Lesson 1: Ecology of the System 1.7 Origin of the Solar System: Clues from Comets Figure 1.7.1 Comet Hale-Bopp The white dust tail trails behind the comet, while the blue ion tail traces the outward direction of the solar wind. Another repository of memories of the early stages of the solar system are the comets. In essence, these are dusty snowballs, the snow being ice made of methane, ammonia and water. When these snowballs come close to the Sun, the ice evaporates and releases vapor, which makes a tail pointing away from the Sun. Comets of course have been seen for thousands of years. Their nature was a complete mystery to Aristotle, who thought of them as a kind of atmospheric phenomenon. Among ordinary folks, comets were simply bad omens; a kind of celestial finger wagging, telling people down below to shape up or else. The Danish astronomer Tycho Brahe (1546-1601) observed a brilliant comet in 1577, noting that it came in from the region of the outer planets, passing through the traditional crystalline celestial spheres without effort. The spheres did not exist! There was nothing but empty space between the planets. Newton showed that the path of comets followed his laws and Edmond Halley, using Newtonian arguments, showed that at least one comet was on a regular schedule, indicating a highly elliptical orbit. He predicted, correctly, when it would return. Figure 1.7.2 The appearance of the comet we now call Halley was recorded in 1054 in the Bayeaux Tapestry. Halley is a short-period comet returning to the inner solar system every 76 years. Comets, we now know, are errant messengers from one of two populations of icy bodies surrounding the solar system. One is the Kuiper belt, which is just beyond Pluto, and may actually include this small and distant planet as one of its members. The other is the Oort cloud, a chaotic assemblage of icy debris swirling around the solar system at some distance, like a cloud of mosquitoes swirling around people on a summer picnic in Minnesota. In essence, the short-period predictable comets are from the Kuiper Belt, the unpredictable one-time visitors from the Oort Cloud. Figure 1.7.3 Impact site of Comet Shoemaker Levy-9 fragment D on Jupiter. The icy bodies become comets when a passing large body, even a star passing at some distance, disturbs their lonely travel in outer space and deflects their path toward the inner parts of the solar system where we can see them. More likely than not, they end up as fodder for the Sun. In rare cases, they hit one of the planets, notably Jupiter. The spectacular Shoemaker Levy-9 event on July 16-20, 1994 was such an occasion. Even more rarely, one of these wanderers can impact the Earth. The Tunguska event on June 30, 1908, apparently was a collision with an asteroid. On that day there was an explosion heard hundreds of miles away, near Tunguska in Siberia. On investigating the site, leveled trees were found in an area more than 30 km in diameter. Figure 1.7.4 Leveled trees at Tunguska in Siberia, believed to be due to an asteroid impact. The largest impact for which there is direct evidence (a crater and disturbed environments on a global scale) was the one 65 million years ago which resulted in the extinction of the dinosaurs and many other organisms on land and in the ocean. Fortunately, the rate of impacts in the inner solar system has greatly diminished since the early days. The Moon's face, riddled by impact craters, is but an ancient memory of this primeval stage. Thus, we put together the clues for the origin of the solar system from the study of ancient objects in this system: meteorites, comets, the Moon. But what about the Sun, how can we study its origin? Mainly, this is done by studying the various stages in the development of stars in the Galaxy. The first step was to show that the elements in these stars are much the same as those in the Sun. The next step was to order the various types of stars seen into evolutionary family trees. It turns out that small stars develop differently from medium-sized ones and big ones have their own special course of evolution. Stars of the size and disposition of the Sun are the most stable and long-lived. But even they run out of fuel at some point, with awful consequences. But stars form all the time (we can observe star formation right now, for example, in the Orion nebula, with a small telescope). So, the death of some stars engenders the birth of a new generation, as long as there is plenty of hydrogen around. Figure 1.7.5 Stellar nursery in the Orion Nebula. The thousand or so newly forming stars emit ultraviolet radiation that gives this region an eerie glow. The Sun, it is thought, will exist as long again as it has existed so far, and will be reasonably well behaved, so that Life in the solar system still has a long future ahead.