Unequivocal evidence for life on Mars, or anywhere beyond Earth for that matter, would raise larger philosophical questions that lie behind the interpretive details of the chemistry, biology, geology, and astronomy. Why did the 1995 announcement capture the public imagination so completely? How could such life have once thrived on the red planet, which today is a frozen desert? And why were different scientists able to view the same evidence and come to quite different conclusions? These are major questions, not just about the Mars rock, but about how science works.
The answer to the question "Why did the public react so strongly to the possible discovery of life on another planet?" is that humans have been wondering for several centuries if life on Earth is unique. What is our role in the universe? Could there be other worlds that have environments like Earth and life forms like ours? Is Earth the apex, or pinnacle, of all creation and the "capital of the universe?" Is it a unique cosmic accident or just one of many planets scattered among the galaxies that supports life? These concerns are part of a broad intellectual movement spanning several centuries.
Until the 1500s, most people thought that Earth was the center of the universe, with the Sun, planets, stars, and other bodies moving around it. But then the Polish scientist Nicolaus Copernicus hypothesized that the Sun, not the Earth, is at the center of our planetary system. Within generations, this was proven correct, and humanity learned that the Sun was just one ordinary star among millions of stars. This discovery became known as the Copernican Revolution. Copernicus showed for the first time that humans were not at the center of creation and spurred people to think about their relationship with the universe. Another aspect of this revolution came in the 1800s when the English scientist Charles Darwin and others showed that humans are just one among many species that have evolved on the Earth. In fact, biologists revealed a multi million-year procession of species, with most becoming extinct. The next milestone in the changing worldview that started with the Copernican Revolution came in the 1920s, when astronomers proved that the Sun is not at the center of our gigantic disk-shaped galaxy, but at its outskirts. Moreover, our galaxy is only one of millions of visible galaxies. Once again, these findings displaced human beings from our central role in the universe.
The question of whether life ever existed on Mars can thus be seen as a current chapter of this long adventure of discovering where we fit in the universe. Planets have been found around many nearby stars. With perhaps a solar system for every star and so many stars to choose from, the number of potential sites for life is enormous. Is it possible that life on Earth is just one of many cases of life scattered through the universe like flowers in a meadow? Or is life so complex and unlikely that it has originated just once in the universe? It's a big question, and we don't yet know the answer. However, after centuries of speculation, we may be on the verge of a breakthrough.
The question "How could life have appeared on Mars?" contains the essence of a larger theme. We are really asking whether the rules that govern our terrestrial experience also apply across vast reaches of space. What are the planets and stars really like? Where do elements come from? How is matter organized? How does it interact with radiation? How do carbon atoms combine with other atoms to create complicated molecules that can reproduce themselves? Do these processes happen on other planets? Answering these questions requires some basic concepts of physics and chemistry. It turns out that a remarkably small set of physical rules governs the behavior of everything in the universe.
The question "How do we know that the results of science are correct?" leads to a broad theme. How do we "know" something? Humans have come to believe that there are physical explanations for the diverse phenomena of the natural world. For example, we know that solar eclipses are caused when the Moon passes in front of the Sun. We have rejected the notion that eclipses express the displeasure of Sun gods, as some ancient cultures believed. If we encounter a phenomenon we do not understand, we can study it by a logical, systematic method. This method has yielded physical ideas that describe everything from the reactions going on at the center of a star to the conditions just after the creation of the universe.