Posted on by Nancy Atkinson

Stunning Views of Venus, All the Way from Saturn

Venus appears just off the edge of the dark disc of Saturn, in the upper part of the image, directly above the white streak of Saturn's G ring. Credit: NASA/JPL-Caltech/Space Science Institute

Two amazing images from the Cassini spacecraft today: We know how brightly Venus shines in our own night sky; now here’s visual proof it shines brightly even in the skies above Saturn. In one image it shines so brightly that it is even visible looking through Saturn’s rings! But in this absolutely stunning shot, above, Venus appears as a morning star, just off the edge of the planet. From Cassini, you’re looking directly above the edge of Saturn’s G ring to see the white dot, which is Venus. Lower down, Saturn’s E ring makes an appearance, looking blue thanks to the scattering properties of the dust that comprises the ring. (A bright spot near the E ring is a distant star, the Cassini CICLOPS team says.)

This beautiful image was taken on January 4, 2013.

On average, Venus and Saturn are about 1,321,200,000 km (820,955,619 mi or 8.83 astronomical units) apart, so that’s a nice, long distance shot! Venus is brighter in Saturn’s skies than Earth is, however, because Venus is covered in thick sulfuric acid clouds, making it very bright.

And here’s the other great shot, showing Saturn and its rings in true color:

NASA's Cassini spacecraft spies the bright, cloudy terrestrial planet, Venus. Credit: NASA/JPL-Caltech/Space Science Institute
NASA’s Cassini spacecraft spies the bright, cloudy terrestrial planet, Venus. Credit: NASA/JPL-Caltech/Space Science Institute

Venus is the white dot, just above and to the right of the image center. Again, its amazing that it shines through the rings.

This view looks toward the unilluminated side of the rings from about 17 degrees below the ring plane, and was taken in visible light (and it is a true-color image) with the Cassini spacecraft wide-angle camera on Nov. 10, 2012.

In an email about these images, Cassini imaging team lead Carolyn Porco said that even though Venus reaches nearly 900 degrees Fahrenheit (500 degrees Celsius) and has a surface pressure 100 times that of Earth’s, Venus is considered a twin of our planet because of their similar sizes, masses, rocky compositions and close orbits.

And so, she pointed out, “Think about Venus the next time you find yourself reveling in the thriving flora, balmy breezes, and temperate climate of a lovely day on Earth, and remember: you could be somewhere else!”

See more about these images at the CICLOPS (Cassini Imaging Central Laboratory for Operations) website.

Posted on by Jason Major

A New Look at Saturn’s Northern Hexagon

Raw Cassini image captured on 26 Feb. 2013 (NASA/JPL/SSI)

Freshly delivered from Cassini’s wide-angle camera, this raw image gives us another look at Saturn’s north pole and the curious hexagon-shaped jet stream that encircles it, as well as the spiraling vortex of clouds at its center.

Back in November we got our first good look at Saturn’s north pole in years, now that Cassini’s orbit is once again taking it high over the ringplane. With spring progressing on Saturn’s northern hemisphere the upper latitudes are getting more and more sunlight — which stirs up storm activity in its atmosphere.

The bright tops of upper-level storm clouds speckle Saturn’s skies, and a large circular cyclone can be seen near the north pole, within the darker region contained by the hexagonal jet stream. This could be a long-lived storm, as it also seems to be in the images captured on November 27.

About 25,000 km (15,500 miles) across, Saturn’s hexagon is wide enough to fit nearly four Earths inside!

The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)
The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)

The hexagon was originally discovered in images taken by the Voyager spacecraft in the early 1980s. It encircles Saturn at about 77 degrees north latitude and is estimated to whip around the planet at speeds of 354 km/h (220 mph.)

Watch a video of the hexagon in motion here.

The rings can be seen in the background fading into the shadow cast by the planet itself. A slight bit of ringshine brightens Saturn’s nighttime limb.

Cassini was approximately 579,653 kilometers (360,180 miles) from Saturn when the raw image above (W00079643) was taken.

Image credit: NASA/JPL/Space Science Institute

 

Posted on by Fraser Cain

Weekly Space Hangout: ScienceOnline 2013 Edition

This week, we broadcast the Weekly Space Hangout from the ScienceOnline 2013 conference in Raleigh, North Carolina. Fraser Cain, Nicole Gugliucci, Alan Boyle, and Amy Shira Teitel were on location in Raleigh, and then Scott Lewis and Dr. Thad Szabo reported from their offices.

This week, we talked about:

We record the Weekly Space Hangout every Friday on Google+ at 12:00 pm PST / 3:00 pm EST / 2000 GMT. You’ll want to circle Cosmoquest on Google+ to find out when we’re recording next. The audio for the Weekly Space Hangout is also released to the Astronomy Cast podcast feed.

