Seeing the icy moon Rhea up close is always a treat. This Cassini image shows us a very battered surface.
Being a moon of Saturn this is a very cold place, the surface temperature ranges from about -220 C to -174 C on a warm day (-364 F to -281 F). It will be fun to compare this to Pluto and its moons which we think is about -229 C (-380 C).
Image Credit: NASA/JPL-Caltech/Space Science Institute.
Gazing off toward the horizon is thought-provoking no matter what body’s horizon it is. Rhea’s horizon is slightly irregular and battered by craters, so thoughts inevitably turn towards the forces that shape these icy worlds.
The surface of Rhea (949 miles or 1527 kilometers across) has been sculpted largely by impact cratering, each crater a reminder of a collision sometime in the moon’s history. On more geologically active worlds like Earth, the craters would be erased by erosion, volcanoes or tectonics. But on quieter worlds like Rhea, the craters remain until they are disrupted or covered up by the ejecta of a subsequent impact.
Lit terrain seen here is on the trailing hemisphere of Rhea. North on Rhea is up and rotated 12 degrees to the right. In this view, Cassini was at a subspacecraft latitude of 9 degrees North. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Feb. 10, 2015.
The view was obtained at a distance of approximately 35,000 miles (56,000 kilometers) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of 76 degrees. Image scale is 1,100 feet (330 meters) per pixel.
NASA’s title for the image was so good I had to use it. Looks can be a little deceiving because the atmosphere we are looking at is moving at 1,800 kmh / 1,100 mph.
Winds are driving storms you can see (click the image) that probably would be anything but serene, except for the fact the lack of a solid surface. The lack of a solid surface means no drag to slow down the winds which contribute to the speed. Saturn like Jupiter actually generates more heat internally than it receives from the Sun and this also contributes to the wind speed and convection to feed the storms.
You can see a moon in is image too, Mimas can be seen at about the one o’clock position just above the planet. Mimas was brightened by a factor of 2 so it would show up aside the brightness of Saturn.
Cassini to Saturn distance was about 2.5 million km / 1.6 million miles.
Image Credit: NASA/JPL-Caltech/Space Science Institute
Saturn’s atmosphere is very active and the storms are much less noticeable than Jupiter but they are no less interesting.
From NASA: Saturn’s surface is painted with swirls and shadows. Each swirl here is a weather system, reminding us of how dynamic Saturn’s atmosphere is.
Images taken in the near-infrared (like this one) permit us to peer through Saturn’s methane haze layer to the clouds below. Scientists track the clouds and weather systems in the hopes of better understanding Saturn’s complex atmosphere – and thus Earth’s as well.
This view looks toward the sunlit side of the rings from about 17 degrees above the ringplane. The image was taken with the Cassini spacecraft wide-angle camera on Feb. 8, 2015 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 752 nanometers.
The view was obtained at a distance of approximately 794,000 miles (1.3 million kilometers) from Saturn. Image scale is 47 miles (76 kilometers) per pixel.
If you click the image you will get a larger version. There is an odd feature in the dark spot towards the upper part of the planet. Is it a part of the cloud wall? An updraft of some sort?
The Cassini spacecraft took this image of Iapetus on 27 March from a distance of about a million km (621,000 miles). This is the second-closet approach to the moon in this mission phase. The closest was in 2011.
The image was taken with the narrow angle camera and gives a good view of the bright terrain in the north polar region.
Iapetus is 1,471 km / 914 miles in diameter. Iapetus is two-toned so the color shading isn’t shadow. The color is pretty close to natural color thanks to RGB filters, the image was brightened to bring out the darker features.
NASA’s description of the features: The large basin at lower right, within the dark terrain, is named Turgis. The slightly smaller crater at the nine o’clock position is Falsaron. The two prominent craters just above image center are Roland and Turpin. At the limb around the three o’clock position is the darkened rim of the crater Naimon.
The image was produced by Tilmann Denk at Freie Universität in Berlin. Image Credit: NASA/JPL-Caltech/Space Science Institute
The Cassini spacecraft made a close flyby of Rhea on 9 February 2015 and returned one of the highest resolution color views of the Saturn moon Rhea so far.
From NASA: Images taken using clear, green, infrared and ultraviolet spectral filters were combined to create these enhanced color views, which offer an expanded range of the colors visible to human eyes in order to highlight subtle color differences across Rhea’s surface. The moon’s surface is fairly uniform in natural color.
