Plan B：Life on Mars①？
In the 1960s and 1970s, the greatest fear was [1]that the human race, and possibly all advanced② life forms on the planet, could be wiped③ out by nuclear④ missiles, just at the push of a button⑤.Today, however, environmental problems have taken over as the greatest risk to life on Earth.Scientists are thinking of ways to lower this risk, such as replacing coal and oil with forms of renewable energy⑥.But they are also preparing for the worst：what can we do if the terrifying⑦ scenes in films such as The Day After Tomorrow happen in real life? What is our Plan B for Earth?
[1]that引导表语从句。
One option is [2]to explore other planets to see if we could live on them.The most likely choice is Mars, [3]which is relatively⑧ close to Earth and has an environment less hostile⑨ than that of other planets.Mars has fascinated⑩ people since ancient times, and today our interest in Martian exploration is greater than ever before.Films such as The Martian enjoy worldwide popularity.More governments and organisations are making efforts? to educate the public on the Red Planet, for example, the Mars Desert Research Station in the Utah desert of the US and the Mars Village in Northwest China's Qinghai Province.
[2]动词不定式短语to explore other planets作表语；动词不定式短语to see if we could live on them作目的状语；if引导宾语从句。
[3]which引导非限制性定语从句，修饰名词Mars。
Since the 1960s, we have been sending unmanned? spacecraft? to Mars.Our probes have orbited? the planet, [4]sending back valuable data and stunning? images.Our robots have explored the Martian surface, [5]testing the soil and searching for? resources, water and signs? of life.[6]Encouraged by discoveries over the years, space agencies? of various countries are planning manned missions to Mars [7]that could take place within the next 25 years.
[4]现在分词短语作状语。
[5]现在分词短语作伴随状语。
[6]过去分词短语作原因状语。
[7]that引导定语从句，修饰名词missions。
There is no doubt [8]that humankind is drawn? towards Mars, with dreams of making it our second home.However, sending people there will require all the skill, courage and intelligence of the human race.[9]While the Moon can be reached within days, it would take months to reach Mars, [10]travelling through dangerous solar radiation?.[11]And even if the first settlers do reach Mars safely, they may not be able to return to Earth — ever.Staying alive will be a daily challenge, but as proved by the Biosphere 2 experiment, not impossible.As early as the 1980s, scientists were building Biosphere 2 in the Arizona desert.It consisted of a closed space [12]in which people, animals and plants could live together.The “closed” concept meant that the space was designed to function with its own oxygen, food and water, needing nothing from the outside world.Although the twoyear experiment was not a success, it did provide us with a better understanding of how humans might be able to live on another planet.More recently, scientists have succeeded in growing a variety of plants in an environment similar to that on Mars.That definitely is a big step forward.
[8]that引导同位语从句，对名词doubt作进一步解释。
[9]While意为“而，然而”，表示对比。
[10]现在分词短语作伴随状语。
[11]even if引导让步状语从句，even if意为“即使，尽管”；do强调谓语动词reach。
[12]in which引导定语从句，修饰名词space。
For now, human settlement of Mars is still decades away.In the meantime， scientific research shows that the planet Earth is getting warmer.This change is being caused by human activity and is having a terrible effect on the biosphere.Until we are finally able to live on another planet, we need to take much better care of our own.Right now, it's the only one we have！
