More than a decade ago the Cassini probe entered orbit around Saturn. It was a risky mission. We had never orbited such a distant planet, which meant there was plenty to go wrong. Cassini also carries a companion probe, known as Huygens, which had a mission to land on Titan. Cassini has been a remarkable success, and has given us an unprecedented view of Saturn and its moons. But now the aging spacecraft is running out of power, so it’s time for Cassini to have one last mission. 

It would be easy to let Cassini simply run out of power, letting it drift around Saturn like a cold rock. But some of Saturn’s moons, such as Enceladus, have conditions that could be suitable for life. If Cassini were to crash on a moon hundreds of years from now, there’s a small chance it could contaminate it with terrestrial life. So a better option is to bring it into a close orbit with Saturn, eventually letting the spacecraft burn up in Saturn’s atmosphere. This is the idea behind the Grand Finale mission.

Several orbits will take Cassini between Saturn and its rings. Credit: NASA / Jet Propulsion Laboratory – Caltech / Erick Sturm

During the Grand Finale, Cassini will pass between Saturn and its rings several times. It’s a tricky maneuver, since the gap between planet and rings is small on an astronomical scale. It’s also a region where no spacecraft has ever explored, so we aren’t entirely sure what to expect. For example, as Cassini passes between the rings and Saturn, the gravitational tugs of both will shift Cassini’s orbit slightly. The amount of shift will let us calculate the mass of Saturn’s rings. We have a basic idea of the rings’ mass, but this will give us a far more precise answer. It will also give us a view of the inner region of Saturn’s ring system, which we haven’t had yet. If Cassini survives all the way into Saturn’s atmosphere, we could also get a better understanding of Saturn’s composition.

This is a high risk mission, and it’s possible that Cassini will fail to complete it. That’s why it hasn’t been tried until now. But Cassini has reached a point where it has little to lose, and a Grand Finale now holds a lot of promise.

The post Grand Finale appeared first on One Universe at a Time.

The rings of Saturn are incredibly thin. The most visible portions of the rings span 280,000 kilometers, and yet they are only about a kilometer thick. The rings aren’t solid, but rather a collection of icy particles and moonlets. Because of this, starlight can pass through the rings. We don’t normally notice it because the rings are so bright, but when the Cassini spacecraft passes into the shadow of the rings, it can watch a star as its light twinkles through the rings. 

The optical depth of Saturn’s rings compared to their visible appearance.

Stars have a known brightness, so when a star is seen passing through Saturn’s rings, the amount the light dims is a measure of how much light is absorbed by the rings. Known as the optical depth, it is a good measure of how thick and dense the rings are at different distances. This allows us to look for patterns that aren’t easy to see just by looking at the rings. While we’ve done this type of thing with stars as seen from Earth, Cassini has the advantage of watching different stars as their light passes through the rings at different angles, which gives us a much richer picture than Earth-based observations alone.

The post To The Edge Of Night appeared first on One Universe at a Time.

We don’t generally think of Saturn’s moon Titan as an Earth-like world. It has no breathable atmosphere, and with a surface temperature of about 90 Kelvin life as we know it is out of the question. But there are many parallels between Titan and Earth, and so we can see the moon as a kind of colder, smaller cousin to our own planet.

Both Earth and Titan have thick nitrogen atmospheres. Earth’s also has about 20% oxygen, but the atmospheric dynamics are similar. On Titan, methane plays a similar role to water on Earth. Titan has clouds, rain and large lakes or seas. It has seasons following the changing tilt of its orbital plane relative to the Sun. As a result, the terrain of Titan is interestingly similar to Earths, with rivers, flood plains, and mountains.  It even has ice volcanoes, and so is geologically active.

Because of its lower temperature, and the way methane obscures visible light, we have to look in the infrared to see much of these details. A recent image by the Cassini mission does just that. Shown above, the false-color image gives infrared wavelengths more Earth-like hues. The result is Titan as an Earth-like world. It’s a great example of how sometimes a world so different from our own can also be hauntingly similar.

The post Titan As An Earth-like World appeared first on One Universe at a Time.

Seismology is the study of vibrations and waves through the Earth’s interior. By studying how vibrations are transmitted through the Earth we can study the structure of the Earth as a whole. Similar methods have been applied to the Sun, known as helioseismology, and through it we have an understanding of things such as the temperature and pressure of the Sun’s core. We’ve also been able to study some stars in this way (asteroseismology) from which we can determine things like a star’s age. While it’s a powerful tool, seismology methods pose a challenge for gas planets. But because Saturn has a complex ring system, its vibrations can be measured indirectly. The process is known as Kronoseismology.

