January 11 – Oberon and Titania

January 11th 1787 was a good day for William Herschel.  Not many of us get to discover even one major moon in a lifetime, but on this particular day in history Herschel was fortunate enough to find both the largest moon of Uranus, Titania, and the second largest, Oberon.

Oberon is the outermost major Uranian moon, and the ninth most massive moon in the solar system.  It is about 50/50 rock and ice, with the rock probably forming the core.  It’s quite “moonish” in appearance, being fairly dark and rather cratered by impacts like our own companion.  However, unlike our dry Moon, as well as being partly icy, Oberon may well have a layer of liquid water between the core and the mantle.

Oberon from Voyager 2 (image: NASA)

Oberon from Voyager 2 (image: NASA)

Titania is similar in appearance and composition to Oberon, but is less heavily cratered, possibly as a result of internal processes that removed some of the older cratering.  Both moons were probably formed in situ from an accretion disc.

Titania from Voyager 2 (image: NASA)

Titania from Voyager 2 (image: NASA)

Only Voyager 2 has ever been close enough to Uranus to capture good shots of Oberon and Titania.  It made its closest approach to the planet on January 26th 1986, but had already got to about 300,000 miles of Titania and 400,000 miles from Oberon two days earlier, at which time it took these photographs.


1865   –   German astronomer Johannes Franz Hartmann born.


 

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