Friday, February 13, 2009

Questions about the History of our Planet

In the email which started it all, Ben wrote:
Dear Planetary Astronomer Mike:

I was watching a show on the Science Channel yesterday, and it was this super-dramatic documentary about the history of our planet. The whole story was told from the tabloid angle of "our world was created via a series of terrifying catalysmic events!!" Cheesy hype aside, I'm wondering which of the events below is considered well-accepted/mostly-agreed-upon by planetary scientists, which are controversial theories, and which are laughable. (The show presented every event as an amazing "fact" in glorious CGI.)
Ok, there's a lot here, so let's take it step-by-step.

First, let me make a caveat, though: cataclysmic change is all the rage in the sciences these days, from biology to astronomy. One-hundred years ago this was not the case...Barringer had a very difficult time trying to prove to people that Meteor Crater in Arizona was caused by an impact. Back then, people believed geological events only occurred on geological timescales.

Nowadays, possibly due to a massive philosophical shift resulting from the World Wars, the science world has embraced the concept of sudden change. This is not to say that it doesn't happen - there's very good evidence to suggest that it does - just that the pendulum may have swung a little too far in the other direction.

So, point by point, then:

Ben wrote:
The timeline went something like this:

After cooling down from the nebula, Earth was a big chunk of dead, lifeless rock.
I think this first one is actually the least scientific claim of all. Soon after the early Earth formed, there'd be a lot of self-gravity holding the thing together...easily enough to melt rock and metals due to pressure heating. Moreover, a lot of the relatively short half-life radioactive materials (such as aluminum-26) would still be hot from the last supernova from which our solar system formed...these radioactive materials act as another energy source to melt material in larger planetesimals.

All this molten rock swimming around with molten metal would cause differentiation - heavier materials like iron would sink to the core, lighter materials like silicates would rise to the surface. You don't have to be *that* big for differentiation to occur. Some of the larger planetesimals in the asteroid belt differentiated before they were rended apart due to Jupiter's tidal forces, leaving us with almost entirely metallic asteroids such as Vesta.

This ends up leaving a lot of hot magma on the surface exposed to the vacuum of space...the vacuum pressure alone would cause serious outgassing from the lava. This early Earth would probably have had a pretty significant CO2 and water vapor atmosphere, so I guess I'm not seeing the "dead, lifeless rock" here.

Ben continued:
An anonymous mars-sized planet collided with Earth, and the planets *fused together*. This caused Earth to get much bigger, and the sheer pressure and force of the collision caused our planet to become super geologically-active -- molten iron core, lava-ey mantle, drifting tectonic plates on top.
Okay, so technically it's not anonymous - they've actually provisionally named it "Theia".

Current n-body simulations (remember "orbit7"?) have shown it'd be pretty likely for an early Earth to accrue Trojans. These are essentially mini-planetesimals which orbit at the same distance from the Sun as a parent body (in this case, the early Earth), but at Lagrangian points 60 degrees ahead and 60 degrees behind the parent body's position in its orbit. We've actually seen tons of asteroids 60 degrees ahead and 60 degrees behind Jupiter, but in the same orbit.

Here's the catch, though: those objects at the Lagrangian points are only in stable orbits if they're much less massive than their parent body. These same simulations have shown that it's quite possible for these early Trojans to start accreting to a mass over the stability limit...at which point they come spiraling to the Earth.

Getting to your later question, then, we are pretty sure this event is what formed our moon. A mars-sized object (roughly 1/10th the mass of the current Earth) hit us in a sidelong collision, mostly fusing, but leaving a whole bunch of debris in Earth orbit. This debris later coalesced into our Moon.

This scenario explains two seemingly contrary properties of the moon:
  1. The moon composition is surprisingly close to Earth's rocks.
  2. The moon's orbit is really close to the plane of the solar system.
Statement 1 implies that the Earth and Moon formed side-by-side (the "sister theory")...but if this were true, we'd expect the Moon's orbit to be aligned with Earth's rotation axis, 23 degrees tilted to Earth's orbital axis. Statement 2 implies that the moon was captured from elsewhere in the solar system (the "capture theory")...but if this were true, we'd expect the Moon to be significantly different in composition.

The "Giant Impact Hypothesis" neatly ties both observations together. Statement 1 can be true because the orbital debris which formed the Moon came from the Earth and Theia. Statement 2 can be true because the initial angular momentum to establish the Moon's orbit was in line with Theia coming in from elsewhere in the solar system.

Now, why they didn't cover this in the documentary, I don't know...it'd make for some pretty sweet CGI. Again, the Earth was already pretty geologically active prior to this impact, but definitely way moreso afterwards.

Ben said:
Somehow (?) this caused water to form and cover the whole planet.
Umm, again, the water should already have been there, though possibly in vapor form depending on Earth's pressure and temperature prior to impact. Simulations I've seen at planetary conferences have actually shown that after the impact the Earth's surface was hot enough that, at least for a few thousands years, a significant component of the atmosphere was *rock vapor*. Whoa.

Eventually it'll cool down enough for rock, and then even water to rain out of the atmosphere. Note that there's still some modest debate about why Earth has *so much* water...some people point to a continual barrage of comets. Other people point to those first people and laugh.