Posted on by Nancy Atkinson

Saturn’s Mini-Moons Align for Family Portrait

Saturn, its rings and three moons are visible in this image from Cassini. Credit: NASA/ESA

It’s a good thing NASA labeled the moons in this image of Saturn, because they are pretty hard to see. But they are there, keeping each other company in this Cassini spacecraft image of Saturn’s night side. And as the Cassini team says, it seems fitting that they should do so since in Greek mythology, their namesakes were brothers.

In Greek mythology these three were all sons of Iapetus (another of Saturn’s moons), and supposedly Prometheus and Epimetheus were tasked with creating humans. Prometheus was a pretty good sort, and gave gifts to humans like fire; Epimetheus gave humans evil – not so good. And famously, Atlas ended up having the weight of the world on his shoulders.

But in science, Prometheus the moon is about 86 kilometers across (53 miles) and is located just inside the F ring in this image, while Epimetheus is about 113 kilometers across (70 miles) and is farther from the rings, due right of Prometheus in this image. Atlas is the tiny guy (30 kilometers across (19 miles) and can be just barely seen between the A and F rings almost right below Epimetheus,

This view looks toward the unilluminated side of the rings from about 30 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Sept. 19, 2012.

The view was obtained at a distance of approximately 2.2 million kilometers (1.4 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 96 degrees. Image scale is 128 kilometers (80 miles) per pixel. Epimetheus has been brightened by a factor of 1.5 and Atlas’ brightness has been enhanced by a factor of 3 relative to the rings and Prometheus to improve visibility.

You can see an unlabeled version here.

Posted on by Nancy Atkinson

What Happened During the Huygens Mission?

Artist depiction of Huygens landing on Titan. Credit: ESA

It was eight years ago on January 14, 2005 that the Huygens spacecraft descended through Titan’s murky atmosphere and touched down – if a bit precariously – by bouncing, sliding and wobbling across the surface of Saturn’s largest moon Titan. This was the first time a probe had touched down on an alien world in the outer Solar System.

But that surface wasn’t quite what we expected.

While earlier studies of data from Huygens determined the surface of Titan to be quite soft, scientists now think the surface consisted of a hard outer crust but is soft underneath, so that if an object put more pressure on the surface, it sank in significantly.

“It is like snow that has been frozen on top,” said Erich Karkoschka, a co-author of a paper published in October 2012. “If you walk carefully, you can walk as on a solid surface, but if you step on the snow a little too hard, you break in very deeply.”

The scientists think that Huygens landed in something similar to a flood plain on Earth, but that it was dry at the time. The analysis reveals that, on first contact with Titan’s surface, Huygens dug a hole 12 cm deep, before bouncing out onto a flat surface.

The probe, tilted by about 10 degrees in the direction of motion, then slid 30–40 cm across the surface.

A new animation. top of the event has been created using real data recorded by Huygen’s instruments, allowing us to witness this historical moment as if we had been there.

ESA explains:

The animation takes into account Titan’s atmospheric conditions, including the Sun and wind direction, the behaviour of the parachute (with some artistic interpretation only on the movement of the ropes after touchdown), and the dynamics of the landing itself.

Even the stones immediately facing Huygens were rendered to match the photograph of the landing site returned from the probe, which is revealed at the end of the animation.

Split into four sequences, the animation first shows a wide-angle view of the descent and landing followed by two close-ups of the touchdown from different angles, and finally a simulated view from Huygens itself – the true Huygens experience.

Also, a ‘fluffy’ dust-like material – most likely organic aerosols that are known to drizzle out of the Titan atmosphere – was thrown up and suspended for around four seconds around the probe following the impact. The dust was easily lifted, suggesting it was most likely dry and that there had not been any ‘rain’ of liquid ethane or methane for some time prior to the landing.

Huygens was released from the Cassini spacecraft on Christmas Day 2004, and arrived at Titan three weeks later. The probe began transmitting data to Cassini four minutes into its descent and continued to transmit data after landing at least as long as Cassini was above Titan’s horizon, for about 90 minutes, and radio telescopes on Earth continued to receive Huygen’s signal well past the expected lifetime of the craft.

Cassini was supposed to receive Huygen’s signal over two channels, but because of an operational commanding error, only one channel was used. This means that only 350 pictures were received instead of 700 that were expected. All Doppler radio measurements between Cassini and Huygens were lost as well; however, Doppler radio measurements of Huygens from Earth were made, though not as accurate as the expected measurements that Cassini would have made. But when added to accelerometer sensors on Huygens and VLBI tracking of the position of the Huygens probe from Earth, reasonably accurate wind speed and direction measurements could still be derived.

You can see images from the Huygens mission here.