How Saturn’s oscillations affect its rings. Credit: Matthew Hedman

If Saturn were a static mass, then the motion chunks of rock and ice that make up its rings would depend largely on the gravitational interactions between each other. But because Saturn is vibrating its gravitational field oscillates, and this induces wave patterns within Saturn’s rings. These patterns are subtle and difficult to observe, but in recent years we’ve been able to watch these changing patterns.

Saturn’s main rings. Credit: NASA/JPL

The patterns are measured using the Cassini spacecraft orbiting Saturn. As Cassini orbits Saturn, it’s view of background stars are often occulted by the rings of Saturn. By observing a star as the rings pass in front of it, we can gather information how thick the rings are in a particular area. This is what a team did for Saturn’s C-ring. From the data they were able to deduce 6 wave patterns that oscillated too quickly to be caused by the gravitational tug of a Saturnian moon. Instead they are caused by the oscillations of Saturn itself.

From this data, the team was able to verify that our model of Saturn’s interior is relatively accurate, though further analysis of these patterns will help us refine the model.

Paper: M. M. Hedman and P. D. Nicholson. Kronoseismology: Using density waves in Saturn’s C ring to probe the planet’s interior. The Astronomical Journal, 146:12 (16pp), 2013.

The post Kronoseismology And The Rings Of Saturn appeared first on One Universe at a Time.

Iapetus is a moon of Saturn known for two distinctive features. One is that it has a two-tone coloration, where roughly half of the moon is a dark, reddish-brown color while the other half is white and almost as bright as Jupiter’s moon Europa. It’s not entirely clear what gives Iapetus is yin yang coloring, but the most popular view is that it is cause by sublimation of the moon’s warmer side. Ice evaporates away leaving the dark remnant material. We know, for example, that the dark layer is no more than a foot thick, and has a bright layer underneath it.

Iapetus’ yin yang coloring. Credit: NASA/JPL-Caltech

Another strange feature is the moon’s large equatorial ridge. It’s about 1,300 km long, and 13 km high. We know that the ridge is old because it is heavily cratered. Again, we aren’t entirely sure how such a ridge could have formed, but generally fall into two camps.  One is that it was produced by some type of internal mechanism such as a convective overturn in its youth, the other is that is was caused an external mechanism such as the accumulation of debris from an ancient ring system. A recent paper in Icarus gives support to the accumulation model.

In this work the team made a detailed model of the ridge system based upon observations from the Cassini probe. They then measured the shapes of the mountain peaks in the ridge, and found that they were within the angle of repose. That is, the angle at which accumulated matter tends to form a peak. Any steeper and the material will tend to collapse to a shallower peak. A geologic upheaval would likely produce a wide range of peak angles, so this suggests the ridge was produced by accumulation. Accumulation from a collapsed ring system would explain why the ridge lies along the equator.

Paper: Erika J. Lopez Garcia, et al. Topographic constraints on the origin of the equatorial ridge on Iapetus. Volume 237, Pages 419–421 (2014)

The post Yin Yang Moon appeared first on One Universe at a Time.

In our solar system, the ring system of Saturn dwarfs all others. While such a large and complex ring system is rare for our solar system, it’s reasonable to presume that similar ring systems exist throughout the cosmos. We know, for example, that Saturn’s rings are stable against things like meteor impacts, and that they are likely as old as the planet itself. Now we know of at least one exoplanet with a large and complex ring system.

Brightness of the eclipsed star (red) compared with the ring model (green). Credit: Kenworthy & Mamajek

It is a planet known as J1407b, which is a brown dwarf that orbits a young sunlike star (J1407). In 2007 the star went through a series of eclipses, where something passed in front of it. Earlier studies of these eclipses hinted at the existence of a ring system around J1407b. As the planet passed near the star from our perspective, the rings of the planet occulted the star, causing it to vary in brightness over a period of more than 50 days. Now a new paper being published in the Astrophysical Journal confirms these rings. The new work also demonstrates that the ring system is complex, much like Saturn’s rings.

The team compared the occultation data with models of ring systems to determine the basic structure of the rings. They found it spans a distance 200 times that of Saturn’s rings, and contains about an Earth-mass of material. It even has what appears to be a large gap in the ring system. Such gaps are known to form in Saturn’s rings, for example, when a moon or moons clears its orbital path. If that’s the case for this gap, it could contain a moon somewhere between the size of Mars and Earth.

