Ice on Europa

Europa is the smallest of Jupiter's Galilean satellites. It has a diameter of 3,130 km, about the size of Earth's Moon, and is covered with white and brown colored water ice. With the exception of Earth, Europa currently appears to be the only body in the solar system which potentially harbors a global ocean of liquid water. This putative ocean is hidden under Europa's frozen surface crust. Close-up images returned by the Galileo spacecraft showed areas which have similarities to ice-floe covered arctic oceans on the Earth (1-3). Fractures and apparent movement of the icy surface plates, as in this Galileo image, suggest the existence of water or warm "slushy" ice below a frozen surface.

How do we know Europa's surface is made of ice?

The Voyager and Galileo spacecraft were not equipped with instruments which could directly detect water or water ice. Observations of Europa show the geometric albedo is generally high, indicative of a reflective surface. Earth-based observations of the infrared spectra of Europa and the other Galilean satellites were made in the early 1970's from ground and airborne observatories (4,5). The observations used Michelson interferometer spectrometers and showed that Europa strongly absorbs infrared light with wavelengths of 1.4 and 1.8 microns. This is a distinctive characteristic of water ice, and the strength of the absorption in these bands coupled with the otherwise high albedo is indicative of comparatively uncontaminated ice.

What indicates the surface crust is underlain by water or slush?

The surface of Europa is broken up into large plates and covered with extensive fractures. The plates in many regions appear to have shifted and rotated, and can be fit back together like pieces in a puzzle. The false-color image to the left shows Minos Linea. The long red bands are 10 to 20 km wide and have lighter lines running through the centers. These may be a result of a sequence of water eruptions along a fault, the first clearing dark contaminants to the side, and the later ones leaving a bright central band. Similar banded structure can be seen in the image of Belus Linea, below. The wide fractures and relative movement and rotation of the surface plates indicates the crust is brittle but is underlain by a softer, more fluid, substance. Presumably this would be water or warm, slush-like ice. It is possible that the plate movements occurred in the past and the water layer is now frozen and can no longer move. However, the surface of Europa is not blanketed with impact craters, an indication that it may be relatively young and still mobile. Preliminary estimates range from less than one million years to a billion years, depending on the cratering rate at Jupiter, which is not well known.

Why doesn't the water freeze?

The equilibrium temperature due to solar heating at Europa is about 100 K (-280 F), well below the freezing point of water, so if the Sun were the only source of heat at Europa, all water would be completely frozen. However, Europa is in an orbital resonance with the satellites Io and Ganymede. This resonance keeps Europa in an eccentric orbit around Jupiter, which subjects the moon to gravitational stresses and distorts its shape slightly, generating heat in its interior. This is the same process which is responsible for Io's active tectonics and dramatic volcanic plumes. Io is in a much more eccentric orbit than Europa (e = 0.041 for Io compared to 0.010 for Europa) and is much closer to Jupiter, so the effects of this tidal stressing are much more severe on Io and have probably removed all water from the satellite. On Europa, the effects are gentler. Nonetheless, these tidal stresses are thought to be responsible for the cracked appearance of the surface and may be sufficient to maintain a liquid water or warm ice interior, insulated by a thin crust of solid ice.

How thick is the crust and how deep is the ocean/slush?

Estimating based on the size of the smallest plates (approximately 10 km in diameter), the frozen surface crust may be only a few kilometers thick. Europa has a bulk density of about 3.0 grams per cubic centimeter, 3 times the density of water, so it is believed to be primarily composed of silicates. However, this density still allows for an outer shell of water and/or ice up to 100 km thick surrounding the rocky interior.
The image above shows two views of Europa's trailing hemisphere, the one at left shows Europa in approximately real color, the view at right is a false-color image. Below is an image of a banded feature, Belus Linea, running through a dark 30 km diameter impact crater on Europa.

References

1) Galileo gazes at Jupiter and its moons, Vogel, Science, v. 273, p. 1048, 1996
2) Galileo finds evidence of a watery Europa, Cowen, Science News, v. 150, p. 102, 1996
3) Europa, Sky and Telescope, p. 32, March 1997
4) Galilean satellites: identification of water frost, Pilcher et al., Science, v. 178, p. 1087, 1972
5) Infrared spectra of the Galilean satellites of Jupiter, Fink et al., Astrophysics Journal, v. 179, p. L155, 1973

Water on Europa Press Release - 9 April 1997

Jovian Satellites Fact Sheet

Water on Europa Press Release - 13 August 1996

Galileo

More images of Europa - NSSDC Photo Gallery

Author/Curator:
Dr. David R. Williams, dave.williams@gsfc.nasa.gov
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
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