Europa Glows: Radiation Does a Bright Number on Jupiter's Moon
New lab experiments re-create the environment of Europa and find that the icy moon shines, even on its nightside. The effect is more than just a cool visual.
As
the icy, ocean-filled moon Europa orbits Jupiter, it withstands a relentless
pummeling of radiation. Jupiter zaps Europa's surface night and day with
electrons and other particles, bathing it in high-energy radiation. But as these
particles pound the moon's surface, they may also be doing something otherworldly:
making Europa glow in the dark.
New
research from scientists at NASA's Jet Propulsion Laboratory in Southern
California details for the first time what the glow would look like, and what it
could reveal about the composition of ice on Europa's surface. Different salty compounds
react differently to the radiation and emit their own unique glimmer. To the
naked eye, this glow would look sometimes slightly green, sometimes slightly
blue or white and with varying degrees of brightness, depending on what material
it is.
Scientists
use a spectrometer to separate the light into wavelengths and connect the distinct
"signatures," or spectra, to different compositions of ice. Most observations using a spectrometer on a moon like
Europa are taken using reflected sunlight on the moon's dayside, but these new
results illuminate what Europa would look like in the dark.
"We
were able to predict that this nightside ice glow could provide additional information
on Europa's surface composition. How that composition varies could give us
clues about whether Europa harbors conditions suitable for life," said JPL's
Murthy Gudipati, lead author of the work published Nov. 9 in Nature Astronomy.
That's
because Europa holds a massive, global interior ocean that could
percolate to the surface through the moon's thick crust of ice. By analyzing
the surface, scientists can learn more about what lies beneath.
Shining
a Light
Scientists
have inferred from prior observations that Europa's surface could be made of a
mix of ice and commonly known salts on Earth, such as magnesium sulfate (Epsom
salt) and sodium chloride (table salt). The new research shows that incorporating
those salts into water ice under Europa-like conditions and blasting it with
radiation produces a glow.
That
much was not a surprise. It's easy to imagine an irradiated surface glowing.
Scientists know the shine is caused by energetic electrons penetrating the
surface, energizing the molecules underneath. When those molecules relax, they
release energy as visible light.
"But
we never imagined that we would see what we ended up seeing," said JPL's Bryana
Henderson, who co-authored the research. "When we tried new ice compositions,
the glow looked different. And we all just stared at it for a while and then
said, 'This is new, right? This is definitely a different glow?' So we pointed
a spectrometer at it, and each type of ice had a different spectrum."
To study a laboratory mockup of Europa's surface, the JPL team
built a unique instrument called Ice Chamber for Europa's High-Energy Electron
and Radiation Environment Testing (ICE-HEART). They took ICE-HEART to a high-energy
electron beam facility in Gaithersburg, Maryland, and started the experiments
with an entirely different study in mind: to see how organic material under
Europa ice would react to blasts of radiation.
They
didn't expect to see variations in the glow itself tied to different ice compositions.
It was - as the authors called it - serendipity.
"Seeing
the sodium chloride brine with a significantly lower level of glow was the 'aha'
moment that changed the course of the research," said Fred Bateman,
co-author of the paper. He helped conduct the experiment and delivered
radiation beams to the ice samples at the Medical Industrial Radiation Facility
at the National Institute of Standards and Technology in Maryland.
A
moon that's visible in a dark sky may not seem unusual; we see our own Moon
because it reflects sunlight. But Europa's glow is caused by an entirely
different mechanism, the scientists said. Imagine a moon that glows continuously,
even on its nightside - the side facing away from the Sun.
"If Europa weren't under this radiation, it would look the
way our moon looks to us - dark on the shadowed side," Gudipati said. "But
because it's bombarded by the radiation from Jupiter, it glows in the dark."
Set
to launch in the mid-2020s, NASA's upcoming flagship mission Europa Clipper will observe the moon's
surface in multiple flybys while orbiting Jupiter. Mission scientists are
reviewing the authors' findings to evaluate if a glow would be detectable by
the spacecraft's science instruments. It's possible that information gathered by
the spacecraft could be matched with the measurements in the new research to
identify the salty components on the moon's surface or narrow down what they
might be.
"It's
not often that you're in a lab and say, 'We might find this when we get there,'"
Gudipati said. "Usually it's the other way around - you go there and find
something and try to explain it in the lab. But our prediction goes back to a
simple observation, and that's what science is about."
Missions such
as Europa Clipper help contribute to the field of astrobiology, the interdisciplinary research on the variables
and conditions of distant worlds that could harbor life as we know it. While
Europa Clipper is not a life-detection mission, it will conduct detailed
reconnaissance of Europa and investigate whether the icy moon, with its
subsurface ocean, has the capability to support life. Understanding
Europa's habitability will help scientists better understand how life developed
on Earth and the potential for finding life beyond our planet.
More
information about Europa and Europa Clipper can be found here:
europa.nasa.gov
|
