Earth as a Planet: Atmosphere and Oceans 187
these emissions were steady. The individual components
had complicated modulations in their amplitude that have
never been detected in naturally occurring emissions. The
simplest explanation is that these signals were transmitting
information, which implies that there is advanced techno-
logical life on Earth. In fact, the radio, radar, and television
transmissions that have been emanating from Earth over the
last century result in a nonthermal radio emission spectrum
that broadcasts our presence out to interstellar distances.
[SeeThe Solar System at Radio Wavelengths.]
6.1.2 SURFACE FEATURES
During its first encounter with Earth, the highest-resolution
mapping of the surface byGalileo’s Solid-State Imaging Sys-
tem (SSI) covered Australia and Antarctica with 1–2 km
resolution. No usable images were obtained from Earth’s
night side on the first encounter. The second encounter
netted the highest resolution images overall of Earth by
Galileo, 0.3–0.5 km per pixel, covering parts of Chile, Peru,
and Bolivia. The map of Australia from the first encounter
includes 2.3% of Earth’s total surface area, but shows no
geometric patterns that might indicate an advanced civiliza-
tion. In the second encounter, both the cities of Melbourne
and Adelaide were photographed, and yet no geometric
evidence is visible because the image resolution is only
2 km. The map of Antarctica, 4% of Earth’s surface, reveals
nearly complete ice cover and no signs of life. Only one
image, taken of southeastern Australia during the second
encounter, shows east–west and north–south markings that
would raise suspicions of intelligent activity. The markings
in fact were caused by boundaries between wilderness ar-
eas, grazing lands, and the border between South Australia
and Victoria. Studies have shown that it takes nearly com-
plete mapping of the surface at 0.1-km resolution to obtain
convincing photographic evidence of an advanced civiliza-
tion on Earth, such as roads, buildings, and evidence of
agriculture.
On the other hand, many features are visible in the
Galileoimages that have not been seen on any other body in
the solar system. The SSI camera took images in six differ-
ent wavelength channels. A natural-color view of Earth was
constructed using the red, green, and violet filters, which
correspond to wavelengths of 0.670, 0.558, and 0.407μm,
respectively. The images reveal that Earth’s surface is cov-
ered by enormous blue expanses that specularly reflect sun-
light, and end in distinct coastlines, which are both easiest
to explain if the surface is liquid. This implies that much of
the planet is covered with oceans. The land surfaces show
strong color contrasts that range from light brown to dark
green.
The SSI camera has particular narrowband infrared fil-
ters that have never been used to photograph Earth be-
fore, and so they yielded new information for geological,
biological, and meteorological investigations. The infrared
filters allow the discrimination of H 2 O in its solid, liquid,
and gaseous forms; for example, clouds and surface snow
can be distinguished spectroscopically with the 1μm fil-
ter. False-color images made by combining the 1μm chan-
nel with the red and green channels reveal that Antarctica
strongly absorbs 1μm light, establishing that it is covered by
water ice. In contrast, large regions of land strongly reflect
1 μm without strongly reflecting visible colors, which con-
flicts with our experience from other planetary surfaces and
is not typical of igneous or sedimentary rocks or soil. Spec-
tra made with the 0.73 and 0.76μm channels reveal several
land areas that strongly absorb red light, which again is not
consistent with rocks or soil. The simplest explanation is that
some nonmineral pigment that efficiently absorbs red light
has proliferated over the planet’s surface. It is hard to say
for certain if an interstellar explorer would realize that this
is a biological mechanism for gathering energy from sun-
light, probably so, but certainly we would recognize it on
another planet as the signature of plant life. We know from
ground truth that these unusual observations are caused
by the green pigments chlorophyll a (C 55 H 72 MgN 4 O 5 ) and
chlorophyll b (C 55 H 70 MgN 4 O 6 ), which are used by plants
for photosynthesis. No other body in the solar system has
the green and blue colorations seen on Earth. [SeeThe So-
lar System at Ultraviolet Wavelengths;Infrared
Views of the Solar System from Space.]6.1.3 OXYGEN AND METHANE
Galileo’s Near-Infrared Mapping Spectrometer (NIMS)
detected the presence of molecular oxygen (O 2 ) in Earth’s
atmosphere with a volume mixing ratio of 0.19±0.05.
Therefore, we know that the atmosphere is strongly oxidiz-
ing. (It is interesting to note that Earth is the only planet in
the solar system where one can light a fire.) In light of this,
it is significant that NIMS also detected methane (CH 4 )
with a volume mixing ratio of 3±1.5× 10 −^6. Because
CH 4 oxidizes rapidly into H 2 O and CO 2 , if thermodynam-
ical equilibrium holds, then there should be no detectable
CH 4 in Earth’s atmosphere. The discrepancy between ob-
servations and the thermodynamic equilibrium hypothesis,
which works well on other planets (e.g., Venus), is an ex-
treme 140 orders of magnitude. This fact provides evidence
that Earth has biological activity and that it is based on or-
ganic chemistry. We know from ground truth that Earth’s
atmospheric methane is biological in origin, with about half
of it coming from nonhuman activity like methane bacteria
and the other half coming from human activity like grow-
ing rice, burning fossil fuels, and keeping livestock. NIMS
also detected a large excess of nitrous oxide (N 2 O) that is
most easily explained by biological activity, which we know
from ground truth comes from nitrogen-fixing bacteria and
algae.
The conclusion is that the interplanetary spacecraft we
have sent out to explore our solar system are capable of