Saturday Snow Moon

Credit: NASA/JPL/Space Science Institute
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A nice, full-on view of Enceladus against the clouds and rings (shown here in monochrome) of Saturn.

Christmas a la Cassini

Since it's Saturnalia, or thereabouts, let's take a look at some of the Cassini images I missed over the past few months:

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The familiar icon of a crescent moon is rendered alien in this view of Enceladus bisecting a slender crescent of Saturnian daylight.

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Another snap from the same flyby shows Enceladus' characteristic geysers.

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Cassini caught a good portrait here of one of Saturn's many smaller, potato-shaped moons: Helene.

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Saturn's second largest moon, Rhea, eclipses Titan in a grab for the limelight. Rhea is about a million kilometres away in this image, and Titan 2 million.

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Saturn has many cratered, icy moons, but Tethys here shows off one of its most remarkable features: Ithaca Chasma, a massive canyon dwarfed only by Valles Marineris on Mars.

Happy Christmas everyone - whatever planet, moon or ring-system you may call home.

(All images credit NASA/JPL/Space Science Institute.)

Million Kilometre Siblings

Credit: NASA/JPL/Space Science Institute
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This view provides a stark view of Mimas' Herschel crater. Dione, in the foreground, looks quite plain by comparison, its "wispy" terrain hidden from view.

Both moons were over a million kilometres from Cassini when this image was taken.

Space Rocks, Clouds

Credit: NASA/JPL/Space Science Institute

Cassini captured this group photo of two of the Solar System's best looking moons, with the varied terrain of Dione standing out especially sharply against the thick clouds of Titan.

Parasol Rings

Credit: NASA/JPL/GSFC/Oxford University

There's an interesting article at the Cassini website about just how much of a cooling effect the shadow cast by Saturn's rings has. Given that the shadow falls - almost by definition - on the winter hemisphere, this may go some way towards explaining the differences observed between Saturn's top and bottom, not least the striking blue hue first observed by Cassini in the wintry north.

Read it here.

Power Pill

Credit: NASA/JPL/GSFC/SWRI/SSI

Mimas seems to be the go-to world in the Solar System for pop culture references. In the visible spectrum it looks like the Death Star, and in infrared it, apparently, resembles Pac-Man eating a dot.

But as much as this may have caught the eye of the mass media for quite simple reasons, Emily Lakdawalla has the lowdown on why this image is actually a scientifically fascinating discovery.

Orbital Skylines

Credit: NASA/JPL/Space Science Institute
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This is one of the images of Enceladus that Cassini took on my birthday last year. This one's cool for featuring Saturn, the icy moon, and the plumes of water ice at its south pole.

Click the image to turn on the bigmatron.

Old Time Spacescape

Credit: NASA/JPL/Space Science Institute
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There's always an interesting arrangement of light, shadow and pastel colours to find around Saturn, and I rather like this Cassini image of Mimas against the rings, with smaller Epimetheus in the background.

Cassini made its all time closest approach to Mimas some time last week.

Dwarves and Giants

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Credit: NASA/JPL/Space Science Institute

This rather elegant Cassini image lends something of a sense of scale to Saturn as we see it dwarfing 1500km wide Rhea.

Sunlight on an Alien Lake

Cassini sends us this image of something scientists have been seeking for a while now: a tell-tale glint of sunlight, reflected on the surface of one of its hydrocarbon lakes.

“This one image communicates so much about Titan -- thick atmosphere, surface lakes and an otherworldliness,” said Bob Pappalardo, Cassini project scientist, based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “It’s an unsettling combination of strangeness yet similarity to Earth. This picture is one of Cassini’s iconic images.”

Read the rest here.

Snowball Fountain

Image credit: NASA/JPL/Space Science Institute
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So it seems that on my birthday Cassini dove close to Enceladus for its eight flyby. The eighth, huh? Well, there won't be much different or exciting to look at, for this one, surely? Just more scientific data that may eventually contribute towards unravelling the mysteries of this very interesting world.

Or so you might have thought! Instead, we find that the Cassini imaging team have snapped some gorgeous close-up (well, within a few thousand kilometres) shots of the moon's geysers in action. This is also our last peak at the south pole of Enceladus before it enters winter and several years of darkness.

You can find some more images and some amateur-assembled mosaics at the Planetary Society blog here.

