Steven Scribbins, February 2005 The European Space Agency’s highly publicised mission to successfully land a probe on the surface of Titan, Saturn’s largest moon, has reached its climax. The Huygens lander separated from NASA’s Cassini spacecraft on 25th December 2004, and entered Titan’s atmosphere on 14th January 2005. Launched in 1997, Huygens was to become the first space probe to land on a world in the outer solar system, all previous landers having been sent to Earth’s moon, Venus and Mars.
The mission itself was an ambitious undertaking, from start to finish. Because of the size and mass of the craft (almost 7m long and 4m wide, and weighing 5600kg), the launch of the probe could not be achieved by artificial mechanisms. Instead, it was decided to utilise the gravitational effects of planets to accelerate the spacecraft. This method requires very precise calculations and the margin for error is very low, especially over interplanetary distances, and the mission could have failed at any stage. Moreover, landing a probe on the surface of any extra-terrestrial body is extraordinarily difficult; the greater majority of missions to land probes on other worlds have been failures. The Huygens mission was even more complicated, due to the phenomenal distances involved. Fortunately for the organisations involved (ESA, NASA and ASI, the Italian Space Agency), the mission was a success, and the Cassini-Huygens spacecraft entered the Saturnian system in July 2004.
The Cassini craft, which is to remain in orbit around Saturn for at least 4 more years, is equipped with a bewildering array of instruments for a variety of experiments. Scientists hope to obtain data regarding several aspects of the Saturnian system as well as Titan, including:
The origin of the rings
The reason for the variations in their colour
The “heat source” responsible for the emission of energy from Saturn itself
The nature of the striking colour difference that can be seen on the surface of Iapetus, another of Saturn’s satellites (see photograph, next page)
The reason for the apparently smooth surface of Enceladus, yet another of Saturn’s numerous satellites (see photograph)
Cassini also performed a series of minor observations during transit, including a Jupiter flyby to aid with analysis of the planet’s atmosphere, a test of Einstein’s general relativity which increased the accuracy of theoretical predictions significantly, the discovery of two new Saturnian satellites, now called Methone and Pallene, and also further photography of Saturn, its ring system, Titan and Phoebe.
Titan itself is the second largest known satellite in the solar system (the largest being Ganymede), and is larger than Mercury and Pluto. It was known prior to the arrival of the probe that Titan possessed a thick methane and nitrogen atmosphere, with traces of other elements and some heavier organic compounds such as ethane. The Huygens probe transmitted data from the surface of Titan for approximately 90 minutes after successfully landing on the surface. Atmospheric data collected provided additional confirmation of the composition, as well as atmospheric conductivity and electromagnetic wave activity. The conductivity of the surface material was measured, and acoustic events recorded. Huygens also photographed the moon’s surface and revealed what appear to be lakes and rivers of hydrocarbon compounds. The surface itself seems to have a clay-like texture. Huygens data also suggests precipitation, but of liquid methane and ethane, rather than water. Prior to Cassini-Huygens, it was believed that conditions on Titan resemble those of the early Earth. It is hoped that, once analysed, the Huygens data will provide a clearer picture of whether or not this is the case, and that it will provide some insight into the origin of life on Earth.
The darker area of the moon, which appears to the bottom left of the photograph, is believed to be organic material. It is hoped that data from Cassini’s instruments will shed some light on the nature of this material.
Photograph: www.pa.msu.edu Enceladus
The area to the left side of the image is particularly smooth, especially for a satellite with no atmosphere. It is not known why this is the case. Observations made by Cassini may provide some clues.
This photograph, taken by Huygens, shows rock-like objects which are believed to be blocks of water ice, resting on a clay-like material. Originally a greyscale photograph, the colour is constructed from the line spectra obtained by Huygens.
The dark lines to the left of this photograph are believed to be “drainage channels”, for liquid hydrocarbons. The dark region to the right is possibly an ocean, also of liquid hydrocarbons, or perhaps more viscous, tarry substances.