The largest crater in the view above is 123 km / 76 miles in diameter and is named Wakonda.
The names of surface features follow a naming convention adopted by the International Astronomical Union. In the case of craters on Rhea names are from People and places from creation myths. In this case Wakonda is a term used by Native Americans of the Omaha people when praying, and also applied by them to objects or phenomena regarded as sacred or mysterious.
The image was produced by Heike Rosenberg and Tilmann Denk at Freie Universität in Berlin, Germany.
Cassini captured this image of the Saturn rings and the little moon Prometheus. I got to thinking about the early drawings, in particular the famous 1666 drawing by Robert Hooke showing the shadows from the planet and rings.
As an aside, Robert Hooke is probably one of the most famous scientists many school kids have never heard of.
From afar, Saturn’s rings look like a solid, homogenous disk of material. But upon closer examination from Cassini, we see that there are varied structures in the rings at almost every scale imaginable.
Structures in the rings can be caused by many things, but often times Saturn’s many moons are the culprits. The dark gaps near the left edge of the A ring (the broad, outermost ring here) are caused by the moons (Pan and Daphnis) embedded in the gaps, while the wider Cassini division (dark area between the B ring and A ring here) is created by a resonance with the medium-sized moon Mimas (which orbits well outside the rings). Prometheus is seen orbiting just outside the A ring in the lower left quadrant of this image; the F ring can be faintly seen to the left of Prometheus.
This view looks toward the sunlit side of the rings from about 15 degrees above the ringplane. The image was taken in red light with the Cassini spacecraft wide-angle camera on Jan. 8, 2015.
The view was obtained at a distance of approximately 566,000 miles (911,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 37 degrees. Image scale is 34 miles (54 kilometers) per pixel.
Here’s an image of the Saturn moon Iapetus from the Cassini spacecraft. It is the third largest moon of Saturn with a diameter of 1,417 km / 914 miles or about 40 percent of our moon . Iapetus has density of 1.1 so we know it is made largely of ices.
It’s a strange moon. Take note of the dark spot, that’s not a shadow. Iapetus has a dark side to it that has nothing to do whether or not it is facing the Sun. What causes the asymmetry? There are a few theories, the latest (and my new favorite) is due to ice migration in what is known as the Thermal Runaway Model.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
Cubism is an artistic movement that featured surfaces of geometrical planes in France beginning in the early 1900’s.
Cassini got into the act back in December 2014 with this image. Usually I find cubist art pretty straightforward, this one is a little confusing so here is the description from JPL:
Sometimes at Saturn you can see things almost as if from every angle at once, the way a Cubist might imagine things. For example, in this image, we’re seeing Saturn’s A ring in the lower part of the image and the limb of Saturn in the upper. In addition, the rings cast their shadows onto the portion of the planet imaged here, creating alternating patterns of light and dark. This pattern is visible even through the A ring, which, unlike the core of the nearby B ring, is not completely opaque.
The ring shadows on Saturn often appear to cross the surface at confusing angles in close-ups like this one. The visual combination of Saturn’s oblateness, the varying opacity of its rings and the shadows cast by those rings sometimes creates elaborate and complicated patterns from Cassini’s perspective.
This view looks toward the sunlit side of the rings from about 19 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 5, 2014.
The view was obtained at a distance of approximately 1.2 million miles (2 million kilometers) from Saturn. Image scale is 7 miles (11 kilometers) per pixel.
Cassini took this image of Mimas from just 50,000 km / 31,000 miles on 05 June 2012. North is up and rotated four degrees to the left.
Mimas shows a surface with numerous impact craters. Generally the more craters there are, the older the terrain is. As always there are other factors like surface renewal processes which can erase craters on a few moons. In most cases the rule of thumb prevails.
Saturn’s main rings, seen here on their “lit” face, appear much darker than normal. That’s because they tend to scatter light back toward its source — in this case, the Sun.
Usually, when taking images of the rings in geometries like this, exposures times are increased to make the rings more visible. Here, the requirement to not over-expose Saturn’s lit crescent reveals just how dark the rings actually become. Scientists are interested in images in this sunward-facing (“high phase”) geometry because the way that the rings scatter sunlight can tell us much about the ring particles’ physical make-up.