The J1407 system is only about 16 million years old, so it is quite possible that thing ring system is in the process of forming moons. If that’s the case the rings may gradually diminish as the moon system forms. For now, it is the greatest ringed planet we know.

Paper: Matthew A. Kenworthy & Eric E. Mamajek. Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons? arXiv:1501.05652 [astro-ph.SR] (2015)

The post Shadow of the Rings appeared first on One Universe at a Time.

Saturn is everyone’s favorite ringed planet. It’s ring system is both complex and extraordinarily bright. Just why it is so bright has been a bit of a mystery. It’s brightness is due to the fact that the rings are composed almost entirely of water ice, and the fact that there isn’t a great deal of dust in the rings. Because of this, it was once thought that Saturn’s rings were generally young. If the ring system were old, one would expect it to darken over time as dust and other debris from the solar system accumulate over time. This also agreed with the idea that such a complex ring system would likely be unstable over millions of years.

Since then we’ve come to understand how complex interactions can occur within Saturn’s rings, and computer simulation have shown that complex ring systems can be stable over billions of years, and observational evidence has even shown Saturn’s rings can withstand cometary or asteroid bombardment. Spectral analysis of the rings show that its composition varies with distance, just as the moons of Saturn do, and this suggests the ring system formed around the same time as Saturn itself. But if Saturn’s rings are indeed old, how is it that they remain so bright? We’ve also observed that ice particles within the rings are continually clumping and breaking apart, which would help keep the rings looking “fresh”, but this doesn’t seem to be enough to keep the rings bright over billions of years.

Now new results announced at the University of Colorado may have solved the mystery. They analyzed the number dust particles striking the Cassini probe as it orbits Saturn. By determining the trajectories of these particles, the team could determine the rate at which material from the solar system is captured by Saturn’s rings. What they found was that the accumulation rate is smaller than previously supposed by a factor of 20. This means Saturn’s rings could remain bright for much longer than we had thought.  Based upon the accumulation rate, the team estimated Saturn’s rings to be about 4.4 billion years. This is within the range of Saturn’s formation.

So it seems that Saturn has always been a ringed planet.

The post Ancient Rings appeared first on One Universe at a Time.

The image above is a processed color image of Earth from Saturn. As Carl Sagan once wrote, “That’s here. That’s home. That’s us.” The view of Earth as a pale blue dot demonstrates the rarity of our world, and the fragility of our lives. It is easy to feel small and humbled by such a visage.

But this image of our homeworld was taken by a probe 900 million miles away, a distance so vast that light takes 80 minutes to traverse. And it was built by us. Human hands crafted a machine capable of travelling to Saturn and looking back. Human minds designed it. We dreamed of a way to explore our solar system and we made that dream a reality.

In a single lifetime we have climbed out of our nursery crib. We’ve walked upon the surface of the Moon, sent rovers to explore Mars, built telescopes to gaze upon the farthest reaches of the universe. We’ve scattered probes across our solar system, from our closest neighbors to its outer edge.

This is what humans do. We seek, explore and dream. We devise, and build, and learn.

This picture of the Earth is an image not of our past, but of our future. Yes, this is an image of home, but so is every picture of the starry heavens. The universe is our home. We are a part of it, and it is a part of us.

Here is where we began, and this image is just the beginning.

The post Home appeared first on One Universe at a Time.

Janus is a small moon of Saturn. It is somewhat oval in shape and has a diameter of about 180 kilometers. Epimetheus is another moon of Saturn, with a diameter of about 120 kilometers. The two moons are very similar, even down to their orbits. They share the same orbital plane, and at the moment the orbit of Janus is only about 50 kilometers closer to Saturn than that of Epimetheus. In other words the gap between the orbits is less than the size of the moons.

You might think this is a recipe for unpleasantness. After all, since the orbit of Janus is closer to Saturn, Janus moves around in its orbit faster than Epimetheus. So over time Janus will catch up to Epimetheus, and would overtake its sister moon if it weren’t for that fact that it is in the way. Surely it’s only a matter of time before the two moons collide.

Except that isn’t what happens. Instead of an imminent collision, the two moons do a little dance. Janus and Epimetheus are not only of similar orbits, they are of similar mass. Similar in this case means that Janus is only about four times more massive than Epimetheus, rather than hundreds or thousands. So as Janus begins to approach Epimetheus, the gravitational pull of Janus will cause the orbit of Epimetheus to get a bit smaller. As a result, the speed of Epimetheus will increase. Likewise the gravitational pull of Epimetheus will embiggen the orbit of Janus a bit, causing it to slow down. You can see this in the figure.

