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Pictured here are all three terrestrial planets other than Earth, accompanied by the seven largest moons of the Solar System with a selection of smaller moons. 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). In reality, Venus is covered by a thick cloud layer that renders it as featureless as Titan.
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).
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). Rhea has 50% of Europa's diameter, but only 5% of its mass.
Triton may actually be a captured Kuiper Belt object rather than a co-formed moon of Neptune. Triton is somewhat larger and more massive than Pluto and Eris, the largest known Kuiper Belt objects.
Rhea's surface looks remarkably like the highland regions of Luna, with a radiating crater reminiscient of Luna's Tycho. However, Rhea is composed primarily of ice, while Luna consists of silicates.
See also Galilean Moons of Jupiter.
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