Compare the Planets
Comparing the physical characteristics of the worlds in our solar system (and beyond)
The worlds of our solar system come in all shapes, sizes, and colors. Red-eyed Jupiter, ringed Saturn, and frigid Uranus and Neptune are giant gassy globes containing nearly all of the matter in the solar system. These Jovian planets, or gas giants, are huge worlds of air, clouds, and fluid that may have no solid surfaces no matter how deep you go. Everything else in the solar system is just rock, ice, and dust. The largest rockballs are known as the terrestrial planets: Mercury, Venus, Earth, and Mars, with our Moon usually considered part of the club, and now Vesta is applying for membership. Earth is the biggest of all the rocky worlds.
But the planets are not the only worlds of the solar system. All but two of the planets are orbited by moons, each of them a world unto itself. The largest moons are bigger than the smallest planets, and 16 or 17 would qualify as dwarf planets if they orbited the Sun. There are more than 100 Kuiper belt dwarf planets, but only one among the asteroids, Ceres.
Six solid worlds -- Venus, Earth, Mars, Titan, Triton, and Pluto -- have atmospheres dense enough to produce weather. Eris likely does, when it is near its perihelion. We have witnessed active geology on four worlds -- Earth, Io, Enceladus, and Triton -- and we suspect it on Venus, Europa, and Titan. Comparing the same processes across many worlds helps us to understand how each planet's unique composition and history influence its present state, and will help us predict what to expect on Earth in the future.
Pretty Pictures with Many Worlds
On March 3, 2010, Cassini flew within about 2,000 kilometers of Dione's co-orbital moon Helene, snapping a series of images with its narrow-angle camera. Most of those photos missed Helene entirely; toward the end of the encounter, some caught Helene in the corner of the image. In the background are the cloud tops of Saturn. This image was artificially colorized; the data is from a single green-filter image of Helene and Saturn.
Dione passed in front of Titan on March 12, 2010 as Cassini snapped photos. Late in 2009, Cassini planners started commanding the spacecraft to take these "mutual event" movies not through a clear filter but instead through red, green, and blue filters sequentially; the raw data exists to produce an animation of Dione passing across Titan in color.
Cassini captured this picture-postcard view of the moons Mimas (top) and Tethys (bottom) floating near the rings on March 15, 2010.
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