Shadows touching in the outer solar system

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Credit: NASA/JPL/Space Science Institute

With Saturn at equinox in this Cassini image, a thin line with a blip on it marks the conjoined shadows of Enceladus and the rings.

A Postcard from Saturn at Equinox

Image Credit: NASA/JPL/Space Science Institute

Cassini has assembled a great, high resolution mosaic of Saturn as it appeared during its recent equinox. Grab a larger version and read the details here.

One of the most notable features of the Saturnian equinox is that its rings are edge on to the sun, which means that as well as this being the only time you won't see their shadows strewn across the gas giant's cloudtops, it's also a time when the rings become pretty much invisible. This image has therefore obviously been tweaked slightly to afford us a better view.

Cloud Depths

Credit: NASA/JPL/Space Science Institute
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Cassini used a wavelength of infrared light that passes easily through methane to obtain this image of the turbulent depths beneath the serene pastel cloud-tops of Saturn.

On another note, I've often wondered just how much fuel Cassini has left after all these years of steadily adjusting its orbit. Todd Barber, the mission's lead propulsion engineer, discusses the probe's fuel gauge here.

Edge on to the sun

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Credit: NASA/STScI/AURA

So today's the day (perhaps not the Saturnian day, but close enough) that Saturn is at equinox. The gas giant's axis - and therefore also its ring plane - is neither tilted towards or away from the sun. With the sun's light striking them edge on, the Cassini imaging team's favourite frame of reference - the illuminated/unilluminated side of the rings - will be briefly meaningless, before turning on its head.

The Cassini homepage has a nice article on the event here, but as always, you're probably best off checking out Emily Lakdawalla's eloquent description at the Planetary Society blog here.

Disclaimer: the Hubble image above actually shows Saturn from 1996-2000, leaving an equinox and heading into the northern winter that would greet Cassini.

Messy Moon

Credit: NASA/JPL/Space Science Institute
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The influence that Saturn's moons have on its rings is nicely illustrated by this Cassini image of the feathery mess Prometheus has made of the F-ring. (There's a good description of Prometheus' shenanigans here.)

Moon-Moon Eclipse

Credit: NASA/JPL/Space Science Institute

So Cassini has shown us eclipses of one kind or another enough times. Saturn is a lot bigger than its moons, so they're always throwing their little shadows onto its clouds, or wandering into the shade of their gargantuan parent.

But around Saturn's equinox, something new becomes possible. With the sun and Saturn's moons all on a similar plane, an observer on one moon could now potentially see one of the other moons blotting out their view of the sun. Which is what's happened here, with Enceladus casting its shadow onto Mimas.

There's a time-lapse video of the event that you can watch here.

Happy Earth Birthday, Cassini!

NASA's Cassini mission has been orbiting Saturn for five Earth years as of June 30, 2009. That's about one sixth of a Saturnian year, enough time for the spacecraft to have observed seasonal changes in the planet, its moons and sunlight's angle on the dramatic rings.

Visit the Cassini homepage here.

Enceladus Knowledge +1

Credit: NASA/JPL/SWRI/University of Colorado

"Our measurements imply that besides table salt, the grains also contain carbonates like soda. Both components are in concentrations that match the predicted composition of an Enceladus ocean," [Frank] Postberg [Cassini scientist for the cosmic dust analyzer at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany] said. "The carbonates also provide a slightly alkaline pH value. If the liquid source is an ocean, it could provide a suitable environment on Enceladus for the formation of life precursors when coupled with the heat measured near the moon's south pole and the organic compounds found within the plumes."

However, in another study published in Nature, researchers doing ground-based observations did not see sodium, an important salt component. That team notes that the amount of sodium being expelled from Enceladus is actually less than observed around many other planetary bodies. These scientists were looking for sodium in the plume vapor and could not see it in the expelled ice grains. They argue that if the plume vapor does come from ocean water, the evaporation must happen slowly deep underground, rather than as a violent geyser erupting into space.

Read the rest here.

It also well worth taking a peek at the explanation for the diagram above, which outlines several proposed models for Enceladus' plumes. These recent observations have ruled out model A, but the others (or some combination of them) are all candidates of various plausibility.

Sepia Equinox

Credit: NASA/JPL/Space Science Institute
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Here's another image of an equinox shadow on Saturn's rings, cast by Mimas as before, but this time in natural colour.