Life in the Universe

Life in the Universe Life on Earth Is there life beyond Earth? Great for science fiction plots, but at present there is no undeniable evidence that aliens have been here. Many astronomers of the past have suggested that life existed elsewhere. Kepler – thought there were inhabitants on the Moon. Herschel – claimed life existed on nearly all the planets. Lowell – thought he saw canals on Mars. Before searching for life elsewhere in the universe we must first look at how life arose on the Earth. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth Three recent developments have led to the idea that life may not be so uncommon beyond Earth. Life arose quite early in Earth’s history. Laboratory experiments show that the chemical make-up of the young Earth could readily combine to form organic molecules, suggesting that life formed from naturally occurring chemistry. We have discovered microorganisms which live in extreme Earth conditions which may be similar to those found on other worlds © Sierra College Astronomy Department 36

Life in the Universe Life on Earth When did life arise on the Earth? After the Earth formed 4.5 billion years ago, life had little chance to survive. First several hundred million years was era of heavy bombardment. These bombardments could vaporize oceans killing any life that might have formed. Studies of the craters on the Moon suggest that the heavy bombardment ended 4.2 to 3.9 billion years ago. Remarkably life may have been thriving 3.85 billion years ago – consequently it appears lifer arose in a geological blink-of-an-eye once conditions became hospitable. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth Fossils and Geologic Time Scale The key to understanding ancient life is to look for fossils, relics of organisms which lived and died long ago. These fossils can be found under layers of sediments which are carried by rivers or lie under ocean floors. Grand Canyon shows billions of years of Earth’s history. Relative ages of rocks and fossils are easy to determine: each deeper layer formed earlier. Radiometric dating confirms this and estimates the absolute ages of the material. Based of the layering of the rocks and fossils Earth’s history can be divided into several distinct intervals or geological time scales. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth Fossil Evidence of Early Life Since the Earth is so geologically active, it is difficult to find rocks which are over 3 billion years old. The evidence of the existence of life 3.5 billion years ago is suggestive and subtle since microscopic fossils are difficult to detect. What we see today is what was left behind by ancient bacteria in rocks called stromatolites. Also, the ratio of carbon-13 to carbon-12 in rocks containing fossils has been found to be lower relative to rocks that do not contain fossils. Some rocks dating back to 3.85 billion years ago have the lower ratio and this is suggestive that ancient life formed at this distant time in the past. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth How did life progress on Earth? Theory of Evolution In the 19th century, most biologists agreed that species change through time. In 1859, Charles Darwin explained how life might undergo those changes based on two observations: Overproduction and struggle for survival. Individual variation. Conclusion: unequal reproductive success  what is often termed natural selection or survival of the fittest. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth The Mechanism of Life DNA (deoxyribonucleic acid) is the genetic material of all life on Earth. It consists of 4 chemical bases: adenine (A), thymine (T), guanine (G), cytosine (C) which pair up in two long strands and wind together in a double helix. DNA is self-replicating, which is the key to heredity. Any change (small or large) to this sequence from imperfect duplication is called a mutation. Most mutations are lethal and kill the cell with the mutation. But some are beneficial and these can be passed to offspring. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth The First Living Organism DNA has the same basic chemical nature in all life. Also, all organisms build proteins from the same set of amino acids. This suggests that all life had a common ancestor which arose some 3.85 billion years ago. The present day black smokers which thrive deep underwater at high temperatures may resemble this early form of life. High temperatures may promote faster and more diverse chemistry which allows life to form quickly. Looking at DNA from all life, biologists have composed a “tree of life”, which suggests how the DNA changed to other types of life. Consists of 3 main domains: bacteria, archaea, and eukarya. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth The Origin of The First Living Organism Where did this first life come from? Some experiments done in the 1950s (and since) showed a mixture of early-Earth organic molecules plus lightning can produce all the major molecules of life including amino acids and DNA bases. Strands of RNA (ribonucleic acid) which resemble single strands of DNA have been reproduced in the laboratory. Many biologists presume that RNA came first followed by DNA. Microscopic enclosed membranes can form which may have surrounded self-replicating RNA on the early Earth. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth Could life have migrated from elsewhere? The idea that life started elsewhere and then came to the Earth (via meteor impacts) is called panspermia. While the prospect of life traveling and surviving through space seems difficult, we have evidence of organic molecules in meteorites and tests that show microbes can survive space for years. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth A Brief History of Life on Earth As Earth formed, chemical reactions form the first organic molecules. After the heavy bombardment ended, the common ancestor of life formed. Life rapidly grew and diversified, but remained single cell organisms for 1 billion years. The land was still inhospitable until the ozone layer formed – this required atmospheric oxygen © Sierra College Astronomy Department 36

