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The whole question of the existence of extra terrestrial life, especially extra terrestrial intelligence is one I that I actually think is becoming less clear cut the more we learn.
One the one hand, the baseline chemistry needed for it all seems to be quite pervasive in the universe. Quite complex organic molecules have been detected everywhere from the murky atmosphere of Titan to (slightly more fundamental examples) even in stellar gas clouds. Despite the massive gap in our knowledge regarding the origin of life itself (i.e. abiogenesis, not evolution, which requires a working biology to begin with), the substances we do know to be minimally required are, it seems. everywhere. When you start throwing in findings of rocky extrasolar planets in their parent star's habitable zones, it's tantalising to believe that life must be as pervasive.
On the other hand, the more we learn about our planet, it's history and development, the more unique it seems. Once again, compare our world to Venus. As we know, Venus is more terrestrial (in terms of basic size, mass and composition) than any other body in the solar system. However, the conditions there are utterly hostile. The question is, why are Earth and Venus so different?
The simplistic answer is that Venus is closer to the sun and consequently just too hot. However, that's actually rather too simplistic. Venus orbits within the inner edge of the "goldilocks" zone and without the greenhouse effect, Earth would be pretty much frozen over. Were we rather closer, increased insolation would increase evaporation of water, which in turn increases cloud cover, which in turn raises the planetary albedo and subsequently results in a reduction in insolation. In short, there's nothing to preclude an Earth-like planet (albeit one which would be rather warmer, but not completely hostile to life as we know it) occupying Venus' orbit.
One key difference between Earth and Venus is that the Earth slightly denser. The reason that's important will become clear. It has been suggested that this discrepancy is down to the formation of the Moon (see giant impact hypothesis). This theory (which is reasonably supported by the available evidence) suggests that the lower density materials in the outer layers of the early Earth were blasted into orbit in a massive collision, to coalesce into the Moon, leaving the Earth itself relatively enriched in denser elements. These migrated inwards during gravitational differentiation and this has led to a difference in internal structure (relative to Venus) that gives the Earth its's planetary dynamo and geomagnetic field. It's believed that our inner core is solid, surrounded by mobile liquid outer core, the motion of which is in part responsible for our magnetic field. Venus, on the other hand, has a core region that is suspected to be entirely molten and comparatively quiescent. Venus has no significant planetary magnetic field at all. The only field that does exist (orders of magnitude weaker than ours) are the result of currents in it's upper atmosphere caused by the ionisation of gases there.
Earth's magnetic field is the strongest of all terrestrial planets in the solar system and is instrumental in preventing the lighter molecules (water in particular) from being stripped by the solar wind. Indeed, it would appear that Venus has lost a large amount (read oceanic volume) of water over it's history, inferred from the elevated ratio of deuterium in the hydrogen bearing compounds that remain. Losing an ocean results in a loss of plate-tectonic activity and the planetary sequestration of carbon bearing minerals that subduction gives. This allows it to accumulate in the atmosphere instead.
You can continue this chain of cause and effect and come up with the surprising conclusion that the catastrophic event which gave Earth it's abnormally large moon also left it in a unique position in which it could retain it's surface water, something neither of our neighbours have managed. Moreover, the Moon itself has had a stabilising effect on the planet's rotation, preventing unchecked changes in our axial tilt.
To cut this long story slightly shorter, it may be that our planet represents something extremely rare by having all the "right things" in the "right place" at the right time. Not just the chemistry or distance from the sun, but the planetary structure and stability brought on by a chance event in our planet's history.
One the one hand, the baseline chemistry needed for it all seems to be quite pervasive in the universe. Quite complex organic molecules have been detected everywhere from the murky atmosphere of Titan to (slightly more fundamental examples) even in stellar gas clouds. Despite the massive gap in our knowledge regarding the origin of life itself (i.e. abiogenesis, not evolution, which requires a working biology to begin with), the substances we do know to be minimally required are, it seems. everywhere. When you start throwing in findings of rocky extrasolar planets in their parent star's habitable zones, it's tantalising to believe that life must be as pervasive.
On the other hand, the more we learn about our planet, it's history and development, the more unique it seems. Once again, compare our world to Venus. As we know, Venus is more terrestrial (in terms of basic size, mass and composition) than any other body in the solar system. However, the conditions there are utterly hostile. The question is, why are Earth and Venus so different?
The simplistic answer is that Venus is closer to the sun and consequently just too hot. However, that's actually rather too simplistic. Venus orbits within the inner edge of the "goldilocks" zone and without the greenhouse effect, Earth would be pretty much frozen over. Were we rather closer, increased insolation would increase evaporation of water, which in turn increases cloud cover, which in turn raises the planetary albedo and subsequently results in a reduction in insolation. In short, there's nothing to preclude an Earth-like planet (albeit one which would be rather warmer, but not completely hostile to life as we know it) occupying Venus' orbit.
One key difference between Earth and Venus is that the Earth slightly denser. The reason that's important will become clear. It has been suggested that this discrepancy is down to the formation of the Moon (see giant impact hypothesis). This theory (which is reasonably supported by the available evidence) suggests that the lower density materials in the outer layers of the early Earth were blasted into orbit in a massive collision, to coalesce into the Moon, leaving the Earth itself relatively enriched in denser elements. These migrated inwards during gravitational differentiation and this has led to a difference in internal structure (relative to Venus) that gives the Earth its's planetary dynamo and geomagnetic field. It's believed that our inner core is solid, surrounded by mobile liquid outer core, the motion of which is in part responsible for our magnetic field. Venus, on the other hand, has a core region that is suspected to be entirely molten and comparatively quiescent. Venus has no significant planetary magnetic field at all. The only field that does exist (orders of magnitude weaker than ours) are the result of currents in it's upper atmosphere caused by the ionisation of gases there.
Earth's magnetic field is the strongest of all terrestrial planets in the solar system and is instrumental in preventing the lighter molecules (water in particular) from being stripped by the solar wind. Indeed, it would appear that Venus has lost a large amount (read oceanic volume) of water over it's history, inferred from the elevated ratio of deuterium in the hydrogen bearing compounds that remain. Losing an ocean results in a loss of plate-tectonic activity and the planetary sequestration of carbon bearing minerals that subduction gives. This allows it to accumulate in the atmosphere instead.
You can continue this chain of cause and effect and come up with the surprising conclusion that the catastrophic event which gave Earth it's abnormally large moon also left it in a unique position in which it could retain it's surface water, something neither of our neighbours have managed. Moreover, the Moon itself has had a stabilising effect on the planet's rotation, preventing unchecked changes in our axial tilt.
To cut this long story slightly shorter, it may be that our planet represents something extremely rare by having all the "right things" in the "right place" at the right time. Not just the chemistry or distance from the sun, but the planetary structure and stability brought on by a chance event in our planet's history.
