Astronomy Chapter 30 - Life in the Universe
30.3 Searching for Life beyond Earth
The search for life beyond Earth offers several intriguing targets. Mars appears to have been more similar to Earth during its early history than it is now, with evidence for liquid water on its ancient surface and perhaps even now below ground. The accessibility of the martian surface to our spacecraft offers the exciting potential to directly examine ancient and modern samples for evidence of life. In the outer solar system, the moons Europa and Enceladus likely host vast sub-ice oceans that may directly contact the underlying rocks—a good start in providing habitable conditions—while Titan offers a fascinating laboratory for understanding the sorts of organic chemistry that might ultimately provide materials for life. And the last decade of research on exoplanets leads us to believe that there may be billions of habitable planets in the Milky Way Galaxy. Study of these worlds offers the potential to find biomarkers indicating the presence of life.
30.2 Astrobiology
The study of life in the universe, including its origin on Earth, is called astrobiology. Life as we know it requires water, certain elemental raw materials (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), energy, and an environment in which the complex chemistry of life is stable. Carbon-based (or organic) molecules are abundant in space and may also have been produced by processes on Earth. Life appears to have spread around our planet within 400 million years after the end of heavy bombardment, if not sooner. The actual origin of life—the processes leading from chemistry to biology—is not completely understood. Once life took hold, it evolved to use many energy sources, including first a range of different chemistries and later light, and diversified across a range of environmental conditions that humans consider "extreme." This proliferation of life into so many environmental niches, so relatively soon after our planet became habitable, has served to make many scientists optimistic about the chances that life could exist elsewhere.
What makes the Earth different from the other worlds that might have life in the solar system (and easier to detect from far away) is that only Earth a.has liquid water on it; no other world in our solar system has water b.has temperatures in some places where chemical change is possible c.has a solid surface d.has a biosphere on its surface where photosynthesis can take place e.has the chemical ingredients from which life might begin
has a biosphere on its surface where photosynthesis can take place
When they talk about the Copernican principle, philosophers and astronomers mean that: a.the idea that Copernicus was the greatest astronomer who ever lived and the model for astronomers ever since b.the idea that everything in the universe revolves around the Sun c.the idea that everything in the universe rotates and revolves (i.e. has angular momentum) d.the idea that there is nothing special about our place in the universe e.the idea that the universe is expanding in every direction that we look
the idea that there is nothing special about our place in the universe
30.1 The Cosmic Context for Life
Life on Earth is based on the presence of a key unit known as an organic molecule, a molecule that contains carbon, especially complex hydrocarbons. Our solar system formed about 5 billion years ago from a cloud of gas and dust enriched by several generations of heavier element production in stars. Life is made up of chemical combinations of these elements made by stars. The Copernican principle, which suggests that there is nothing special about our place in the universe, implies that if life could develop on Earth, it should be able to develop in other places as well. The Fermi paradox asks why, if life is common, more advanced life-forms have not contacted us.
In a globular cluster, astronomers (someday) discover a star with the same mass as our Sun, but consisting entirely of hydrogen and helium. Is this star a good place to point our SETI antennas and search for radio signals from an advanced civilization? a.yes, because such a star is probably old and a technological civilization will have had a long time to evolve and develop there b.yes, because we have already found radio signals from another civilization living near a star in a globular cluster c.no, because such a star would most likely not have a stable (main-sequence) stage that is long enough for a technological civilization to develop d.no, because such a star (and any planets around it) would not have the heavier elements (carbon, nitrogen, oxygen, etc.) that we believe are necessary to start life as we know it e.yes, because globular clusters are among the closest star clusters to us, so that they would be easy to search for radio signals
no, because such a star (and any planets around it) would not have the heavier elements (carbon, nitrogen, oxygen, etc.) that we believe are necessary to start life as we know it
30.4 The Search for Extraterrestrial Intelligence
Some astronomers are engaged in the search for extraterrestrial intelligent life (SETI). Because other planetary systems are so far away, traveling to the stars is either very slow or extremely expensive (in terms of energy required). Despite many UFO reports and tremendous media publicity, there is no evidence that any of these are related to extraterrestrial visits. Scientists have determined that the best way to communicate with any intelligent civilizations out there is by using electromagnetic waves, and radio waves seem best suited to the task. So far, they have only begun to comb the many different possible stars, frequencies, signal types, and other factors that make up what we call the cosmic haystack problem. Some astronomers are also undertaking searches for brief, bright pulses of visible light and infrared signatures of huge construction projects by advanced civilizations. If we do find a signal someday, deciding whether to answer and what to answer may be two of the greatest challenges humanity will face.