Life in the Universe Life on Earth Nearly all of the original oxygen was released via photosynthesis from single-celled cyanobacteria some 3.5 billion years ago. For more than 1 billion years, this oxygen reacted with surface rocks and little stayed in the atmosphere. Eventually, some 2 billion years ago, the oxygen began to accumulate, but would not be “breathable” until just a few hundred million years ago. This allowed new species (e.g. plants and animals) to form. Many other species did not need oxygen or even found oxygen to be toxic. © Sierra College Astronomy Department 36

Life in the Universe Life on Earth About 540 million years ago, tiny plants and animal organisms changed dramatically in about 40 million years and formed into all the basic plants (phyla) that we find on Earth today. The dramatic change in the diversity of life is called the Cambrian explosion. Early dinosaurs and mammals arose at the same time (225 to 250 million years ago). Dinosaurs died off 65 million years ago (by a meteor impact?) and the mammals then rose to dominance. The earliest humans formed only a few million years ago (after 99.9% of Earth’s history). © Sierra College Astronomy Department 36

Life in the Universe Life on Earth What is necessary for Life to exist? While animals need moderate temperatures and abundant oxygen, simpler life can live under much more extreme conditions and locations (extremophiles). Underground, high and low temperatures. There seem to be 3 basic requirements. A source of nutrients Energy to fuel the activities of life Liquid water (the biggest constraint) © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System To look for life elsewhere, we need to search for places where the basic necessities of life exist – the habitable worlds. This eliminates most of worlds in our solar system. Moon and Mercury are barren and dry. Venus too hot for liquid water. Jovian planets are gaseous. This leaves Mars and a few of the moons orbiting the Jovian planets, notably Europa. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System Mars Percival Lowell thought he saw canals on Mars, but we are quite confident now that there are no civilizations on Mars. Nevertheless, we have good evidence the liquid water once flowed on the Martian surface. Today it contains subsurface ice which could be heated to form areas of liquid water underground. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System Missions to Mars, looking for life The Viking missions took soil samples and looked for chemical changes that could be attributed to biological processes. 3 experiments suggested that life may be present, but also ordinary chemical reactions could have caused the same results. A fourth experiment found little organic material, the opposite of what one would expect if life were present. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System The Mars Phoenix mission detected water under the surface, though soil was basic and may have trouble harboring life (perchrolates) Pathfinder, Spirit and Opportunity studied the Martian conditions to see if life might have existed. The Mars Express orbiter detected methane gas. Methane should disappear within a few centuries due to chemical reactions. So, something is supplying Mars with methane. It could come from comet impacts, volcanoes, or life. Volcanism seems to be the most likely candidate. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System Martian Meteorites One meteorite which landed in Antarctica 13,000 years ago and found in 1984 was clearly of Martian origin. Inside the meteorite were complex organic materials and structures which looked like nanobacteria , very small bacteria which have been discovered on Earth. These structures can also be made by chemical and geological means. Contamination from being on the Earth may also explain the presence of organic materials. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System Life on Europa Europa has enough tidal heating to possibly form a subsurface ocean underneath its icy crust. Life there could form like the “black smokers” on Earth. Larger life forms could exist in the vast oceans, but energy sources are limited and this would tend to limit the size of any life there. © Sierra College Astronomy Department 36

