Are we alone in the universe? Scientists have spent years     1     (search) deep into space for signs of life beyond our planet. Now they've discovered what might be life on Venus.

Venus is the second planet from the sun in our solar system. It's one of     2     brightest objects in the night sky. Several billion years ago, the surfaces of Venus and Earth may have been     3     (similar) than now. But today, the surface of Venus is too hot for life. And until recently, Venus's thick clouds were thought to be too poisonous for a living thing     4     (survive) there.

    5     September 14, in the journal Nature Astronomy, scientists reported the discovery of a chemical     6     (call) phosphine(磷化氢)in the clouds of Venus. On Earth, the phosphine gas     7     (give) off by living things. Scientists aren't yet able to prove that life on Venus is     8     is producing the phosphine. But they can't explain the source of the gas in any other way.

The discovery of life beyond Earth would teach us more about the universe     9     our place in it. To Sousa-Silva, an astrochemist of Harvard University, it would say something about our     10     (important). “I like finding out that we as humans aren't that special,” she says.

7 . A study confirmed that the cracks (裂缝) found on the surface of Mars last year by the Curiosity Rover are evidence of ancient lakes that likely dried up about 3.5 billion years ago. The new study provides further evidence of what the climate on the Red Planet may have been like in its ancient past.

The study, published online in Geology, proved that cracks on Mars’s surface previously photographed by Curiosity are dry mud cracks which could have only been formed when wet ground was exposed to the air. This conclusion was based on an analysis of a single area of rock known as “Old Soaker.”

Researchers used the Curiosity Rover and information from its many tools including the Mars Hand Lens Imager, ChemCam Laser-Induced Breakdown Spectrometer (LIBS) and the Alpha Particle X-Ray Spectrometer (APXS) to study both the physical appearance and the chemistry of the rock, which is described as no bigger than a coffee table.

The analysis showed that cracks on the rock were formed by exposure to air, rather than heat or the flow of water. In addition, the shape of the cracks suggests it experienced a single drying event on the planet, rather than getting wet and drying over repeatedly. The position of the cracks, closer to the center of the ancient lake rather than alongside it, also suggests that the lake levels changed often, rising and falling over time.

“The mud cracks are exciting because they help us to understand this ancient lake system,” lead study author Nathaniel Stein, a geologist at the California Institute of Technology in Pasadena, said, referring to the ancient lake system on the planet.

Scientists have known of the existence of ancient water on Mars for years. A 2015 NASA study that measured water in Mars’s atmosphere suggested that ancient oceans may once have had more water than our own Arctic Ocean. However, because the planet has less gravity and a thinner atmosphere than Earth, this water evaporated into space over the course of several billion years.

8 . There are nearly 2,800 working satellites in space, which we depend on for technology we use every day, such as video calls, online maps, satellite TV, and weather tracking. Scientists use them to study space and learn more about our planet.

But there are many other satellites in orbit (轨道) that are no longer working. They're among the objects cluttering (使杂乱) up space. Some of these eventually fall back toward Earth, either landing or burning up in the atmosphere. But much of this space junk (垃圾) circles Earth for years. Space junk is a problem. Debris (残骸)   floating around Earth puts technology and future space tasks at risk. Experts are working on ways to solve it.

Orbital debris, a type of space junk, is any human-made object that has slopped working but continues to float around the Earth. This includes satellites that are no longer used and pieces of spacecraft, such as rocket stages. It has been a large problem since the 1960s.

Space junk also includes broken pieces of objects. These occur when satellites hit against things. They also result from an object crashing into an old rocket stage that still contains fuel, causing an explosion (爆炸) . There have been more than 250 space explosions since the 1960s. These tiny broken pieces can damage working satellites, which can affect research in space.

Companies all over the world are working to clean up the area surrounding our planet. A company based in Japan will test the method of using magnets (磁石) to collect space debris. Another mission is led by a company based in Switzerland. It plans to carry out a debris-removal spacecraft in 2025. The craft will get hold of a piece of an old rocket, slow it down, and move it back to Earth. Eventually, the debris will burn out like a shooting star.

Governments are trying to help too. Some are updating their country's space guidelines to limit the amount of debris created. Space is so large that the problem won't be solved by a single organization or a single country. We have to work on this together.

9 . The company SpaceX has already launched hundreds of its Star-link satellites, with plans to put as many as 42,000 of them in Earth orbit. Its goal is to provide high-speed Internet to billions of people. Moving toward that kind of access is important, but it comes at a cost. Glittering with reflected sunlight, these first orbiters, sent up in the past year, are brighter than 99 percent of the 5,000 or so other satellites now circling Earth, and obviously there are going to be a lot more. This sudden increase is bad for astronomy: the probability of a Star-link satellite crossing a telescope’s field of view and ruining an observation will be quite high near sunset. For that reason, my fellow astronomers have signed a petition (请愿书) calling for governments to protect the night sky from this invasion.

In response to protests, SpaceX has promised to address the visibility problem by, for example, applying experimental coatings — essentially painting the satellites black — but the company’s aggressive launch schedule remains unchanged. And the satellites’ illuminated (被照亮) surfaces are mostly their solar panels — exactly the part that cannot be painted over.

Unfortunately, at present no regulations govern how bright a single satellite can be, let alone thousands of them together. Even if there were such regulations, one nation’s laws can not hinder (阻碍) another country’s launches. Space literally has no borders, and the sky will need to be protected at an international level. As a consequence, we hope that the United Nations will find a way to think outside of the box to save the sky for everyone.

When I was growing up in Montana, it was a game to be the first to find a moving satellite among the host of stars in the night sky. Soon it could be a game to recognize the constellations (星座) behind a swarm of moving points of light.

Water doesn't currently exist on Mars' surface, but it used to. About 4 billion years ago, something happened to Mars' atmosphere, and most of the liquid water     1    （disappear） due to harmful radiation. But some of     2     may still be underground. Could those ancient water holes contain life?

China's Tianwen-1 mission will search     3    the existing water by radar fixed on the rover （月球车）.The European Space Agency's spacecraft found evidence for subsurface water,     4    （use） radar from space, but this will be the first time a rover has searched from the ground.

Tianwen-1 will also give China     5    （value） Mars experience and lay foundation for a sample return mission planned for     6    end of the 2020s. Getting Martian samples back to Earth is a top priority for the scientific community. Despite the impressive advances     7    （make） in placing science instruments on spacecraft, only the technology on Earth,     8    can test samples accurately, is to verify the presence or absence of life in a sample.

As of 2020, only NASA has landed and operated spacecraft on Mars     9    （successful）. More countries exploring Mars means more     10    （opportunity） for global cooperation. Space exploration brings out the best in us all. When more nations work together, everyone wins.