Planets colliding artistic mock-up

Space rocks! Meteorite analysis and the building blocks of life

Image credit: Bristol University

Meteorite analysis is giving us new insights into the formation of our solar system and the building blocks of life.

Early in the morning of 15 February 2013, a blinding white fireball streaked across the sky of Russia’s Ural Mountains and exploded in the air near the town of Chelyabinsk, showering the ground with fragments of rock.

Captured by dashboard cameras and mobile phones, the Chelyabinsk meteorite was later identified as a huge fragment of rock (around 20 metres across before it broke up) that came from the asteroid belt between Mars and Jupiter.

Chelyabinsk is one of a number of meteorites recently analysed using high-precision methods that is changing our understanding of how planets in the solar system evolved and how the building blocks of life were created.

Scientists think our solar system formed 4.5 billion years ago out of a swirling disc of gas and dust whose huge gravity pulled more than 99 per cent of the material into the centre to form the Sun. What is less clear is how the leftover matter clumped together (‘accreted’ is the official term) to form planets, asteroids, little moons and comets.

Clues from meteorites now suggest that ‘accretion’ was messier and more violent than previously thought, and these dramatic beginnings were central to the development of complex organic molecules (and life).

The Chelyabinsk rock is a primitive chondrite formed from dust and small grains present in the early solar system. Unusually, it was shot through with cracks filled with what had been molten metal.  

Early investigations by Kevin Righter and colleagues at Nasa’s Johnson Space Center concluded that the parent body asteroid survived about a dozen different impact events over its 4.5-billon-year life, ranging from 300 million years to just 27 million years ago, which is relatively recent (dinosaurs were lumbering around on Earth from 230 million years ago).

Last year, new research by Philippe Schmitt-Kopplin and his team at the Analytical BioGeoChemistry (BGC) research unit at the Helmholtz Zentrum in Munich, showed that Chelyabinsk had an abundance of novel organomagnesium compounds called dihydroxymagnesium carboxylates.

Such compounds had never been described before the group published their results in the Proceedings of the National Academy of Sciences in March 2017. Schmitt-Kopplin and his colleagues found the unusual compounds in a total of 61 different meteorites.

BGC is interested in the molecular diversity of organic systems, ranging from composition of biological metabolites (such as those found in blood or urine from humans and organisms) to the make-up of organic molecules produced by geochemistry in reactions caused by sunlight or heating and pressure (as found underground or in cooked food).

“Geochemistry integrates a mixture of bio- and abiotic chemical transformations. A pond full of algae, for example, would contain chemical biosignatures of biological origin. Complex organic signatures found in meteorites, on the other hand, show the end-point of a whole history of purely chemical processes that integrate the whole period of its formation to the point it fell down to earth,” explains Schmitt-Kopplin.

The Helmholtz BCG team discovered the novel magneso-organic compounds by using tools like high-field nuclear magnetic resonance (NMR)-spectroscopy and ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) that can follow mass differences in molecules to a resolution of just half an electron mass.

Organic molecules have commonly been > < found in meteorites rich in carbon: a decade ago, Schmitt-Kopplin’s team found millions of organic molecules in the Murchison meteorite, a carbonaceous chondrite that fell in Australia in 1969. Yet to discover complex organics in meteorites that have a history of heating, shock, melting, and collisions, gives us another level of understanding into the origins of life.

“What our measurement techniques show is that the more a meteorite has been thermally altered or shocked [for example in the melt veins of the Chelyabinsk meteorite], the greater the abundance of these organomagnesium compounds,” says Schmitt-Kopplin.

While organomagnesium compounds have nothing directly to do with life, what they represent, says Schmitt-Kopplin, is the chemical coevolution of minerals and organics and their interactions between one another. “They are an important marker on the continuum of the organic-mineral evolution.”

Know your meteorite

Identification

Most meteorites are categorised as ‘stony’, and the majority of these are primitive rocks called chondrites, thought to be bits of undifferentiated (no core or crust) shattered asteroids. Chondrites contain grains called chondrules that scientists think condensed out of the solar nebula 4.5 billion years ago. They have never experienced melting.

Achondrites are stony meteorites that lack chondrules and look more like earth rocks. They come from planetary bodies that are differentiated (with a core and crust like Mars or the Moon and asteroids like Vesta), and were broken off by asteroid collisions.

Iron meteorites consist mainly of an iron-nickel alloy similar to the Earth’s core. They probably started life in the molten cores of large asteroids whose outer rocky crust has been ripped off during collisions.

Stony-iron meteorites have a mixture of iron-nickel and silicate minerals that are thought to come from differentiated asteroids.

Magnesium is one of the major elements that make up the terrestrial planets Earth, Mars, Mercury and Venus (it accounts for around 15 per cent of the Earth’s mass). Measurements of magnesium isotope ratios in meteorites can be used as a ‘tracer’ for changes in the bulk composition of these planets, shedding light on the earliest stages of planetary creation.

A study by scientists at the University of Bristol’s School of Earth Science, led by Remco Hin, has recently compared magnesium isotope ratios in carbonaceous chondrite meteorites (representing the most ancient solar system building material) with ratios found in meteorites from Mars and Vesta and in volcanic rocks from Earth (representing the crusts of ‘finished’ planets) to a new level of precision.

Their results, published in Nature in September 2017, have provided proof of the huge violence of planetary accretion, and evidence that the way the planets collided, melted, and vaporised would have shaped their different compositions. Earth’s unique composition was, in effect, a “cowboy building job” they said.

“We’ve confirmed that planetary crusts are significantly heavier by about 25ppm [parts per million] than the starting materials,” explains Tim Elliott, professor of the School of Earth Sciences at the University of Bristol. “To shift 25ppm of magnesium away from a planet, you need to do something pretty significant. Creating a vapour would do it because it always prefers the lighter isotope, leaving behind more heavy isotopes.

“What this tells us is that as planets smashed into each other, they kicked bits off each colliding body and the energy of the impact is enough to completely melt the planet, producing a magma ocean with a vaporised rock atmosphere above it,” he explains. “For small bodies with a small gravitational field, some of the vaporised silicate molecules would have had enough thermal energy to escape.”

Hints in data from earlier studies had pointed to this isotopic imbalance. Until now, no-one has managed to tease out the tiny differences with sufficient accuracy to distinguish real ‘signals’ from noise caused by the idiosyncrasies of each mass spectrometer and impurities in the rock samples.

By applying their own rather elegant version of a method called Double Spiking that involves adding precise mixtures of magnesium isotopes to the rock samples (before measuring them in the mass spectrometer), Hin and colleagues were able to remove the ‘noise’.

Recent measurements of traces of ancient magnetic fields in iron meteorites tell a similar story. Magnetic fields are generated in planetary bodies with hot iron cores. As molten iron churns around due to pressure, density differences and convection, the iron generates electric currents and in turn creates magnetic fields.

“By studying the intensity and stability of these fields, we can learn about the amount of heat moving through the planet and the type of motion of the core,” says James Bryson at the Department of Earth Sciences at University of Cambridge. For example, faster cooling generates strong convection currents to produce a more powerful magnetic field.

Planetary bodies are assumed to be either molten inside like Earth or cold like many asteroids. Yet there are iron meteorites with no signs of melting that appear to be magnetised by a planetary field.

Bryson’s theory is that these iron meteorites broke off the surface of a proto-planet with a molten core that had its rocky mantle smashed off by collisions with asteroids. The remaining exposed nickel-iron core is Psyche, a cold metal planet in the asteroid belt between Mars and Jupiter.

To test this, he has pioneered a technique, detailed in a paper in Earth and Planetary Science Letters (June 2017), that can reveal traces of the original parent-asteroid magnetic field by examining particles of an iron-nickel alloy called tetrataenite that crystallises into a stable structure if cooled very, very slowly (at a rate of 100°C per million years). Tetrataenite can hold a magnetic memory going back billions of years.

Bryson used a magnetic microscope at the synchrotron at Lawrence Berkeley lab in California to image magnetisation of individual magnetic carriers. He has also made more conventional measurements of the magnetism using a magnetometer.

His analysis of several iron meteorites that appear to be from Psyche indicate their parent asteroid had a strong and unstable magnetic field, supporting the hypothesis that it is an exposed planetary core that cooled quickly due to absence of a rocky mantle. Bryson is hoping he will be proved right by data gathered from a Nasa mission, planned for the late 2020s, to send a satellite to orbit Psyche.

Unbelievable rocks

History

Before the early 1800s, reports of burning rocks plummeting from the sky were dismissed as fanciful because witnesses were often agricultural workers who were considered uneducated and unreliable.

Then in 1794, Ernst Chladni, a German polymath and lawyer, published a book called ‘On the Origin of Ironmasses’. He detailed witness statements of meteorite sightings from contemporary and historical records that led him to conclude these falling iron and stone fragments were probably extra-terrestrial.
Chladni was ridiculed for his claims until two events changed public opinion.

One was a large fall of stones in Siena, Italy, in 1794, seen by the ‘right kind of people’, including British travellers who took back samples. Then in 1795 a meteorite fell near Wold Newton in Yorkshire. Pieces from both falls were analysed by British chemist Edward Howard and French mineralogist Jacques de Bournon, who, in a paper published in 1802, concluded they came from outer space.

Finally, studies of a shower of rocks that fell in 1803 near l’Aigle in France led to wider acceptance of Chladni’s theories.

Compared to a cold metal asteroid, Mars looks friendly and inviting, with features we recognise from Earth such as volcanoes and (dry) river channels.

New studies on meteorites known as the SNCs (shergottite, nakhlite, chassignite) – part of the Natural History Museum’s huge meteorite collection in London – are providing detailed insights into the planet’s volcanic and watery past.

SNCs were once volcanic rocks on Mars. They range from 180 million years old for the shergottites to 1.3 billion years for the nakhlites.

It was thought that nakhlites were knocked off one region of a Martian volcano by a single asteroid impact, but they are subtly different in their chemistries and mineral content.

“Despite people thinking they were the same age, you can put them in four or five different groups according to their chemistry. This led to various complicated hypotheses about what it means for the evolution of Mars,” explains Caroline Smith, head of Earth Sciences collections and principal curator of meteorites at the Natural History Museum.

“But when we were chatting over a few pints with Ben Cohen [at the University of Glasgow School of Geographical and Earth Sciences], we concluded that the problem goes away if their ages vary.”

Cohen and his team subsequently showed, using a new approach to a common rock-dating technique, that six nakhlites in the museum’s collection were formed during a protracted series of at least four different volcanic eruptions spanning 94 million years. The study was published in Nature Communications in October 2017.

Atoms of the radioactive isotope of potassium (potassium-40) in a rock will decay at a known rate to form stable atoms of argon-40. By determining the amount of argon-40 in a sample, you can calculate the sample’s age. This dating method has been used to measure meteorites as old as 4,560,000,000 years, and volcanic rocks younger than 2,000 years.

Yet it turns out that different parts of rock release argon gas at different temperatures. To improve the accuracy, Cohen and colleagues laser-heated samples of each of the six meteorites incrementally over a 40-step range to get an age spectrum, and then repeated the process with up to five samples per meteorite to ensure reproducibility. “This is in more detail than has ever been done before,” he says.

In addition, they corrected for other sources of argon in the samples that come from the Martian atmosphere and from bombardment by cosmogenic rays when the meteorite has been flying through space for millions of years.

“From our analyses, we can see the Martian volcano was less active than terrestrial volcanoes. In the area sampled by the nakhlite meteorites, on average only around half a metre of lava was put on the surface in over a million years. In contrast, a big Hawaiian volcano would produce around 1,000 metres in a million years,” says Cohen.

Earth volcanoes are active for just a few million years, after which the tectonic plates move and a new volcano pops up elsewhere. Mars has no plate tectonics so its volcanoes stay put.

Nakhlites have little veins filled with minerals formed by the action of water, so these new dates suggest there was water on Mars much earlier than previously thought, increasing chances that the environment was suitable for generating life.

Research led by Oliver Tschauner and Christopher Adcock, from the Department of Geoscience at University of Nevada, Las Vegas (UNLV), points to the possibility that this water was also more plentiful than previous data has suggested.

They have carried out a series of shock-compression experiments (outlined in Nature Communications March 2017) to simulate conditions of impacts that ejected meteorites from Mars.

They tested a lab-made sample of whitlockite (a calcium phosphate-based mineral) that loses water and turns into merrillite under shock-compression. Merrillite is commonly found in Martian meteorites, but does not occur naturally on Earth.

“If even a part of merrillite had been whitlockite before, it changes the water budget of Mars dramatically,” says Tschauner.

Now the big test for many of these findings is whether they can be matched with data and rock samples from Nasa space missions, in particular the next-generation Mars 2020 rover, which arrives on the planet in 2021.

The Barwell meteorite

Inside story

30 years ago, Bob Hutchison, a curator at the Natural History Museum in London, changed our view of how planets evolved by cutting a serendipitous slice from the Barwell meteorite, a 44kg chondrite from the asteroid belt that fell in Leicestershire on Christmas Eve in 1965.

“Inside, he found a strange stony pebble that had came from an object with igneous textures. He thought it had come from a differentiated parent body, one that had already melted and formed its metal, silicate and crust like a planet but one that was inside a more primitive asteroid,” explains Dr Natasha Almeida, meteorite curator at the Natural History Museum.

Prior to Hutchison’s discovery, planets were thought to develop in ordered steps: Chondrules formed, metal grains formed, asteroids formed, they grew, coalesced and eventually formed planets. Yet Hutchison showed that planets were melting and forming large differentiated bodies prior to the formation of primitive asteroids.

Almeida is leading the museum’s work on scanning extraterrestrial material using computed tomography (CT), a non-​destructive first step for looking inside meteorites. “The smallest samples we provide for research might be only a few milligrams, but to use such samples to answer your scientific questions, you need to be sure the sample is representative of a particular meteorite or feature,” she says. 

CT scanning involves shooting a cone beam of X-rays (at energies around 50 times that used to scan human tissue) at a meteorite sample placed on a rotating holder in front of a detector panel. As the sample turns, the detector collects from 3,000 to 6,000 images of slices to build up a series of greyscale pictures based on attenuation of the X-rays. Putting these image slices together creates a 3D representation of the meteorite sample’s interior.
 
In a recent CT scan of the Barwell meteorite, Almeida found at least 20 more pebbles, allowing for targeted subsampling of the meteorite for ongoing elemental and isotopic analyses. This also illustrates how CT scanning can be used in the future, to examine samples brought back by space missions without breaking open the material, or exposing it to terrestrial atmosphere.

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