INTERNATIONAL SPACE SCIENCE INSTITUTE No 5, June 2000 SPATIUM Published by the Association Pro ISSI Earth,Earth, Moon Moon andand MarsMars Editorial Mankind finds itself thrown into grateful for his kind permission Impressum a hostile universe. While our bod- to publish herewith an adapted ver- ies are subject to the soulless laws sion of his address at the Fourth of chemistry and physics govern- Anniversary of the ISSI founda- ing physical processes on even the tion. SPATIUM most distant star, our minds, our Published by the hopes and fears find no spiritual Association Pro ISSI equivalent. The intriguing sketch- Bern, June 2000 twice a year es on the walls of the Altamira Hansjörg Schlaepfer cave, for example, may have been the attempt of our early ancestors INTERNATIONAL SPACE to create humanlike beings simply SCIENCE in order to alleviate their spiritual INSTITUTE loneliness. The Greeks later filled Association Pro ISSI their heavens with a great variety Hallerstrasse 6, CH-3012 Bern of deities who, by virtue of their Phone ++41 31 631 48 96 human features, clearly betray the Fax ++41 31 631 48 97 intent of their creators. President Little has changed since. The past Prof. Hermann Debrunner, two thousand years have seen man- University of Bern kind struggling to find a rationale Publisher for its existence, hopefully given Dr. Hansjörg Schlaepfer, by an entity beyond its transitory Contraves Space, Zurich existence. These days, evidence Layout seems to take precedence over be- Marcel Künzi, marketing · kom- lief. Hence, modern science seeks munikation, CH-8483 Kollbrunn to provide answers to these age- Printing old questions. It is only logical to Drucklade AG, CH-8008 Zurich begin with the nearby celestial bodies, i.e. with the Earth, Moon and Mars, to which the present issue of Spatium is devoted, and to search there for traces of life. But, while this endeavour has provided us with deep insight into the his- tory of our own planet, the feasi- bility of life on the Moon has meanwhile been excluded, and corroboratory proof of life on Mars has not yet been found. We are still alone. Prof. Johannes Geiss is the spiritus rector of the International Space Frontcover Science Institute in Bern and its Mars: South of Candor Chasma Executive Director. We are very (NASA/JPL) SPATIUM 5 2 Earth,Moon and Mars *) Johannes Geiss,International Space Science Institute,Bern Introduction of geologic epochs from the In the second half of the 20th cen- Cambrian to the Quarternary was tury, Apollo and other pro- identified. Then, at the turn to the grammes enabled scientists to use 20th century, radioactivity was dis- the experience gained from the The ultimate goal of planetary re- covered, opening up a new era for earth sciences and apply their con- search is to find answers to the big many branches of science, includ- cepts and methodstotheinvestiga- questions: What happened when ing the earth sciences. The decay tion of the Moon. By combining 4’600 million years ago the Sun of uranium, thorium and potassi- observations at the lunar surface and the planets formed out of an um isotopes was found to be the and from orbit with the chemical interstellar cloud? What were the main source of energy in the inte- data obtained from the analyses of external circumstances and specif- rior of the Earth, maintaining there lunar rocks brought back to Earth, ic processes that governed the ori- a high temperature and driving scientists reconstructed a large gin of the Earth and determined geological processes. The applica- part of lunar history. its evolution to the present state? tion of radioactive decay for meas- Is there – or was there – life in the uring ages of rocks was an equally Thirty years after astronauts set past on another planet or on one important breakthrough. By using foot on the Moon and explored its of the moons in the solar system? various “radioactive clocks”, the geology, scientists and space agen- From the presence or absence of age of the Earth was derived and cies are turning their attention in- signs of life outside the Earth, can an absolute time scale was estab- creasingly to Mars, the Red Planet we draw conclusions about the lished for the larger part of its (Figure 1). Let me emphasise that origin of life on our own planet? history (Figure 2). Thus, it became the exploration of Mars for quite a possible to gain a quantitative while will be by robots. Human To an interplanetary traveller, understanding of the causes of exploration will be a great chal- Earth, Moon and Mars would geological evolution and identify lenge, because Mars is at least 200 present themselves as entirely dif- the