Tag: Janus

Saturn's shadow slices the rings

By Phil Plait | August 29, 2012 6:30 am

There is a whole lot of awesome in a picture of Saturn and its rings just released from the Cassini spacecraft. Check this out:

Mmmmm, ringalicious.

Cassini was about 2 million kilometers (1.2 million miles) from Saturn when it took this picture, so we’re seeing a decently wide-angle view. At the time, the spacecraft was below the plane of the rings, looking north (up, if you like). The Sun is off mostly to the left and up a bit.

The first cool thing is obviously the shadow of the planet itself cast on the rings. It cuts across like a black scythe! As I looked at the picture my eyes and brain kept trying to fill in the missing arc of rings, which was amplified by a slight afterimage as my eyes moved around. It’s a difficult illusion to ignore.

Second, I love how you can see all the different rings in the picture, including the thin, lumpy F-ring outside the main band. The big gap is called the Cassini Division; it’s not really an empty space since there are many faint thin rings inside it. They’re just hard to see here. The Cassini Division is fairly easy to spot even through a small telescope, looking from Earth like someone took a knife to the rings and sliced them.

Third, you can see the tiny moons Janus (below the rings on the left) and Epimetheus (above the rings on the left) as well. I wonder how hard it is to get a picture like this without seeing any moons in it? Saturn has quite the fleet of them.

Fourth, look to the left, just where the inner arc of the rings cuts across Saturn. You can see the planet right through the rings! The rings aren’t solid; they’re composed of gazillions of particles of nearly pure water ice. There are spaces between the particles, so we can partially see through them, like looking through a screened window.

Fifth, and perhaps most cool of all: the part of Saturn we’re seeing here is the night side, entirely unlit by the Sun. The bottom (southern) part of Saturn is only noticeable by its absence! But what’s that glow in the north?

That, my friends, is ringshine! Although this part of Saturn is in nighttime, the Sun is still shining on the rings (wherever you don’t see Saturn’s shadow across them). The ring particles are very bright and shiny. They reflect the sunlight, which then illuminates the northern hemisphere of Saturn. The southern half is still dark because the ice particles tend to reflect light back up, like a mirror. Since the Sun is coming from the north, that’s the way the light gets reflected. I’ll note that most of the light gets reflected away from Saturn, to the upper right in this picture, but enough is reflected back to make the cloud tops glow softly.

This happens on Earth too, when sunlight reflects off the Earth and illuminates the dark part of the Moon. This is called Earthshine, also poetically called "the old Moon in the new Moon’s arms." It’s quite lovely.

And it’s science! Which is lovely, too.

Image credit: NASA/JPL-Caltech/Space Science Institute


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Saturn, raw (this one’s a jaw-dropper)

CATEGORIZED UNDER: Astronomy, Pretty pictures

A trillion and five moons

By Phil Plait | September 13, 2011 9:45 am

On July 29, 2011, the unending dance of Saturn’s moons lined them up perfectly for a stunning view by the Cassini spacecraft: five moons arrayed for your pleasure:

[Click to embiggen.]

From left to right that’s Janus, Pandora (in the rings), Enceladus, Mimas, and Rhea. Perspective plays a role here; Rhea is three times bigger than Enceladus, but was much closer to Cassini when this picture was taken, so it looks even bigger.

But the moons themselves are so different from each other! Janus is a lump, too small to have enough gravity to crush itself into a sphere. Enceladus is mostly ice, so it appears very bright in this image compared to its rocky siblings. You can just barely see part of the monster crater Herschel peeking out of the dark side of Mimas, while Rhea is peppered with smaller craters. And Pandora orbits inside Saturn’s rings themselves, its meager gravity enough to entrain the particles in the thin F ring and keep it in place.

And, of course, the rings themselves, composed of countless tiny ice crystals. Over millions of years, collisions have ground them into pieces ranging in size from barely big enough to see to perhaps 10 meters across, the volume of a roomy two-car garage.

Amazing. And this vista was taken just a couple of weeks after Cassini’s seventh anniversary in orbit around Saturn. Even after all that time, and tens of thousands of images, it still has the capability to take our breath away.


Related posts:

Cassini’s Pentaverate
Cassini’s slant on the rings
A little weekend Saturn awesomeoness
The real Pandora, and two mooning brothers

CATEGORIZED UNDER: Astronomy, Pretty pictures

Perspective on four moons

By Phil Plait | September 8, 2010 7:00 am

The Cassini probe orbiting Saturn returned an interesting picture yesterday. It shows four tiny moons and the rings seen nearly edge-on. Take a look:

cassini_4moons_rings

[Click to enjovianate.]

