Servo - May 2015

Think again. Our seas are not alone — Oceanography is no longer just an Earth science. Saturn’s giant moon, Titan has been surveyed for the last decade by the formidable Cassini-Huygens mission. That

$4B project (on which I started my career a quarter century ago as an aerospace engineer straight out of college, working for the European Space Agency, ESA) saw a launch from Florida in 1997 and a seven year journey past Venus (twice), Earth, and Jupiter — epic before its science mission even started!

Shortly after arrival at Saturn in 2004, the four ton orbiter spacecraft (built primarily at NASA’s Jet Propulsion Laboratory in Pasadena, CA) released the 319 kg Huygens probe to parachute down to Titan’s surface. It did so 10 years ago (January 14, 2005), hitting the ground near Titan’s equator in what turned out to be damp sand.

However, it was damp with something other than water. Titan — being ten times further from the Sun than is the Earth — is a frigid 94K (-189°C), where water is ice and is as hard as many rocks on Earth. Instead, it was damp with hydrocarbons, liquid methane, and ethane, which we know on Earth as the main components of natural gas. Indeed, natural gas is transported around the Earth in large refrigerated ships which keep the gas in liquid form at conditions rather close (~100K, 1.1 bar pressure) to Titan’s surface conditions.

Among Titan’s many remarkable features is that it has an atmospheric pressure somewhat larger than Earth’s (1.5 bar), with the atmosphere made mostly of nitrogen like our own. That composition, temperature, and pressure mean the air on Titan is four times denser than air on Earth, making Titan an easy place on which to fly. Easier still, because Titan’s gravity is about the same as that of the Earth’s moon, or 1.35 m/s — a seventh of Earth’s. Titan is the only moon in the solar system on which a parachute is of any use!

The atmosphere has no oxygen, but does have a few percents of methane vapor — much like the

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Figure 1. Titan at lower left (infrared false-color view, showing dark dune fields near equator), compared with the Earth and moon at the same scale.

Courtesy Wikipedia.

Figure 2. The author in 2005 at the European Space Operations Center in Darmstadt, Germany for Huygens' historic descent to Titan's surface. This full-scale model of the Huygens probe was used in a stratospheric drop test to verify the heat shield separation and parachute performance in 1995.