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Telescopes and space missions

Telescopes and space missions

Pathfinder leads the way to red planet

01 Feb 1998

In The Crystal Egg, a short story written in 1897, HG Wells describes how a remarkable object in Mr Cave’s Curiosity Shop allows its owner to gaze at a real-time image from the surface of Mars while sitting in a darkened room on Earth. A century later, we can all do much the same thing, courtesy of NASA and its new line of bargain-basement planetary missions.

The programme was spearheaded last year by Mars Pathfinder, largely developed by scientists and engineers at the Jet Propulsion Laboratory (JPL) in California. They are proud of the fact that Pathfinder cost about the same as Titanic, the latest Hollywood blockbuster, and delivered at least as many thrills. It has also made a another small step in mankind’s quest for knowledge of its origins and its destiny, as the first scientific results have shown (Science 1997 278 1743-1768).

To appreciate the scale of Pathfinder’s achievement, it is necessary to understand its background and goals. Despite appearances, the name was not chosen because Pathfinder is the first in a series of missions that will explore Mars in the next decade, which will hopefully include Mars Express, Europe’s first contribution to the “faster, cheaper, better” revolution. It was instead a test flight for the Mars Environmental Survey (MESUR), an ambitious project that would have landed 20 or so stations on Mars at the same time. Even in the old days, implementing so many stations would have required a rethink of the expensive landing technology used for Viking in 1976. When MESUR was cancelled because of its cost – pushing into billions of dollars – NASA decided to carry on with the development flight. Science was never really in the driving seat.

How wonderful, then, that Pathfinder has achieved so much in terms of public interest, and scientific results as well. The dramatic landing, the ability of the probe to move on Mars for the first time and the intrinsic interest of seeing the Martian surface all contributed to its success. For example, important insights can be gained from the photographs Pathfinder took of the region where it landed, the Ares Vallis. This is one of many regions on Mars that appears, from orbit at least, to have contained running water in the geological past. The scene around the lander showed rocks and pebbles rounded by erosion, as well as a layered material that was probably deposited by running water. These and other clues provide almost indisputable evidence that Mars was once warm and wet with a thick atmosphere.

Tracking the lander on the surface of Mars has given new information about the precession of the planet’s axis of rotation and variations in the rate at which the planet turns on its axis. This has provided a new value for the moment of inertia, which constrains models of the interior. The latest indication is that Mars has a central metallic core 1300-2000 km in radius, with the exact value depending on the composition of the core and the overlying crust. It seems that the core of Mars contributes less to the planet’s total mass than is the case for Earth.

A large fraction of the Martian atmosphere (about 30%) condenses on the winter pole and then sublimes back into the atmosphere in the summer, which changes the planet’s moment of inertia and causes the length of the Martian day to vary with season. Nearly a trillion tonnes of carbon dioxide are transferred in this way every year, a phenomenon that has no significant analogue on Earth but has a large effect on the Martian weather. Combining the new observations of rotation and precession with those from Viking 20 years ago has provided a much better picture of the dynamics of the Martian globe. This provides good agreement with a model of the dynamics in which the rate of mass exchange is estimated from the change in pressure over a Martian year.

Pathfinder investigated the atmosphere of Mars with a package of meteorology instruments that collected data on the surface and on the way down to the planet. The MESUR network would have monitored the atmosphere in three dimensions, and a single station is much less useful, especially when short-lived. However, the data provide a valuable one-off test for general circulation models of Mars, currently under development in the UK, France and the US. The day-night cycle was much as predicted, as were the trends in temperature (a chilly -10 °C at best on the surface, and as low as -96 °C at night), pressure (varying between 6.55 and 6.85 millibars) and winds (about 10 m s-1, with the direction swinging through a full 360° during the course of a day). Martian weather forecasting may soon be a viable proposition.

The Sojourner rover was also primarily an engineering test bed for more sophisticated missions planned for the next year or two, and was small and dumb compared to the prototypes that are now running around JPL, Arizona, and Death Valley in California. However, it did carry a camera and an a-proton X-ray spectrometer to analyse and examine the soil and rocks. Preliminary results show that the dusty soil does not have the same composition as the rocks, so it was not formed by the grinding up of local material. However, the soil is like that found by Viking at locations elsewhere on Mars, indicating that it is a global mixture of weathered surface material from an unknown location, distributed around the planet by stormy winds.

Magnets mounted on the lander trapped some of the airborne dust and showed that it has a strongly magnetic component. The rocks visited by Sojourner on its 52 m trek around the lander station (now called Sagan) are not identical to each other or to the Martian meteorites found on Earth. Mars is certainly a diverse world.

The philosophy behind Pathfinder meant that it was not intended to last long – a week was planned and the mission actually lasted 83 sols (Mars days, 24.7 hours each). But despite its short life, it opened a new fast track to Mars and did some key science. Most of all, perhaps, it captured the public imagination and provided a boost to the forthcoming programme of intensive research planned for the Martian environment and climate. How appropriate that the landing station should be named after Carl Sagan, who in his spare time was a modern disciple of HG Wells and shared his vision.

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