Huygens is currently the most distant landing of any craft launched from Earth. Cassini has been in orbit around Saturn since July 2004, and will continue operations until 2017.

Sources: ESA, Wiki

Posted on by Jason Major

A Color View of Darling Dione

Color-composite of Dione made from raw Cassini images acquired on Dec. 23, 2012. (NASA/JPL/SSI. Composite by J. Major.)

Although made mostly of ice and rock, Saturn’s moon Dione (pronounced dee-oh-nee) does have some color to it, as seen in this color-composite made from raw images acquired by Cassini on December 23.

700 miles (1120 km) wide, Dione is covered pole-to-pole in craters and crisscrossed by long, bright regions of “wispy line” terrain — the reflective faces of sheer ice cliffs and scarps that are too steep for darker material drifting in from Saturn’s E ring to remain upon.

The composite  was assembled from raw images captured in red, green and blue visible light wavelengths by Cassini from a distance of 154,869 miles (249,238 km).

The view above looks at a region on Dione’s mid-northern hemisphere. The bright-walled crater in the center surrounded by warmer-hued terrain is named Creusa, and the long rift system next to it is Tibur Chasmata, which runs north-to-south. Dione’s north pole is to the upper left.

Dione’s heavily cratered areas are most common on its trailing hemisphere. Logically, a moon’s leading hemisphere should be the more heavily cratered, so it has been hypothesized that a relatively recent impact spun Dione around 180 degrees. The moon’s small size mean that even a modest-scale impact could have done the job.

Relative sizes of Earth, Moon and Dione (J. Major)

Dione orbits Saturn at a distance of 209,651 miles (377,400 km), closer than our Moon is to us.

See more images and news from the Cassini mission here. And for more on Dione, see some of my previous posts on Lights in the Dark.

Posted on by Jason Major

The Rings on the Planet Go ‘Round and ‘Round…

Raw wide-angle Cassini image of Saturn’s rings (NASA/JPL/SSI)

Recently I posted an image of two of Saturn’s shepherd moons, Pandora and Prometheus, captured by Cassini in a face-off across the spindly F ring. Now here’s a much wider-angle view of the gas giant’s rings, seen by Cassini  two days later on December 20, and the same two moons can still be seen staring each other down… two tiny points of light visible across the wavering line of the F ring at lower center.

This is just one raw image in a series of 56 that Cassini captured on the 20th, and I’ve combined them together to make a GIF animation — click below to watch:

Animation of Saturn’s rings made from raw images acquired by Cassini on Dec. 20, 2012 (NASA/JPL/SSI. Animation by J. Major)

In the animation you can see Pandora and Prometheus promenade around Saturn (detail at right) as well as a “spoke” of light material moving within the inner dark edge of the A ring. Also many clumps are visible in the thin F ring — caused by embedded moonlets and the gravitational influence of the shepherd moons.

Saturn’s enormous shadow engulfs the entire ring system at the top of the scene.

Cassini was moving relative to Saturn while these images were captured so some background stars make brief appearances, as well as a couple of pixel flares and a cosmic ray hit. These are common in Cassini images.

See more news and images from the Cassini mission here.

 

Posted on by Jason Major

Shepherd Moon Face-Off!

Raw Cassini image acquired on Dec. 18, 2012 (NASA/JPL/SSI)

Two of Saturn’s shepherd moons face off across the icy strand of the F ring in this image, acquired by the Cassini spacecraft on December 18, 2012.

In the left corner is Pandora, external shepherd of the ropy ring, and in the right is Prometheus, whose gravity is responsible for the subtle tug on the wispy ring material. (Please don’t blame the moon for any recent unsatisfying sci-fi films of the same name. There’s no relation, we promise.)

Similar in size (Pandora is 110 x 88 x 62 km, Prometheus 148 x 100 x 68 km) both moons are porous, icy, potato-shaped bodies covered in craters — although Prometheus’ surface is somewhat smoother in appearance than Pandora’s, perhaps due to the gradual buildup of infalling material from the F ring.

Check out some much closer images of these two moons below, acquired during earlier flybys:

Here’s Pandora, as seen by Cassini on September 5, 2005:

False-color image of Pandora (NASA/JPL/SSI)

…and here’s Prometheus, seen during a close pass in 2010 and color-calibrated by Gordan Ugarkovic:

 Prometheus casting a shadow through F ring haze (NASA/JPL/SSI/Gordan Ugarvovic)

The external edge of the A ring with the thin Keeler gap and the wider Encke gap can be seen at the right of the top image. Both of these gaps also harbor their own shepherd moons — Daphnis and Pan, respectively.

These moons keep their gaps clear, as well as maintain the crisp edge shapes of the nearby rings — hence the term “shepherd.”

