Saturn’s Moon Tethys

Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.

This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.

Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.

Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.

An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain

The modern names of all seven satellites of Saturn come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus). In his 1847 treatise Results of Astronomical Observations made at the Cape of Good Hope, he suggested that all should be named after the Titans – the brothers and sisters of Cronos – from Greek mythology.

Size, Mass and Orbit:
With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.

Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.

Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding
Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding

Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

Composition and Surface Features:
Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.

It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).

This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.

The Odysseus Crater, a Credit: NASA/JPL/SSI
The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI

The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.

The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.

Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.

Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.
Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.

Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were  captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.

Exploration:
Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.

Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.

On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.

This set of global, color mosaics of Saturn's moon Tethys was produced from images taken by NASA's Cassini spacecraft during its first ten years exploring the Saturn system. Credit: NASA / JPL-Caltech / Space Science Institute / Lunar and Planetary Institute
Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute

The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.

At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.

We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.

Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Could the Death Star Destroy a Planet?

In the movie Star Wars, the Darth Vader’s Death Star destroyed a planet. Could this really happen?

You’ve watched Star Wars right? Is that still a thing? With the Starring and the Warring? Anyway, there’s this classic scene where the “Death Star” sidles up to Alderaan, and it is all like “Hey Planetoid, you lookin’ fine tonight” and then it fires up the superlaser and destroys the entire orb in a single blast. “BOOM”. Shortly followed by some collective group screaming on the interstellar forceway radio.

This is generally described as “science fiction”. And when you’re making up stories, anything you like can happen in them. George Lucas’ hunger for your childhood toy money wasn’t hampered by the pesky constraints of physics in any meaningful way.

Here at the Guide to Space, we get to take our own flights of fancy and pointlessly speculate for your amusement. That’s our job. Well, that and snark. Let’s consider what it would actually take to destroy a planet with a ‘pew pew’ style laser beam, and what kinds of energy would need to be harnessed in a fully armed and operational battle station.

Let’s go back and carefully review our “evidence”. The Death Star drifts in, charges up all its lasers into a superlaser blast focused on Alderaan. The planet then detonates and chunks fly off in every direction just like the pie eating contest in “Stand By Me”.

What we saw was every part of Alderaan given enough of a kick so that it was traveling at escape velocity from every other part of the planet. If the Death Star hadn’t delivered enough explosive energy, the planet might have fluffed up for a moment, but then the collective gravity would suck it all back in together, and then the slightly re-arranged, and likely now uninhabited planet would continue orbiting its star.

You can imagine doing this the slow way. Take each continent on Alderaan, load it up into a rocket and blast that rocket off into space as though it was on escape trajectory from the planet. Sure, you’d would need an incomprehensible number of rocket launches to get that material off the planet. But hey, midichlorians, blue finger lightning and ESP.

Fortunately, as you carted away more and more of the busted up rock, it would have less mutual gravity, and so the rocket launches would require less and less energy to get the job done. Eventually, you’d just be left with one last chunk of rock that you could just force ninja kick into the neighboring star.

Death Star beam. Credit: Lucasfilm
Death Star beam. Credit: Lucasfilm

So how much energy is that going to take? Well, there’s an “easy” calculation you can make. The energy you’d need is equal to 3 times the gravitational constant (6.673 x 10^-11) times the mass of the planet squared divided by 5 times the planet’s radius. Do this math for an Earth-sized/mass world, and let’s see that’s, two and one, carry the 5… and you get 2 x 10^36 joules. That’s a two followed by 36 zeros in joules. Is that a lot? That sounds like a lot.

Well, our own Sun puts out 3 x 10^26 joules per second. So, if you poured all the energy from the Sun into the task of tearing apart the Earth, it wouldn’t have enough energy to do it. In fact, you’d need to focus the light of the Sun for a full week to get that level of planet destruction done.

According to ancient Star Warsian dork scholars, the Death Star (SOLUS MORTIS) is powered by a hyperreactor with the output of multiple main sequence stars. So there you go, problem solved. It’s the size of a small moon, but it’s more powerful than many stars. Of course it can destroy a planet.

Exploding planet. Credit: ESO
Exploding planet. Credit: ESO

The Death Star clearly destroyed Alderaan. We watched it explode. I saw it, you saw it. We heard the screams of millions of souls cry out. It happened. But what if it wasn’t a beam thingy?

Our math is good, but clearly we’re not enlightened enough to comprehend the true wisdom hidden within the Lucasian scriptures. Perhaps the Death Star’s superlaser was just a targeting laser. Directing the placement of gigantic antimatter bomb. According to Ethan Siegel, from “Starts With a Bang,” you’d only need 1.24 trillion tonnes of antimatter.

Imagine you made a bomb out of that much antimatter iron – if that’s even a thing – you’d only need a sphere about 3 km across. If the Death Star is 150 km across or so, they could carry a bunch of these. Very carefully. Like super carefully. Okay, maybe it’d be a good idea if everyone took off their boots, and make sure they only talked with their inside voices.

