Fifty years ago, a meteorite crashed near the Murchison village in Victoria, Australia. What lay upon the ground was 100kg of dark scorched rock, emitting fumes akin to methylated spirits. It was identified as a very special type of meteorite, having once been part of a rocky object in space that formed one of the basic building blocks of our Solar System.
Meteorites are incredible rocks. For generations, their dramatic entrances into Earth’s atmosphere have been revered by civilisations as signs from deities of vast cosmic power. These space rocks act like time capsules from the very beginning of our Solar System, some 4.5 billion years ago, preserving information about the conditions at that time.
Murchison is a special type of carbonaceous chondrite meteorite known as a CM2. These rocks are relatively rare in meteorite collections, but they contain information about how the planets formed, and even possibly where our water came from. As such, Murchison has been the focus of intense scientific study for over half a century. But in 2019, meteorite scientists were once again blessed by the cosmic gods when a fireball crashed into the Aguas Zarcas region of the Alajuela province in Costa Rica , depositing yet another CM2. With a recovered mass in excess of 25kg, it is the largest recorded CM fall since Murchison.
Billions of years ago the Earth was pummelled by asteroids when the Solar System was still in its infancy and the planets had not yet settled down into the stable orbits they inhabit today.
Many fragments of Aguas Zarcas were recovered immediately. This is important for meteorite scientists because it removes the possibility of any contamination from rainfall. As a result, opening a sample in the laboratory is quite the experience. It has a pungent smell comparable to soot, or compost, and a dark friable texture that must be handled with great care (think extra-terrestrial shortbread biscuit). It is certainly a very ‘fresh’ sample of a 4.5-billion-year-old rock, containing all the ingredients it takes to form a planet.
An image from The Graham Ensor Collection of a small fragment of the Aguas Zarcas meteorite. On the right hand side a fusion crust is observed, which is the charred black surface as a result of entry into Earth’s atmosphere. On the left is a fresh surface, showing the plethora of materials within including the dark black matrix, the area where water-rich minerals can be found.
Like most CM2 meteorites, Murchison and Aguas Zarcas contain water-rich minerals called phyllosilicates, formed by the alteration of common rock-forming minerals such as olivine that were already present in the rock. The phyllosilicates tell scientists that water must have been present in significant amounts within the original rocky body from which the meteorite originated. Meteorite scientists have used these observations as the ‘smoking gun’ to suggest that the parent bodies of CM2 meteorites — the water-rich asteroids in space — could represent a potential source of water on Earth. But you might wonder how the water would get from a rock in space to our planet.
Billions of years ago the Earth was pummelled by asteroids when the Solar System was still in its infancy and the planets had not yet settled down into the stable orbits they inhabit today. Scientists think these impacts to Earth could have seeded our planet with water, and even carbonaceous material; the basic building blocks for life! But knowing how much water was held in these asteroids, and to how much they may have contributed to the water budget on Earth is a complex question. No two meteorites are the same and scientists have very few special samples like Murchison and Aguas Zarcas to base their scientific hypotheses upon.
As space exploration has continued to progress over the last 50 or so years, scientists have decided that instead of waiting for the Solar System to send rocks to Earth, they could go out and get them themselves. We are now awaiting the return of samples from C-type asteroids Bennu and Ryugu with NASA’s OSIRIS-REx mission and JAXA’s Hayabusa-2 mission , respectively. Telescope observations have suggested these asteroids may be similar in composition to Murchison and Aguas Zarcas. Images returned from the spacecraft also look startlingly similar on a variety of scales .
Here at The Open University, we are doing our part by studying Aguas Zarcas and other CMs to prepare for the samples that will be returned to Earth from what may be their ‘parent’ asteroids. We are using the element oxygen to study the role of water in these rocks, allowing us to trace the evolution of water through time and understand the temperatures and volumes that may have been present in the early Solar System. This will help scientists understand, and prepare for, the sample material brought back from Bennu and Ryugu in the next few years.
And who knows, perhaps this “sky rock” from Costa Rica may help us find out more about the origin and evolution of life in the early Solar System too!
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