From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Like I said, tiny.

When I see images like this I like to amuse myself by fiddling in my mind with their perspective. For example, is Epimetheus closer to us (well, to Cassini when the picture was taken) than Janus was? Even more interestingly, are we looking down on, or up at the rings?

Images like this don’t give us the clues we usually get here on Earth to figure out distance. Look at the picture: the rings make a tight curve across the field. We know we’re seeing a circular ring nearly edge-on… but are we looking down on it, so that the top of the curve is farther away, or are we looking up at it, so that the bottom of the curve is farther away?

For example, take a DVD and hold it so that you’re looking at it almost edge-on. Tilt the near edge down a bit so you’re looking down on the top side. Now tilt the near side up so you’re looking up on the bottom side. See the issue? Without lighting, focus, or other cues, it’s hard to tell which way you’re seeing an object.

So for the Saturn picture, which is it? I’ll tell you below, but see if you can figure it out.
Read More

CATEGORIZED UNDER: Pretty pictures, Science

Cassini's slant on the rings

By Phil Plait | July 2, 2010 9:40 am

Check out this latest image from Cassini at Saturn!

cassini_diagonalrings

[Click to embiggen.]

Oh, wow! This was taken when the spacecraft was almost in the plane of the rings, which are incredibly thin. You can see several different rings, including the broad A ring in the middle and the thin F ring on the outside. There are also two moons: Janus (the larger one, above) and Prometheus (smaller, below). Janus is about 180 km (110 miles) across, which isn’t terribly big, but Prometheus is even smaller: 120 km (75 miles).

cassini_epimetheus_pandoraThis picture made me smile not only because it is carved out of raw coolness, but also because it’s the complement of an earlier image from Cassini (to the right). The earlier shot is of the moon Epimetheus, which shares an orbit with Janus, and Pandora, which shares an orbit with Prometheus! So the two images go together like a pair of gloves, each showing one of a pair of orbit-sharing moons.
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Prometheus

A little weekend Saturn awesomeness

By Phil Plait | May 28, 2010 12:01 pm

If you need a little more awesome in your weekend, then try this:


cassini_rhea_janus

I know I’ve been posting quite a few Saturn images from Cassini, but I really like this one. It shows the big round moon Rhea, the lumpy small moon Janus, and a lovely view of the foreshortened rings. Both moons were on the far side of the rings, well over 1 million kilometers away, when Cassini took this shot. Rhea is over 1500 km (900 miles) in diameter, while Janus is only about 180 km (110 miles) across. For comparison, our own Moon’s diameter is almost 3500 km (2100 miles).
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CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Rhea, Saturn

Cassini eavesdrops on orbit-swapping moons

By Phil Plait | April 8, 2010 2:00 pm

The Cassini spacecraft just had a few close encounters with some of the odder moons in the Saturn system… and given how weird Saturn is, that’s saying something. I was particularly enthralled with these two small worlds:

cassini_janus_epimetheus

On the left is the moon Janus and on the right is Epimetheus. The scales are not quite the same; Janus is roughly half again as big as Epimetheus’ size of 135 x 110 x 105 km (81 x 66 x 63 miles). Cassini was a little over 100,000 km from Epimetheus and 75,000 km from Janus when these images a were taken.
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CATEGORIZED UNDER: Astronomy, Pretty pictures

Ring shadowplay on a Saturn moon

By Phil Plait | December 23, 2009 1:30 pm

This is simply too cool: the shadow of Saturn’s rings moving across the face of its tiny moon Janus.

cassini_janusshadows

This animation is made up of images taken by Cassini (of course) in August. At that time, it was the equinox on Saturn, so the Sun was shining straight down on Saturn’s equator… which happens to be the plane of the orbit for both the rings and the moons. In other words, the Sun was shining straight along the rings. During this brief time, twice per Saturn orbit of 29 years, the moons can cast long shadows across the rings, and the rings can cast shadows on the moons.

Janus really is dinky, just 179 km (111 miles) across, which is why it’s not really all that round. Its gravity isn’t strong enough to crush itself into a sphere. Other moons are bigger, of course, and the Cassini folks just released several other astonishing animations of them as well, showing the moons dancing and eclipsing each other, with Saturn’s rings as the backdrop. This one showing Rhea and Janus is particularly beautiful.

What more can I add? Cassini continues to deliver, over and again. Amazing.

Related posts:
Behold, Saturn
Ringless
Titan’s Shadow
Saturn’s rings do the wave

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Rhea, Saturn
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