A moon of the planet
Jupiter.
Recent observations of the magnetic field around Europa by the [Galileo spacecraft]? suggest that Europa has a crust of water ice, a mantle of liquid water, and a core of rock.
http://www.jpl.nasa.gov/galileo/europa/
It has been suggested that life may exist in this under-ice ocean.
Data for Europa:
- Discovery: January 7, 1610 by Galileo Galilei
- Diameter (km): 3,138
- Mass (kg): 4.8e22 kg
- Mass (Earth = 1) 0.0083021
- Surface Gravity (Earth = 1): 0.135
- Mean Distance from Jupiter (km): 670,900
- Mean Distance From Jupiter (Rj): 9.5
- Mean Distance from Sun (AU): 5.203
- Orbital period (days): 3.551181
- Rotational period (days): 3.551181
- Density (gm/cm) 3.01
- Orbit Eccentricity: 0.009
- Orbit Inclination (degrees): 0.470
- Orbit Speed (km/sec): 13.74
- Escape velocity (km/sec): 2.02
- Visual Albedo: 0.64
- Surface Composition: Water Ice