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Investigate what is needed for life to exist anywhere and the implications for life in our solar system beyond Earth. Consider Mars and some of the moons of Jupiter and Saturn, including Europa, Callisto, Ganymede and Enceladus.

Little Green Men

Hello. My name is <untranslatable>, but you can call me Greenie. I am from outer space.

I don’t want to tell you my exact location, because I don’t like people to visit me. My house isn’t very clean. But I love to visit you! You see signs of my visits nearly every time you stand at a grocery store checkout stand.

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Now, those headlines about me aren’t true; I’m perfectly harmless. Some people don’t think I even exist. Other people don’t think there’s any extraterrestrial life anywhere in your solar system, much less the universe. But as your two renowned scientists – Carl Sagan and Stephen Hawking – have said, it would be improbable for life not to exist somewhere other than Earth. Let me help you examine extraterrestrial life and then take you on a tour of where in your solar system it might exist.To know if life exists anywhere in the universe other than Earth, you need to decide what life is.

Is life an intelligent, thinking being? Or is it just something that can reproduce, adapt to its environment and metabolize things? If you think life is just the latter, then it is much more likely you will find life elsewhere in the solar system. It is thought that bacteria exist elsewhere in your very own solar system!Once you decide what life is, you need to decide what something needs to live. Most scientists agree that life needs liquid water to survive.

With that in mind, let’s take a tour of your solar system, and I’ll show the places in the universe that all seem to have that one criteria that is needed for life! First stop: Mars.


Scientists believe that as recently as 3.6 billion years ago – when life began on Earth – there was liquid water on Mars. Although today the planet is dry and most of its water is locked up in the polar ice caps, scientists believe that at one time oceans covered the planet. During that time, the planet could have evolved life. Any life on Mars today is probably under the surface in a hidden water source.

Moons of Jupiter

Let’s leave Mars and fly to the moons orbiting Jupiter.


This is Europa, the sixth closest moon to Jupiter. Europa has a layer of ice covering it. Scientists believe there is liquid water under the ice and that life may exist there – just like the life found in the deep oceans of Earth.


Callisto is another moon of Jupiter. Callisto’s atmosphere is made of carbon dioxide and probably oxygen. Callisto may have water 100 kilometers under its surface where life may exist.

This is less likely, though, than the possibility of life on nearby Europa.


This moon of Jupiter is larger than the planet Mercury. Ganymede is made of rock and ice and has a thin atmosphere of oxygen and hydrogen. There is possibly a saltwater ocean 200 kilometers under the surface. If this ocean exists, then Ganymede could support life.

Because the water is so far below the surface, however, it isn’t likely. Ganymede – a moon of Jupiter that possibly has a saltwater ocean 200 kilometers under the surface.

Moons of Saturn

Now we’re off to the moons of Saturn.


Enceladus, a small moon of Saturn, may have liquid water under its icy surface. It also has snow on its surface produced by the cryovolcanoes at its poles. The water near the surface of Enceladus encourages scientists to think this may be one of the most likely places for extraterrestrial life in your solar system. Enceladus – a small moon of Saturn that may have liquid water under its icy surface.

Saturn also has moons that could possibly support life.
Enceladus Image


Titan, the largest moon of Saturn, is similar to Earth. It has a dense atmosphere of nitrogen that allows it to keep liquid water on its surface and surface features similar to Earth. Scientists even compare its methane cycle with Earth’s water cycle. The surface liquid, along with the nitrogen atmosphere, suggest that this is a likely place for extraterrestrial life. Titan – the largest moon of Saturn; it has liquid water on its surface.

Lesson Summary

Most scientists believe that the one thing necessary to support life is liquid water. Liquid water is found many other places in your solar system, including Mars, the moons of Jupiter – Europa, Callisto and Ganymede, and Saturn’s moons Titan and Enceladus.Well, I’ve had fun showing you the possibilities for life in your solar system. I hope to see you the next time I visit Earth. Bye!

Learning Outcome

After watching this lesson, you should be able to identify the places in our solar system where life could exist and explain how.

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