Obviously, Star Wars is a story, so anything, ANYTHING can happen. The future is unknown, and we might discover all kinds of weirdo physics and harness them into all kinds of powerful weapons. I’m only suggesting, that a space station capable of deploying a week’s worth of solar energy in a single second might be a stretch. And maybe, George, if you just done a little back of the napkin math, we wouldn’t be talking about this right now. Also, maybe no Ewoks. I’m just saying.

Where do you stand on the feasibility of imaginary space station weaponry? How big a planet can your imagination destroy?

How Could Aliens Blow Up Earth?

Earth. It seems so solid and permanent. But really, all you need to do is expand the Sun enough, and the entire planet would melt away. Or worse, find yourself at the mercy of some seriously powerful and angry aliens.

Actually, the beings who destroy Earth in The Hitchhiker’s Guide to the Galaxy, which first aired on BBC Radio 4 on this day (March 8) in 1978, were not so much angry as logical about their reasons.

In the novel, Earthlings are shocked when extraterrestrial beings — known as the Vogons — arrive with plans to build a hyperspatial express route that runs through Earth’s orbit. The plans for the route were apparently lodged in Alpha Centauri (a star system four light-years away) for the past 50 Earth years, leaving residents of the planet “plenty of time to lodge any formal complaint.”

The Vogons then prepare to do the deed. The book Douglas Adams wrote describes it thusly:

“Energize the demolition beams.” Light poured out of the hatchways … There was a terrible ghastly silence. There was a terrible ghastly noise. There was a terrible ghastly science. The Vogon Constructor Fleet coasted away into the inky starry void.

The situation had us at Universe Today wondering: just how did the Vogons do it? There isn’t much to go on, admittedly; a demolition beam, and then a terrific noise as the planet breaks apart.

We scoured the Internet for some answers and came up with these ideas:

Anti matter

What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss
What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss

Anti matter is most simply, the opposite of matter. If you think of matter as being made up of electrons, neutrons and protons, anti matter has its own particles that have the opposite charge and magnetic moment (a property of magnetism.) You can read more technical details of anti matter in our past story, but here’s the important take-away: when matter and anti matter collide, they kill each other dead and produce gamma rays or other fundamental particles in the process. Phil Plait (author of the blog Bad Astronomy, now at Slate) says it’s indeed possible to blow up the Earth with it, but it would take a trillion tons. That’s not only complicated, but expensive. “Given that it currently costs hundreds of billions of dollars to make a single ounce of anti matter, you might have to work an extra job to cover the expense,” he wrote on Blastr.

Black hole

This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech
This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech

If a black hole were to pop up right next to Earth or inside the planet, this might be a way to shrink the planet down to nothing super-quick. We’re not sure how the Vogons did this, but hey, we’re talking science fiction here. It’s also unclear to us how bright this would look (remember, the Vogons had a light beam), but maybe the Vogons turned on the lights for dramatic effect. And here we should interject with some sobering reality from NASA, too: “Black holes do not go around in space eating stars, moons and planets,” the agency once wrote, adding that even if a black hole appeared where the Sun is today, Earth still wouldn’t be sucked over there. In fact, the gravitational force would be identical and the planets would continue their merry orbits.

A Death Star

The Death Star in Star Wars. Credit: Lucasfilm.
The Death Star in Star Wars. Credit: Lucasfilm.

Yes yes, we know, we’re mixing up our science fiction franchises. This was actually a laser-blasting, planet-destroying machine from Star Wars. But at risk of offending the Internet, a couple of legitimate points: There’s nothing to stop alien civilizations from sharing technology, or perhaps acquiring it, rather than spend the money to develop it themselves. In 2011, three researchers from the University of Leicester suggested that indeed a Death Star could destroy a planet, given an adequate power source. Check out the details in our past Universe Today story.

Do you have some other ideas of how the Vogons destroyed Earth?

Could a ‘Death Star’ Really Destroy a Planet?

[/caption]Countless Sci-Fi fans vividly remember the famous scene in Star Wars in which the Death Star obliterates the planet Alderaan.

Mirroring many late night caffeine-fueled arguments among Sci-Fi fans, a University of Leicester researcher asks the question:

Could a small moon-sized battle station generate enough energy to destroy an Earth-sized planet?

A paper by David Boulderston (University of Leicester) sets out to answer that very question. First, for the uninitiated, just what the heck is a Death Star?

According to Star Wars lore, the DS-1 Orbital Battle Station, or Death Star, is a moon-sized battle station designed to spread fear throughout the galaxy. The image above shows the Death Star as it appeared in Star Wars Episode IV: A New Hope (1977). The Death Star’s main weapon is depicted as a superlaser capable of destroying planets with a single blast.