Image credit: NASA/JPL/Space Science Institute

Posted on by Nancy Atkinson

Gorgeous New Backlit View of Saturn

NASA’s Cassini spacecraft has delivered a glorious view of Saturn, taken while the spacecraft was in Saturn’s shadow. Image credit: NASA/JPL-Caltech/Space Science Institute

The Cassini team has done it again. A new 60-image mosaic of Saturn shows a back-lit view of the giant ringed world in several wavelengths, making Saturn look like a colorful holiday ornament. In October, the Cassini spacecraft was deliberately positioned within Saturn’s shadow, and the cameras were turned toward Saturn and with the Sun behind the planet.

“Of all the many glorious images we have received from Saturn, none are more strikingly unusual than those taken from Saturn’s shadow,” said Carolyn Porco, Cassini’s imaging team lead based at the Space Science Institute in Boulder, Colorado. “They unveil a rare splendor seldom seen anywhere else in our solar system.”


“Looking back towards the Sun is a geometry referred to by planetary scientists as “high solar phase;” near the center of the target’s shadow is the highest phase possible,” the Cassini team explained. Not only does this produce a stunning image, but it is very scientifically advantageous as well, as it can reveal details about both the rings and atmosphere that cannot be seen in lower solar phase.

This is a rare view, as the last time the Cassini spacecraft was able to take a backlit view of Saturn and the rings was 2006. Also captured in this image are two of Saturn’s moons: Enceladus and Tethys. Both appear on the left side of the planet, below the rings. Enceladus is closer to the rings; Tethys is below and to the left.

The black area at the top of Saturn is the planet’s shadow on the rings.

See more info about this image here, as well as get access to really huge versions so you can enjoy it in its full splendor.

Posted on by Jason Major

Titan’s Gravity Indicates a Thicker, Uneven Icy Crust

Color composite of Titan and Dione made from Cassini images acquired in May 2011. (NASA/JPL/SSI/J. Major)

It’s long been speculated that Saturn’s moon Titan may be harboring a global subsurface ocean below an icy crust, based on measurements of its rotation and orbit by NASA’s Cassini spacecraft. Titan exhibits a density and shape that indicates a pliable liquid internal layer — an underground ocean — possibly composed of water mixed with ammonia, a combination that would help explain the consistent amount of methane found in its thick atmosphere.

Now, further analysis of Cassini gravity measurements by a Stanford University team has shown that Titan’s ice layer is thicker and less uniform than originally estimated, indicating a more complex internal structure — and a stronger external influences for its heat.

Titan’s liquid subsurface ocean was previously estimated to be in the neighborhood of 100 km (62 miles) thick, sandwiched between a rocky core below and an icy shell above. This was based on the behavior of Titan in its orbit — or, more precisely, how Titan’s shape changes along the course of its orbit, as measured by Cassini’s radar instrument.

Because Titan’s 16-day orbit is not perfectly circular the moon experiences a stronger gravitational pull from Saturn at certain points than at others. As a result it’s flattened at the poles and constantly changing shape slightly — an effect called tidal flexing. Along with the decay of radioactive materials in its core, this flexing generates the internal heat that helps keep a subsurface ocean liquid.

A team of researchers from Stanford University, led by Howard Zebker, professor of geophysics and electrical engineering, used recent Cassini measurements of Titan’s topography and gravity to determine that the icy layer between the moon’s surface and ocean is up to twice as thick as previously thought — and it’s considerably thicker at the equator than at the poles.

“The picture of Titan that we get has an icy, rocky core with a radius of a little over 2,000 kilometers, an ocean somewhere in the range of 225 to 300 kilometers thick and an ice layer that is 200 kilometers thick,” said Zebker.

Different thicknesses of Titan’s ice layer would mean that there’s less heat being generated internally by the decay of radioactive materials in Titan’s core, because that type of heat would be more or less globally uniform. Instead, tidal flexing caused by the gravitational interactions with Saturn and neighboring smaller moons must play a stronger role in heating Titan’s insides.

Read more: Titan’s Tides Suggest a Subsurface Sea

With Cassini’s new measurements of Titan’s gravity, Zebker and his team calculated that the icy layer below Titan’s flattened poles is 3,000 meters (about 1.8 miles) thinner than average, while at the equator it’s 3,000 meters thicker than average. Combined with the moon’s surface features, this makes the average global thickness of the ice layer to be more like 200 km, not 100.

Heat generated by tidal flexing — which is more strongly felt at the poles — is thought to be the cause of the thinner ice there. Thinner ice would mean there’s more liquid water beneath the poles, which is denser and thus would exert a stronger gravitational pull… exactly what’s been found in Cassini’s measurements.

The findings were announced Tuesday, Dec. 4 at the AGU convention in San Francisco. Read more on the Stanford University news page.