Short answers, pulled from the story.
Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. It was the sixth extraterrestrial moon discovered in human history, and the second moon of Saturn found after Christiaan Huygens spotted Titan in 1655.
The leading hemisphere of Iapetus is dark because it sweeps up dust spiraling inward from outer moons. Once that dark coating created a temperature difference, a thermal runaway amplified it: the darker surface absorbs more heat, ice sublimates from the warm dark areas and deposits on the colder bright regions, making dark areas darker and bright areas brighter over billions of years.
The equatorial ridge is a mountain system running roughly 1,300 km along the center of Cassini Regio, about 20 km wide and 13 km high. Its peaks rise more than 20 km above the surrounding plains, placing them among the tallest mountains in the Solar System. The ridge was confirmed by the Cassini spacecraft on the 31st of December 2004.
Iapetus has an estimated diameter of 1,469 km, making it the third-largest moon of Saturn and the eleventh-largest moon in the Solar System.
Iapetus has a bulging equatorial waistline and compressed poles rather than a spherical or ellipsoidal form. Its equatorial ridge is so prominent that it visibly distorts the moon's silhouette even from a distance, giving it its characteristic walnut-like appearance.
The Cassini orbiter made its closest approach to Iapetus on the 10th of September 2007, passing within 1,227 km of the moon's surface. Cassini approached from the night side and no further targeted flybys were made after that encounter.