New Horizons would soon pass Pluto in July, and the author—along with many admirers of this once-classical planet, while acknowledging its reclassification—was eagerly waiting for the spacecraft to return humanity's first close-range images of Pluto. This article considers what those images might reveal about Pluto's surface. [source]
Triton, Neptune's moon. [source]
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| Image from the original article |
The evidence discussed here suggests a possible mental picture of Pluto resembling what Voyager had earlier shown us at Triton, Neptune's large moon. Why?
Astronomers believe Triton, unlike many other planetary moons, formed somewhere other than its present orbit around Neptune. Most regular moons are thought to have formed from disks of material surrounding their parent planets. Triton does not fit that pattern. Its orbital direction is opposite Neptune's rotation, and its orbit is strongly inclined: the source gives an inclination of about 157 degrees relative to Neptune's equator. Both characteristics suggest that Triton probably formed elsewhere—possibly in the Kuiper Belt—and was later captured by Neptune's gravity. In other words, it is thought to have originated as a trans-Neptunian object. [source] [source] [source]
The article argues that a broadly similar story may apply to Pluto. Pluto's orbital eccentricity, elliptical orbit and inclination, together with its failure to follow the general Solar System pattern in which the outer planets are gas giants, are presented as clues that Pluto may also have formed farther out, perhaps somewhere in the Kuiper Belt. [source] [source] [source]
Other hypotheses exist for the formation locations of both Pluto and Triton, each with strengths and weaknesses. Regardless of exactly where they formed, the idea that the two objects originated in similar regions is strengthened, the author argues, by several additional similarities. [source]
They are approximately the same size, with Triton only somewhat larger than Pluto. The article interprets this as suggesting formation in a region of the primordial Solar System disk whose material density limited their growth. Their physical characteristics are also similar. The source gives surface temperatures of about −237.6°C for Triton and −229°C for Pluto.
Their surface and atmospheric chemistry was also thought to be similar, based on Voyager observations of Triton and Hubble spectroscopy of Pluto. The source states that roughly 55% of Triton's surface, like Pluto's, consists of frozen nitrogen, and that Triton's atmosphere, again like Pluto's, may arise partly from sublimation of surface nitrogen.
Triton's reddish-brown appearance is thought to result from the formation of tholins when solar ultraviolet radiation acts on methane ice. Hubble images of Pluto likewise showed coloration consistent with such material. Taken together, these similarities led the author to predict that New Horizons might reveal a Pluto surface bearing considerable resemblance to Triton's—as if the two bodies were twins.
The author stresses that this was not a claim of certainty and did not imply that New Horizons would return images identical to Voyager's views of Triton. Pluto, like other worlds first encountered by spacecraft, would undoubtedly have its own surprises. The Solar System has a way of producing unexpected sights, and the author eagerly awaited New Horizons to reveal them.