Ben continued:
Plates started drifting around. Their collisions caused them to buckle up, revealing islands, and eventually whole continents.
Well, again, I don't know why there wouldn't have already been plates prior to impact, but whatev'...

Ben then wrote:
Continents aggregated into ever-bigger continents, and eventually the last 2 super-continents collided to form Pangaea
Whoa, super fast-forward! We just skipped *most* of the history of Earth. The Giant impact happened about 3.9 billion years ago...Pangaea formed only 250 million years ago.

It's believed there were actually several supercontinents formed prior to Pangaea...Vaalabara, Columbia, Rodinia, etc. Each of them came together, drifted apart, came together, drifted apart. The best analogy I've heard for this is:

Imagine you put small plates of styrofoam on top of some boiling water...at certain times they'll all lump together as they're pushed by the convective flow. As they drift around as a single mass, they might pass over a convective plume...when this happens, they'll get pushed apart again, and the cycle repeats.

Ben wrote:
The collision which form Pangaea created super-mountains across the center, bigger than the Himalayas. They were so high that they blocked all clouds and weather. This meant that the coastlines were lush and rainy. but the center of Pangaea (inner 60% land mass) was one giant "super desert", more than 10x the size of the Sahara. The rainy coasts set the stage for the development of multicellular marine life.
Ok, it probably wasn't Pangaea this happened on, but the prior supercontinent Rodinia (750 million years ago, roughly). It *is* a general property of supercontinents that they form deserts in their interior...you don't need to have big mountains, but it helps. The zonal flow picks up moisture passing over the super-ocean, and it rains out as it passes over the landmass. In the case of supercontinents, though, there's just more landmass than available rain.

I'm not sure what "bigger than the Himalayas" means...you really can't form a mountain much taller than Everest, since rock liquefies under the pressure of anything taller. Only on planets with lower gravity can you do this - for example, Mars has 1/3 the gravity of Earth, and Olympus Mons (tallest mountain on Mars) is 3 times taller than Everest.

By bigger do you just mean the range was more extensive? For ther record, the Himalayas already push up into the prevailing jet stream, causing vortices further downwind. and alternating highs and lows that we experience here in N. America.

Ben continued:
Later on (250 million years ago?), some Big Nasty Event caused 90% of all life to go extinct -- most likely planet-wide volcanic activity which poisoned the atmosphere with methane and warmed the planet excessively. This set the stage for tiny lizards (which survived) to evolve into dinosaurs and cover the pangaeic "hot earth".
Ah, the Permian-Triassic extinction event - the largest extinction event *ever*. Over 95% of marine life died.

There's still heavy debate about the cause. There were definitely huge crazy volcanoes unlike any seen before...the "Siberian Traps". It wasn't so much a single volcanic explosion from a mountain as it was a giant fissure in the crust of the Earth...lava just oozed from the gash for upwards of a million years, paving over most of what is now current-day Siberia with basalt. That kind of event would release massive, massive amounts of CO2 in the air and easily kill off lots o' life.

What precipitated the eruption is the debated part. There's extensive evidence of iridium at this geological layer, which suggests an impact event. Iridium is one of the two densest materials, so most of the iridium the Earth formed with quickly sunk to the center of the core during the early differentiation period. In general, if you find iridium on the surface, it came from an extra-terrestrial source.

However, there doesn't seem to be any really good candidate crater from this time period. It's interesting to note, though, that there are similar formations to the Siberian Traps on the Moon, known as maria - the large, dark flat regions you see when looking at it. These seem to have been caused by impact events so massive that the impactor pierced through the crust, creating the fissure from which lava pours out...it's possible there's no obvious impact crater because most of the impactor went all the way through the Earth's crust and melted in the mantle.

Ben added:
65 million years ago, an asterioid hit earth, wiped out the dinosaurs, left the mammals.
There's very little debate about this now...Chicxulub crater is the smoking gun, and the geological evidence shows iridium planet-wide at exactly 65 million years ago.

Ben then wrote:
Pangaea starts to break apart over the next 60 million years. The great plain of North America is a giant inland ocean.

An ice age happens about 12,000 years ago. Ice caps cover 1/3 of the planet, causing ocean levels to fall and the N. American inland ocean to completely drain. Glaciers carve the hell out our landscape.
Well, ice ages are super common...the one 12,000 years ago was just the most recent. The theory is they should happen every 41,000 years, as the period of Earth's precession comes into phase the period of change of Earth's perihelion (known as the "Milankovitch" cycle).

Ben concluded with:
There's an ongoing cyclical process of continents drifting apart and coming back together into a supercontinent (why??). Folks hypothesize a 'new' pangaea happening in the future, whereby Asia rotates clockwise, pushing the UK into the polar cap and Siberia into the equator. Africa smashes into Europe, replacing the Meditarrean sea with a huge mountain range: see here and here.
Yup, "Pangaea Ultima". It happens for the same reason that this cycle has always occured...unstable plates floating along magma convection flows.

Mike.

1 comment:

  1. Such an educational writing piece. I will need to write my paper soon, that is why I won't be able to follow your blog on a daily basis!

    ReplyDelete