Liquid water, not anything else, carved the canyons, valleys and outflow plains of Mars, according to the latest research.
By Dr David Whitehouse
BBC News Online science editor
A rival theory, that liquid carbon dioxide or rocks supported by CO2 gas could have made the surface features, does not work, according to Dr Neil Coleman, of the US Nuclear Regulatory Commission.
Did Mars once have ice-covered oceans?
It could put an end to the "White Mars" hypothesis, which favoured CO2 and made the prospect for life on Mars more remote.
Vast lakes of water accumulating under sheets of ice until they overflowed crater and canyon walls is the way many Martian features formed, says Dr Coleman.
White or wet Mars
Neil Coleman has studied some of the most dramatic and significant scenery on Mars: the enormous outflow channels of Chryse Planitia.
Chryse Planitia may once have been an ocean. Today it is a huge plain where many of Mars' largest outflow channels converge. It was also Viking 1's landing site in 1976.
But what caused the outflow channels? Many scientists say obviously running water, but in recent years a few researchers have questioned this, arguing that liquid carbon dioxide might be responsible.
The White Mars hypothesis contends that gaseous or liquid carbon dioxide carried debris down Martian slopes and eroded the features some attribute to fluvial processes.
Advocates of the theory say it solves many problems with the water model, such as where the water came from and where it went.
Neill Coleman's work, published in the Journal of Geophysical Research, is, he believes, conclusive evidence that water, not carbon dioxide, is responsible for the features seen on the planet's surface.
Water, water, everywhere
Dr Coleman says his latest study shows that nowhere is the evidence for vast quantities of running water clearer than the outflow channels of Chryse Planitia.
Consider the associated Kasei Valles. He suggests it is a system of intertwined flood-carved channels more than 2,000 kilometres long that begins in near the equator in Echus Chasma and ends in Chryse Planitia.
He says the White Mars theory is "inconsistent with studies of terrestrial and Martian mass movements" because, "put simply, rocks can 'float' on air, but not for long".
Evidence for water flow?
This means that only water had the stamina to carve large-scale Martian features like Kasei Valles.
Advocate of the White Mars idea, Nick Hoffman of Australia's La Trobe University, says that Coleman's work is interesting but that he does not feel the White Mars idea is mortally wounded by it.
Oceans under ice
Another factor in favour of the water hypothesis is the discovery of reservoirs of water ice beneath the surface at Martian mid-latitudes and the fact that outflow channels on Mars are related to the planet's volcanic activity in the past.
This latest research indicates the outflow channels may have been carved by lakes of water trapped under ice that burst through crater walls and flowed downhill in torrents.
Only a liquid like water, says Dr Coleman, could have slowly accumulated under ice cover, and ultimately filled a basin until it overflowed.
Chryse Planitia - significant scenery
Water-carved channels are good for the prospect for life on Mars hanging on at the present having been formed in the Martian oceans of three billion years ago.
Neil Coleman says: "In reviewing the White Mars hypothesis, we find even more evidence of water than expected, which bodes well for the search for life on Mars."