A.S.I. - Agenzia Spaziale Italiana - The road to Space goes through Italy.

29 Feb 2008

Titan: Saturn's largest moon, and the second largest in the solar system. A fascinating celestial body with unique features. At a distance of more than a billion kilometres from the Sun (nine times the Earth-Sun distance), its surface, a mixture of water ice and liquid hydrocarbons, is at a temperature of -180° C. T. In such a cold environment, hydrocarbon lakes fill large areas. The satellite is surrounded by a misty nitrogen-methane atmosphere (the ground pressure is slightly larger than that of the Earth), where hydrocarbon rain sporadically occur. Atmospheric methane is likely formed by a unique phenomenon called criovolcanism: an outpour of water and hydrocarbons, forming cones hundreds of meters high and several kilometres wide.

Titan is one of the main targets of the mission Cassini-Huygens, launched from Cape Canaveral on Oct. 15, 1997. A collaboration between NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI), it is the most complex planetary mission ever conceived, and one of the richest in discoveries.

Because of its cloudy and dense atmosphere, Titan's surface cannot be easily observed by cameras. The determination of the surface morphology requires a Synthetic Aperture Radar (SAR), an instrument that illuminates the ground with a radio beam. The reflected radio waves are received onboard, sampled and transmitted to ground, where they are processed to generate images of the surface.

If observing Titan's surface isn't easy, gaining information on the interior is much more difficult. The measurement cannot be but indirect, and the only available tools are gravity and rotation. It is Titan's rotation, in particular the determination of the length of day, that provided the first, strong indication of a global ocean: a liquid water (or water-ammonia) layer, maybe 100-200 km deep, hidden beneath the icy surface.

This result has been independently obtained by two Italian scientists from Sapienza Università di Roma, Paolo Persi del Marmo and Luciano Iess (Infocom Department and Department of Aerospace and Astronautical Engineering), and a team of US scientists from Jet Propulsion Laboratory and other institutions. The length of day has been determined by tracking the motion of surface landmarks, such as mountains, lakes and valleys, shown in the images of the Cassini SAR. "Titan rotation is much slower than the Earth: a day on Titan lasts about 16 Earth days", says Dr. Persi del Marmo. "Were the interior made only by rocks and ice, Titan should behave like the Moon: its spin period would be exactly equal to the rotation period about the Saturn. Surprisingly, Titan spins about 0.36 deg/year faster than expected, and the length of day is shorter by about 60 seconds".

Explaining this tiny discrepancy in Titan's spin period is simple if one assumes that the rocky core is separated from the icy surface by a liquid layer covering the whole satellite. This conclusion was reached by considering the action exerted by the seasonal winds on the external icy shell. For 14 years the winds blow predominantly in west-east direction at certain latitudes, and reverse their prevailing direction in the next 14 years. Indeed, Titan has its seasons just like the Earth. The atmospheric circulation acts on the satellite, slightly changing its rotation velocity. Were Titan entirely solid, the effect would be very small, but if the surface floats on an ocean, the winds are able to affect it much more effectively. "With a simple analogy, pushing a heavy gate results in a slow rotation, while the same push applied to a light gate results in a much faster rotation", prof. Iess explains. "On the Earth a similar effect takes place, but the length of day changes with seasons by only a millisecond."

This mechanism was proposed in 2005 by two scientists from the University of Cologne, who predicted a rotation faster by 0.6 deg/year on the basis of their circulation model. The measurements from Cassini indicate a value very close to the theoretical predictions.

The model entails also that Titan's length of day has seasonal variations: the minimum should be reached in 2009, to increase gradually until 2023. Tracking these variations would provide the unequivocal proof of this mechanism and of an internal ocean. Enrico Flamini, Cassini project manager for ASI, says: "The Cassini mission was due to end in three months from now, but a first extension has already been approved by NASA until 2010, with the concurrence from ESA and ASI. However, the puzzling discoveries and the interest in seasonal effects on the atmospheric circulation have led the science team to propose a further extension to 2017. This proposal will be evaluated in the next months." Additional observations will improve also the knowledge of the interior structure, thanks to the measurement of the gravity field and tidal deformations. These measurements are planned at the Department of Aerospace and Astronautical Engineering of Sapienza University of Rome, in collaboration with other Cassini scientists.

This research will be published in the March 21, 2008 on the journal Science:
R.D. Lorenz, B. Stiles, R.L. Kirk, M. Allison, P. Persi del Marmo, L. Iess, J.I. Lunine, S.J. Ostro, S.Hensley : "Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds".