2007年VOA标准英语-US Spacecraft Gathers Hints of Underground Wate（在线收听）

By David McAlary
Washington
15 February 2007

A U.S. spacecraft has provided fresh evidence that water once flowed on Mars. In this case, it is water that ran deep underground through rock fissures, places where scientists say life could have thrived hidden from the harsh Martian surface environment. VOA's David McAlary reports.

 
Satellite photo showing erosion of rock formations on Mars
The U.S. space agency NASA's newest satellite around the red planet, the Mars Reconnaissance Orbiter, has taken color pictures showing a landscape called Chasma Canyon composed of dozens of alternating layers of dark and light toned rocks and crossed by dark sand dunes.

Within those layers, the detailed images show fractures, called joints, surrounded by light colored bedrock that was once deep underground before erosion exposed it. University of Arizona researcher Chris Okubo argues that this brighter bedrock is clear evidence that a liquid flowed through it and washed the dark minerals away, as water has done in similar situations on Earth.

"So the unaltered rock has this dark brown color. However, along the joints you see that the dark brown minerals have been chemically removed by the fluids that were flowing within the joints, so the area has been bleached," he said.

 
Chris Okubo

In a paper in the journal Science, Okubo and a colleague say the bleaching would have taken a long time to occur - perhaps hundreds of millions or a few billion years. He explained at a scientific meeting in San Francisco that an underground setting for water could have been conducive for life on Mars.

"The fact that these areas were underground is important because the overlying several kilometers of rock would have acted as buffers against any sort of harsh environmental conditions that may have existed at the surface of Mars at this time. So these areas would be nice protected areas for any sort of biologic processes to occur," he said.

Previous findings by NASA robot rovers on Mars have shown that water once flowed on the surface and may have formed lakes and even oceans. Recent photos of fresh gullies from another U.S. satellite indicate it might still be percolating up from underground.

Mars water specialist Stephen Clifford of the Lunar and Planetary Institute in Houston, Texas, says Okubo's research supports the notion that ground water remains beneath the Martian surface crust.

"The fact that there is such persuasive evidence of joints and fractures in the crust also suggests that this groundwater had an ability to flow enormous distances. That has implications for understanding the processes that led to the redistribution of water from surface environments like lakes and seas into the subsurface, into that increasingly thick layer of frozen ground that was the result of both the external climate cooling and the decline of the planet's internal heat flow," he said.

Okubo says his analysis awaits confirmation by a future generation of Mars rovers, which could check the chemical composition of the rocks up close.