Tuesday, February 17, 2009

More Followups on the History of our Planet

Mike said:
...years after the impact to do so), but would certainly have been obliterated once there was nothing but hot lava to stand on.
Roland wrote:
How certainly and how obliterated? Do the temperatures involved clearly preclude the survival of any molecules more complex than those that existed during the earlier "hot" period? I find it slightly more romantic to imagine that some little step of material complexity on the way towards what we now call life might have carried over from an older world.
Estimates of surface temperature post-impact that I've seen are around 2500K, or roughly the surface temperature of your average red star. The only molecules that survive those temps are pretty simple...metal hydrides and metal oxides, mostly. This is definitely not a regime where amino acids can exist, and depending on the the exact temperature, even H2O can dissociate. One of the few surviving minerals from before the impact is zirconium dioxide (from which we radiometrically date Earth's formation)...it has a melting point around 3000K, so that does place an upper limit, at least.



Mike said:
Well, the whole Precambrian era was not the most exciting time-period, kinda like how in a Western Civ class they'll skip straight from "End of the Roman Empire" to "The Renaissance".
Roland then asked:
Was there something amazingly cool going on simultaneously next door (whatever that means) at the time? (The geological analogue to how the heights of the West's sister societies under Islam was taking place during this "boring period".)
Well, Mars certainly had *something* going on around 3 billion years ago. It's pretty much settled at this point that Mars had an abundance of liquid water on its surface, and probably even had a massive northern ocean. Just look at a surface topography map and you'll notice that not only is the entire northern hemisphere much lower in elevation, but also relatively devoid of craters. Moreover, there are very obvious drainage patterns, e.g. Warrego Valles. Moving from the southern highlands into the northern lowlands, which abruptly fade away right where you'd expect there to be a "beach".

Now, what happened to all the water is another matter, and still somewhat debated. The likely scenario involves a few key observations...Mars definitely had active volcanoes in the past, but they're all extinct now. Remember that Mars is significantly smaller than the Earth, about half the radius. This means its surface area-to-volume ratio is about twice as large as Earth's, so it should cool approximately twice as fast. As far as we can tell from a lack of magnetic field, there is no longer any molten material in Mars' interior...so it looks like the volcanoes simply shut off once the mantle cooled.

Once your volcanoes stop working, it's a very bad day for your planet. Atmospheres continually "sputter" off into space - high-energy photons for the Sun will hit gas molecules, which then have enough energy to make escape velocity and leave the planet forever. On Earth and Venus, our active volcanoes are the only things which continually resupply the atmosphere to keep it in a quasi-steady state. Add to that Mars' escape velocity, which is less than half of Earth's, and atmospheric sputtering becomes a big deal.

Another significant observation in this scenario is Mars' lack of a big moon. Earth's habitability depends strongly on having a relatively large moon...from regular torsion forces exerted by our Moon on Earth's slightly oblate shape, our North Pole precesses every 26,000 years, just like a top which both spins (rotation) and wobbles (precession). The downside to this is that we don't always have a pole star to navigate by (we happen to be lucky to be born at at a time when our North pole points to Polaris). The upside is way more important, though - we have a relatively constant axial tilt of 23.5 degrees. Since axial tilt determines the strength of a planet's seasons, ours have been relatively constant since the moon formed.

Mars is a different story, though. It gets tidally pulled in a very non-regular manner...sometimes by the Sun, sometimes by Jupiter. Although its current axial tilt is only 24 degrees, numerical simulations have shown its axial tilt in the past to be anywhere from 0 to 60 degrees over the course of millions of years. This would mean crazy seasons...with an axial tilt of 60 degrees, your arctic circle will be down at 30 degrees latitude. Constant sunshine during the summer over most of a hemisphere would only help the sputtering phenomenon, further decreasing the atmospheric pressure.

Recall that the temperature range at which liquid water can exist decreases as atmospheric pressure drops...this is why there are things like high-altitude baking directions. Eventually, at a pressure right around Mars' current atmosphere, it can't exist at any temperature. Goodbye, ocean.

Mike

2 comments:

  1. One point worth noting is that, although this may seem counterintuitive, an Earth with an axial tilt of 60˚is like to be much more hospitable than a zero-tile Earth.

    On a zero-tilt Earth glaciers would be able to form almost anywhere snow might fall, since the absence of seasons precludes summer melting - which occurs even with glaciers that are not shrinking. This would allow glaciers anywhere with sufficient precipitation and a mean temperature below 0˚C (unless topography was unsuitable).

    The result would be that with zero tilt the west coasts of Europe, the Americas and New Zealand would be covered with ice as far as 35˚N and 30˚S. Glaciers would also cover the entire Appalachian system, and much of Japan. They would rapidly cause the climate to cool so much that even the sun-drenched equatorial regions would be much drier because huge amounts of very cold water would flow equatorward, possibly in a cycle quite different to what we have today.

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  2. While you make a good point, I'm dubious that a zero-tilt Earth would have glaciers at latitudes as low as 35˚N and 30˚S. I think the missing key piece in your idea is the circulation of heat carried by the oceans and atmosphere.

    Let me offer two examples. First, the planet Venus. It's a good laboratory for this test case, because Venus has almost exactly a zero-degree tilt. However, there's very little difference between the temperature at the pole and the equator, because the atmosphere is very good at taking the sunlight soaked up at the equator and propagating that heat towards the pole in a planet-wide circulation.

    The second example is Northern Europe. Inverness, Scotland is located at 58˚N, and has an average high in January of 39˚F. Meanwhile, the town of Churchill, on the shores of Hudson Bay in Canada - and roughly at the same latitude as Inverness - has an average January high of -11˚F. Since they're at the same latitude, they each receive the same amount of sunlight. So, this 50 degree temperature difference is due entirely to the mitigating effects of the Atlantic Ocean circulating heat from the tropical Gulf of Mexico to warm the shores of Scotland.

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