根据以下资料，回答20~23题。 At the first sight the planet Mars does not appear very welcome to any kind of life．It has very little oxygen and water, the temperature at night is below -50~Cand winds of 100 miles per hour cause severe dust storms．However, the surface of the planet seems to show that water flowed across it some time in the past, and it is believed that there is enough ice at the poles to cover the planet with water if it melts．Although there is no life on the Mars now, some scientists think that there may have been some form. of life long time ago．At that time, the planet had active volcanoes the atmosphere was thicker and warmer and there was water．In fact, in some ways the Mars may have been similar to the Earth, where life exists. Some people believe that the Mars could support life in the future if the right conditions were produced．The first step would be to warm the planet using certain gases which trap the Sun's heat in the planet's atmosphere．With warmth, water and carbon dioxide (二氧化碳), simple plants could begin to grow．These plants could slowly make the Mars more suitable for living．It is estimated that the whole process might take between 100,000 and 200,000 years．In the meantime, people could begin to live on the planet in special closed environments．They would provide a lot of useful information about conditions on the Mars and the problems connected with living there. Some scientists think there may have been life on the Mars in the past because __.