But it's much less than on Earth, of course. I didn't know this, but Mars has higher gravity than the Moon. Student 2: Gravity is another difference. Student 1: Right, and it's mostly carbon dioxide. It's got about one per cent as much air as Earth. Student 2: Mars doesn't really have air, not compared with Earth. Most of Earth's air is nitrogen and oxygen, but Mars …? I don't think we can say the air is the same, though. It's just that most of the water there now is probably frozen. Student 2: Well, Earth is 70 per cent water and Mars probably had huge oceans in the past. Student 2: I suppose you could say they have water in common. Earth's day is 24 hours but Mars's is about half an hour longer. Student 2: What about this? It looks like the days on both planets are almost the same length. Student 1: Their sizes are pretty different. Student 2: Yeah! Earth is called the blue planet and Mars is called the red planet for pretty obvious reasons! Student 1: Let's see … not the colour, obviously! What other similarities are there between the two planets? Jupiter is 5.2 astronomical units so I guess you're right. The difference doesn't sound so big when you look at it that way. Earth is 1 astronomical unit from the Sun and Mars is 1.3. That's why we use astronomical units when we talk about distances in space. Jupiter is, like, almost 780 million kilometres.

Student 1: Yes, but in space that's not that far. Student 2: No way! Mars is much further away! It says here it's about 228 million kilometres, while Earth is about 150 million. Well then, I suppose you could say both planets are a similar distance from the Sun? Student 2: Yeah, but do we know that's actually true? I mean, Mars is much colder than Earth, isn't it? It says here it's about minus 55 degrees most of the time, whereas on Earth only places like Antarctica get that cold. Student 1: So, as far as I know, the main similarity between Mars and Earth is that they can both support human life. Teacher: So you've got a few minutes to discuss with your partner.

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