12 ASTRONOMY • MARCH 2020
QUANTUM GRAVITY
Jupiter’s moon Europa has an icy
shell that astronomers have long
thought conceals a liquid water ocean
beneath. Now, scientists have finally made
the first direct measurement of water
vapor trapped in Europa’s atmosphere,
which serves as the most compelling evi-
dence yet that water plumes are indeed
erupting from the icy moon’s surface.
The new measurements also imply
that, outside of plume events, Europa’s
atmosphere likely has less water vapor
overall than previously thought. The
findings were published November 18
in Nature Astronomy.
Scientists have known since the 1960s
that Europa is likely home to water ice
and a liquid water ocean beneath its sur-
face. They even predicted that radiation
from Jupiter would bombard the moon’s
icy surface, thereby releasing water vaporinto its atmosphere. Previous observa-
tions have spotted plumes erupting
into the moon’s atmosphere; however,
researchers have been unable to confirm
whether they contain water vapor. In
the new study, researchers used Keck
Observatory to observe the moon on 17
different dates from February 2016 to
May 2017, searching for specific infrared
wavelengths of light emitted by water
vapor. On one night, April 26, 2016, they
saw it when their instruments measured
a surprisingly large amount or water
vapor — roughly 2,000 metric tons.
Although the researchers failed to
find signs of water vapor on the other 16
nights, that doesn’t mean it wasn’t hiding
there all along. In fact, the team believes
some water vapor exists in Europa’s
atmosphere all the time due to Jupiter’s
radiation. So, the only reason they didn’tconsistently spot it, they said, is that the
water vapor is probably usually just too
scarce for instruments to detect.
Overall, the latest measurements
imply that the typical amount of
water vapor in Europa’s atmosphere
at any given time is probably less than
previously thought. This means that the
standout measurement in April 2016
likely came from a one-time event, like
a plume of water erupting from the
moon’s surface. Fortunately, upcoming
space missions like Europa Clipper and
the JUpiter ICy moons Explorer, or
JUICE, will take a much closer look at
Jupiter’s smallest Galilean moon.
“I’m really looking forward to
follow-up studies of Europa and other
ocean worlds,” Lucas Paganini, a NASA
planetary scientist and one of the
recent paper’s authors, tells Astronomy.
“It has been difficult to detect water in
liquid form. These detections of water
in vapor form, I think, [are] the closest
thing we have in the search for liquid
water environments.” — E.K.C., J.P.Water vapor detected in
Europa’s atmosphere
WHERE THERE’S WATER. The suspected
global ocean of liquid water beneath the
surface of Jupiter’s moon Europa is one
place humans plan to search for life. This
artist’s concept shows a massive plume of
water erupting from the moon’s surface.
NASA/ESA/K. RETHERFORD/SWRI