underlying physical processes. times more distant than the Moon. ferent worlds (Figure 1). For earth and planetary scientists, however, these three members of the solar system have important similari- ties. Their sizes, their chemical compositions and their distances to the Sun are relatively similar, so that comparative studies yield results on their origin and their evolution. Moreover, Earth, Moon and Mars are the only objects in the universe where we have detailed geological observations and at the same time possess rock samples for laboratory analysis. Figure 1 Nineteenth century scientists real- Earth, Moon and Mars. The “Blue Planet" Earth, the “Red Planet” Mars and the ised that evolution is central to bi- grey Moon. The relative sizes are to scale. We have evidence that on Mars there were large quantities of water at some time in the past. Today we see only small ice caps of ology and geology. On the basis of CO2 and H2O, which are seasonally distilled from pole to pole, giving rise to strong characteristic fossils, the sequence winds. *) Keynote address at the Fourth Anniversary of the International Space Science Institute, 21th October 1999 SPATIUM 5 3 We have on Earth a dozen Mar- The Early Days tian meteorites. There is little doubt that their origin is the Red of the Planet, but we do not know the exact location they came from. Solar System The Mars meteorites were ex- pelled from the planet by a large impact, as a result of which they were badly shocked. This makes Sun and planets radioactive dating difficult. Still, ages of Mars meteorites have Stars are created when an inter- given us a general idea of the stellar molecular cloud collapses. times of formation of geological We can observe this process in the features on Mars. However, sam- Orion Nebula, where star forma- ples returned by spacecraft from a tion is presently going on. The few well-defined geological units Sun was born 4’600 million years will probably be needed to obtain ago in a collapsing interstellar a definite time scale for Martian cloud that was dispersed long ago. geology. Once initiated, it took only a few million years to form the Sun and to allow the nuclear fusion pro- cess in its interior to start working and hence for the Sun to shine. Ages of Rocks Mars 6’860 km As predicted by Pierre Laplace Moon early in the 19th century, a portion 3’476 km Earth of the gas and dust of the protoso- 12’756 km lar cloud remained in a rotating 4’600 disc from which the planets were 0 1’000 2’000 3’000 4’000 5’000 formed by accretion (Figure 3).This Million Years process followed the principle of Figure 2 “big fish eats small fish", well The ages of Earth, Moon and Mars. known in the behavioural and This diagram shows the distribution of social sciences. It began with dust ages of rocks from Earth, Moon and The Earth Mars. Earth has been geologically very particles attaching to each other. active throughout history. As a result, After they reached a certain size, the geological record of the early times gravitation took over, and larger In the case of the Earth, the accre- has largely been overwritten. The Moon bodies attracted and swallowed tion process did not go so smooth- is much smaller than the Earth, and con- sequently, the geological activity of the smaller ones. In the end, a number ly. A second object, about the size Moon ended about 3’000 million years of planets were left, most of them of Mars, formed on a nearby orbit, ago. We have, however, a good geologi- well separated from each other, so so that the two were bound to col- cal record with absolute times by radio- active dating for the epoch 4’100–3’200 that they could safely orbit the lide.All the evidence indicates that million years ago for our companion. Sun for more than four billion this collision did occur, and it cre- For 3’000 million years, the Moon has years. ated the pair of Earth and Moon as been geologically dead. For Mars, we we know it, circling around the have only a rudimentary time scale of geological events, since we only have 13 common center of mass, thus Mars meteorites for radioactive dating. avoiding any further collision. SPATIUM 5 4 The Origin of the time of their formation. The exceptional size of our Moon, the Earth–Moon however, calls for an entirely dif- System ferent origin. The Giant Collision For centuries, scientists have won- Among the various hypotheses dered about the large size of the about the origin of our Moon, Moon. Many planets in the solar there is only one, the “Giant Col- system are circled by moons, but, lision Theory" that explains all with the exception of the satellites relevant observations.