Life on Earth

• In the beginning, the Earth was not life-friendly:
  • Very high volcanic activity, with lots of outgassing.
  • The atmosphere was composed primarily of CH₄, NH₃, H₂, and H₂O.
  • No oxygen, ozone—high UV radiation.
  • Lots of collisions, which dropped off dramatically after about 800 million years.
  • This early period of so-called heavy bombardment included the massive collion with a Mars-sized object which ultimately produced our Moon.
• The oldest fossils are from cyanobacteria, dated to roughly 3.5 billion years ago, shortly after the period of heavy bombardment.

Landmarks in our understanding of life

• Before the 1600’s, the notion that life can form from non-living matter was an accepted idea. People believed that:
  • open wounds produce maggots
  • garbage produces mice
  • rotting meat produces flies
  • sweat produce fleas and mice
  • mud produces frogs and snakes
  • rotting logs at the bottom of rivers produce crocodiles
• In 1668, Francesco Redi (1626–1697), an Italian physician, established that rotten meat does not produce flies; only flies produce flies. The experiment is regarded as one of the first steps in refuting "spontaneous generation" -- a theory also known as Aristotelian abiogenesis.
• In the 1800's, Louis Pasteur debunked the widely accepted myth of spontaneous generation for microbes, thereby setting the stage for modern biology and biochemistry.
• In 1953, Stanley Miller, a graduate student working the laboratory of Harold Urey, built an apparatus to demonstrate that biological molecules, on which all life is based, can result under the conditions of primeval Earth. After mixing methane, ammonia, water, and a spark, Miller produced a variety of bio-organic compounds, including amino acids, nucleic acids, and ATP.
• In 1969, the Murchison meteorite that fell near Murchison, Victoria, Australia was found to contain over 90 different amino acids, nineteen of which are found in Earth life. This fueled the notion that life here on Earth may have been originated from the complex organic molecules (as well as water) brought here by space debris. This could also imply an origin of life outside of Earth, which then migrated here (see Panspermia).

Drake equation

• In 1961, astronomer Frank Drake proposed his famous equation for the probability of intelligent life in the Universe.
  • The Drake equation attempts to answer the grand question of the number of communicating civilizations in our galaxy by considering simpler questions which are more easily quantifiable.
  • While most of the factors in the Drake equation can be somewhat quantified, the factor which suffers from the greatest uncertainty is the longevity of an advanced civilization. Clearly, we have only one data point--us--and even that does not tell us how long our civilization will persist.
  • Because of the uncertainty in the various factors in the Drake equation, particularly the longevity factor, the Drake equation cannot currently tell us how many advanced civilizations are out there. However, the Drake equation remains a reasonable starting point for discussion.
  • You can play around with this estimator and come up with your own estimate.
• Fermi’s paradox: If extraterrestrial intelligent life is common, it is very likely that this life would have colonized much of our Galaxy (and Universe) by now, after billions of years of cosmic evolution. So where are they? There are several possible responses, although the last one is favored by most astrobiologists:
  • We have not made contact because civilizations do not leave their home worlds—either because they are uninterested in interstellar travel or because they destroy themselves before achieving it.
  • There are galactic civilizations out there, but they have deliberately avoided revealing their existence to us.
  • We have not made contact because advanced civilizations are not common.
  • This Kurzgesagt video discusses these issues in a lively and (literally) animated way.
• The Rare Earth Hypothesis argues that complex (or intelligent) life is not common in the Universe, though the Universe might well be teeming with primitive life forms, such as bacteria.

