Week Five:火星探險

Mars Rover Finds Changing Rocks, Surprising Scientists

As NASA’s Curiosity rover treks up a three-mile-high mountain on Mars, the rocks are changing. That says something about how the planet’s climate and environment changed more than three billion years ago — but scientists are not sure what.

Since it landed more than three years ago in a 96-mile-wide depression known as Gale Crater, Curiosity has made a number of discoveries, notably that the crater once held lakes of fresh water. For most of that time, the rocks it encountered were generally basaltic, a volcanic composition typical on Mars.

“Now in the recent few months, that has changed,” Ashwin R. Vasavada, the project scientist for the mission, said at a news conference on Thursday at a meeting of the American Geophysical Union in San Francisco, where researchers were presenting some of their newest results.

They have surprising clues but no definitive story, yet.
Each layer of sedimentary rock tells something about the geological conditions at the time the rock formed, meaning that Curiosity, which arrived at the base of the mountain in September 2014, is in a sense moving forward through the geological history of Mars as it climbs.

What has caught the attention of Dr. Vasavada and his colleagues lately is silica, a class of minerals made of silicon and oxygen. The evidence points to the action of liquid water even after the lakes disappeared.

“Groundwater passed through the rock multiple times, leaving different chemical signatures behind,” Dr. Vasavada said.

Basalt is generally half silica. Curiosity has been examining two rock units: one a mudstone of lake bed deposits, among the oldest rocks the rover will examine, and the other a sandstone of coarse grains that were blown in and draped onto the mountain. “It probably is among the youngest rocks we’ll encounter on the mission,” Dr. Vasavada said.

In the mudstone and the sandstone, Curiosity found much higher levels of silica, up to 90 percent more than it had observed previously in basaltic rocks.

“All of this we’re just beginning to piece together and understand,” Dr. Vasavada said.

After arriving at a spot the scientists named Marias Pass, an intersection between the older mudstone and younger sandstone near the base of the mountain, Curiosity spied a patch of light-toned bedrock, part of the mudstone. It fired a laser to vaporize the rock in several places; the instrument identifies the constituent elements from the colors of light given off. Then Curiosity drove off to do more science elsewhere.

Back on Earth, scientists analyzing the data realized this was something different: It turned out to be the first of the high-silica rocks.

http://www.nytimes.com/2015/12/18/science/mars-rover-finds-changing-rocks-surprising-scientists.html?_r=0



Structure of the Lead：
WHO-  NASA’s Curiosity rover
WHERE- MARS
WHAT- Finds Changing Rocks
WHEN- not given
HOW- Surprise

Keywords：
basaltic 玄武
mudstone 泥岩
vaporize 蒸發
drove off  擊退
high-silica 高硅