Sky & Telescope - November 2023 - 40

IMAGE DATA: NASA / JPL-CALTECH / SWRI / MSSS, IMAGE
PROCESSING: BJÖRN JÓNSSON / CC BY-NC-SA 2.0
Of Brines and Biology
Spectral Search
We usually locate salts on other worlds through remote
sensing. Spacecraft or Earth-based telescopes measure
the amount of reflected sunlight from the surface, and
then we look for the spectral fingerprints of various salts.
Collecting laboratory spectra of mixtures is also important,
because research shows that certain salts (such as
gypsum) can overwhelm the signal and make identification
of other salts tricky.
Salts are also impacted by the harsh radiation environment
on worlds with little to no atmosphere. With laboratory
experiments, we try to determine how salts, ice, and
their mixtures react to different types of radiation, and
how that changes their stability and spectral features.
SALT-ENCRUSTED WORLD This enhanced-color image from
NASA's Juno spacecraft shows the rugged, fractured surface
of Jupiter's moon Europa. Spectral observations have revealed
salts across the moon's surface, especially in jumbled, geologically
young areas called chaos regions.
interact with liquid water. Unfortunately, life as we know it
has a harder time utilizing a very salty brine. (Interestingly,
much of the work to understand life in very salty water conditions
has been done by the food industry, which wants to
find ways to inhibit microbial growth in foods.) Thus we have
this tension in the scientific community between wanting to
find liquid water and making sure that water is habitable.
In any case, we are very unlikely to find liquid water on
other worlds without salt. So perhaps our new strategy should
be " follow the salt? " Many current and future missions
plan to do just this. NASA's Perseverance mission is caching
samples on Mars that will eventually be returned to Earth.
The samples being collected are primarily those that indicate
interactions with liquid water in the past, which often
includes salts.
Then there are two spacecraft on their way to the Jupiter
system: ESA's Juice mission, arriving in 2031, and NASA's
Europa Clipper, launching in late 2024 and arriving in 2030.
Juice will investigate Europa while in Jupiter orbit before
eventually circling Ganymede - the largest moon in the solar
system, which also has a subsurface salty water ocean. Europa
Clipper will perform numerous close flybys of Europa with
enhanced cameras, spectrometers, and a magnetometer to
vastly increase our understanding of this fascinating moon
(S&T: Apr. 2022, p. 14).
Scientists also have their eyes on the other salty ice worlds
as they consider potential missions. Researchers have proposed
that NASA send a spacecraft to Enceladus to sample
40 NOVEMBER 2023 * SKY & TELESCOPE
its plumes in our effort to understand the internal ocean,
whether and how there is any interaction between the ocean
and the surface, and the ocean's potential habitability.
Another proposed mission would return samples from
Ceres, which would collect a small amount of exposed salt
and return it to Earth. The type of salt we've seen - primarily
sodium carbonate - means there is both carbon and oxygen
on the dwarf planet, a marker for habitability. Returning
samples to Earth is more cost-effective than in situ measurements
due to the low gravity and our close proximity to
Ceres. Laboratories will be able to use state-of-the-art instruments
to understand the composition of the samples and how
friendly Ceres might be to life.
All of these targets have been picked due to the astrobiological
implications from finding salts and water on them.
Hardy microorganisms have been found in places like Antarctica
and the Atacama Desert that can tolerate high salt
concentrations and/or cold temperatures and/or low water
amounts. But when we add all these extreme conditions
together, we don't know of any life as of yet that could thrive
on another planet. Work continues in the astrobiology community
to understand the limits of living organisms. For now,
we can better appreciate just how special Earth is - a blue
temperate world with a dash of salt.
¢ JENNIFER HANLEY is a planetary scientist at Lowell Observatory
in Flagstaff, Arizona. She loves exploring the outdoors,
especially with her family.
Sky & Telescope - November 2023

Table of Contents for the Digital Edition of Sky & Telescope - November 2023
