This post was contributed by Paola Bernoni and Anja Lanin, students on Birkbeck’s BSc Geology.
What can asteroids tell us about the formation of the solar system about 4.6 billion years ago and how are we able to extract such information from objects that are located in a region, the Main Asteroid Belt, somewhere between Mars and Jupiter, several hundred million miles from the Sun? This was the subject of the lecture delivered by Professor Hilary Downes of Birkbeck’s Department of Earth and Planetary Sciences for this year’s Science Week on Thursday 3 July, her debut talk for the event despite Professor Downes’ long association with Birkbeck.
Asteroids: what, where, when?
First of all, what are asteroids? Remnants of cosmic material unable to accrete and form a planet-sized object. In the Main Asteroid Belt this was due to the gravitational pull of the giant planet Jupiter: hence asteroids are a “failed planet”, not – as one might be led to believe – fragments of broken-up ones. We are mainly interested in asteroids whose orbits cross that of the Earth and Mars as they are most likely to yield useful information about our own planet.
What do they look like? “Potato shaped”, or long and thin, but invariably irregularly shaped, their surfaces pock-marked with impact craters … not volcanic craters as, unlike the volcanically active Earth, asteroids are dead bodies that have lost all of their internal heat.
What do we know about asteroids and how do we know it?
Near-Earth asteroids are occasionally knocked out of the Main Belt and can even end up colliding with the Earth: these are meteorites. In 2008, for the first time ever, an asteroid was detected prior to impact and predicted to land in North Sudan, where researchers flocked to recover 600 fragments, after it had exploded in the atmosphere. The more recent impact at Chelyabinsk in the Southern Urals, Russia, was even filmed.
Space missions have collected useful information: there has even been a landing on asteroid Itokawa in 2005, which managed to collect some dust material. The ongoing Dawn mission, departed in 2007, reached Vesta in the Main Belt in 2011, orbited around the asteroid for one year and then departed for Ceres, where it is expected to arrive in 2015. Why the interest in Vesta and Ceres? These are two of the largest surviving protoplanetary bodies that nearly became planets and therefore can help us gain a better understanding of the evolution of the solar system and of the processes that led to the formation of differentiated, layered bodies like the Earth (and Vesta) and less differentiated bodies (Ceres).
Measurements of radioactive decay of different isotopes performed on meteorite fragments have yielded consistent results on their age: they are as old as the solar system (4.6 billion years), a result matched by the results on the oldest terrestrial zircons. Yet there are some younger meteorites and they come from the Moon or Mars.
How do we classify asteroids and why?
The traditional classification of meteorites based on composition – iron, stony and stony iron – does not really tell us much, a discrimination based on provenance might be a better option: whether meteorites come from a layered body, such as the Earth, with a nickel-iron core, an olivine-rich mantle and a silicate feldspar-rich outer shell, the crust, or not … hence the interest in the layered Vesta and the less layered Ceres, which is made of a rocky core, a water-ice layer and a thin crust. But many of the recovered meteorites, especially from Antartica, do not show signs of provenance from a layered body: called “chondrites” as they containing small globules, chondrules, which are some of the earliest materials formed in our solar system, they are unfortunately not very useful in the quest for a better understanding of our layered Earth. Iron meteorites, compositionally similar to the Earth’s core, are thought to represent the core of small asteroids that blew apart and lost the encasing mantle. We have some 50 specimens, but it is a biased sample: they are more resistant passing through the atmosphere and easier to detect on the ground. Stony-iron meteorites are very rare instead: as they also contain an iron-nickel alloy, and olivine, one of the main components of the Earth’s mantle, they are thought to represent the core-mantle boundary of the parent asteroid, which was hot enough to commence differentiation.
Asteroids and Research at Birkbeck
Professor Downes then gave some highlights on the research underway at Birkbeck where stony meteorite samples from a very old, unknown asteroid are studied to establish similarities with the Earth’s mantle. Their olivine and other silicates are surrounded by carbon, including tiny diamonds, and nickel-iron rims, whilst on Earth these metals have segregated into the core and carbon is found in organic matter. The meteorite minerals show evidence of shock from impact and the carbon component also shows that graphite has been shocked into diamond. Compositional analyses have shown the presence of a known mineral, Suessite and an unknown mineral made of 91% iron and 9% silica, which is the most likely composition of the Earth’s core whilst the composition of meteorites originated from the outer shell of layered asteroids is similar to that of the basaltic rocks we find at the Earth’s surface.
Professor Downes finally underlined the uniqueness of the Earth amongst the rocky planets with the continued presence of water – lost on Venus and Mars – and especially of life, which is not known to have ever developed in any of the other terrestrial planets. The question of where Earth’s water came from is still open. A “meteor shower” of questions then followed, on the provenance of water and life on Earth, the age of meteorites found in Antartica and what drives differentiation: for some of these matters the audience was referred to courses offered by the Department of Earth and Planetary Sciences … for others to Birkbeck’s astrobiologists. Finally, the talk and the Q&A session came to an end but the opportunity was available to carry on with discussions and queries helped by a nice glass of wine and nibbles.. .