Seismic waves in Saturn’s rings reveal the strange ‘fuzzy core’ interior of the planet within.
NASA’s Cassini spacecraft continues to uncover amazing facts about the ringed planet Saturn. A recent study in the August edition of Nature: Astronomy highlighted an intriguing method to indirectly probe the interior of the planet.
Typically, missions to the gas giant planets Jupiter and Saturn rely on direct measurements of gravity fields during flybys and orbits to characterize planetary interiors. This method, however, has a chief drawback, as it doesn’t describe whether the planet has a small, solid rocky or large liquid core.
For the recent study, astronomers took a different approach. Specifically, the Cassini mission looked at stellar occultations of distant stars passing behind the rings of Saturn. These observations were carried out in the final days of the mission known as the risky Grand Finale ring plane-crossing phase, done before the final destructive disposal of the spacecraft in the atmosphere of Saturn on September 15th, 2017.
The final orbits for Cassini during the Grand Finale phase for the mission, during which astronomers took advantage of radio occultations of the spacecraft as seen from Earth, and visual occultations of stars from behind the ring particles. Credit: NASA/JPL-Caltech
During the occultations, Cassini observed very low frequency spiral waves traversing the inner C ring. The ring system itself is complex: thought to be part of a shredded moon, the major moons of Saturn pull and braid the outer ring system, while tiny shepherd moons are seen carving out paths in the rings, like the groves in a record. But it’s the planet itself that tugs on the inner sections such as the C-ring.
Astronomers soon realized that it was the surface of Saturn itself that was quivering to the tune of a meter every few hours, exerting tidal forces rippling through the ring system.
A Fuzzy Core
And what’s more, this oscillation says something about what’s going on inside Saturn itself. The best fit model suggests that rather than having a rocky distinct core, Saturn has a ‘fuzzy’ indistinct core extending about 60% of the way to the planet’s surface. Computer models suggest that the core is 55 times as massive as the Earth (Saturn iteself is 95 times as massive as Earth), with rock and ice only making up 17 Earth masses worth.
A new model for the interior structure of Saturn. Credit: Caltech/R. Hurt (IPAC).
To stabilize a sloshing liquid core, astronomers realized that it had to be gradually mixed with heavier material, like layers in a cake. The Earth’s oceans produce a similar stabilized scenario, as salinity increases with depth.
“The fuzzy cores are like a sludge,” says Christopher Mankovich, a Caltech research associate in planetary science in a recent press release. “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.”
Standard gravitational measurements made by Cassini also back up the ‘fuzzy/sloshy core’ model for Saturn. Many of us remember the mantra we were all taught as school kids, that Saturn has a low enough density to float in water… if you could somehow construct a pool for it that would not collapse into a planet in its own right.
A simulation of a stellar occultation, versus Saturn’s rings. Credit: NASA/JPL/University of Colorado.
Here another strange and (fun) fact that Cassini sought to solve before its destructive demise in 2017: we don’t actually know the exact rotational period of Saturn to as high a degree of precision as you might think. Like Jupiter and the Sun itself, Saturn is mainly a ball of spinning gas rather than a single solid
The Milky Way’s Mass is Much Lower Than We Thought
How massive is the Milky Way? It’s an easy question to ask, but a difficult one to answer. Imagine a single cell in your body trying to determine your total mass, and you get an idea of how difficult it can be. Despite the challenges, a new study has calculated an accurate mass of our galaxy, and it’s smaller than we thought.
One way to determine a galaxy’s mass is by looking at what’s known as its rotation curve. Measure the speed of stars in a galaxy versus their distance from the galactic center. The speed at which a star orbits is proportional to the amount of mass within its orbit, so from a galaxy’s rotation curve you can map the function of mass per radius and get a good idea of its total mass. We’ve measured the rotation curves for several nearby galaxies such as Andromeda, so we know the masses of many galaxies quite accurately.
But since we are in the Milky Way itself, we don’t have a great view of stars throughout the galaxy. Toward the center of the galaxy, there is so much gas and dust we can’t even see stars on the far side. So instead we measure the rotation curve using neutral hydrogen, which emits faint light with a wavelength of about 21 centimeters. This isn’t as accurate as stellar measurements, but it has given us a rough idea of our galaxy’s mass. We’ve also looked at the motions of the globular clusters that orbit in the halo of the Milky Way. From these observations, our best estimate of the mass of the Milky Way is about a trillion solar masses, give or take.
The distribution of stars seen by the Gaia surveys. Credit: Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt
This new study is based on the third data release of the Gaia spacecraft. It contains the positions of more than 1.8 billion stars and the motions of more than 1.5 billion stars. While this is only a fraction of the estimated 100-400 billion stars in our galaxy, it is a large enough number to calculate an accurate rotation curve. Which is exactly what the team did. Their resulting rotation curve is so precise, that the team could identify what’s known as the Keplerian decline. This is the outer region of the Milky Way where stellar speeds start to drop off roughly in accordance with Kepler’s laws since almost all of the galaxy’s mass is closer to the galactic center.
The Keplerian decline allows the team to place a clear upper limit on the mass of the Milky Way. What they found was surprising. The best fit to their data placed the mass at about 200 billion solar masses, which is a fifth of previous estimates. The absolute upper mass limit for the Milky Way is 540 billion, meaning that the Milky Way is at least half as massive as we thought. Given the amount of known regular matter in the galaxy, this means the Milky Way has significantly less dark matter than we thought.
Reference: Jiao, Yongjun, et al. “Detection of the Keplerian decline in the Milky Way rotation curve.” arXiv preprint arXiv:2309.00048 (2023).
The post The Milky Way’s Mass is Much Lower Than We Thought appeared first on Universe Today.
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Is it Life, or is it Volcanoes?
