Rhea Before Titan

Craters appear well defined on icy Rhea in front of the hazy orb of the much larger moon Titan in this Cassini spacecraft view of these two Saturn moons.
Lit terrain seen here is on the leading hemispheres of Rhea and Titan. North on the moons is up and rotated 13 degrees to the left. The limb, or edge of the visible disk, of Rhea is slightly overexposed in this view.
The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Dec. 10, 2011. The view was acquired at a distance of approximately 1.2 million miles (2 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 109 degrees. The view was acquired at a distance of approximately 810,000 miles (1.3 million kilometers) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of 109 degrees. Image scale is 8 miles (12 kilometers) per pixel on Titan and 5 miles (8 kilometers) per pixel on Rhea.
Image credit: NASA/JPL-Caltech/Space Science Institute

Saturn moon’s atmosphere forged by comet impacts

Titan is the only moon in the solar system with much atmosphere

The unique atmosphere of Saturn’s moon Titan could have been created by comets blasting gases out of its icy crust.
Titan is the only moon in the solar system with much atmosphere, and the origin of its nitrogen-rich air is a puzzle. There are several theories: volcanic activity may have belched it out, or sunlight may have broken up a primordial atmosphere’s ammonia molecules. But these suggestions assume that the young Titan was a warm world, whereas measurements by the Cassini spacecraft imply that Titan has always been fairly cold.
The latest idea is that the atmosphere was created 3.9 billion years ago in a period known as the late heavy bombardment, when comets swarmed through the solar system. “Huge amounts of cometary bodies would have collided with outer icy satellites, including Titan,” says Yasuhito Sekine of the University of Tokyo, Japan.
To mimic the effects of such high-speed impacts, Sekine and his colleagues fired projectiles into a mixture of ammonia and water ice similar to Titan’s crust. The impacts converted some of the ammonia into nitrogen gas, and Sekine’s team calculates that ancient comet impacts could have liberated enough nitrogen to build Titan’s atmosphere. Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1147
newscientist.com