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Pictured here are all three terrestrial planets other than Earth, accompanied by the seven largest moons of the Solar System, the two largest Kuiper Belt objects (Eris and Pluto), two smaller moons (Titania and Dione), and the largest asteroid (Ceres). The image of Venus was created by compositing radar scans of the planet's surface and projecting them onto a globe (courtesy Magellan Project/NASA/JPL-Caltech). Venus is actually covered by a thick cloud layer that renders it almost featureless. Titan, the largest moon of Saturn, is shown in an infrared view to reveal more detail. This moon is also enveloped in thick hazy clouds, like Venus but orders of magnitude colder.
Although this smooth gradation of sizes might suggest an equally smooth range of masses, terrestrial planets and moons reveal unexpected extremes.
- Venus has less than twice the diameter of Mars but more than seven times its mass (0.81 M
EARTH vs. 0.11 MEARTH). This may indicate that Venus has a greater concentration of heavy metals in its deep interior.
Mercury is slightly smaller than Ganymede and Titan, yet it is more than twice as massive. Mercury is 0.060 MEARTH, Ganymede is 0.0250 MEARTH, and Titan is 0.0225 MEARTH. This disparity is an indication that Mercury is composed entirely of heavy elements, while Ganymede and Titan, like all the moons of the outer Solar System, contain a large proportion of ices.
Ganymede is about 40% volatiles and 60% silicate rock and metals. Much of its volatile mass may exist in the form of water, as ice and perhaps as subsurface liquid (Kuchner 2003). A similar icy envelope and subsurface ocean are predicted for Europa.
Beginning with Triton, the smaller moons of the Solar System become still more icy. Triton is less than half as massive as Europa, although its diameter is similar (1,675 miles vs. 1,940 miles).
Triton may actually be a captured Kuiper Belt object rather than a co-formed moon of Neptune. It is somewhat larger and more massive than Pluto and Eris, but otherwise quite similar to them.
Although Ceres was regarded as a planet for the first few decades after its discovery in the nineteenth century, a number of additional, smaller objects were soony detected on similar orbits, demonstrating that Ceres is simply the largest of the asteroids. These are mostly rocky objects left over from the primeval Solar System. While many moons and Kuiper Belt objects are spherical, Ceres is evidently the only asteroid massive enough to assume sphericity.
See also Galilean Moons of Jupiter.
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