Astronomers are working hard to understand biosignatures and how they indicate life’s presence on an exoplanet. But each planet we encounter is a unique puzzle. When it comes to planetary atmospheres, carbon is a big piece of the puzzle because it has a powerful effect on climate and biogeochemistry. If scientists can figure out how and where a planet’s carbon comes from and how it behaves in the atmosphere, they’ve made progress in solving the puzzle.
But one of the problems with carbon in exoplanet atmospheres is that it can send mixed signals.
Carbon, in this context, means all of the major species of carbon, things like carbon dioxide, carbon monoxide, and methane (CO2, CO, and CH4.) A new study investigates the diversity of these chemicals in the atmospheres of exoplanets similar to Earth orbiting stars similar to the Sun.
The study is “Relative abundances of CO2, CO, and CH4 in atmospheres of Earth-like lifeless planets.” It’s been submitted to The Astrophysical Journal and is available on the pre-press site arxiv.org. The authors are Yasuto Watanabe and Kazumi Ozaki. Watanabe is affiliated with the Department of Earth and Planetary Science at the University of Tokyo, and Ozaki is affiliated with the Department of Earth and Planetary Sciences at the Tokyo Institute of Technology.
The study is particularly concerned with CO. “We focused on the conditions for the formation of a CO-rich atmosphere, which would be favourable for the origin of life,” the authors write.
There’s no escaping carbon’s importance. Earth life is carbon-based, and there’s no particular reason to think it’ll be different on other planets. This illustration shows carbon molecules in space. Credit: IAC; original image of the Helix Nebula (NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner, STScI, & T.A. Rector, NRAO
In Earth’s present atmosphere, CO can’t build up because chemical reactions destroy it. But in the deep past, three billion years ago, when the oceans were teeming with simple life, CO could’ve accumulated in Earth’s atmosphere. It’s because there was very little oxygen in the atmosphere, and the Sun was dimmer.
So when we’re searching for biosignatures, an atmosphere with CO could indicate simple life. That’s because it can be an important source of both carbon and oxygen for life. But it’s not that cut and dried. This study aims to untangle some of the details of exoplanet atmospheres so we can identify which mixtures of carbon molecules, including carbon monoxide, might be a biosignature.
“Consequently, a detailed understanding of those factors that govern the relative abundances of CO2, CO, and CH4 in planetary atmospheres has far-reaching implications in the search for habitable planets beyond our solar system,” the paper states.
A key concept in this research is called CO runaway. In an atmosphere like early Earth’s, which contained very little oxygen, CO is produced by photodissociation from UV radiation. On the other side of the equation, it’s destroyed by chemical reactions stemming from the photodissociation of water. When conditions are right, more CO is produced than destroyed, and that can lead to CO runaway.
Understanding CO runaway is critical in the appearance of life because prebiotic chemicals necessary for life—especially peptides—are more readily created in a CO-rich atmosphere than in a CO2-rich atmosphere. Evidence from Mars bolsters this point.
The pair of researchers used atmospheric chemistry models to investigate the details behind CO runaway and how it might help us discern which exoplanets could shelter life.
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Colliding Moons Might Have Created Saturn’s Rings
If we could wind the clock back billions of years, we’d see our Solar System the way it used to be. Planetesimals and other rocky bodies were constantly colliding with each other, and new objects would coalesce out of the debris. Asteroids rained down on the planets and their moons. The gas giants were migrating and contributing to the chaos by destroying gravitational relationships and creating new ones. Even moons and moonlets would’ve been part of the cascade of collisions and impacts.
When nature crams enough objects into a small enough space, it breeds collisions. A new study says that’s what happened at Saturn and created the planet’s dramatic rings.
The research is “A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons,” and it’s published in The Astrophysical Journal.” The lead author is Luis Todorow, a Research Fellow at the School of Physics and Astronomy at the University of Glasgow.
Saturn’s rings are so iconic that even schoolchildren can identify them. Astronomers have puzzled over them for a long time, trying to figure out how they formed and when. We know they’re mostly made of ice, but a consensus for their formation has been hard to reach.
This study, conducted by NASA and its partners, says a collision between two icy moons is responsible, and the debris is still circling the planet.
We don’t have to wind the clock back too far to find the impact the research identifies. It occurred only a few hundred million years ago, maybe even more recently than that. The research team says that it was triggered by “resonant instabilities in a previous satellite system.”
The research is based on detailed simulations of Saturn and its system of moons (it has 146 confirmed satellites) and rings.
NASA’s Cassini mission laid the groundwork for this research. The spacecraft spent more than ten years in the Saturn system. One of its main discoveries was that the gas giant’s rings and moons are not very old in astronomical terms. The larger ones are probably old, and their cratered surfaces are a clue to their ages. But some of the planet’s smaller moons are likely much younger.
An annotated picture of Saturn’s many moons captured by the Cassini spacecraft. Image Credit: By Kevin Gill from Los Angeles, CA, United States – Saturn – September 9, 2007 – Annotated, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=131463918
A moon’s distance from its planet plays a role in this. The gravitational struggle between a planet and its moon tends to drive moons away. Earth’s Moon is receding a tiny yet measurable amount each year. Some research shows that if the moons nearest to Saturn’s rings were old, they would’ve been pushed away by now. Since they’re still there, they must be young.
But it’s not that cut and dry because the smaller inner moons also have cratered surfaces.
Saturn’s moon Mimas is covered in craters, including the dramatic Herschel crater that gives the moon its “Death Star” nickname. But it’s close to Saturn. What’s going on? Image credit: NASA/JPL/SSI
So Saturn is still mysterious.
Adding to the intrigue is our fascination with icy moons. Saturn’s moon Enceladus, as well as other moons like Jupiter’s Europa, contain vast oceans underneath icy shells. They’re prime targets in the
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