2) Magma oceans and
marauding asteroids
When we look at the moon, the dark gray "face" that we see is the remnants of a vast magma ocean, so if we could go back in time, there was a time when looking up at the moon you would see a hot, red sea of churning vulcanism. Now things are cooler, both on the Moon and here on Earth.
 |
| Late Heavy Bombardment as imagined by Timwether and the evolution of the moon movie from NASA |
The early Earth was hot…or was it? The early earth is quite
mysterious. The sun was dimmer at only ~70% of today’s levels, meaning that
less energy was coming to the Earth from a colder Sun. Yet the early atmosphere
lacked oxygen, and this anaerobic mixture was high in greenhouse gases that
would trap in warmth. On top of that, if the moon was born from an early impact
4.5 Ga (4.5 billion years ago), that might have left the Earth surface with a
magma ocean, similar to the one the moon had, evident in the dark patterns of
lava rock still seen on the Moon today. And a magma ocean, as awesome as that sounds, is totally
unsuitable for life.
So the window for life doesn’t begin until about 4.4 Ga,
when the earth cooled enough to allow water vapor to rain down on to the planet
and begin to form oceans. The crust of the earth was still undergoing heavy
volcanic activity, the magma covered moon was ten times closer, causing the
planet to spin faster, such that a day would only last 6 hours. And just to
make things even more fun that early period for life to begin was also subject
to the Late Heavy Bombardment, a terrific name for a period that saw hundreds
of millions of years where the early Earth was subjected to marauding asteroids
at a higher frequency of impact than any other time in our planet’s history.
The story of the early Earth is far from complete, and
further research will likely continue the debate about how things got started,
but life probably got started here while things were still hot and heavy.
Indeed, the earliest cellular life is thought to be super heat-loving
hyperthermophiles, that thrive at temperatures that could boil water. Plenty of thermophiles still exist today, and their DNA
places them close to the origin of cellular life at ~3.9 Ga. That means that
the 500 million year period from 4.4 – 3.9 Ga is where the building blocks for
cellular life had to come together -- or arrive from somewhere else -- all while dodging asteroids and magma flows.
Go on to #3 The First Language
When you said "that might have left the Earth surface with a magma ocean, similar to the one the moon had, evident in the dark patterns of lava rock still seen today", are the dark patterns of lava rock that are still seen today found on earth or on the moon. Is there any additional evidence that the earth was a magma ocean? How would you be able to tell that there was a magma ocean that surrounded the entire earth from lava rock? Is it possible that lava covered only part of the earth's surface? I guess the idea is difficult to wrap my head around. Is there evidence on how the ozone formed around the earth? I am assuming this occurred after the magma ocean cooled. Did the formation of ozone contribute to the formation of life? Is it part of the reason life began on this planet, and not others? Or did the ozone form after living cells?
ReplyDelete-Steph Triggas
Good point Steph, and I edited my post above to make it more clear. There is evidence for magma oceans on Earth, but it's nowhere near as obvious as the dark spots left on the Moon. So, its very difficult to pinpoint exactly how much of the earth could've been magma ocean, or for how long. But the Late Heavy Bombardment period must have been spectacular, from a geological perspective. I'm glad things are much calmer now.
ReplyDeleteThe primary source of ozone is from O2 (dioxygen) reacting with UV light, so the ozone layer we have today probably didn't form until after life started engaging in oxidative photosynthesis to produce the oxygen rich atmosphere we have now.
A magma ocean is unsuitable for life yet there are theories that life originated near deep sea hydrothermal vents. Either the magma ocean was much hotter than the heat from hydrothermal vents or life was not able to be detected in a magma ocean because fossils could not be formed or found from such conditions. Whichever way life did originate, I cannot help but wonder if life could be originated spontaneously in the same way today and throughout history. If it happened once, couldn’t it happen again? Is it possible for life to originate like it first did on Earth before reproduction was able to happen?
ReplyDeleteMagma oceans are indeed much hotter, they have no water just molten rock that would certainly be too extreme for any organic molecules to stay together.
ReplyDeleteMany people have wondered the same thing -- could life start more than once, or were there some very special fleeting conditions required? If life has started more than once, how do we find evidence for multiple starts to life? Hydrothermal vents are definitely an exciting environment to explore these questions.
If thermophiles and other forms of early life were able to spontaneously originate then there likely were several separate events of these developments of life created by an entirely different 'cell of origin'. Why is it then that no events like this have been seen in the genetic or the geological record? Perhaps these forms of life with a different originator 'cell' would be so extraneous to our observable lineage that we would not be looking in the right manner. Or maybe this separate cell lineage was absorbed into our own originator cells.But wouldn't you expect an imprint in the genetic record?
ReplyDeleteThis question of looking in the wrong places raises greater questions of what can life truly be expressed as and what other forms of unobservable life exist in the universe. I have heard of theories of gaseous forms of life on other planets that persist with genomes constructed out totally different of compounds. For example it is hypothesized that silicon could be a substitute for carbon... anyways this is starting to sound like science fiction.