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https://astronomy.com/news/2022/11/saturns-moon-enceladus-may-be-more-habitable-than-thought

Saturn's moon Enceladus may be more habitable than thought

New research suggests the subsurface soda ocean on Saturn's moon could
increase the availability of phosphorus — a key ingredient for life as
we know it.
By Sharmila Kuthunur | Published: Thursday, November 10, 2022
RELATED TOPICS: ENCELADUS | ALIEN LIFE
Enceladusgeysers
This artist’s concept shows the plumes of ice particles erupting from
fractures in the south polar region of Saturn’s moon Enceladus.
NASA/JPL-Caltech.
In our solar system, Saturn’s moon Enceladus is unique: It provides
scientists direct access to a global ocean hidden beneath the moon’s icy
surface. That’s because Enceladus is constantly venting its subsurface
water out into space.

Numerous geysers spew water vapor hundreds of miles into space through
cracks that seem clawed into the moon’s south polar region. During its
flybys, the Cassini spacecraft “tasted” some of the plumes, and its
analysis indicated the water may have come from a carbonate-rich
subsurface ocean.

A few of the plume’s contents — salty ice particles, ammonia, carbon
monoxide, and other organic materials — strengthened the case for the
potential habitability of Enceladus’ ocean. However, Cassini’s analysis
did not reveal phosphorus, a key ingredient for life.

But now, a group of researchers have, for the first time, estimated the
presence of a form of phosphorus called orthophosphate, which Earth life
uses in many contexts. The researchers also found the phosphorus is both
highly soluble and stable in Enceladus’s salty subsurface ocean.

The researchers published their findings Sept. 19 in the Proceedings of
the National Academy of Sciences.

Evidence for phosphorus on Enceladus
Enceladusplumes
Enceladus’s plumes are seen erupting from the famous tiger stripes near
the moon’s south pole in this image captured by Cassini. The
spacecraft’s survey of Enceladus ultimately revealed more than 100 geysers.
NASA
The study’s authors performed geochemical modeling, constrained by
Cassini data, to predict how much phosphorus could be present in
Enceladus’ ocean. They found that the ocean water should be rather rich
in dissolved phosphorus, lending credence to the notion that the moon’s
subsurface ocean could be a habitable environment.

“The most interesting and promising aspect of this work is that the
abundance of phosphorus may be quite high in the ocean of Enceladus —
anywhere from 0.1 to 1000 times that of Earth's oceans,” Manasvi Lingam,
an assistant professor of astrobiology at Florida Institute of
Technology who was not involved in the study, told Astronomy. “If so,
the case for habitability (and life) on Enceladus gets stronger than
before.”

The chemical reactions that generate the “energy currency” in cells and
build RNA and DNA are crucial to the functioning of every living
organism. And while these reactions require phosphorus, the mineral is
relatively scarce in natural waters because it reacts with cations
(positively charged atoms or molecules) such as calcium to form calcium
phosphate. That reaction makes the phosphorus inaccessible for use by
potential life.

The key to abundant availability of dissolved phosphorus in Enceladus’s
ocean is that it “has a ‘soda’ ocean that is rich in sodium bicarbonate
(baking soda),” Christopher Glein, a planetary scientist and geochemist
at the Southwest Research Institute and one of the study’s authors, told
Astronomy.

Enceladusocean
Based on Cassini data, this illustration shows how researchers think
water might interact with rock at the bottom of Enceladus’ subsurface ocean.
NASA/JPL-Caltech/Southwest Research Institute
Soda lakes on Enceladus
Commonly known as soda lakes or oceans, carbonate-rich waters are great
at letting phosphorus accumulate. That’s because carbonates are more
prone to bond with calcium or other cations, forming carbonate or
phyllosilicate minerals and leaving phosphorus available for life.

This geophysical phenomenon is seen on Earth, too, where soda lakes such
as the Mono Lake in California have 50,000 times higher phosphorus
levels than other waters. That abundant phosphorus supports complex
ecosystems, including several species of fish, algae, and fungi.

“[Phosphorus] serves as a critical medium between solid Earth and its
biosphere, as it is widely thought to control the level of biological
production across a geological timescale,” Jihua Hao, a senior research
scientist in China and one of the study’s authors, told Astronomy.

As to where the phosphorus on Enceladus came from, carbonaceous
chondrite meteorites are a likely suspect. These remnant rocks date back
to the formation of the outer solar system.

Our Sun has “an unusually high amount of [phosphorus] compared with
nearby stars,” planetary astrophysicist Natalie Hinkel told Astronomy,
“which has been beneficial for life on Earth.” And not surprisingly,
stars, planets, moons, and meteoroids from the same stellar cloud often
have similar compositions.

“If you look at how much [phosphorus] is in the most primitive
carbonaceous chondrites and how much rock we think is inside Enceladus,”
Glein told Astronomy, “the conclusion is that Enceladus should easily
contain enough phosphorus feedstock to generate a relatively
phosphorus-rich ocean.”

PhosphorusEnceladus
The layers of Enceladus.
Southwest Research Institute
The habitability of Enceladus’ subsurface ocean
As important as phosphorus is, it’s only one piece of a fascinating
puzzle when it comes to the habitability of Enceladus.

Sulfur, for example, is another important ingredient for life as we know
it. But scientists are currently unsure how much sulfur is in Enceladus’
ocean, as well as the chemical forms it may take. The biggest obstacle
in finding out, according to Glein, is limited data.

“We have a wonderful data set from Cassini, and folks are still
searching for treasures in those data,” he says. “I think we still have
some exciting discoveries that await us. In the long term, we are going
to need more data to really understand Enceladus as a geochemical system
— and, of course, to test for the presence of life.”

When the ocean trapped underneath Enceladus’ icy surface was first
discovered, it added an intriguing option to the list of potential
habitable environments where life could exist. Future missions would
undoubtedly benefit from visiting the moon’s south polar region, where
the plumes bursting through the surface provide unfiltered access to the
subsurface ocean.

And fortunately, such a mission is already in the works: NASA’s
Enceladus Orbilander. Currently a flagship mission concept with a stated
launch in the late 2030s, Orbilander will focus on hunting for signs of
life on Enceladus from orbit — at least at first.

According to NASA, Orbilander is eventually expected to land near the
moon’s south pole shortly before 2052. Once there, it would scoop snow
from the plumes that fell back to the surface. And testing that snow
might just provide the first direct evidence of phosphorus on Enceladus.

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