Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Monday, April 4, 2011

Paradox Earth: III

Understanding the climate and overall environment of the very young Earth continues to be an extremely tricky business. Previous posts on several issues (I, II) surrounding the so-called Faint Young Sun paradox have discussed some of the sticking points. In a nutshell; 4 billion years ago the Sun was about 30% fainter than it is today, a direct consequence of the fundamentals of stellar evolution. So the puzzle is that as far as we can tell the surface environment harbored liquid water, yet today's atmospheric composition would have resulted in a vastly colder climate. Boosts to greenhouse gases might solve the problem, but it remains at the hairy edge of plausibility.

Now a new study by Court and Sephton casts an even murkier pall over the problem, literally. We have high confidence (from the record of lunar cratering, as well as the orbital evolution of the outer planets) that some 4.1 to 3.8 billion years ago the Earth was subjected to period of sustained impact over about 100 million years by asteroidal-type material. The so-called Late Heavy Bombardment (LHB) was quite a pounding. It likely provided the major constituents of the juvenile Earth's outer layers. Court and Sephton have studied the effect of the sand-grain sized components of material that may have poured into the Earth's atmosphere as micrometeorites during this era. Atmospheric friction as these tiny particles raced into the upper atmosphere produces high temperatures and the grains ablate, releasing sulfur dioxide - among other gases.

Sulfur dioxide is great for making particulates in a planetary atmosphere. This increases reflectivity, and can dramatically lower the solar radiation reaching the surface. Net result; planet cools. During the LHB roughly 20 million tonnes of sulfur dioxide a year may have been dumped into the atmosphere by this flux of tiny meteorites. That's equivalent to having a massive volcano erupt into the stratosphere every year for a hundred million years. The problem of keeping the Earth warm is greatly exacerbated. Court and Sephton also point out that Mars would have received a significant flux of these sulfur-bearing micrometeorites, seemingly creating an even bigger problem for an early temperate martian climate.

There are still a lot of questions. Was the sulfur content of these particles really as high as claimed? Do we really know the rate at which such tiny grains hit the Earth? Could the atmospheric chemistry of the young Earth have mitigated the production of sulfate aerosols?

Understanding what happened on the young Earth is a major issue. It seems for every solution to keeping the planetary surface warm there is an opposing mechanism that will plunge it into deep freeze. Yet the evidence remains for the presence of substantial liquid surface water during at least the tail end of the LHB and likely much earlier. Clearly somewhere we're missing a piece of the equation, or perhaps several pieces. Being able to study the deep geological history of Mars could help enormously, since it would allow us to separate out some of the planet-specific mechanisms at play. It may also be time to think a little more radically. Putting aside the mineralogical evidence for an early aqueous environment then perhaps a deep-frozen young Earth offers some advantage for the subsequently rapid emergence of life?

6 comments:

Eniac said...

When we look at Venus and Mars, we see atmospheres rich in carbon dioxide. Why would the Earth not have had a lot of it too? I do not recall ever seeing an explanation for that.

Absent one, to me this is the easiest and best explanation: Before life arose to fix the carbon, Earth was like a cooler Venus, with a dense CO2 atmosphere and a strong greenhouse effect.

I even think it is likely that there was a multigigayear homeostatic equilibrium, where increased temperature leads to increased growth leads to decreased carbon dioxide leads to cooler temperature....
That is pure speculation, but it seems so plausible.

Anyone know the evidence against it?

Caleb Scharf said...

The basic issue is that the rock//mineralogical record indicates CO2 levels only a factor of a few higher than today, whereas to solve the temperature problem it would have had to be much, much higher. An excellent summary is by Jim Kasting:
http://www.geosc.psu.edu/~jfk4/PersonalPage/Pdf/Kasting_Nature_N&V_10.pdf

Other greenhouse gases (e.g. methane) are options, but one runs into other side effects. So if this Court & Sephton idea is correct it further exacerbates an already very difficult situation.

Eniac said...

Fair enough, although that geological evidence had better be very direct and convincing before we junk such an obvious and satisfying explanation.

What about internal heating? As I understand, even today the deep ocean is heated from inside the Earth more than by the sun, and on land you don't need to drill very deep until geothermal energy dominates the temperature balance.

Because this internal heat is coming from radioactive decay, the early Earth must have been a lot hotter inside than today. Could this have made up the difference?

Eniac said...

Oh, also, I am still lacking an explanation why Earth should have had so much less CO2 than Venus and Mars. I do not mean geological evidence, but a causal explanation. I think one is owed, because the difference is pretty stark and surprising. Has one ever been put forward?

Caleb Scharf said...

Been a long day so these are brief comments!

Re CO2 on Earth. Obviously while Mars today has mostly CO2 for atmosphere it's still much lower pressure and therefore net amount is not so very much I think? Main off-the-cuff difference must be to do with geophysical differences. Carbon Cycle on Earth keeps CO2 long term close to the amount 'required' for 0-100C surface temps. Venusian geophysics very different. If you shut off subduction on Earth then I think a million or so years and CO2 would be significantly higher.

Today Earth's geophysical energy flux is a tiny fraction of the solar input (about 0.0001 times the power) - so while its true that at mid-ocean ridges you could probably maintain 'pockets' of liquid water, or even conceivably a true sub-surface ocean, overall I don't think you could maintain the kind of marine environments necessary to produce the mineralogical signs of liquid water (zircons etc).

I'd agree that the geothermal flux must have been a lot more in the past - both from radioisotopes but also from the trapped gravothermal energy of planet formation. So perhaps there is a chink in the armor there.

Eniac said...

That is very interesting about the low geothermal power. I would have thought it to be larger. Thanks for clarifying that.

About the CO2, yes, the carbon cycle keeps CO2 low. But, that carbon cycle is dominated by life, which is responsible for the fixation of carbon. Mostly through the formation of calcium carbonate shells in the ocean, as I understand. Without that biological fixation, there is nothing to subduct, and CO2 levels in the atmosphere would be much higher, I would think.