186 Encyclopedia of the Solar System
out in the solar system, we know that Europa, a satellite
of Jupiter, has a smooth icy surface with cracks and flow
features that resemble Earth’s polar ice fields and suggest
a liquid-water interior, while its larger sibling, Ganymede,
exhibits a conductive reaction to Jupiter’s magnetic field
that is most easily explained by a salty liquid-water inte-
rior [SeeMars: Surface and Interior;Meteorites;
Planetary Satellites.]
However, to date we have no direct evidence for ex-
traterrestrial life. This includes data from landers on Venus,
Mars, and the Moon, and flyby encounters with 8 planets, a
handful of asteroids, a comet (Halley in 1986), and over 60
moons. Are the interplanetary spacecraft we have sent out
capable of fulfilling the goal of detecting life? This question
has been tested by analyzing data from theGalileospace-
craft’s two flyby encounters with Earth, which, along with a
flyby encounter with Venus, were used by the spacecraft’s
navigation team to provide gravity assists to sendGalileo
to Jupiter. The idea was to compare ground-truth infor-
mation to what we can learn solely from Galileo. [SeeAt-
mospheres of the Giant Planets;Io: The Volcanic
Moon;Planetary Satellites.]
Galileo’s first Earth encounter occurred on December
8, 1990, with closest approach 960 km above the Caribbean
Sea; its second Earth encounter occurred on December
8, 1992, with closest approach 302 km above the South
Atlantic. A total of almost 6000 images were taken of Earth
byGalileo’s camera system. Figure 5 shows the Earth–
Moon system as seen byGalileo. Notice that the Moon
is significantly darker than Earth. The spacecraft’s instru-
ments were designed and optimized for Jupiter; neverthe-
less, they made several important observations that point
to life on Earth. These strengthen the null results encoun-
tered elsewhere in the solar system. The evidence for life
on Earth includes complex radio emissions, nonmineral
surface pigmentation, disequilibrium atmospheric chem-
istry, and large oceans.6.1.1 RADIO EMISSIONS
The only clear evidence obtained byGalileofor intelligent
life on Earth was unusual radio emissions. Several natural
radio emissions were detected, none of which were unusual,
including solar radio bursts, auroral kilometric radiation,
and narrowband electrostatic oscillations excited by thermal
fluctuations in Earth’s ionospheric plasma. The first unusual
radio emissions were detected at 1800 UT and extended
through 2025 UT, just before closest approach. These were
detected by the plasma wave spectrometer (PWS) on the
nightside, in-bound pass, but not on the day side, out-bound
pass. The signal strength increased rapidly as Earth was
approached, implying that Earth itself was the source of
the emissions. The fact that the signals died off on the day
side suggests that they were cut off by the day side iono-
sphere, which means we can place the source below the
ionosphere.
The unusual signals were narrowband emissions that oc-
curred in only a few distinct channels and had average fre-
quencies that remained stable for hours. Naturally occur-
ring radio emissions nearly always drift in frequency, butFIGURE 5 The Earth–Moon
system as observed by theGalileo
spacecraft.