Saturday, 31 March 2012

The Secrets Of Ancient Rain

Rain is something we all deal with, sometimes on a daily basis. It can range from small showers to torrential downpours. Both types, however, derive their energy from a temperature difference of just a few degrees Celsius. When the Earth first formed, all the water vapor contained within the rocks, was gaseous in form due to the incredible temperatures of the young lava planet. The first ever downpour sourced its energy from a radioactive world shrouded in super heated steam.

This storm was on a global scale and lasted for millions of years. Whole continents may have been eroded into the sea by the unstoppable force of the falling water. Our planet never has, or never will, experience anything like it again. Yet the rain that followed, during the Archaean and Proterozoic eons, was also vicious. The Earth was still hot and the equivalent to a monsoon could have occurred on a regular basis. Now researchers from the University of Washington have begun to reconstruct the weather forecast of the young planet.

They began by looking at the fossilised impressions of raindrops created when they struck layers of volcanic ash. These 2.7 billion year old trace fossils came from South Africa, a country with some of the oldest geological formations on the planet. By pouring latex over the impressions and hardening them with chemicals, the researchers were able to make very accurate negatives. Then, using mathematical calculations, involving impact forces and speed, combined with laser measurements of the droplet 'craters' they reconstructed the size of the raindrops.

A meerkat sits on one the 2.7 billion year old slabs of
 rock bearing impressions created by ancient droplets
of rain
They found that some of the downpours were composed of droplets double the size of what we experience today. What was more interesting, however, was the atmosphere in which they fell.

Raindrops today fall at around 30 feet per second. To get an idea of their effect, the researchers dropped water from a  pipette into a tray 90 feet below which contained ash with the same chemical composition and particle size as the ancient South African samples.

The giants of 2.7 billion years ago falling in an atmosphere with the same density and air pressure as today would create far larger marks than the artificial craters made by the researchers. Instead the impressions made by the South African samples were actually smaller than expected. This could only mean one thing: the Proterozoic atmosphere was far denser than the one we recognize today. The concentrations of greenhouse gases would have been incredibly high, so high in fact that they actually slowed down the raindrops, leading to the smaller impact craters.

The reconstruction of the early atmosphere may have also laid the first stone in the pathway to resolving the faint young sun paradox. This is the contradiction between the two observations that billions of years ago, the sun's heat output was far lower, but liquid water still existed on the surface of the planet, despite the cooler temperatures. A high concentration of greenhouse gases would have trapped what little heat did reach the Earth, keeping the planet warm.

For this to work, the pressure would need to have been higher in order to achieve an effect known as 'pressure broadening.' This allows greenhouse gases to absorb far more solar radiation than under normal conditions. It is likely that as oxygen appeared in the atmosphere in significant quantities around 2.4 billion years ago, the greenhouse gases were forced out, leading to extreme heat loss and triggering the Snowball earth glaciation event just 800 million years ago.

Rain is one of the most powerful forces on the planet. It has the capacity to sculpt the face of the earth and keep ecosytems alive even in the most hostile of environments. Yet as this study shows it is vital to the inner workings of the planet.