Europe is the fourth largest of Jupiter's 79 moons. It is
the sixth largest moon in the solar system, just after
Earth's own moon. Europe is the most likely place to find
liquid water in the solar system except here on Earth. The
surface is covered by a several-kilometer-thick layer of
water, and there is probably an even thicker layer of liquid
salt water under the ice.
Europe has a diameter of 3121 km and an average mass
density of 3.01 g/cm 3, which is about 90% of
the diameter and density of our own moon. The structure of
Europe is believed to be similar to that of the Earth.
Abbreviationfinder, Europe has a metallic core surrounded by a mantle of
silicate rocks. The surface is covered by a layer of 15 to
25 km thick water. Among other things, measurements of how
the magnetic field of Jupiter is changing around Europe, it
is estimated that there is a 60 to 150 km thick floating
layer of salt water under the ice, covering the entire moon.
Europe is so far away from the Sun that the surface
temperature is very low, below –160 °C, which means that
the water under the ice should also be frozen. But the seas
of Europe are probably kept afloat due to heat generated by
the tidal forces of Jupiter and made with ice insulating
from above. In addition, there are potential radioactive
elements in the core of Europe that can heat the sea from
the inside through deep-sea valves, similar to the hot
springs we have on the seabed here on Earth.
Because Europe's underground ocean is so deep, it can be
up to twice as much water in Europe as all the water we have
here on Earth, even though the diameter of Europe is only a
quarter of Earth's diameter.
Europe walks in an almost circular orbit around Jupiter
with eccentricity of 0.0094, average distance of 670 900 km
and average orbit speed of 13.7 km/s. Europe has a fixed
rotation, which means that the rotation period is equal to
the turnaround time of 3.55 days around Jupiter. This means
that the same side of Europe always points towards Jupiter,
just as our moon always faces the same side towards Earth.
The orbits of the three Jupiter moons Io, Europa and
Ganymedes are locked to each other in such a way that their
turnaround times are in a 1: 2: 4 ratio (triple resonance).
Together with Jupiter's tidal forces, this contributes to
tensions that create Io's volcanism and make Europe's
surface burst. Because Jupiter and Europe have coherent
orbits and rotational axes that are almost normal on the
orbit, Europe has almost no seasonal differences during the
12 years Jupiter uses in its orbit of the Sun.
Jupiter itself orbits the Sun with an average distance of
780 million km, 5.2 times longer than the distance between
the Sun and the Earth. Thus, it takes about 42 minutes for
light to travel from the Sun all the way to Jupiter and
Europe, and the light level is about 1/25 of the light level
Exploration and surface
Europe, together with the three neighboring moons Io,
Ganymede and Callisto, was discovered by Galileo Galilei in
1610 when he aimed his homemade telescope at the sky. This
was the first time objects were observed in orbit around
celestial bodies other than the Earth, which in the
prevailing geocentric worldview at that time was believed to
be the center of the universe. Galileo's observation of
Jupiter's moons was an important argument for Nicolaus
Copernicus ' heliocentric model, and helped launch the known
conflict between Galileo Galilei and the Catholic Church.
Along with the neighboring moons, Europe was imaged up
close in 1979 by the two American Voyager probes. During the
period 1995–2000, the moons have been mapped with a
resolution of at least one km from the Galileo space probe.
Europe's surface appears as a deserted, reddish-brown ice
surface, intersected by a complicated network of cracks and
grooves that stretch for hundreds of miles. The color
probably comes from salts and sulfur compounds that have
been mixed into the ice and modified by high-energy Jupiter
radiation. The complicated structure is probably due to the
cracking of the ice mainly due to tidal forces from Jupiter,
similar to plate tectonic movements here on Earth.
The surface of Europe is considered very young in a
geological context. Europe has very few craters, which
indicate that the surface is no more than 40 to 90 million
years old. This is very young compared to Earth's own moon,
which is clearly full of craters, and is estimated to be 4.5
billion years old. One way to understand this is that the
surface must be constantly changing, so that older craters
are smoothed and erased by the movement of the ice. Without
mountains and notable craters, the surface of Europe is the
smoothest surface of all known bodies in the solar system.
Because of the icy surface of Europe, the moon is very
reflective. With an albedo of 0.64, it reflects about 5.5
times as much light as the surface of our own moon does. So
even though the light level is much lower at Jupiter, Europe
will be relatively well visible in the night sky for a
hypothetical observer standing on Jupiter.
Potential for life
Life, at least as we know it, requires at least three
main ingredients: liquid water, appropriate chemical
elements and an energy source. Here on Earth we have
discovered closed, independent ecosystems in extreme
conditions at the bottom of the ocean. These do not depend
on the energy of the Sun, but survive on the energy of hot
deep-sea valves. In Europe, there are precisely strong
indications of large volumes of liquid water, with potential
deep-sea valves, salt and sulfur compounds. Thus, Europe is
often regarded as the best candidate we have for finding
extraterrestrial life in the solar system. The icy moon
appears to have the necessary building blocks for life to
occur, and it is also close enough to Earth that we can
examine it more easily with both telescopes and space