Mars is the only other planet in our solar system that has or had the correct conditions for life as we know it to develope. Discovering life on another object in our solar system could give insight into the age-old question of the origins of life. The graviy of a planet or moon determines that planet or moon's escape velocity. Escape velocity is the velocity an object need to have in order to escape a certain gravitational field. Molecules and atoms of an atmosphere have a velocity that depends on their mass and temperature.
Heavier elements have a slower speed than lighter elements.
Hotter gas has a higher velocity than cooler gas of the same composition. The direction of these velocities are random. If the velocities of certain elements are greater than the Moon's escape velocity, then that element will eventually escape from the Lunar surface, once the velocity direction is upwards from the surface. Earth's gravity has held the heavier elements like nitrogen and oxygen, but let the lighter elements like hydrogen and helium escape. Jupiter, with its huge gravitational field, and therefore high escape velocity, is able to hold onto even the lightest elements like hydrogen and helium.