The sixth of Jupiter's known moons and the fourth largest; it is the second of the Galilean satellites. With a diameter of 3140 km, Europa is slightly smaller than Earth's Moon. Its mass is 4.80 × 10^22 kg, i.e. 1.5 times less massive than Earth Moon. Its distance to Jupiter is 670,900 km, or about 9 Jovian radii. Its orbital period is 3.55 Earth days which equals its rotation period. Europa's density is 3.0 g cm^-3, typical of a mixture of rocks including ice. Its high albedo (0.67) suggests that its surface is mostly water ice. The surface temperature of Europa ranges between about 125 K (-150 °C) at the equator and about 50 K (-220 °C) at the poles. There are few impact craters on Europa, because its surface is too active and therefore young. The most striking features of Europa's surface are structures called lineae and lenticulae. The thickness of the ice crust could range between a few kilometers to a few tens of kilometers. It is now believed that there is an ocean of salty water, up to 100 km deep, flowing under Europa's ice. Europa's ocean is kept liquid due to tidal heating by Jupiter.