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

Friday, June 4, 2010

Paradox Earth: II

A few weeks ago I wrote about one of the most slippery topics in understanding the evolution of environment on the Earth, with huge implications for other terrestrial-type planets. The so-called 'Faint Young Sun Problem' is that 3-4 billion years ago the Sun was about 30% fainter than it is today - a consequence of really quite well understood physics in stellar cores; stars get hotter and brighter as they age. The Earth teeters close to a distance from the Sun beyond which the planet would freeze up, yet multiple lines of evidence point towards a temperate environment back in our youth - despite a significantly less lush solar output. What kept things warm on the planet?

Stuffing more greenhouse gases, like carbon dioxide or methane, into the atmosphere could be one solution. Problems arise though in matching what you'd need to keep things toasty to the geological record of atmospheric composition billions of years ago. Now here comes another possible answer. A new paper by Wolf and Toon in this week's Science (together with an opinion piece by Chyba) suggests that a hydrocarbon rich atmosphere would form a fairly delicious photochemical haze. A haze would shield the lower atmosphere from destructive ultra-violet photons and allow for very potent greenhouse gases like ammonia to survive for far longer than they would otherwise.

The idea of soupy hazes in the young Earth is not new. The problem with hazes is that they end up reflecting away more sunlight altogether, which can completely negate any enhanced greenhouse effect. Wolf and Toon provide a clever solution to this. They point out that everyone has assumed haze particles - greasy little microscopic things - were spherical. More realistically they are probably 'fluffy' or fractal in nature - like malformed snowflakes. Remarkably, this transforms the optical properties of the haze - ultraviolet photons are still blocked, but lower energy visible and infrared photons are scattered and still make it through. So, nasty ultra-violet light is kept out, strong greenhouse gases like ammonia can form, and the warming rays of visible and infrared light can get to work and keep the young Earth at a nice temperature.

It's another interesting take on the problem, with implications for when we start coming across Earth-analogs around other stars that are young compared to us.


Anastasia said...

This is interesting - but is there any way to prove or show that the haze particles had a more snowflake-like geometry instead of spherical? Another question may be what may have caused the geometry of haze to take on this particular shape?

Caleb Scharf said...

Right, these authors simply quote another work that shows that hydrocarbon aerosols tend to make 'fluffy aggregates' - I'm assuming that has an experimental basis. From the physics of it I can see that this might be the case - a bunch of different carbon based molecules, probably little charge separation compared to (say) water - and so sphericity may not be the lowest energy configuration. I think the weak point in the Wolf & Toon paper may be that the photon scattering is all based on theoretical models - it also presumes a particular 'monomer' (i.e. aggregate building block) size, so I guess overall it's really more of a demonstration that if you had hazes with *these properties* then you could alter the chemistry/greenhouse accordingly - it doesn't actually provide a case for the existence and properties of the haze.