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 its 956th sol (day) on Mars, the rover Curiosity used its Mastcam to image a stunning sequence of the Sun (Sol) setting over the local horizon at Gale Crater, where it has been exploring since August 2012. The individual images taken by Curiosity covered a period of about 6 minutes. The sequence of images was too intermittent to make smooth movie of the sunset on its own. Using the foreground and horizon from one image and then recreating the sky and Sun in Photoshop, Glen Nagle used Adobe Premier to create a near real-time sunset sequence as if you could stand on Mars and see it for yourself. With the addition of a little 'lens flare' and the haunting music 'Lux Aeterna' by György Ligeti, this imagining of a sunset on Mars evokes a sense of awe and reverence.
There are four moons in the solar system that are known to have collisional atmospheres. In order of decreasing atmospheric column density, they are: Saturn's Titan, Neptune's Triton, and Jupiter's Io and Callisto.
On March 25, 2015, the Cassini spacecraft captured three of Saturn's moons in the same shot. From left to right: Rhea, Titan, and Mimas. (Mimas is barely visible below and to the right of Titan.)
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