same; their shapes and names are dependent on their locations.

[61] A climate model, reported in the journal Science in 2006, found that large amounts of ice should accumulate in the Hellas region, in the same places where glaciers are observed.

[65][66][67][68] Indeed, a study published in June 2017, calculated that the volume of water needed to carve all the channels on Mars was even larger than the proposed ocean that the planet may have had.

Recent studies describe using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine gullies at 356 sites, starting in 2006.

Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon dioxide frost and temperatures that would not have allowed for liquid water.

[84][85][86][87][88][89][90] Thick deposits of ice were found by a team of researchers using instruments on board the Mars Reconnaissance Orbiter (MRO).

[91] The scientists found eight eroding slopes showing exposed water ice sheets as thick as 100 meters.

[92][93][94] Shane Byrne of the University of Arizona Lunar and Planetary Laboratory, Tucson, one of the co-authors remarked that future colonists of the Red Planet would be able to gather up ice with just a bucket and shovel.