Discovery:
Europa was discovered by Simon Marius and Galileo Galilei in 1610. Europa is located approximately 671,000 kilometers from Jupiter, and it is Jupiter's second moon.
Astonomers at Johns Hopkins University and the Space Telescope Science Institute, both of which are located in Baltimore, Maryland, used the Hubble Space Telescope to find a thin atmosphere of molecular oxygen around Europa. So far Mars and Venus are the only other planets in our solar system that also have molecular oxygen in their atmospheres.

So how thin is the atmosphere?
The astronomers say that the layer of oxygen on Europa is so thin that if it was brought to Earth and subjected to the atmospheric pressure existing on a our planet, it would compress itself to fit into about 12 indoor football domes.

What about life?
Does this discovery mean there is the possiblity of life on Europa? Not really. Europa is almost 500 million miles away from the Sun, and its surface is -230 degrees Fahrenheit. Too cold to suppport life as we know it.

Any other reason?
The oxygen on Earth is produced by living organisms, whereas the oxygen on Europa is produced by a complicated non-biological process, which produces both hydrogen and oxygen that gets emitted into the atmosphere. The hydrogen escapes because it's so light. The oxygen gets trapped in the atmosphere, because it's heavier, but it slowly leaks out and has to be replenished.