The Isidis Valley Basin on Mars showing possible fluvial features eroded by precipitation. Credit: Mars Odyssey THEMIS public data archive

A pair of studies published this week reveals that while Mars may not have been as warm and wet as once thought, it does have substantial amounts of water ice, including newly-discovered deposits near the planet's south pole.

The two papers, published in Friday's issue of the journal Science, are the latest in an ongoing effort to understand how much water the planet once had, and how much it retains today, information that may be key in the search for life as well as plans for eventual human exploration of the Red Planet.

One study by scientists at the University of Colorado and NASA's Ames Research Center reopened the question of what the environment was like on the early Mars. Images of channels, flood plains, and even a possible ocean basin in the planet's northern hemisphere had led many planetary scientists in recent years to speculate that Mars was considerably warmer early in its history, warm enough to support liquid water - and possibly life - on is surface.

That hypothesis has been challenged by the Colorado and NASA Ames research. Scientists modeled the flux of impacts of large asteroids and comets into the planet 3.5 billion years ago, when the flowing water features seen on the planet today were formed. They believe that large icy bodies, between 100 and nearly 200 kilometers across, struck the planet 25 times during this period, about once every 10 to 20 million years. While the impacts would temporarily create warm, wet conditions, the planet would revert to a cold, dry state long before the next impact.

"When the river valleys on Mars were confirmed in the 1970s, many scientists believed there once was an Earth-like period with warmth, rivers and oceans," said Owen Toon, a professor at the University of Colorado and a coauthor of the Science paper. "What sparked our interest was that the large craters and river valleys appeared to be about the same age."

Another piece of evidence arguing against a condition warm, wet period on Mars are images of river channels without any sign of tributaries flowing into the main channel. "We definitely see river valleys but not tributaries, indicating the rivers were not as mature as those on Earth," said Toon.

Without enough carbon dioxide in the atmosphere to act as a greenhouse gas, the planet cooled after each impact and the water would freeze, a condition the authors of the paper called "an almost endless winter broken by episodes of scalding rains followed by flash floods." "There apparently were some brief warm and wet periods on Mars, but we believe that through most of its history, Mars has been a cold, dry planet," said Teresa Segura, a University of Colorado graduate student and lead author of the Science paper.

While Mars may not have had a wet past, it does still have significant deposits of water that have survived to the present day in the form of ice. Such ice has been previously found in the northern ice cap and, more recently, buried below the surface of the southern hemisphere. In a separate paper also published in Science, a team of scientists from the US Geological Survey (USGS) and Arizona State University present the first evidence of water ice at the surface near the Martian south pole.

Scientists used data from the Thermal Emission Imaging System (THEMIS) instrument on the Mars Odyssey spacecraft and the Thermal Emission Spectrometer (TES) on Mars Global Surveyor to search the surface of the south polar cap for evidence of water ice. Although water ice had been previously discovered in the north polar cap, scientists had detected only carbon dioxide ice in the southern cap.

"When we first saw the images from THEMIS, we noticed that areas that were dark were not all the same temperature, which suggested that the areas were composed of different stuff, perhaps even water ice," said Timothy Titus of the USGS.

One area in particular, dubbed Unit I, warmed only slowly in summer after a covering of dry ice sublimated away. The temperature of the area, about -70 degrees Celsius, was similar to temperatures in the water ice portions of the northern polar cap during summer and is about as hot as ice gets on Mars before sublimating. The temperature of the area also varied little between day and night, further suggesting that the area was made of water ice.

An adjacent region went through somewhat different changes, warming in the day but cooling off at night. Titus believes that there is water ice here, too, but under a layer of dust 2-7 millimeters thick. "The cool nighttime temperatures are what one would expect from having a layer of water ice underneath the thin layer of dust," he said.

Titus and his colleagues believe that there may be much more water ice waiting to be discovered in the southern regions of Mars. "In some ways, this water ice may just be the Œtip' of the iceberg. The speculation is that there may be a whole mass of water ice underneath the southern polar cap."