Servo - May 2015

spacecraft was acting as a data relay set on the horizon. That low transmission path led to some interesting radio multipath effects (see the May 2006 issue of SERVO).

Not all deserts have sand dunes, but, off in the distance in its descent images, a couple of long, narrow dark sand dunes can be seen. These dunes and tens of thousands more were mapped over subsequent years by Cassini’s radar instrument (since the project began, I’d morphed from aerospace engineer into planetary scientist and geologist — I led planning of the radar observations and reported the discovery of Titan’s dunes in 2006) and remarkably cover about 15% of Titan’s surface area — a much bigger fraction than on Earth or Mars (with a diameter of 5,150 km, Titan itself is larger than the planet Mercury, and is only just the second largest moon in the solar system).

Titan’s dunes are the size and shape of the largest dunes on Earth — in the Arabian and Namib deserts, tens to hundreds of km long, regularly spaced 2-3 km apart, and often over 100 m high!

So, Titan is a desert world, right? Well, no. It turns out that Titan — which has an equatorial tilt ( 26. 7 degrees) much like the Earth’s — has seasons, and gets much more rainfall at higher latitudes than low ones. Its seasons are long, since Saturn and Titan take 29. 5 Earth years to go around the sun. When Cassini arrived, it was just past the peak of southern midsummer (2002), and Cassini’s cameras and big telescopes on Earth (like the Keck) snapped methane clouds puffing up around the south pole where

46 SERVO 05.2015

Figure 8. Sun glinting off the dead flat surface of the hydrocarbon lake, Jingpo Lacus on Titan in 2009, as seen in the near-infrared (five microns wavelength) by Cassini's visual and infrared mapping spectrometer.

Figure 7. Cassini camera color-composite of Titan in 2014. This portrayal is slightly false-color — the red channel in the picture here is actually of light in the near-infrared (940 nm — the same wavelength as a typical TV remote control) which pierces the orange smog better than visible light. The North pole is towards the upper left, and the irregular dark spots there are Titan's seas.

The fainter, larger dark areas at the right of the image are the equatorial sand seas.

Resources

Author www.lpl.arizona.edu/~rlorenz Author talk (SETI Institute) about exploring Titan www.youtube.com/watch?v=cgyWaoaIW3Q NASA Cassini website, with latest pictures of Titan http://saturn.jpl.nasa.gov NASA Institute for Advanced Concepts (NIAC) www.nasa.gov/directorates/spacetech/niac/ Huygens descent videos Dashboard version showing radio data and image/sensor acquisition sequence www.youtube.com/watch?v=QbmcoL3OqPk Narrated version for 10th Anniversary www.youtube.com/watch?v=9L471ct7YDo Related N&V /SERVO articles R D Lorenz, “Demonstrations of Multipath Interferences,”