Boulderston claims that it is possible to estimate how much energy the Death Star would need in order to destroy a planet with its superlaser. There are a number of assumptions made, however, in order to come up with the energy requirement.

For starters, Boulderston assumed that Alderaan did not have any sort of planetary “deflector” shield. A second assumption is that the planet is a solid body of uniform density – essentially ignoring the complex interior of planets, due to lack of information on Alderaan itself. Using the idealized sphere model based on Earth’s mass and diameter, it was possible to determine the gravitational binding energy of Alderaan, using a simple equation of:

U= 3GMp2
——
5Rp

Where G is the Gravitational Constant (6.673×10-11), Mp is planet mass, and Rp is the planet’s radius. Using Earth’s mass and radius, the required energy comes out to 2.25 x 1032 Joules. Using Jupiter’s data, the energy required goes up to 2 x 1036 Joules.

Boulderston asserts that (according to Star Wars lore) the Death Star is powered by a ‘hypermatter’ reactor, possessing the energy output of several main-sequence stars. Given that the power output of our Sun is about 3 x 1026 Joules per second, it’s a reasonable assumption the Death Star’s reactor could power the superlaser.

Despite using a simplified model of a planet, Boulderstone states the simplified model is reasonable to use since the Death Star’s main power reactor has the energy output equal to several main-sequence stars. Even if Earth’s exact composition were used in the equation above, the required energy to destroy a planet would only be affected by a few orders of magnitude – well within the Death Star’s power budget.

Boulderstone reiterated that the energy required to destroy a Jupiter-sized planet would put considerable strain on the Death Star. To destroy a planet like Jupiter, all power from essential systems and life support (no re-routing from the auxiliary EPS conduits – that’s a Star Trek hack!) would be required, which is not necessarily possible.

Boulderstone’s conclusion is that the Death Star could indeed destroy Earth-like planets, given its main power source. While the Death Star could destroy an Earth-sized planet, a Jupiter-sized planet would be a tough challenge, and the Galactic Empire would need to resort to using a Suncrusher to destroy stars.

If you’d like to read Boulderstone’s paper, you can access it at: https://physics.le.ac.uk/journals/index.php/pst/article/view/328/195

On New Year’s Eve, Cassini Will Stare at the Death Star’s Superlaser

OK, it’s actually the Herschel crater on Mimas, a smallish moon of Saturn (and it’s the eve of Chinese New Year, February 13th, 2010), but it’s a cool headline, don’t you think?

Cassini will be very busy that day, which begins with a rare sunrise – the Sun goes behind Saturn (from Cassini’s perspective) – followed by a rare blackout, as the Earth goes behind Saturn. Then there’s three “Forward shields up!” moments, as Cassini tries to dodge a Klingon missile flies through regions of “increased ring particle concentration”, a couple of distant flybys (Epimetheus, Janus; ~100,000 km each), a ring-plane crossing, another “Shields up!” moment, and a 9,500 km close approach to Death Star Mimas. And the day ends with a distant (112,000 km) flyby of Tethys. Whew!

“Mimas bears the mark of a violent, giant impact from the past – the 140-kilometer-wide Herschel Crater – and scientists hope the encounter will help them explain why the moon was not blown to smithereens when the impact happened. They will also be trying to count smaller dings inside the basin of Herschel Crater so they can better estimate its age,” JPL’s Jia-Rui C. Cook said, “The Mimas flyby involves a significant amount of skill because the spacecraft will be passing through a dusty region to get there. Mission managers have planned for the Cassini spacecraft to lead with its high-gain antenna to provide a barrier of protection.”

To date, the best images of Mimas – and its Herschel crater – were obtained on August 2nd, 2005, during Cassin’s distant flyby.

Mimas is an inner moon of Saturn that averages 396 kilometers in diameter. The diameter of Herschel Crater is about one-third that of the entire moon. The walls of the crater are about 5 kilometers high, and parts of the floor are approximately 10 kilometers deep.

Map of Mimas (Credit: NASA/JPL/Space Science Institute )

Mimas would have beaten another of Saturn’s moons, Rhea, for the record of “most battered moon”, but for the fact that it was warmer – and so softer – for longer than Rhea (because it’s closer to Saturn), so many of the earliest craters were more degraded.

Have you heard of the “Mimas paradox”? Mimas’ orbit is more eccentric than Enceladus’, and is in resonance with Dione and Enceladus – so it should be heated, tidally, more than Enceladus – but its surface has not, apparently, changed for a very long time (while geysers on Enceladus show that it is still quite active). Further, the two moons seem to have similar compositions.

On this flyby, Cassini’s composite infrared spectrometer will be working to determine the thermal signature of the moon, and other instruments will be making measurements to learn more about the surface composition. Perhaps that will shed some light on the Mimas paradox.

Sources: Cassini Set to Do Retinal Scan of Saturnian Eyeball, Mimas (NASA/JPL)