Astronomy – September 2019

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2016

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32 ASTRONOMY • SEPTEMBER 2019

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 heating

creates 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