FAST forward to a billion years AD. On the shores of an ocean of liquid methane, under an impenetrable orange haze, a creature stoops to pick up a small diamond disc. Locked beneath the surface of the disc, engraved in letters only a few micro-metres high, is a message from planet Earth.
It鈥檚 science fiction, of course, but only in part. There is every chance that just such a message disc will be carried by the spacecraft Cassini when it sets off for Titan, Saturn鈥檚 fogbound moon, in 1998. 鈥淚t鈥檒l be our memorial,鈥� says Gregory Benford, a radio astronomer at the University of California at Irvine.
Benford proposed the message disc in collaboration with space artist Jon Lomberg, who designed the message disc on the Voyager spacecraft of the late 1970s. Their proposal is being discussed by NASA and the European Space Agency, which are collaborating on the $3.2 billion Cassini project. 鈥淲e鈥檙e hoping to get approval at an ESA meeting in May,鈥� says Benford. 鈥淚鈥檓 quite optimistic.鈥�
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Cassini is the largest unmanned space mission ever. The spacecraft is so heavy that it will have to swing round Venus, Earth, Venus again, then Jupiter in a game of interplanetary billiards that will give it enough energy to reach its destination. The route is so circuitous that the probe will take more than seven years to reach Saturn, finally arriving in 2004.
Cassini will consist of a Titan orbiter and a Titan lander, called Huygens. The orbiter will map Titan鈥檚 surface in infrared light which penetrates the moon鈥檚 thick clouds, and identify a spot for Huygens to land. In case 鈥渓anding鈥� means a splashdown, Huygens has been designed to float.
Benford and Lomberg are proposing to put a message on both the orbiter and the lander. There are three reasons why the mission should carry the message, says Benford. The first is to ignite public interest in Cassini in the same way as the Voyager discs and the plaque aboard the earlier Pioneer spacecraft did. The second is to provide a 鈥渢ime capsule鈥� which could be recovered by another mission in a few centuries. But the last reason is the most optimistic. 鈥淭he message will be aimed at any life forms that may evolve in the organic soup of Titan in the next billion years,鈥� says Benford. 鈥淚t鈥檚 a long shot, I admit.鈥�
Despite having an average surface temperature of 鈭�178 掳C, Titan is considered the most likely place in the Solar System for life to evolve in the future. Radar measurements indicate the presence of oceans of liquid hydrocarbons and a single giant continent the size of Australia. The pressure in the dense organic atmosphere of nitrogen and methane is one and a half times that on the Earth鈥檚 surface.
Creating a message that will survive a billion years will be no mean feat, especially as Titan鈥檚 weather is harsh and the temperatures cryogenic. 鈥淭he plan is to use the most durable material we know of 鈥� diamond,鈥� says Benford. Specifically, the idea is to use a thin, single-crystal diamond disc, perhaps 1 or 2 millimetres thick and 2 centimetres in diameter.
There are two possible ways of creating the microscopic message inside the diamond disc. One is to write it in boron, deposited as a vapour through a titanium mask. Diamond vapour would then be deposited on top of this to a depth of about a millimetre, creating an 鈥渁rtificial fossil鈥� encapsulating the message. The other idea is to cut the message into the diamond with an excimer laser, effectively turning diamond into graphite, and then deposit a layer of diamond on top. 鈥淚n principle, the writing could be only a few micrometres high,鈥� says Benford. 鈥淭here will be room to write a lot.鈥�
Benford suggests that the message could contain greetings from the contributing countries. 鈥淭here could also be a map of Earth and our present Solar System, which cannot be seen from Titan鈥檚 surface,鈥� says Benford. 鈥淚f anyone has any more ideas they would be much appreciated.鈥�
The Cassini team is discussing the fabrication of the diamond discs with Drukker Diamond, a company based in Amsterdam. Benford says five discs will be needed 鈥� one each for the orbiter and lander, with two more for the backup spacecraft, plus one for vibration testing and so on. The total cost will be about $60 000. 鈥淭he message will far outlast the lander,鈥� says Benford.