Remote observations of planetary surfaces from orbiting spacecraft have produced a plethora of details and much exciting new information. But this technique, being remote and ‘hands-off’, still has nearly everything in common with earth-based observations. A unique aspect of space exploration comes when we actually land on other bodies. So far we have managed this on the Moon, and on planets Venus and Mars.
The Moon has become the best known celestial body in the solar system after Earth itself. It is the only body from which rock samples have been obtained, and some 381 kg of rock have been returned, the vast majority from the six landing Apollo Missions but some very much smaller samples from USSR robotic vehicles. Much was revealed about the age of the Moon and of specific lunar features. Also the rocks provided vital clues as to the tectonic and magnetic development of our nearest neighbour and the conditions on its surface. Most of the analysis techniques were non-destructive and used only very small sample masses. In fact the vast majority of the returned Moon rock is still available for the scientists of the future to experiment with. All that is needed are new ideas and more sensitive analysis techniques.
Samples have also come from Mars, in the form of meteorites such as Shergotty (this rock falling in India in 1865), Nakhla (an Egyptian fall in 1911), Allan Hills 84001 (that was picked up on a glacier in Antarctica in 1984) and Chassigny (that hit France in 1815). These rocks were formed volcanically between 1300 and 200 million years ago and have been blasted off the surface of Mars by asteroidal impact cratering events. They subsequently spent many millions of years in space before suffering a scorching and erosive retarding transit through the Earth’s atmosphere. The problem with these exciting Martian meteorites is that we have little idea as to exactly where they came from on the surface of Mars. That they actually came from Mars was confirmed by the Viking landers, these finding that the nitrogen isotopes in the Martian atmosphere are similar to those found absorbed in the meteoritic rocks.
The history of lunar landing is rather sad, because history is very much the apposite word. The seven USA Surveyor probes were working on the surface between May 1966 and January 1968. The six USA Apollo missions landed between July 1969 and December 1972. The USSR’s Luna 20 and Luna 24 sample return missions brought back some 30 and 170 gm respectively of rather randomly selected soil, in 1972 and 1976. There have been no landers in the last 28 years. And this is not for want of good scientific justification. The mechanism that led to the formation of the Earth-Moon system is still essentially uncertain. Even though the suggested theories have vociferous advocates they have many aspects that are difficult to accept. Much more needs to be understood about the structure and interior of the Moon. We are not even sure whether it initially became so hot that it differentiated and now has a small iron core. The dynamic history of the lunar orbit is also problematic. The change in the Earth-Moon distance (and thus the rotation speed of the Earth) seems to have been discontinuous, with periods of fast evolution separated by epochs of stability. The Moon, through its tides, has played a critical role in the evolution of the Earth’s surface and on the origin and speed of the development of life.
Mars exploration has been dogged by disasters. Of 31 robotic missions to the red planet only 11 have been completely successful. After six initial failures, the fly-by in 1964 by the Mariner 4 spacecraft paved the way, but the first major success was the one-billion dollar Viking 1 and 2 landers and orbiters. The first lander touched down on the western slopes of Chryse Planitia (the Plain of Gold) in August 1975 and the second in Utopia Planitia of the other side of Mars, close to the edge of the north polar cap, in September 1975. Both were searching for water and life. Their cameras revealed a gently rolling freeze-dried ferric-oxide-red wasteland, with wind-blown fine dust drifting into impact craters and clinging to eroded rocks, all shivering beneath a thin carbon dioxide atmosphere. The Martian surface was blasted by cosmic radiation because Mars has only a negligible magnetic field which is incapable of shielding the planet from incoming ionised particles. It was also illuminated by life-threatening doses of ultraviolet light. The Martian soil was apparently antiseptic and organics-free and was rich in peroxides and superoxides. Cameras on board the Viking probes enabled scientists on Earth to select specific soil samples from the surroundings. These were then collected by manipulating scoops and sucked into miniature onboard biology laboratories. There the soil was exposed to water, and liquid food laced with radioactive carbon. The results indicated that the planet Mars was lifeless.
Observations of the surrounding landscape continued for six years. Little changed. The banks of wind-blown Martian dust seemed to be much more permanent than Earth’s sand dunes. What was really exciting, however, was the simple fact the Earth-made Viking probes were out there, on the surface of a neighbouring planet, standing on the soil and observing features in millimetre detail, whilst around them the wind roared, and the dust hissed, and occasion rumbles from nearby landslides and active volcanoes rent the thin poisonous air.
On 4 July 1997 Mars Pathfinder, a $250 million, ‘faster, better, cheaper’ mission, landed close to the mouth of a canyon, Ares Vallis, that some 3.6 to 4.5 billion years previously had been carved out by a flash flood of water. A new epoch of Mars exploration was launched. Mars Pathfinder was carrying with it Sojourner, a small, 10.6 kg, half a meter long, six-wheeled roving robot. This could be guided from Earth, rather like a child can guide a radio–controlled toy car. Sojourner, moved away from Pathfinder at about 0.6 metre per minute and eventually examined 250 square meters of the nearby soil and rocks. Surprisingly there was little evidence of chemical diversity. The rocks were volcanic and consisted of a mixture of basalt and quartz, very similar to the Andes Mountains in South America.
Sojourner added much excitement and drama to the mission, and showed that a Martian rover could be effectively driven and manipulated from Earth. This remote robotic rover could explore the nearby planetary surface effectively, quickly and relatively cheaply. It could steer round or climb over rocks, and maintain a steady course. If it fell over, however, there was nobody around to put it back on its wheels again.
2004 has seen two more Mars rovers exploring the planet. Opportunity and Spirit, vital components of the Mars Exploration Rover mission have acted like robotic human field geologists. They have travelled over 0.6km climbing 25° rocky slopes on their way, used abrasion tools to drill into various rocks, returned images of the rock surfaces using on-board microscopes and taken (together with the lander) coloured stereo panoramas of their surroundings
Spirit landed in the 140-km diameter Gusev Crater and the returned data clearly show that this was once a lake. Data from Opportunity indicates that its landing site, Meridiani Planum, was once a salty swamp. The climate of Mars has changed drastically over time mainly due to slow variations in the eccentricity of the orbit and the angle of inclination of the spin axis to the orbital plain. It is quite likely that it will again become warm and wet some time in the future.
One of the next steps in Martian exploration, midway between a lander and an orbiter is NASA’s proposed remote controlled ‘model’ aeroplane ARES (short for Aerial Regional-scale Environmental Survey). This will fly around searching for possible sources of methane, trying to check whether they are volcanic or biotic.
Landing on Venus has always been a well-known problem ever since the USSR’s Venera 7 touched down in December 1970. Concentrated sulphuric acid from the all-enveloping cloud layers corrode any descending space probe and it quickly becomes inoperable. The last probes to land there were Vega 1 and Vega 2 in June 1985. No samples have been returned and no Venusian meteorites are known. It is certainly time to try again. Modern technology should surely be able to shield a space probe during descent. That said, with a surface temperature of about 460°C and a surface pressure 92 times that at the surface of Earth there are still serious challenges.
Landing on Mercury is also a job for the future. The lander that was to be part of the joint European / Japanese mission to the planet has been cancelled due to lack of funds.
Landers are important. They can be used to monitor atmospheric pressure, composition, winds and weather. It is also very important to have networks of landers scattered over planetary surfaces. Atmospheric parameters clearly vary with latitude and longitude and time of day and year. And having seismometers at different places enable the source and strength of planet-quakes to be monitored. Even though it is relatively easy to see what is on the surface of a planet, the interiors remain extremely mysterious and our knowledge of the composition and volumes of planetary cores is extremely rudimentary.