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The Frosted Craters of Northern Spring and Southern Autumn - Lowell Crater from NASA's Planetary Photojournal.It is spring in the northern hemisphere, and frost that accumulated during the most recent 6-month-long winter has been retreating since May. It is autumn in the southern hemisphere, and frost was seen as early as August in some craters; later the frost line moved farther north, and we began to see frost in Lowell Crater in mid-October.

People could land on Mars in the next 20 to 30 years provided scientists can find water on the red planet, the head of NASA's surface exploration mission said on September 16.

Two partially solar-powered "robot geologists" -- Mars Exploration Rovers, or MERs -- have been trundling across 3 miles of the planet and into craters since January, beaming back data about the makeup of what scientists believe is Earth's sister planet.

Asked how long it could be before astronauts land on Mars, Arthur Thompson, mission manager for MER surface operations said, "My best guess is 20 to 30 years, if that becomes our primary priority."

The two MER robots, dubbed Spirit and Opportunity, have found ancient evidence that water was once plentiful -- important for scientists hoping to know if there was once -- or could still be -- life on Mars.

Without water, the dream of sending astronauts to the often dusty planet, which has rust-colored rocks and where the sky is red and sunsets are blue, couldn't unravel.

"If we cannot find water，it really makes it difficult to send humans. Water is the key," said Thompson, who was attending a mining engineers' conference.

Such a mission would take 11 to 12 months to get to Mars and it would be impossible to carry enough water for the astronauts, plus the water needed to make rocket fuel for the return journey, to cool the spacecraft and to generate energy.

Thompson said scientists had found a canyon on Mars "that makes the Grand Canyon look like a small canyon," where water could still be present.

"There are indications that there is actually water that seeps out the side of the canyon, and going down the side it evaporates. We believe it's an ongoing process," he said.

Three satellites now orbiting Mars are constantly gathering information, and Thompson said, "If there is water, we believe the chances of finding life are greatly increased."    

(Agencies)

trundle: to spin; twirl（轉動、旋轉）

astronaut: a person trained to pilot, navigate, or otherwise participate in the flight of a spacecraft（宇航員）