Vast regions of dark dunes also extend across Titan's exotic landscape, especially around its equatorial regions. Unlike Earth's sand, the "sand" that creates Titan's dunes is composed of dark grains of hydrocarbon that resemble coffee grounds. The tall linear dunes of this misty moisty moon-world appear to be quite similar to those seen in the desert of Namibia in Africa. Because Titan's surface is pockmarked by relatively few impact craters, its surface is considered to be quite young. Older surfaces display heavier cratering than more youthful surfaces, whose craters have been "erased" by resurfacing. This resurfacing is caused by processes that cover the scars left by old impacts as time goes by. Our own planet is similar to Titan in this respect. The craters of Earth are erased by the ongoing processes of flowing liquid (water on Earth), powerful winds, and the recycling of Earth's crust as a result of plate-tectonics. These processes also occur on Titan, but in modified forms. In particular, the shifting of the ground resulting from pressures coming from beneath (plate tectonics), also appear to be at work on this veiled moon-world. However, planetary scientists have not seen signs of plates on Titan that are analogous to those of our own planet. Until 2004, no spacecraft had visited Saturn for more than twenty years. Pioneer 11 had snapped the very first up close and personal images of Saturn when it traveled past it in 1979, Voyager 1 flew past Saturn about a year later, and in August 1981, Voyager 2 had its own fleeting but very productive close encounter. At last, on July 1, 2004, NASA's Cassini spacecraft went into orbit around Saturn, and started taking very revealing photographs of this bewitching planet and its many lovely and bizarre moons. Astronomers are still debating Titan's origin. However, its intriguing atmosphere does provide a hint. Several instruments aboard the Huygens spacecraft measured the isotopes nitrogen-14 and nitrogen-15 in Titan's atmosphere. The instruments revealed that Titan's nitrogen isotope ratio most closely resembles that seen in comets that exist in the remote Oort Cloud--which is a sphere composed of hundreds of billions of icy comet nuclei that circle our Star at the amazing distance of between 5,000 and 100,000 AU. This shell of icy objects extends half way to the nearest star beyond our own Sun.