Mean diameter: approximately 3,122 km · Orbits Jupiter.
Europa is smaller than Earth’s Moon, but its ice shell makes it a major target for investigating potentially habitable environments. Evidence supports a salty ocean beneath the ice; that does not establish that life exists there.
The reddish lines crossing its surface distinguish it from heavily cratered Callisto. Its place here comes from size, while its scientific appeal comes from what might lie below the visible crust.
Sources: NASA moon guide · JPL size data.
Image source: NASA/JPL-Caltech/SETI Institute; reuse terms. Resized and JPEG-compressed.