This delightfully detailed false color image of Saturn was earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January 1998 and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn’s upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn’s many moons have also put in an appearance, Tethys just beyond the planet’s disk at the upper right, and Dione at the lower left.
Image credit: E. Karkoschka (University of Arizona), HST, NASA
NASA’s Cassini spacecraft revealed Dione’s atmosphere during a close flyby of the Saturnian satellite. Cassini’s data showed that Dione leaves behind “fingerprints” as it sweeps through Saturn’s huge magnetic field.
The moon is about 1.5 times as dense as liquid water, leading scientists to surmise that it’s made mostly of water ice with a rocky core.
However, Dione isn’t massive enough to hold on to a substantial atmosphere in the same way Earth does. Earth and other large bodies boast strong gravitational fields, which prevent atmospheric particles from escaping into space. Dione’s atmosphere lacks this gravitational aid—the moon’s thin layer of air exists only because it’s constantly being recharged.
Saturn is surrounded by a belt of highly energetic particles, akin to the Van Allen belts around Earth. Dione is located in this belt, and the reason it possesses an atmosphere is that these hot and very fast particles continuously splatter on the moon’s surface. When the particles hit Dione, they cause the moon’s surface ice to break apart chemically, releasing molecules that become the moon’s atmosphere.
This animation is composed of sixteen wide-angle frames captured by Cassini on July 29, 2007. The images were taken through a filter in a methane “window,” so details in Saturn’s clouds and on Titan’s surface that are usually obscured by methane haze are visible. Each image has been colorized based upon other natural-color Cassini images.
Titan transits Saturn from left to right as much smaller, brighter Dione moves from right to left at the extreme right edge of this image. Saturn’s rings are seen edge-on so are almost invisible. The rings cast broad black shadows on Saturn’s northern hemisphere. The full-size version is best viewed as a Quicktime movie (264k)
Credit: NASA / JPL / SSI / color and animation by Gordan Ugarkovic
Tetradrachm of King Pyrrhus of Epirus, Lokroi Epizephyrioi mint, c. 297-272 BC
A masterpiece of Hellenistic engraving and one of the finest known.
The tetradrachm shows Dodonean Zeus, whose sanctuary was at Dodona in Epirus. Epirus achieved fame during the reign of Pyrrhus, whose campaigns against Rome are the origin of the term “Pyrrhic victory”. Pyrrhus’ army suffered irreplaceable casualties in defeating the Romans at Heraclea in 280 BC and Asculum in 279 BC during the Pyrrhic War. After the latter battle, Plutarch relates in a report by Dionysius:
The armies separated; and, it is said, Pyrrhus replied to one that gave him joy of his victory that one more such victory would utterly undo him. For he had lost a great part of the forces he brought with him, and almost all his particular friends and principal commanders; there were no others there to make recruits, and he found the confederates in Italy backward. On the other hand, as from a fountain continually flowing out of the city, the Roman camp was quickly and plentifully filled up with fresh men, not at all abating in courage for the loss they sustained, but even from their very anger gaining new force and resolution to go on with the war. —Plutarch
On the coin, Dodonean Zeus faces left, wearing oak wreath; Θ and monogram below. On the reverse, BAΣIΛEΩΣ ΠYPPOY, Dione seated left, holding staff in right hand and lifting her veil with her left; A in exergue.
Ancient Epirus was a region located in southeastern Europe, shared between Greece and Albania.