Researchers from McMaster University in Canada have recently published evidence that supports the hypothesis that meteorites could have brought extraterrestrial amino acids to Earth. Their research looked at the frequency with which amino acids occur in a certain class of meteorites and under what conditions these amino acids are likely to form. If such meteorites did indeed bring their amino acids to Earth, then they may have helped create the environmental conditions necessary to begin life as it is known today.
Meteorites come in many different varieties that are characterized primarily by the chemical composition. One type of meteorite is the carbonaceous chondrites (also referred to as “C chondrites”) which have a chemical composition very similar to that of the sun. C chondrites may contain a rich chemical diversity that includes water, minerals, and even organic compounds such as amino acids.
In the McMaster study researchers examined the frequencies with which different amino acids could be found in different classes of meteorites. The greatest variety of amino acids brought to Earth was found in the CM2 and CR2 subclasses of C chondrite meteorites. CM2 and CR2 meteorites experience a wide range of temperatures that, when combined with water, may have created the conditions necessary to spontaneously form amino acids within the meteorite.
The study also found a correlation that indicated that more complex amino acids were relatively rare to find within meteorites. While not altogether unexpected, this finding led researchers to conclude that a certain type of meteorite—called planetesimals—could produce amino acids.
Planetesimals are objects formed out the collision of dust, rocks, and other materials. Plantesimals can measure only a few meters hundreds of kilometers in size. Researchers have proposed these bodies contain different heated layers that are described as following an “onion shell” model of heat distribution. While the center of the onion-shelled planetesimal is too hot to store or form amino acids, the cooler outer regions are more forgiving. On the cooler surface of the meteorites produce relatively simple amino acids that do not require much modification. The farther into the hot center of the meteorite, the more complex the amino acids are created. An intermediate zone within the depths of the meteorite is proposed as the area most conducive to both the creation and maintenance of complex amino acids.
The finding that amino acids could spontaneously form within planetesimals is very exciting when one considers the question of how life on Earth began. Life requires some key chemical building blocks to begin. At the top of the list of necessary components are amino acids because of the multitude of different ways in which they can be combined to form different proteins. In order to answer the question “how did life on Earth begin?” researchers must first understands where the amino acids found on pre-biotic Earth came from.
The hypothesis that amino acids and other organic molecules may have been brought to Earth on meteorites was proposed decades ago. However the extraterrestrial origins of organic molecules only seemed to displace the question “where did organic molecules on Earth come from” to “where did extraterrestrial organic molecules come from?” However, in the light of these new findings that meteorites can forge their own amino acids, perhaps this question has been answered.
By Sarah Takushi