In the same way that earthquakes cause our planet to rumble, oscillations in
the interior of Saturn make the gas giant jiggle around ever so slightly.
Those motions, in turn, cause ripples in Saturn's rings.
In a new study accepted in the journal Nature Astronomy, two Caltech
astronomers have analyzed those rippling rings to reveal new information
about the core of Saturn. For their study, they used older data captured by
NASA's Cassini, a spacecraft that orbited the ringed giant for 13 years
before it dove into the planet's atmosphere and disintegrated in 2017.
The findings suggest that the planet's core is not a hard ball of rock, as
some previous theories had proposed, but a diffuse soup of ice, rock, and
metallic fluids—or what the scientists refer to as a "fuzzy" core. The
analysis also reveals that the core extends across 60 percent of the
planet's diameter, which makes it substantially larger than previously
estimated.
"We used Saturn's rings like a giant seismograph to measure oscillations
inside the planet," says co-author Jim Fuller, assistant professor of
theoretical astrophysics at Caltech. "This is the first time we've been able
to seismically probe the structure of a gas giant planet, and the results
were pretty surprising."
"The detailed analysis of Saturn's rippling rings is a very elegant form of
seismology to infer the characteristics of Saturn's core," says Jennifer
Jackson, the William E. Leonhard Professor of Mineral Physics in the
Seismological Laboratory at Caltech, who was not involved in the study but
uses different types of seismic observations to understand the composition
of Earth's core and to potentially detect seismic events on Venus in the
future.
The lead author of the study is Christopher Mankovich, a postdoctoral
scholar research associate in planetary science who works in Fuller's group.
The findings offer the best evidence yet for Saturn's fuzzy core and line up
with recent evidence from NASA's Juno mission, which indicates that the gas
giant Jupiter may also have a similarly diluted core.
"The fuzzy cores are like a sludge," explains Mankovich. "The hydrogen and
helium gas in the planet gradually mix with more and more ice and rock as
you move toward the planet's center. It's a bit like parts of Earth's oceans
where the saltiness increases as you get to deeper and deeper levels,
creating a stable configuration."
The idea that Saturn's oscillations could make waves in its rings and that
the rings could thus be used as a seismograph to study Saturn's interior
first came about in studies in the early 1990s by Mark Marley (BS '84) and
Carolyn Porco (PhD '83), who later became the leader of the Cassini Imaging
Team. The first observation of the phenomenon was made by Matt Hedman and
P.D. Nicholson (PhD '79) in 2013, who analyzed data taken by Cassini. The
astronomers found that Saturn's C-ring contained multiple spiral patterns
driven by fluctuations in Saturn's gravitational field and that these
patterns were distinct from other waves in the rings caused by gravitational
interactions with the planet's moons.
Now, Mankovich and Fuller have analyzed the pattern of waves in the rings to
build new models of Saturn's sloshing interior.
"Saturn is always quaking, but it's subtle," says Mankovich. "The planet's
surface moves about a meter every one to two hours like a slowly rippling
lake. Like a seismograph, the rings pick up the gravity disturbances, and
the ring particles start to wiggle around," he says.
The researchers say that the observed gravitational ripples indicate that
the deep interior of Saturn, while sloshing around as a whole, is composed
of stable layers that formed after heavier materials sunk to the middle of
the planet and stopped mixing with lighter materials above them.
"In order for the planet's gravitational field to be oscillating with these
particular frequencies, the interior must be stable, and that's only
possible if the fraction of ice and rock gradually increases as you go in
toward the planet's center," says Fuller.
Their results also indicate that the core of Saturn is 55 times as massive
as the entire Earth, with 17 Earth-masses of that being ice and rock and the
rest a fluid of hydrogen and helium.
Hedman, who is not part of the current study, says, "Christopher and Jim
were able to show that one particular ring feature provided strong evidence
that Saturn's core is extremely diffuse. I am excited to think about what
all the other ring features generated by Saturn might be able to tell us
about that planet."
In addition, the findings pose challenges to current models of gas giant
formation, which hold that rocky cores form first and then attract large
envelopes of gas. If the cores of the planets are indeed fuzzy as the study
indicates, the planets might instead incorporate gas earlier in the process.
Reference:
A diffuse core in Saturn revealed by ring seismology by Christopher R.
Mankovich and Jim Fuller, 16 August 2021, Nature Astronomy.
Tags:
Space & Astrophysics