Europa

Europa is the sixth closest moon of the planet Jupiter, and the smallest of its four Galilean satellites, but still one of the largest moons in the Solar System. Europa was discovered in 1610 by Galileo Galilei and possibly independently by Simon Marius around the same time. Progressively more in-depth observation of Europa has occurred over the centuries by Earth-bound telescopes, and by space probe flybys starting in the 1970s.

• Europa is the smoothest surface of any planet/moon in our solar system.
• Most of Europa's surface is highly reflective, because it is mostly made of ice.
•  Europa has a very thin tenuous atmosphere mainly made up of molecular oxygen.
•   The Galileo probe found our that Europa has a slight magnetic signature that it picks up from Jupiter's magnetic field but the signature could only be created if Europa was covered with something that conducted electricity much like salt water dose. 

Europa may be 485,000,000 miles away from the sun but it isn't actually that cold, turns out that Europa is caught in a constant gravitational tug of war with Jupiter its self and its sister moons IO and Ganymede causing friction on Europa meaning its sub surface oceans are probably quite warm and liquid.