Life in the Universe Life in the Solar System Life on Ganymede, Callisto and Titan Ganymede and Callisto might have subsurface oceans, but their internal heat is small and liquid water would not be terribly abundant. Titan has no native liquid water, but an abundance of organic materials. Could life evolve from the lakes of methane? Water might be brought in from comets, but this would eventually freeze. © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars Beyond the Solar System Where in the Galaxy might we find life? Since technology might allow us to obtain surface pictures or spectra, we restrict ourselves to considering extrasolar planets with habitable surfaces. So far all detected extrasolar planets (except maybe one or two) are gaseous giants and are unlikely to have surface life. However, they may be surrounded by moons which may support life. © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars Constraints on Star Systems A star must be stable and live long enough to allow a planet to develop life. Stars greater than a few solar masses are ruled out (but this is only about 1% of all stars). A star must allow stable planet orbits. Binary and multiple star systems are much less likely to have this – about 50% of all star systems. © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars A third constraint is a planet must form in the habitable zone. This is a region where a terrestrial type planet would have the right surface temperature for liquid water to exist. Stars less massive than the Sun have smaller zones. A star like the Sun (or more massive) would have the largest zone. Even if we restricted our search to Sun-like stars, we would still have to consider billions of stars in our Galaxy. © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars Finding Habitable Planets Two upcoming missions may be able spot Earth sized planets. Kepler will look for transits of planets across other stars. The Space Interferometer Mission (SIM) may be able to detect Earth sized planets. A decade or so from now, the Terrestrial Planet Finder (TPF) or something like it may be able to image extrasolar planets. Infrared spectra from future telescopes can look for signatures of life © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars Rare Earth? Some feel that an Earth type planet (with its complex type of life) is rare: Galactic constraints Too close to the galaxy’s center and the rate of supernovae are too great. Too far from the center and “metal” content is too low. This leaves about 10% of the galaxy’s disk that might be habitable. A stellar system needs a Jupiter-like planet to sweep-out and deflect meteors that might wipe out life on Earth. Climate stability Plate tectonics and the carbon dioxide cycle. Earth’s large Moon keeps axial tilt relatively stable. © Sierra College Astronomy Department 36

Life in the Universe Life Around Other Stars Rare Earth? Counterarguments to the Rare Earth Hypothesis The above conditions may not affect the creation and advancement of complex life as much as we think. There may be other overlooked conditions and processes that could assist the creation and advancement of complex life. © Sierra College Astronomy Department 36

Life in the Universe The Search for Extraterrestrial Intelligence What About “Intelligent” Life beyond the Solar System? SETI (Search for Extraterrestrial Intelligence) is trying to find signs of alien communication How many civilizations out there? The (modified) Drake Equation suggests the number of civilizations we might be able to contact: # of civilizations = NHP × flife × fciv × fnow where NHP is the number of habitable worlds flife is the fraction of these world which actually have life fciv is the fraction of these worlds which have interstellar communications fnow is the fraction of these worlds which have a civilization at the present time It is hard to know exactly what any of these numbers are at the present time. © Sierra College Astronomy Department 36

Life in the Universe Interstellar Travel If one is restricted to going no faster than the speed of light, then interstellar travel will be difficult. In any event, vast new energy sources must be used to propel a ship. Hydrogen scoopers. Nuclear bombs or nuclear power Matter-antimater. © Sierra College Astronomy Department 36

Life in the Universe Interstellar Travel If there is advanced alien life out there, why haven’t we seen them (Fermi Paradox)? We are alone and there is no other advanced life out there. Civilizations are common, but no one has colonized the galaxy because Technology prevents a widespread travel. The desire to explore is unusual. Civilizations destroy themselves before they can colonize the stars. There is a galactic civilization, but it has not revealed itself!!! © Sierra College Astronomy Department 36

THE END 36