compiled by Wm. Robert Johnston
last updated 10 January 2003

Left banner artwork is imaginary view from 3,700 km above Europa, showing (right to left) Europa, Jupiter, and Io. Right banner artwork is imaginary view from surface of Europa with Jupiter and Io above. Both images © 2001 by Wm. Robert Johnston (see notes below).

Europa is one of the four largest natural satellites (moons) of Jupiter. These four satellites, the Galilean satellites, are (in order of increasing distance from Jupiter) Io, Europa, Ganymede, and Callisto. They are planet-sized, ranging from slightly smaller than the Earth's Moon (in the case of Europa) to larger than the planet Mercury (in the case of Ganymede).

Europa's surface is covered by bright water ice and transversed by linear streaks and other features colored brownish orange. There are no large impact craters--very unusual for a planetary satellite, indicating that some geologic process has been at work to eliminate craters. (The largest unambiguous crater is 48 km across.) It is mostly rock underneath a 70-km to 200-km thick layer of water ice. The rocky interior is probably separated into a silicate mantle and a metal-rich core. Europa has no atmosphere except for trace amounts of gases such as oxygen.

Images of Europa's surface suggest that this water ice layer is mostly "warm" slushy ice or even liquid water. The linear streaks appear to be cracks in a solid ice layer 10 km thick or less. The absence of large craters is also explained by a relatively thin ice layer. It is possible that an ocean of water--90 km deep--could exist under the ice layer. The gravitational tides from the other Galilean satellites have melted Io's interior, giving it the most active known volcanoes in the solar system. The same effect could heat Europa's interior enough to maintain an ocean.

Since liquid water is essential for the existence of life, the recognition in 1980 that Europa could have an ocean made it the most likely abode for extraterrestrial life in the solar system. Recent discoveries of life on the Earth's ocean floor thriving around thermal vents lends to obvious comparisons to a possible dark ocean on Europa.

Europa was discovered by Galileo in 1610, but was little more than a point of light in telescopes until the Pioneer probes transmitted fuzzy images in 1973 and 1974 while swinging past Jupiter. Flybys in 1979 by Voyagers 1 and 2 produced the first images to provide details about the surface. Voyager 2 in particular provided enough information to lead to speculation about an ocean. In 1995 the Galileo orbiter entered orbit around Jupiter for long-term study of Jupiter and its satellites. Several close passes by Europa have provided evidence that below the surface most of Europa's ice is soft or liquid (i.e. an ocean). NASA hopes to send a spacecraft to orbit Europa exclusively from 2008 to 2012.

Data on Europa:

Primary: Jupiter
Semimajor axis (mean orbital distance): 671,100 km
Orbital period: 3.551181041 days (= 0.009722 years)
Orbital eccentricity: 0.0101
Orbital velocity: 13.74 km/second
Orbital inclination: 0.464° to Jupiter's equator

Diameter: Mass: 4.7998x1022 kg, or 0.008035 times the Earth's mass
Average density: 2,989 kg/m3
Gravitational acceleration at surface: 1.315 m/s2, or 0.134 times that at the Earth's surface
Gravitational acceleration 100 km below surface: 1.41 m/s2, or 0.144 times that at the Earth's surface
Escape velocity: 2.026 km/second at the surface

Geometric albedo: 0.64
Amount of sunlight at surface relative to Earth (above atmosphere): 0.037
Visual magnitude (as seen from Earth): 5.29
Equilibrium surface temperature (average over one day-night cycle): 103 K ( -180° C )
Daytime surface temperature: 125 K ( -148° C)
Nighttime surface temperature: 85 K ( -188° C)
Apparent size of Sun as seen from Europa: 0.102°
Apparent size of Jupiter as seen from Europa: 12.04°

Discovery: 7 January 1610 by Galileo Galilei, named by Simon Marius
Space probes which have observed Europa, with closest approach date and distance:


general overviews:

missions to Europa:

maps and related information:

less technical articles:

more technical articles:

Europa artwork:

Banner artwork: © 2001 by Wm. Robert Johnston. Left image is a 3-D graphic based on a USGS/NASA/JPL map of Europa (from Galileo and Voyager 1/2 images) and a JPL/NASA map of Jupiter (from Cassini images); right image is based on a JPL/NASA map of Jupiter (from Cassini images) and a 1989 photograph of Athabasca Glacier in Canada by Linda Johnston.

Image credits: NASA/JPL (from top to bottom: Galileo image of Europa; Voyager 2 image of Europa, 1979; Galileo image of Europa, 1997; Galileo close-up image of Europa, 1997; Cassini image of Jupiter with Europa in foreground, 2000)

© 2001, 2003 by Wm. Robert Johnston.
Last updated 10 January 2003.
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