Did life’s building blocks crash land?

In 1969, on September 28, the skies near Murchison, Victoria (not to be confused with Murchison, Western Australia) were illuminated by a dramatic sight. A spectacular fireball blazed its way through Earth’s atmosphere, its outer layers heated to extreme temperatures by its speed. Residents of the town reported seeing the fireball split into three pieces before it faded from view, leaving a trail of smoke in its wake. Seconds later, a tremor was heard as meteorite fragments crashed into the ground, signalling the arrival of what would become one of the most well studied meteorites ever.

A meteorite streaks across the Australian night sky, in front of the Milky Way. But just what might it be carrying with it? Image credit: Alex Cherney/terrastro.com

If you like to watch the sky at night, chances are good that you’ve seen a meteor or two streaking across the night sky, and while many of them burn up in the atmosphere, it’s quite possible that one which you’ve seen might have eventually reached the ground. Meteorites strike Earth a lot more frequently than most people realise. By most estimates, a few hundred tons of meteorite material make it to our planet’s surface every day. Large meteorites like the Murchison meteorite, however, are a lot less common.

A fragment of the famous Murchison meteorite. Image credit: Art Bromage, Wikimedia Commons

While the Murchison meteorite shattered into fragments before it landed (known as an “airburst”), over 100 kg of meteorite have been collected from around Murchison, and scientists have been analysing those fragments ever since. This particular meteorite is a specific type known as a carbonaceous chondrite. These meteorites are fascinating to scientists, because carbonaceous chondrites are chemically very primitive – they’re thought to be very close in composition to the solar nebula from which the Sun and planets condensed 4.5 billion years ago. In other words, the meteorite which crash landed in Murchison 43 years ago was probably older than our entire planet!

Several things about the Murchison meteorite are very interesting. For one, it shows evidence that it was altered by water. This would have happened a long time ago, wherever this meteorite originally formed, and certainly a long time before it landed on Earth. Secondly, it’s peppered with Calcium-Aluminium-Inclusions (CAIs). These humble crystals are older than the Sun itself. When they formed, the Sun itself was little more than a huge cloud of warm hydrogen gas. Most interestingly to some scientists, however, is the fact that the Murchison meteorite is full of amino acids.

Uracil, one of the four "nucleobases" used by DNA to encode genetic information, was discovered inside the Murchison meteorite.

Amino acids are one of the basic building blocks of all living things. The proteins which make up almost everything in your body are made from these small molecules. To date, over 100 amino acids have been found inside the meteorite, including many of those used by life on Earth. Several things about the chemical and isotopic compositions of these molecules suggest that they didn’t come from Earth, but were in this meteorite when it landed. As an example, amino acids have two forms, referred to as left-handed and right-handed. Earth life only uses the left-handed forms, while the acids discovered in this meteorite are a mixture of the two (known to chemists as a racemic mixture). Other Earthly molecules which frequently show up as contaminants were absent from the samples analysed, suggesting that these molecules, the bare essentials of life, are extraterrestrial in origin.

These amino acids aren’t the only familiar molecules in the Murchison meteorite either. Amongst over 14000 different molecules found inside the meteorite, the chemists who were analysing the meteorite discovered ring-shaped molecules called purines and pyrimidines. These ring molecules are from the same family as the four nucleobases which make up DNA.One of the molecules found was one called uracil, which is actually used by DNA. This same molecule is in every strand of DNA in your body.

While some still argue over the validity of these studies, if they’re correct then the overall conclusion is a breathtaking one. This space rock is older than the Sun, and it already contained all of the basic ingredients for life to form back when Earth was nothing more than a patch of interstellar dust. We might never know exactly how life started on Earth. Though maybe in the distant past, life’s raw materials crash landed here on Earth in meteorites, just like one meteorite did in Murchison that night 43 years ago.

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