The work is based on new modeling and explores how oceans could exist in unlikely places in our solar system.
Uranus is surrounded by its four major rings and 10 of its 27 known moons in this color-added view that uses data taken by the Hubble Space Telescope in 1998. A study featuring new modeling shows that four of Uranus’ large moons likely contain internal oceans. Credits: NASA/JPL/STScI
Re-analysis of data
from NASA’s Voyager spacecraft, along with new computer modeling, has led NASA
scientists to conclude that four of Uranus’ largest moons likely contain an
ocean layer between their cores and icy crusts. Their study is the first to
detail the evolution of the interior makeup and structure of all five large
moons: Ariel, Umbriel, Titania, Oberon, and Miranda. The work suggests four of
the moons hold oceans that could be dozens of miles deep.
In all, at least 27 moons circle Uranus, with the four largest
ranging from Ariel, at 720 miles (1,160 kilometers) across, to Titania, which
is 980 miles (1,580 kilometers) across. Scientists have long thought that
Titania, given its size, would be most likely to retain internal heat, caused
by radioactive decay. The other moons had previously been widely considered too
small to retain the heat necessary to keep an internal ocean from freezing,
especially because heating created by the gravitational pull of Uranus is only
a minor source of heat.
The National Academies’ 2023 Planetary
Science and Astrobiology Decadal Survey prioritized exploring Uranus.
In preparation for such a mission, planetary scientists are focusing on the ice
giant to bolster their knowledge about the mysterious Uranus system. Published
in the Journal of
Geophysical Research, the
new work could inform how a future mission might investigate the moons, but the
paper also has implications that go beyond Uranus, said lead author Julie
Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Southern California.
“When it comes to small bodies – dwarf
planets and moons – planetary scientists previously have found evidence of
oceans in several unlikely places, including the dwarf planets Ceres and Pluto,
and Saturn’s moon Mimas,” she said. “So there are mechanisms at play that we
don’t fully understand. This paper investigates what those could be and how
they are relevant to the many bodies in the solar system that could be rich in
water but have limited internal heat.”
The study revisited findings from NASA’s
Voyager 2 flybys of
Uranus in the 1980s and
from ground-based observations. The authors built computer models infused with
additional findings from NASA’s Galileo, Cassini, Dawn, and New Horizons (each
of which discovered ocean worlds), including insights into the chemistry and
the geology of Saturn’s moon Enceladus, Pluto and its moon Charon, and Ceres –
all icy bodies around the same size as the Uranian moons.
New modeling shows that there likely is an ocean layer in four of Uranus’ major moons: Ariel, Umbriel, Titania, and Oberon. Salty – or briny – oceans lie under the ice and atop layers of water-rich rock and dry rock. Miranda is too small to retain enough heat for an ocean layer. Credits: NASA/JPL-Caltech
What
Lies Above and Beneath
The researchers used that modeling to
gauge how porous the Uranian moons’ surfaces are, finding that they’re likely
insulated enough to retain the internal heat that would be needed to host an
ocean. In addition, they found what could be a potential heat source in the
moons’ rocky mantles, which release hot liquid, and would help an ocean
maintain a warm environment – a scenario that is especially likely for Titania
and Oberon, where the oceans may even be warm enough to potentially support
habitability.
By investigating the composition of the
oceans, scientists can learn about materials that might be found on the moons’
icy surfaces as well, depending on whether substances underneath were pushed up
from below by geological activity. There is evidence from
telescopes that
at least one of the moons, Ariel, has material that flowed onto its surface,
perhaps from icy volcanoes, relatively recently.
In fact, Miranda, the innermost and fifth
largest moon, also hosts surface features that appear to be of recent origin,
suggesting it may have held enough heat to maintain an ocean at some point. The
recent thermal modeling found that Miranda is unlikely to have hosted water for
long: It loses heat too quickly and is probably frozen now.
But internal heat wouldn’t be the only factor
contributing to a moon’s subsurface ocean. A key finding in the study suggests
that chlorides, as well as ammonia, are likely abundant in the oceans of the
icy giant’s largest moons. Ammonia has been long known to act as antifreeze. In
addition, the modeling suggests that salts likely present in the water would be
another source of antifreeze, maintaining the bodies’ internal oceans.
Of course, there still are a lot of
questions about the large moons of Uranus, Castillo-Rogez said, adding that
there is plenty more work to be done: “We need to develop new models for
different assumptions on the origin of the moons in order to guide planning for
future observations.”
Digging into what lies beneath and on the
surfaces of these moons will help scientists and engineers choose the best
science instruments to survey them. For instance, determining that ammonia and
chlorides may be present means that spectrometers, which detect compounds by
their reflected light, would need to use a wavelength range that covers both
kinds of compounds.
Likewise, they can use that knowledge to design instruments that can probe the deep interior for liquid. Searching for electrical currents that contribute to a moon’s magnetic field is generally the best way to find a deep ocean, as Galileo mission scientists did at Jupiter’s moon Europa. However, the cold water in the interior oceans of moons such as Ariel and Umbriel could make the oceans less able to carry these electrical currents and would present a new kind of challenge for scientists working to figure out what lies beneath.
Source: New Study of Uranus’ Large Moons
Shows 4 May Hold Water | NASA
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