Titan: Looking Back to the Future

Titan descent by Huygens probe leaving Cassini storage, Christmas 2004. Image Credit: JPL/Space Science Institute

As the Huygens probe begins its descent through Titan’s thick haze, few can offer the unique perspective of those who were there in the room when the daring concept for the mission was conceived.

University of Arizona Emeritus Professor, Don Hunten, was present in the beginning more than two decades ago, and this week’s culmination is another milestone in his remarkable career as a planetary scientist. When Hunten received the prestigious Fleming Award from the American Geophysical Union in 1998, he was recognized by his peers for his "original research and technical leadership in geomagnetism, atmospheric electricity, aeronomy, and related sciences".

His unique role in the current Cassini-Huygens mission was noted then: "Don developed a model for the atmosphere of Titan prior to the Voyager 1 encounter in 1980 that was so good it became the standard after the data came in confirming it. With his extraordinary intuition and insight, he had correctly surmised that Titan must have a massive, molecular nitrogen (N2) atmosphere, well before there was any detection of N or N2 on this intriguing satellite. In the following decade, Don used his excellent grasp of physics together with his extensive experience in deep space missions to play a critical role in the design of the Cassini-Huygens mission, now safely on its way to Saturn and Titan."

The haze of an atmospheric layer on Saturn’s moon, Titan. Credit: Voyager Project, JPL, NASA

Professor Hunten talked with Astrobiology Magazine about both the past and near-future might hold for the bold idea to land on the tiny Earth-like moon, Titan, so far away in the outer solar system.

Astrobiology Magazine (AM): Few may realize that the mission planning for Cassini-Huygens began more than 2 decades ago. Can you provide some perspective on its genesis as an exploration initiative and how Titan became the first target for a lander mission in the outer solar system?

Don Hunten (DH): Saturn has been a long-term objective of NASA’s planetary program at least since the encounter by Pioneer 11 in 1979, and Saturn and Titan were important targets of Voyagers 1 and 2.

It was studied by a Science Advisory Group, chaired by A. G. W. Cameron, in 1971 (organized by JPL at NASA’s request).

Again at NASA’s request, I organized a Titan Atmosphere Workshop in 1973.

True color and surface infrared images of Titan Image Credit: NASA/JPL

Another study, called "Symposium on Outer Planet Exploration" and again chaired by Cameron, met in 1974 – 75.

A Saturn System Workshop, chaired by myself and David Morrison, met in 1978 and issued a report in 1979.

When I was chair of COMPLEX (Space Science Board’s Committee on Planetary and Lunar Exploration) in 1982 – 85, we studied a mission concept that was essentially Cassini-Huygens plus a Saturn entry probe, and recommended it to NASA with high priority.

Follow-ons to this, concentrating on a Cassini-like mission, were two joint studies with European scientists and managers: Joint Working Groups, first of the Space Science Board with the European Science Foundation (1982 – 83) and then of NASA and the European Space Agency (1984 – 85).

primordial soup
Miller’s classic experimental setup, with a simulated ocean, lightning and broth of hydrogen, methane, ammonia and water.

AM: Carl Sagan and his colleagues began in earnest the replication of Miller-Urey primordial soup experiments specialized to an atmosphere expected on Titan (nitrogen, methane, ethane, ethylene and acetylene). After he published the production of amino acids, did this finding bear on the mission planning for sampling the atmosphere?

DH: Khare and Sagan’s work had very little influence on anyone else’s opinions on the importance of Titan. The tiny amounts of these substances that are realistically expected to exist in Titan’s atmosphere are far less than the capability of any real instrument to detect.

Of course, the Huygens [Gas Chromatograph-Mass Spectrometer] or GCMS (and the Galileo [Mass Spectrometer] MS) will be, (and was) on the lookout for them just in case, and we hope it will continue after landing.

AM: Was Cassini originally conceived as a nuclear-fueled probe, or were alternatives considered?

DH: Alternative power sources are always considered, but they don’t exist for missions so far from the Sun. In fact, Huygens is powered by batteries, as was the Galileo Probe, but they are only good for a few hours once they start to be used. "Nuclear power" is incorrect; there is no nuclear reactor involved, only radioactive decay. Yes, that is a nuclear process, but it is totally different from what happens in a reactor or a weapon.

AM: NASA Administrator, Dan Goldin, once called the protests about Cassini as a nuclear probe to be one of his most difficult launch decisions. Without getting into the environmental politics, can you comment on the past or future scientific requirements for powering probes so far from the Sun?

Titan continues to puzzle scientists who speculate about whether the darker regions are oily lakes.
Credit: NASA/JPL

DH: Radioactive decay, or an actual tiny reactor, are the only options for generating hundreds of watts so far from the Sun for periods of years.

AM: There seems to be some surprise that the first radar assessments of Titan have not revealed yet the obvious smooth surfaces (on the order of 20 cm) that might signal a liquid surface. Some scientists have put on their future wishlist some instrument to assess elevation to connect the dark and light regions with their phase as liquid, solid, or sludge. Can you comment on any historical perspectives in this discussion?

DH: The FIRST radar measurement, by the Arecibo telescope, DID yield strong evidence for specular reflection from some spots. This is a strong reason for the surprise at the negative result (so far) from the Cassini radar. The prediction of abundant liquid was a good one, on the assumption that the atmosphere has been similar to the present one for several billion years. Maybe it was much less dense for most of that time, or maybe it contained much less methane, or maybe the pools are there and Cassini missed them on its first try.

AM: Will you personally be surprised or thrilled if Huygens makes it to the surface for what might extend its mission a few hours and provide data about any surface liquid?

DH: I will be thrilled at whatever result is found upon landing. I will not be surprised if the probe survives its landing, as did one of Pioneer Venus probes. It will be wonderful if the GCMS continues to work and gives good data, but that will be something of a surprise.

Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Prebiotic Laboratory
Planet Wannabe
Where is Cassini Now?