Genesis by Comets?

"At very low angles, we think that some water ice from the comet would remain intact as a liquid puddle concentrated with organic molecules," ideal for the development of life, Blank said. "This impact scenario provides the three ingredients believed necessary for life: liquid water, organic material and energy."

Though comet hunter Eugene Shoemaker estimated that in Earth's early history only a few percent of comets or asteroids arrived at low enough angles, the bombardment would have been heavy enough to deliver a significant amount of intact organic material and water, according to Blank's estimates.

One well-known model for the beginnings of life on Earth posits that terrestrial life sprang from complex molecules such as amino acids and sugars produced by electrical discharges in a primeval atmosphere replete with gases such as methane, hydrogen, ammonia and water. The famous Miller-Urey experiment in 1953, conducted by Stanley Miller and Harold Urey of the University of Chicago, demonstrated that a lightening-like discharge in a test tube filled with these molecules could produce amino acids.

Other scientists believe that the building blocks of life on Earth arrived from space. Astronomers have detected many kinds of organic molecules in space, floating in clouds of gas or bound up in dust particles. They range from the simplest - water, ammonia, methane, hydrogen cyanide and alcohols, including ethyl alcohol - to more complex molecules.

Credit Jennifer Blank, UC Berkeley. In this schematic diagram of Blank et al.'s apparatus the red arrow indicates the projectile, which is fired from the breach toward the stationary target (inset). The 3 smaller red triangles indicate transducer pins that measure the velocity of the projectile as it passes. "Upon impact, the sample container and a similarly-sized metal plug fly backwards into the recovery area, where they're trapped as gently as possible in layers of felt," adds Blank.

Interestingly, of the more than 70 amino acids found in meteorites, only eight of them overlap with the group of 20 which occur commonly as structural components of proteins found in humans and all other life on Earth.

To test whether water and organic compounds could survive the high pressures and high temperatures of a collision, Blank and her colleagues worked for three years to design a steel capsule that would not rupture when hit with a mile-per-second (1.6 kilometre-per-second) bullet fired from an 80-mm bore cannon at the University of Chicago and later at Los Alamos National Laboratory. The target she and her team developed - a two-centimetre diameter stainless steel disk about a half-centimetre thick - was able to withstand about 200,000 times atmospheric pressure without bursting.