This image from NASA’s Cassini spacecraft shows three moons — Rhea (top), Titan and Mimas (bottom). Titan appears fuzzy because we only see its cloud layers and light refracts within its atmosphere, extending the crescent. Image credit: NASA/JPL-Caltech/Space Science Institute.The three moons shown here — Titan (3,200 miles or 5,150 kilometres across), Mimas (246 miles or 396 kilometres across), and Rhea (949 miles or 1,527 kilometres across) — show marked contrasts. Titan, the largest moon in this image, appears fuzzy because we only see its cloud layers. And because Titan’s atmosphere refracts light around the moon, its crescent “wraps” just a little further around the moon than it would on an airless body. Rhea (upper left) appears rough because its icy surface is heavily cratered. And a close inspection of Mimas (centre bottom), though difficult to see at this scale, shows surface irregularities due to its own violent history.
This view looks toward the anti-Saturn hemisphere of Titan. North on Titan is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on March 25, 2015.
The view was obtained at a distance of approximately 1.2 million miles (2.0 million kilometres) from Titan where the image scale is 75 miles (121 kilometres) per pixel. Mimas was 1.9 million miles (3.1 million kilometres) away with an image scale of 11.4 miles (18.4 kilometers) per pixel, while Rhea was 2.2 million miles (3.5 million kilometres) away with an image scale of 13.1 miles (21.1 kilometres) per pixel.
As avid skywatchers will already know, all of the bright naked-eye planets are currently visible in the pre-dawn sky — the first time in eleven years that such an alignment has occurred. At 6am GMT on Monday, 1 February, the last quarter Moon in the constellation Libra lies just 2½ degrees from magnitude +0.8 planet Mars low in the south for UK observers.
The existence of a fifth giant gas planet at the time of the solar system’s formation — in addition to Jupiter, Saturn, Uranus and Neptune that we know of today — was first proposed in 2011. Now astrophysicists at the University of Toronto have found that a close encounter with Jupiter about four billion years ago may have resulted in the fifth giant planet’s ejection from the solar system altogether.
We may be losing Jupiter in the west at dusk, but two other planets are well placed in the late evening. Skywatchers in the UK and Western Europe should look low in the southern sky around 12am local time on 17, 18 and 19 June to see the waxing gibbous Moon in the vicinity of planets Mars and Saturn, plus first-magnitude star Antares in the constellation of Scorpius.