So instead of colliding, the two moons do a gravitational dance where they effectively exchange orbits. Janus catches up to Epimetheus (to within about 10,000 kilometers), they do their gravitational dance, and then Epimetheus races ahead of Janus. Eventually Epimetheus catches up to Janus and another dance brings them back to where they started. This exchange happens about once every four years.

A magic dance, driven by the force of gravity.

The post Dance Magic Dance appeared first on One Universe at a Time.

The rings of Saturn are among the more beautiful objects in our solar system. With their subtle variations they seem fragile, as if the slightest disruption would cause them to fall apart like a house of cards. In fact the rings are quite old, dating back about four billion years or more. They have evolved over time, and were likely much more extensive in the past, but they are hardly fragile.

Just how disturbances can affect the rings isn’t fully understood, but we have observed some disturbances in action. One of these you can see in the image below. The image below shows a section of the rings of Saturn, in particular the inner portion known as the C-ring (the brighter part in the middle and upper left) and the D-ring (the dimmer part in the lower right).

Credit: NASA/JPL/Space Science Institute

You might notice that in the central region there are these bands of lighter and darker regions. These variations are not caused by varying thickness within the ring, but rather by ripples in the ring itself. As the orientation of the ripples vary, the brightness of the ring varies. What’s interesting is that these variations weren’t observed by Voyager I and II when they passed Saturn in 1980 and 1981, which means something must have caused them between then and now. It must have been something pretty dramatic to cause such a large effect.

So what could it be? The most obvious suspect is an asteroid or comet, but to have such an effect it would need to have struck a wide region of the rings. The observed ripples span the entire ring at that distance, so it couldn’t have been a small, local collision. A wide collision would mean that the rings would become just slightly tilted relative to Saturn. The tiny difference in orientation between Saturn and its rings would then cause a ripple to form in the ring over time. You can see how this effect would work in the animation here.

While it might seem odd that a small comet or asteroid could shift the rings enough to have an effect, we’ve actually seen this happen before. In 1994 the comet Shoemaker-Levy 9 collided with Jupiter. By the time Galileo reached Jupiter, a similar spiraling pattern had formed in the faint rings of Jupiter. By modeling these ripples, it could be determined that they had started forming between June and September of 1994. The Shoemaker-Levy 9 collision occurred in July.

If the same model is applied to the ripples in Saturn’s inner rings, you get a collision sometime in 1983, which confirms that it would have occurred after the Voyager missions. The collision was likely similar to Shoemaker-Levy 9, which broke apart before reaching Jupiter so that its material was extended over a wide region. A similar collision with Saturn and its rings would have caused the ripples we now observe.

So rather than being a fragile thing, the rings of Saturn are an old relic with a history of collisions written upon it.

The post Ripples appeared first on One Universe at a Time.

Titan is the largest moon of Saturn, and the second largest moon in our solar system. It has a greater diameter than Mercury. It is also the only moon with a thick atmosphere. It has liquid methane rivers and lakes, and has a seasonal climate.

And like our moon, we have landed a probe on its surface. In 2005 the Huygens probe made a one-way journey to the surface of Titan. You can see a video of that landing above.

The post Titan Fall appeared first on One Universe at a Time.

Mimas is a small moon of Saturn, about 400 km in diameter.  It’s surface is dominated by Herschel crater, which gives the moon a passing resemblance to a fictional space station.  For this reason it is often used in memes and jokes, but Mimas is actually an interesting puzzle.

Mimas has a very low density, not much higher than water.  It would seem that Mimas is composed almost entirely of water ice.  It is quite close to Saturn, orbiting just outside the planet’s rings. So close that it is tidally locked with Saturn, so that only one side faces the planet. It also seems to be entirely frozen solid. There is no evidence of any geyser activity, and its surface is heavily cratered unlike other moons like Enceladus. And therein lies the mystery.

Enceladus is more distant from Saturn, but does exhibit geysers and ice flows. Thus Enceladus maintains liquid water in its mantle. This is thought to be due to tidal heating, where Saturn’s gravity stretches and compresses the moon slightly, which keeps it warm.  This effect is similar to squeezing a lump of clay in your hands, which causes it to get warm.  Mimas is closer to Saturn, and its orbit is more elliptical.  Mimas is also oblate (it’s wider at its equator than it is from pole to pole), so it should experience tidal heating even more than Enceladus.

So why is it frozen solid? We aren’t sure.  In fact some have called this puzzle the Mimas test, since any model that can explain the liquid water in Enceladus must also explain why Mimas is frozen solid.

The post That’s No Space Station appeared first on One Universe at a Time.