Tethys Takes a Look at Saturn

Tethys and SaturnIn this picture taken by the Cassini spacecraft the two large craters on Tethys near the line where day fades to night seem to be looking at Saturn. (Click the image to embiggen it.)

The shadowing on the craters caused by being near Tethys’ terminator throws their topography into sharp relief. The larger, southernmost of the two shows a more complex structure. Its central peak is  probably the result of the surface reacting to the violent post-impact excavation of the crater. The northern crater doesn’t have a similar feature. The impact was likely too small to form a central peak, or the composition of the material in the immediate vicinity couldn’t support the formation of a central peak.

Image Credit: NASA

Mimas

Mimas is one of Saturns moons. It’s about 130 km in diameter, one of the smallest bodies in the Solar System with sufficient gravity to pull itself into a spherical shape. This picture was taken in 2010 by the Cassini spacecraft.

The Gentle Reader may make his own moon-not-a-space-station or AT&T-naming-rights jokes.

Image Credit: NASA

Hyperion

HyperionHyperion is one of Saturn’s moons. It is named for one of the Titans who was the Greek god of watchfulness and observation and the older brother of Cronus. Saturn was the analog of Cronus in Roman mythology.

Hyperion is one of the largest irregularly shaped bodies in the Solar System, and it rotates chaotically, tumbling unpredictably as it orbits Saturn. That made it  challenging to target a specific region of the moon’s surface for observation by the Cassini spacecraft, and most of Cassini‘s approaches saw the same side of the craggy moon. The view above is from a closest encounter in 2005.

BTW, the first time I saw this picture, I was reminded of a wasps’ nest.

Image Credit: NASA

That’s No Space Station. It’s a Moon.

TethysSaturn’s moon Tethys’s trailing side shows two terrains that tell a story of a rough past. To the north (up in this picture) is older, rougher terrain, while to the south is new material dubbed “smooth plains” by scientists. The smooth plains are roughly antipodal to the large impact crater Odysseus. Odysseus, which is on the far side of Tethys, is out of view. The leading theory is that the impact that created Odysseus also created the smooth plains, although exactly how this happened is not yet clear.

Image Credit: NASA

We Can’t See This *On* The Earth

aurora uvThis is the first image of Saturn’s aurora that was taken by the Hubble Space Telescope in 1997 when Saturn was 1.3 billion km from Earth. Saturn’s auroral displays are caused by an energetic wind of charged particles from the Sun that sweeps over the planet. Unlike the Earth’s, Saturn’s aurora is only seen in ultraviolet light. Because the UV doesn’t penetrate our atmosphere, Saturn’s aurora can only be observed from space.

Image Credit: NASA

A Speck in the Corner

Saturn & TethysAt 116,500 km across, Saturn is roughly 10 times the diameter of Earth. The planet is much larger in relation to its moons than our Earth to its Moon. Saturn’s moon Tethys (which is a bit more than 1,000 km in diameter and could be counted as a dwarf planet it orbited the Sun by itself) can be seen as a speck in the lower right of the picture.

Image Credit: NASA

The Star of Bethlehem

Roughly every twenty years, the paths of Jupiter and Saturn line up in the night sky, and the planets appear close together, an event called the Grand Conjunction. One occurs this evening. Look toward the southwest just after sunset, and if the sky is clear, you’ll see Jupiter and Saturn almost perfectly aligned, only about 0.1 degree apart. They haven’t come this close since 1623, but they were nearly aligned with the Sun and hard to see that year. The last time they were this close and relatively far from the Sun was in 1226.Grand Conjunctions occurred three times in 7 BC and again as a triple conjunction with Jupiter, Saturn, and Mars in early 6 BC. You can find more about those conjunctions and the Star of Bethlehem here.

A Lunar Lineup

Enceladus_Tethys_bullseyEnceladus and Tethys line up almost perfectly in this shot from the Cassini spacecraft. Since the two moons are not only aligned, but also at nearly the same distance from Cassini, their apparent sizes are a reasonable approximation of their relative sizes. Enceladus is 504 km across, and Tethys is 1,062 km in diameter.

Image Credit: NASA

A Pair of Moons

Dione_EnceladusAlthough Saturn’s moons Dione (in the foreground) and Enceladus are made of more or less the same stuff, Enceladus has a considerably higher reflectivity than Dione. Therefore, it appears brighter against the blackness of space.

Enceladus has a constant rain of ice grains from its south polar jets which cover its surface with a bright snow. Dione’s older, weathered surface has slowly gathered dust and radiation damage, darkening through a process known as “space weathering.”

Image Credit: NASA

Look Closely, You’re In This Picture

Click the image to embiggen it. No, really, do it, and click on the new image a second time. You can use your BACK button to return.saturn_full_annotated

On 19 July, 2013,  the Cassini spacecraft slipped into Saturn’s shadow and turned to image the planet, seven of its moons, its inner rings,and—in the background—Earth.

With the Sun eclipsed by Saturn, Cassini‘s cameras were able to take advantage of this unusual viewing geometry. A panoramic mosaic of the Saturn system was taken that allows details in the rings backlit by the sun to be seen. This event was the third time Earth was imaged from the outer solar system.

Cassini captured 323 images in just over four hours. This final mosaic uses 141 of them. Images taken using the red, green, and blue spectral filters of the wide-angle camera were combined to create this natural-color view. This image spans a bit more than 650,000 km.

Make sure you embiggen it and scroll around.

Image Credit: NASA

Titan and Tethys

converted PNM fileSaturn’s moon Tethys with its prominent Odysseus Crater seems to lurk behind Saturn’s largest moon Titan in this image taken by the Cassini spacecraft in 2014.

The Titans were the pre-Olympian gods in Greek mythology. Tethys was a Titan daughter of Uranus and Gaia, sister and wife of the Titan Oceanus, and mother of the river gods and the Oceanids

Image Credit: NASA

Crescent Saturn

On Earth we never see Saturn in a crescent phase because it is farther from the Sun than Earth, and it is always fully illuminated from our point of view. The Cassini spacecraft’s orbits around the planet allowed its cameras to see Saturn in ways not possible from Earth.

Image Credit: NASA

Saturn and Some of Its Moons

The animation shows the orbits of Saturn’s visible moons Tethys, Janus, Mimas, Enceladus, and Rhea over the observing run in June, 2019 (with elapsed time bar).

Video Credits: NASA / ESA / A. Simon (Goddard Space Flight Center) / M.H. Wong (University of California, Berkeley) / J. DePasquale (STScI)