1846

Triton

John Herschel (1792–1871), William Lassell (1799–1880)

The discovery of Neptune in the autumn of 1846 provided a new target for moon hunters—a challenge because of Neptune’s great distance from the Sun. It was the British merchant and amateur-turned-professional astronomer William Lassell who discovered Neptune’s moon just 17 days after Neptune itself was discovered. Lassell had made his money as a beer brewer, and used it to build his own telescope mounts, grinding his own mirrors and assembling a Newtonian reflector 24 inches (61 centimeters) in diameter that was, at the time, the largest functioning telescope in the world. When John Herschel heard about the discovery of Neptune, he suggested to Lassell that he use his telescope to search for moons around the new planet.

It didn’t take long—only eight days of searching—for Lassell to find a moon, and it was a strange one indeed. Tracking its orbit revealed that it was orbiting backward—retrograde—compared to all of the other known moons of the solar system. Further, its orbit is highly tilted to the plane of Neptune’s own orbit, so much so that each of its poles is sometimes pointed almost right at the Sun, like highly tilted Uranus. Lassell didn’t name his discovery; astronomers later agreed on Triton, the Greek sea god and son of Poseidon (the Greek equivalent of Neptune).

Little else was known about distant Triton before the Voyager 2 encounter in 1989, which revealed it to be large (1,678 miles [2,700 kilometers] in diameter), bright (70–80 percent reflective), and with an icy-rocky density of 2.1 grams per cubic centimeter. Most surprising, a very thin nitrogen atmosphere and gently erupting geysers were found, along with a geologically young surface (few craters) that appears to be continually resurfaced through icy cryovolcanism.

Triton’s strange landforms have more recently been found to be composed of nitrogen, water, and carbon dioxide ices only 30 to 40 degrees above absolute zero at the surface but perhaps warmed by radioactive heat in the interior. Triton appears to be a twin of Pluto, and many astronomers believe that it may have formed in the Kuiper Belt and was captured—somehow—by Neptune into its tilted, backward orbit.

Voyager 2 false-color image of Neptune’s large moon Triton, photographed on August 25, 1989. This view shows Triton’s bright southern hemisphere polar cap of nitrogen and methane ices, crisscrossed with ridges and fractures and dotted with streaks from active geyser-like eruptions of nitrogen gas.