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== Physical characteristics ==
== Physical characteristics ==
Pluto has a density of 2,030 kg/m<sup>3</sup>, comparable to that of [[Ceres]]. It also has a brownish-red color, like that of [[iron]] rust. Significantly, [[Charon]], its innermost [[moon]], does not have that color.
[[File:Frozen carbon monoxide pluto.jpg|250px|right]]Pluto has a density of 2,030 kg/m<sup>3</sup>, comparable to that of [[Ceres]]. It also has a brownish-red color, like that of [[iron]] rust. Significantly, [[Charon]], its innermost [[moon]], does not have that color.
Conventional astronomers attribute the rust color of Pluto to a class of substances called "tholins" (literally, "mud-colored stuff") forming from the bombardment of hypothesized atmospheric constituents by ultraviolet radiation. But Walter T. Brown has another theory: the substance giving Pluto its rust color really ''is'' iron rust. Which, he says, formed far closer to the [[Sun]] than Pluto now orbits. (More on that below.)
Conventional astronomers attribute the rust color of Pluto to a class of substances called "tholins" (literally, "mud-colored stuff") forming from the bombardment of hypothesized atmospheric constituents by ultraviolet radiation. But Walter T. Brown has another theory: the substance giving Pluto its rust color really ''is'' iron rust. Which, he says, formed far closer to the [[Sun]] than Pluto now orbits. (More on that below.)