WASHINGTON -- Mars is now dry and cold, but a new study confirms that it once was covered by vast oceans and had more water per square mile than Earth.
In fact, it once had enough water to cover the planet to a depth of almost a mile, researchers say, citing an analysis of data measuring the amount of molecular hydrogen in the atmosphere.
Unlike Earth, Mars lost its water over millions of years through a combination of chemical reaction and the bombardment of asteroids and comets.
There is much evidence now that Mars had an ocean of liquid water, said Vladimir A. Krasnopolsky of Catholic University of America. "But the climate changed. ... Mars became a cold desert."
Krasnopolsky said the proven abundance of water in Mars' early history "improves the prospects" that life could have evolved there.
In a study appearing Friday in the journal Science, planetary researchers Krasnopolsky and Paul D. Feldman of Johns Hopkins University said that Mars' upper atmosphere contains molecular hydrogen, or H2, a finding that confirms earlier theoretical models of the water history of the planet.
The H2 comes from a chemical reaction, called dissociation, that split the hydrogen from water, H20, and allowed the lighter hydrogen to escape to the atmosphere.
"It is a complex chemistry process of which we are detecting only one piece," the molecular hydrogen, said Feldman. But these findings help to support earlier estimates of the amount of water once on Mars.
Donald M. Hunten, an expert on planetary atmospheres at the University of Arizona, Tucson, said the studies by Krasnopolsky and Feldman make an important contribution because they strengthen earlier theories of Mars' water history.
"This is an observational confirmation of a theoretical model," said Hunten. "I am happy to see such a confirmation."
The Krasnopolsky and Feldman study is based on data from the Far Ultraviolet Spectroscopic Explorer, a spacecraft that can detect and identify chemicals in the atmosphere of the distant planets. The data is the first to detect molecular hydrogen in the atmosphere of Mars.
Krasnopolsky said the findings support this scenario of Mars' water history:
The planet was formed 4.6 billion years ago and during its early history had enough water for an ocean about a mile deep. Much of this water, however, was chemically bound to the rocks and soils of the planet. This material was rich in iron. Krasnopolsky said that over a 300 million year period, the water reacted with the iron, releasing molecular hydrogen to the atmosphere in a process called hydrodynamic escape.
"There was a massive escape of molecular hydrogen and the loss of water was very rapid," he said. The iron-water reaction, said Krasnopolsky, helped turn Mars into its rusty-red color.