K.S. RAJGOPAL

As the long quest for alien life goes on and with the discovery of earth-like planets outside the solar system in distant galaxies, here is a surprising find in our own neighbourhood. NASA’s Cassini spacecraft has discovered evidence that points to the existence of an underground ocean of water and ammonia on Saturn’s moon Titan. The findings made using radar measurements of Titan’s rotation appear in the March 21 issue of the journal Science.

With its organic dunes, lakes, channels and mountains, Titan has one of the most varied, active and Earth-like surfaces in the solar system.

Nineteen passes

Members of the Cassini mission’s science team used Cassini’s radar to collect imaging data during 19 separate passes over Titan between October 2005 and May 2007.

The radar can see through Titan’s dense, methane-rich atmospheric haze, detailing never-before-seen surface features and establishing their locations on the moon’s surface. Using data from the radar’s early observations, the scientists and radar engineers established the locations of 50 unique landmarks on Titan’s surface.

They then searched for these same lakes, canyons and mountains in the reams of data returned by Cassini in its later flybys of Titan. They found prominent surface features had shifted from their expected positions by up to 19 miles.

Crust movement

A systematic displacement of surface features would be difficult to explain unless the moon’s icy crust was decoupled from its core by an internal ocean, making it easier for the crust to move.

The team members believe that about 62 miles beneath the ice and organic-rich surface is an internal ocean of liquid water mixed with ammonia. The study of Titan is a major goal of the Cassini-Huygens mission because Titan may preserve, in deep-freeze, many of the chemical compounds that preceded life on Earth. The moon’s atmosphere is 1.5 times denser than Earth’s. The combination of an organic-rich environment and liquid water is very appealing to astrobiologists. Further study of Titan’s rotation will enable understanding of the watery interior better, and because the spin of the crust and the winds in the atmosphere are linked, seasonal variation in the spin in the next few years may be seen.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The mission is managed by the Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena. The Cassini orbiter also was designed, developed and assembled at JPL.

The Cassini radar will be used to investigate the surface of Saturn’s moon Titan by taking four types of observations: imaging, altimetry, backscatter, and radiometry. In the imaging mode of operation, the radar instrument will bounce pulses of microwave energy off the surface of Titan from different incidence angles and record the time it takes the pulses to return to the spacecraft.

These measurements, when converted to distances (by multiplying by the speed of light), will allow the construction of visual images of the target surface. Radar will be used to image Titan because the moon’s surface is hidden from optical view by a thick, cloud-infested atmosphere: radar can ‘see’ through such cloud cover.

Echo measurement

Radar altimetry similarly involves bouncing microwave pulses off the surface of the target body and measuring the time it takes the ‘echo’ to return to the spacecraft. In this case, however, the goal will not be to create visual images but rather to obtain numerical data on the precise altitude of the surface features of Titan.

In the backscatter mode of operation, the radar will act as a scatterometer. That is, it will bounce pulses off Titan’s surface and then measure the intensity of the returning energy.

This returning energy or backscatter, is always less than the original pulse, because surface features inevitably reflect the pulse in more than one direction. From the backscatter measurements, scientists can infer the composition of the surface of Titan.

Passive instrument

Finally, in the radiometry mode, the radar will operate as a passive instrument, simply recording the energy emanating from the surface of Titan.

This information will tell scientists the amount of latent heat (i.e., moisture) in the moon’s atmosphere, a factor that has an impact on the precision of the other measurements taken by the instrument.

Ralph Lorenz, lead author of the paper and Cassini radar scientist at the Johns Hopkins Applied Physics Laboratory in the U.S. and colleagues analyzed several years’ worth of Cassini radar observations and found that some of the geological features on the moon’s surface had drifted from a fixed reference point, implying that the moon’s rotation speed had temporarily increased. The authors show, with modelling, that the winds in Titan’s dense atmosphere may be rocking the moon back and forth around its axis.

The atmosphere accelerates the small moon’s rotation speed, and then decelerates it later in the year, through the exchange of angular momentum.

“The effect (which occurs on the Earth too, but since the Earth is much bigger and more dense than Titan, and its atmosphere is less dense than that of Titan, the effect is much less — our length of day changes by about a millisecond throughout the year) occurs by wind drag on the surface.

“If the winds relative to the surface go west to east, then they drag on the surface to spin Titan’s rotation up. When the winds slow down (or reverse to go east to west) they drag to slow Titan’s rotation down,” said Lorenz in an email communication to this correspondent.

The observed shifts seem large enough that Titan’s crust and core have to be separated by a liquid ocean in order to allow the atmosphere to move the crust around, the authors propose.