Washington, May 16 : New observations from NASA's Mars Reconnaissance Orbiter suggest that any liquid water that might exist below the planet's surface, and any possible organisms living in that water, would be located deeper than scientists had suspected.
Scientists came forward with this hypothesis after the Mars probe observed that the crust and upper mantle of Mars are stiffer and colder than previously thought.
The discovery was made using the Shallow Radar instrument on the spacecraft, which has provided the most detailed pictures to date of the interior layers of ice, sand and dust that make up the north polar cap on Mars.
The radar images reveal long, continuous layers stretching up to 600 miles (1,000 kilometers), or about one-fifth the length of the United States.
"In our first glimpses inside the polar ice using the radar on Mars Reconnaissance Orbiter, we can clearly see stacks of icy material that trace the history of Mars' climate," said Jeffrey Plaut of NASA's Jet Propulsion Laboratory, Pasadena, California.
The radar pictures show a smooth, flat border between the ice cap and the rocky Martian crust.
Even in the weak Martian gravity, that much ice should have caused the bedrock to buckle under the load. But it does not, indicating that the crust is thicker and colder than previously believed.
"This means that the crust is also colder, indicating that liquid water, if it exists, is buried much more deeply than previously suspected," said Phillips. "That has implications for all kinds of things, including the depth at which you might find extant life," he added.
Scientists speculate that any life on Mars associated with deep aquifers also would have to be buried deeper in the interior.
It has long been known that the ice cap is made of many fine layers separated by bands of dust. The ice-penetrating radar revealed that the layering extends all the way across the ice cap and contains four major divisions in addition to the thin layers.
"This means that some very large-scale process is responsible for the layering," said study co-author Roger Phillips, a geophysicist at the Southwest Research Institute in Boulder, Colorado. "The obvious culprit is climate," he added.
According to Phillips, one well-known cycle, operating over about a 100,000-year time period, probably accounts for the fine layers. Another, operating over the course of about a million years, likely explains the major layers, he added. "If this is correct, the observed number of layers suggests the ice cap probably began forming about five million years," he said.