Allevidence points to the fact that Mars once had flowing water, but numerous fl-组卷网

阅读理解-阅读单选 0.4 引用2 组卷125

All evidence points to the fact that Mars once had flowing water, but numerous flybys, orbiters and landers have confirmed one undeniable fact — any liquid (液体的) water that was once on its surface is now long gone.

One idea, the Mars Ocean Hypothesis, suggests that Mars not only had some liquid water, but a lot of it. But the new study’s co-author Kun Wang says his team’s finding pours cold water on it.

“Mars’ fate (命运) was decided from the beginning.” Wang, an assistant professor of Earth and planetary sciences, said in a statement. That’s because the lower mass and gravity of Mars makes it easier for volatile (挥发) elements and compounds such as water to escape from its surface into space.

The researchers looked at 20 Martian meteorites (陨石) ranging in age from about 200 million years old to 4 billion years, dating back to a time when the solar system was still in the chaos of formation. The researchers analyzed water behaves in much the same way, indicating that most of it would have been lost to space during the formation of Mars.

The liquid water that did remain on the Martian surface carved out the riverbeds and other formations that we see there today, Wang says. But that water, too, would likely have disappeared had it not been trapped as ice at the Martian poles as the climate on the planet became colder, he notes.

“This does probably indicate a lower limit on size for a planet to be truly habitable,” Wang says, “Understanding that lower limit is important — there are lines of evidence that small planets are more common than big ones, so if the small ones are dry, then there are fewer habitable (适于居住的) worlds out there than we thought.”

【小题1】What is the function of the first paragraph?