It is hard to imagine that plants or animals could ever exist on Mars. But scientists continue to look for evidence. NASA, America’s space agency, has found evidence that, a long time ago, there was surface water on the Mars. Scientists believe water is necessary for life as we know it. So since that discovery, they have been looking for chemicals that would be present if there once was or still is life on the planet.

At a December 13 meeting in California, NASA scientists reported an important discovery on Mars. They said for the first time that they had found very small amounts of boron(硼). Boron is important because it could help build RNA molecules(分子). And RNA molecules are important because they are one of the basic building parts for life.

One of the next steps in the scientists’ search for life on Mars comes soon. The next spacecraft is planned to launch. It will bring rocks from Mars back to Earth. Scientists in Britain are getting ready for those Mars rocks now. Before this, using a powerful microscope(显微镜), they have already examined 200-million-year-old volcanic rocks found deep in the Pacific Ocean. The microscope showed holes on the rocks caused by tiny living things called microbes(微生物). Microbes are the oldest form of life on Earth.

Next, the scientists in Britain will examine rocks which contain ancient material from Mars. The material comes from a time when Mars would have been more likely to support life. The scientists hope to get the same findings in the rocks from Mars as the ones they saw in the ocean rocks. If they do, they predict that the rocks coming directly from Mars will also show signs of life. That, in turn, could finally prove that we are not alone in the universe.

Like a phoenix (凤凰), some stars may burst to life covered in “ash,” rising from the remains of stars that had previously passed on.

Two fireballs covered in carbon and oxygen, ashy byproducts of helium fusion (氦聚变), belong to a new class of stars, researchers report in the March Monthly Notices. Though these burning objects are not the first stars found covered in carbon and oxygen, they are the first discovered to have helium-burning cores.

“That merger (并合) tells you the star must have evolved differently,” says study author Nicole Reindl.

The stars may have formed from the merger of two white dwarfs (白矮星), the remaining hearts of stars that exhausted their fuel, Reindl further explains. One of the two was rich in helium, while the other contained lots of carbon and oxygen. These two white dwarfs had already been orbiting one another, but gradually drew together. Eventually the helium-rich white dwarf “ate” its partner, leaving carbon and oxygen all over its surface, just as a messy child might get food all over their face.

Such a merger would have produced a star covered in carbon and oxygen to burn nuclear fusion in its core again, says Tiara Battich, a German astrophysicist.

To test this idea, Battich copied the evolution, death and eventual merger of two stars on his computer and simulated (模拟) the process. He found that putting together a carbon-and-oxygen-rich white dwarf and a more massive helium one could explain the compositions of the two stars observed by Reindl and her colleagues.

“But this should happen very rarely,” Battich says. In most cases the opposite should occur, because carbon-oxygen white dwarfs are usually the more massive ones. For the rarer case to occur, two stars slightly more massive than the sun must have formed at just the right distance and the right time.

"The origins story Battich proposes demands a very specific and unusual set of circumstances, " says Simon Blouin, a Canadian astrophysicist. “But in the end, it makes sense.”

Scientists have not found any signs of life on Mars yet,but they say a robotic vehicle called “Curiosity” is helping them learn a lot about the planet’s history and climate.

Curiosity landed on Mars in August 2012 after travelling through space for more than eight months.It was sent to Mars by scientists from NASA in the United States.

Curiosity is about the size of a car and has six wheels.It also has a robotic arm,cameras,and instruments that allow it to examine things it finds on the surface.Then it sends the information back to the earth.

Curiosity’s main task is to find out if anything could live on Mars,either now or in the past.On Nov.2,NASA scientists held a press conference (新闻发布会) to discuss what Curiosity had found in its first two months on Mars.

Curiosity has found soil that is similar to the sand formed by volcanoes (火山) on the earth.Scientists say that studying the minerals in Martian soil will help them understand what conditions were like on the planet in the past.Curiosity also found smooth stones like the ones found on river beds and seashores on the earth,where their rough edges have been worn down by water.Mars is very cold and dry now,but scientists say the smooth stones tell them that a river used to run through the place where they were found.

Curiosity has been testing the atmosphere around Mars for a type of gas called methane (甲烷),but so far it has not found any.On the earth,most methane is produced by plants or animals.Methane on Mars might indicate that some type of tiny plants or animals lived there.

Curiosity is the fourth robotic vehicle to be sent to Mars.It will continue to explore the planet for about two years.