Chapter 19: Life in the Universe

Photograph of Mars taken by the Hubble Space Telescope during opposition in 2003

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 chemistry, biology, geology, and astronomy. Why did the 1995 announcement of possible life in the Martian meteorite ALH 84001 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.

Nicolaus Copernicus

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. A century later, Galileo used his small telescope to show that the Moon was a geological world like Earth. The idea of "many worlds" took root with scientists and philosophers, making it plausible that life was not unique to this planet. 

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 the billions of galaxies that can be seen through large telescopes. Once again, these findings displaced human beings from our central role in the universe.

This collage shows the variety of soils found at landing sites on Mars. The elemental composition of the typical, reddish soils were investigated by NASA's Viking, Pathfinder and Mars Exploration Rover missions, and now with the Curiosity rover, using X-ray spectroscopy.

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 path of the moon arcs due to parallax. The Sun’s lower atmosphere (chromosphere) is shown here

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. The same scientific mode of thinking that displace us in centrality in the universe is now being used to decide if life is scarce or abundant beyond Earth.