Does life improve habitability? (Introduction)

by David Turell , Tuesday, May 03, 2016, 20:09 (3125 days ago)

A very interesting question, because he author points out necessary cycles on Earth that require life to take part:-http://www.scientificamerican.com/article/which-came-first-on-earth-habitability-or-life/?WT.mc_id=SA_DD_20160503-"Some wonder if previously overlooked mechanisms—including life itself—could broaden the habitable zone well beyond its current definition. Colin Goldblatt, a planetary scientist at the University of Victoria in British Columbia, even argues that life's ability to alter a planet's climate poses a new paradox: A planet's habitability could depend on whether life has already made itself at home there, a situation that would place habitability and life in a baffling chicken-or-egg scenario.-***-"It's absolutely essential to keep in mind that habitability is not just where you are in a solar system,” says David Crisp, the lead research scientist for the Orbiting Carbon Observatory 2 at the NASA Jet Propulsion Laboratory (JPL). “It's a property of the planet that you're living on.” Earth, for example, has a built-in temperature control system: the carbon-silicate cycle. Some 2.5 billion years ago the sun was so faint that the oceans should have been frozen—but they were not. The simple explanation is that Earth likely boasted an atmosphere thick with greenhouse gases. Then as the sun's brightness grew, the planet counteracted the warming climate by scrubbing carbon dioxide from the air: Higher temperatures increased rainfall, which pulled the greenhouse gas from the atmosphere and carried it into the oceans, where plate tectonics eventually subducted it into Earth's mantle. -***-"This balancing act has stabilized Earth's climate for billions of years, letting the carbon dioxide swing up or down by more than 1,000 percent in order to keep the planet's temperature steady and thereby increase the size of its habitable zone. And it is not just due to geochemistry; the carbon-silicate cycle depends on biology as well. Carbon dioxide is removed from the ocean when sea creatures convert it into the calcium carbonate they use to build their shells. After those creatures die they sink into the deep ocean where their shells are subducted into the mantle. For an example of this phenomenon, Goldblatt points to the White Cliffs of Dover. These limestone cliffs along the English coastline are composed of calcium carbonate that formed when the skeletal remains of planktonic algae sank to the bottom of the ocean during the Cretaceous period. It appears that levels of both carbon dioxide and nitrogen (which is similarly whipped between Earth's mantle and atmosphere) can be subject to a planet's biosphere. Life creates conditions that help sustain itself.-***-"The habitability of an environment is affected to a certain extent by whether or not it is inhabited by some life form.” Although this is generally agreed on, Goldblatt takes it a step further by saying that we cannot disentangle a habitable planet from the presence of life itself. “The thing that I want to push in this paper is a philosophical point—not a point of technical calculations,” Goldblatt says. “You can't try to address whether a planet is suitable for life or not without considering whether there is already life on the planet.” Whereas most astronomers search for worlds that are suited to host life around other stars, Goldblatt does not think a planet can be called “habitable.” It is either inhabited, or it is not. If we find a lifeless Earth-like planet in the so-called habitable zone and we just plop an egg of life on that planet, there is no guarantee that life will take hold, Goldblatt says. “We have no idea what a planet at that [distance] without life would actually look like,” he says. “It would look nothing at all like the Earth.'”-Comment: A fascinating point. The books describing the Earth as special, point out all the necessary cycles and controls, but don't make this relationship which seems obvious, although I hadn't thought of it. It comes across like the interrelationship of factors that describe the Anthropic Principal, chicken/egg, we're here because we're here.