Surviving Glaciations: Ice or a Slice of Luck?

Ice is a fantastically strange thing. As this BBC documentary on the stuff  explains, it is one of the only materials known which floats on it's liquid form. This unusual property allows water to exist beneath frozen regions such as polar oceans and garden ponds, insulated from the cold above. But here's a thought: could all life on Earth owe its existence to these beautiful hexagonal crystals?

A Geological Perspective

Earth's history is dotted with planet-wide cold periods, most notably at 650 million years ago when huge glaciers on land and ocean extended as far south as the equator. Despite the severity of these 'Snowball Earth' events, life survived. The microscopic organisms that existed could have lived either in cold regions beneath the ice, in warm 'oases' or in equatorial regions where thinner ice and widespread cracks might have let light through allowing photosynthesis. But imagine turning this world upside down and living on a planet where ice sinks into the depths rather than floating to the water's surface. My question is this: could life have coped?

Although it might appear that a thin layer of surface ice helps preserve life, many other effects come into play. For example, the Ice-albedo feedback means ice that forms on the surface of a cooling ocean causes more light and heat to be reflected from the planets surface, leading to ever colder temperatures. The development of sea ice also separates the ocean from the atmosphere. This can shut down climate cycles such as the temperature-regulating carbon silicate cycle on the long term, and can also lead to the dangerous decline in the oxygen and nitrogen levels of the water beneath.

If newly formed ice was to sink, these problems could be avoided. However this bottom-to-top freezing means no warm waters are insulated beneath. What's more, a glaciation event could lead to the solidification of an entire ocean - not exactly a good thing for oceanic life. The build up of thick, ocean-floor ice sheets might also do untold damage to the deposition of carbonates and the subduction of tectonic plates, both of which are involved in long-term climate stability. Many lines of evidence also point to an origin of life at sea-floor vents, a scenario that may have been impossible without floating ice to insulate the depths. All in all, we probably are lucky that ice floats rather than sinks - but has such an effect been pivotal in the development of us?

An Astrophysical Perspective

Like all material properties, the low density of ice can be explained in terms of the chemistry of it's atoms. Water molecules are strongly polar, with two mildly positive hydrogen atoms and one mildly negative oxygen atom. These charges cause two 'hydrogen bonds' to form between adjacent water molecules, and the angle of the hydrogen atoms from each other (104.5 degrees) leads to a regular hexagonal arrangement between water molecules. This bonding results in the classic 6-sided snowflakes, as well as a density slightly lower than in unordered liquid water.

As I have written about before, recent ideas in cosmology suggest fundamental properties of the universe may vary with location, and we exist in a region best suited to the development of intelligent life. For example, varying the fine structure constant (alpha) by only 4% could mean the life-giving element Carbon is not produced in stars. But could the laws of physics our region of the universe also be due to the properties of ice, or is the peculiar nature of H2O merely a by-product of physics?

At 14, the HOH angle drops below

 90° and hexagonal symmetry is lost.

Luckily Rollin A King (what a name!) et al have done the hard work by performing theoretical quantum chemistry for a variety of electron charges and proton masses. They found that a fine structure constant (electron charge) more than 14 times the natural value caused the hydrogen atoms on a water molecule to move closer together. This destroys the unique hexagonal structure of water, and raises the density of ice above that of water. Although this may seem to show that physical conditions are fine tuned to allow ice to float, it must be considered that this change is much less sensitive than much more important properties. For example, this is 280 times higher than the change needed to stop carbon production in stars, and half the change in alpha required to end nuclear fusion entirely.

This leads to the conclusion that, while the density of ice may or may not be important for the development of life on Earth, the peculiar behaviour of freezing water has no great cosmological significance.
Background:
This post was the result of a single idea, so although it produced a negative result (ie, the universe doesn't care about if ice sinks or floats) I wasn't going to spend so much time (3 hours) thinking about and researching this and not write it up!