SETI

• A project now known as the Search for Extraterrestrial Intelligence (SETI) was begun back in the 1960s to search the heavens for radio signals that reach us from some other technological civilization in the Cosmos.
• NASA funded the program in the 1980s but Congress pulled the plug on the project in the early 1990s. They sought private funding for their research and have now established the SETI institute.
• SETI's Arecibo radio-telescope in Puerto Rico, the largest Earth-based telescope in the world, is continuously scanning the sky in search of some signal that would stand out as being unequivocally unnatural. They do this with supercomputers and a network of radio telescopes around the world.
• Radio communication is simple, inexpensive, and the Universe is relatively quiet at most radio frequencies. So it is not unreasonable to suppose that the first choice for an alien civilization to attempt to make contact would be via radio communication.
• Proposals for interstellar languages have taken many forms, with two major strands focusing on either pictorial messages or logic-based messages. Presumably, an advanced civilization would discover the same laws of nature that Earthlings have. The Arecibo message is a good example of how simple math can be converted into a relatively sophisticated message about our location, size, DNA, etc.
• The UC Berkeley SETI team has discovered a way to tap the unused CPU power of idle home computers through a screen saver that analyzes bits of SETI's data and makes you a part of the greatest supercomputer on Earth. Over 2 million people in over 200 countries have now joined SETI@home (also known as project SERENDIP).
• Currently, astronomers use radio waves in the narrow frequency band (1420-1640 MHz) corresponding to a wavelength band (18-21 cm) to search for extraterrestrial signals. This band is known as the water hole.
  • This is the band between the hydrogen spectral line and the strongest hydroxyl spectral line. The combination of hydrogen and hydroxyl yields water (the "water" part of the name); the "hole" part refers to the sudden drop in radio noise within this band.
  • The water hole is also a quiet region bounded by galactic noise on the low-frequency end and by atmospheric nose on the high-frequency end.
  • Since the H and OH lines are visible from anywhere in the cosmos, in the quietest part of the spectrum, one might argue that these markers are not necessarily geocentric.
  • Scientists hypothesize that the water hole would be a good communication channel with extraterrestrial species because water is currently considered the universal prerequisite for life. Consequently, H and OH lines constitute obvious signposts to a natural interstellar communications band, one which would likely occur to other water-based life forms who have some knowledge of the radio sky.
  • In English vernacular, a watering hole is a reference to a common place to meet and talk.
  • The term 'water hole' was coined by SETI pioneer Bernard Oliver. "Where shall we meet our neighbors?" he asked. "At the water-hole, where species have always gathered."
• So far there have been no detections. But the search continues.

Habitable Zone

• Broadly speaking, there are three ingredients upon which life depends: water, energy, and organic molecules. The region where conditions might potentially support life is called the Habitable Zone.
• There are actually two types of habitable zone: galactic (GHZ) and circumstellar (CHZ).
• The circumstellar habitable zone (CHZ), also known as the Goldilocks Zone, is generally defined as the region around a star where water could exist on the surface of an Earth-like planet.
  • The CHZ depends on the mass of the parent star. The more massive (or luminous) the star, the farther and wider the habitable zone since the dependence of temperature on distance is not as steep at greater distances from the star.
  • In our own solar system, the CHZ is thought to lie within the band 0.95-1.37 AU's (from the Sun).
  • As our Sun ages and becomes more luminous, the CHZ will drift outward and will reach Mars in about a billion years.
• The galactic habitable zone (GHZ) is characterized by the following:
  • It is an annular region of our Galaxy in which, according to hypothesis, conditions are best suited to the development and survival of life as we know it. Outside the galactic habitable zone (GHZ) various factors make the existence of complex (multicellular) life difficult, if not impossible.
  • It is sufficiently close to the center of the Galaxy to have a high level of heavy elements, such as carbon, to favor the formation of rocky planets and complex molecules of life. The outer fringe of the galaxy is a difficult place to build life-supporting planets because there are fewer heavy elements.
  • It is sufficiently far from the galactic center to avoid threats to complex life: nearby transient sources of ionizing radiation, including supernovae and gamma ray bursters, impacts with space debris, close encounters with passing stars. Presumably, large amounts of radiation, which occur near the center of a galaxy, make formation of complex molecules more difficult.
  • The current GHZ is said to extend 7-9 kiloparsecs (23,000-29,000 light-years) from the galactic center, is widening with time, and is composed of stars that formed 4-8 billion years ago. The GHZ is analogous to the much more well established concept of the circumstellar habitable zone (CHZ).

Rare Earth Hypothesis

• The Rare Earth hypothesis argues that the emergence of complex life required a host of fortuitous circumstances, the seemingly rare combination of which suggests that intelligent life may be uncommon in the universe.
• Our relatively large Moon exerts a stronger tidal force on Earth that does the Sun and protects us from being tidally locked to the Sun.
• The Earth has a relatively strong magnetosphere that protects us from harmful radiation.
• The Earth has the right combination of size and composition to have plate tectonics, which helps to replenish the atmosphere (some of which constantly escapes into space).
• Jupiter protects us from (reduces the chance of) impacts with comets and asteroids.
• Massive stars may not live long enough to allow biological evolution to produce intelligent critters.
• Smaller stars have habitable zones which are relatively close and narrow. The closeness implies tidal hyperactivity and tidal locking.
• The term "Rare Earth" comes from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist.