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though, creating the jumbled surface of
broken ice blocks astronomers see in
images. Such heating could also generate
the geysers blasting from the surface.
In 1989, NASA launched its Galileo
mission to study Jupiter and the four
Galilean moons — Io, Europa, Ganymede,
and Callisto — in greater detail. With 12
close f lybys, the mission took new mea-
surements that increased scientists’ cer-
tainty of a liquid ocean on Europa.
Perhaps the most conclusive data from
Galileo were the magnetic field measure-
ments. As the spacecraft approached
Europa, it observed a slight “bend” in
Jupiter’s magnetic field, indicating that a
second magnetic field is being created, or
induced, within the moon. The most likely
cause, researchers believe, is the circula-
tion of an electrically conductive global
saltwater ocean beneath the surface.Microbial energy bars
In addition to water, life needs energy.
Most life on Earth derives its energy
from the Sun. Plants use the Sun’s energy
directly, while we — and other animals
— use the products they create. However,
Europa’s harsh conditions likely relegate
life below the surface, where the distant
Sun doesn’t shine.
On Earth, microbes can live near
deep-sea vents where warm, chemical-
rich material bubbles up. Although no
hard evidence yet exists, some scientists
suspect that Europa’s tidal heatingcreates volcanoes and hydrothermal
vents on the ocean f loor, just as tectonic
activity does on Earth. Providing more
than just a heat source, any volcanoes or
vents would also offer an important
source of nutrients. The heat and activity
in the interior would drive chemical
reactions and bring up new material into
the ocean. If Europa’s interior is highly
active, there could be a large exchange of
material, which would provide a steady
f low of nutrients and even the chemical
building blocks for life. Thus, determin-
ing just how active Europa is remains a
key question for scientists investigating
the moon’s potential habitability.
Above the surface, intense radiation
from Jupiter also helps break down mol-
ecules. These chemical bits and pieces
can then reform to create new com-
pounds that could also be useful to
microbial life. Open fissures on the sur-
face might allow these compounds to
eventually circulate below.Gathering intel
Finding hard evidence of life is difficult
— particularly on Europa, where it
would likely lie under layers of ice.THESE IMAGES OF EUROPA target a warm region (enclosed by the blue oval) spotted by Galileo
and identified as the location where plumes erupted in 2014 and 2016. At right, color indicates temperature
in kelvins, with the warmest areas in red. The presence of the plumes and their eruption from a warm area
on the surface support the idea that the moon hosts a relatively warm, liquid subsurface ocean. NASA/ESA/
W. SPARKS (STSCI)/USGS ASTROGEOLOGY SCIENCE CENTER
IN 2014 AND 2016, the Hubble Space Telescope spotted plumes spouting from Europa’s surface,
visible in infrared wavelengths. The 2014 plume (left) was estimated to reach about 30 miles (50 km) above
the surface; in 2016, a plume rose from the same location to about 62 miles (100 km). The detailed images
of the moon are composites of Galileo photos, superimposed over the Hubble data. NASA/ESA/W. SPARKS (STSCI)/USGS
ASTROGEOLOGY SCIENCE CENTER
JUPITER FILLS
THE SKY as seen
from Europa’s surface
in this artist’s concept,
which shows the moon’s
rough, white-and-red
terrain in the foreground
and the Sun in the far
background. NASA/JPL-CALTECH