Iapetus, moon of Saturn, discovered 1671

Iapetus, the third largest moon of Saturn, was discovered by Giovanni Cassini, October 25, 1671.  It is unique for several reasons.  First, it has a dark region, known as Cassini Regio, which is contrasted with the brightness from ice on the rest of the moon.  Ice covers most of the moon.

Second, Iapetus has an inclined orbit, more than any of the regular, i.e. largest, moons.  The rings of Saturn would be clearly visible because of its inclined orbit.  

Third, it has an equatorial ridge that runs through the center of  Cassini Regio.  Mountains in this ridge rise over 12 miles of the surrounding plains, making them among the tallest mountains in the solar system.  Mons Olympus on Mars is the tallest in the solar system. 

The source of the dark material and why the equatorial ridge exists only in Cassini Regio are unknown.

The spacecraft Cassini passed by at a range of 1,227 km (762 miles) on September 10, 2007. 

Cassini-Huygens probe launched 1997

A storm in the northern hemisphere photographed by Cassini in 2011.

The Cassini-Huygens spacecraft, launched October 15, 1997, was a collaboration between NASA, the European Space Agency and the Italian Space Agency. Cassini was the fourth space probe to encounter Saturn but the first to enter into orbit. 

The Cassini probe was named after Giovanni Cassini, an Italian astronomer who discovered four of Saturn's moons and noted the separation of her rings.

The lander Huygens lander was named after Dutch astronomer Christiaan Huygens, who discovered Titan and is noted for his studies of Saturn's rings.

Cassini arrived in July 2004 and spent the next 13 years, far beyond its intended mission, studying the planet.  It was intentionally sent plunging into Saturn's atmosphere September 2017, to destroy it, preventing it from contaminating other celestial bodies.

The lander Huygens descended to the surface of Titan January 2015.  It took the first pictures from the surface of another moon besides ours.