Tag Archives: Earth and Planetary Sciences

Asteroids

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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.

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Science Week: Earthquakes in Italy

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This post was contributed by Bryony Stewart-Seume, of Birkbeck’s Department of Biological Sciences.

Professor Gerald Roberts

Professor Gerald Roberts. Photo: Harish Patel

Science Week continued with a popular lecture about the widespread damage and complicated scientific questions arising from earthquakes.

Professor Gerald Roberts, of Birkbeck’s Department of Earth and Planetary Sciences, delivered a talk, entitled Earthquakes in Italy: the role of the historical record of earthquakes and geology, on 18 April. He began with a little history. The 1915 Avezzano earthquake killed a reported 30,000 people, and destroyed all but one building.

On  6 April 2009 an earthquake with its epicentre close to the town of L’Aquila in central Italy killed “only” 309 people. However, 30-50 per cent of the buildings in the town were badly damaged or razed to the ground, including a halls of residence in which eight students lost their lives. To better get an idea of the extent of the damage to the town (the centre of which has still not been repopulated), Prof. Roberts asked us to imagine half of the city of Bath being damaged beyond repair.

In an unfortunate twist the Town Hall of L’Aquila, which contained plans for dealing with such situations, was also badly damaged. Several 13-15th century cathedrals and churches were damaged and part of the modern hospital fell into the underground car park below it. The older masonry buildings proved especially vulnerable.

The financial cost of the 2009 earthquake has been estimated at €16 billion .

Scientists in the dock
There is, however, more to the story of the L’Aquila earthquake of 2009 than damage and a number of deaths. Prior to the earthquake citizens concerned by a number of tremors that had been rocking the city called upon the National Commission for the Forecast and Prevention of Major Risks to give an idea of the potential danger. What followed was unfortunate, in that initially the answer was along the lines of “it is not possible to predict earthquakes but this area has a long history of earthquakes and you should be vigilant”; a correct statement, but subsequently one of the members said in a TV broadcast that there was “no danger” which was not correct. 

When the earthquake did hit, police had reportedly told people that as there was “no danger” they should return to their houses. Seven members of the National Commission were subsequently tried, and convicted, for involuntary manslaughter. Their conviction was met with disgust by parts of the scientific community, although it was stated by the judge that it was not science that was judged, or the inability to predict an earthquake, but the failure to communicate consistently. This highly controversial conviction has led to concern amongst scientists about the future for those studying and communicating earthquake science.

So what can be done about this? Earthquakes will not stop happening. The African plate will not stop pushing into the plate containing Italy. And Italy will not stop being pulled apart. So how can we better communicate what we do know, and what we can do?

Asking the right questions
The question “when will there be an earthquake here?” is not one that can be answered. When an earthquake happens along any given fault is unpredictable. That an earthquake will happen along any given fault is inevitable. Earthquakes are caused by the movement of the plates of the Earth’s crust. Professor Roberts demonstrated through use of a model with springs and metal blocks moving on a sandpaper surface just how chaotic the movement is. The elastic crust pulls apart (or pushes together), tension builds up and the ‘elastic’ releases. This is what causes the earthquake itself. 

Professor Roberts took us through a more useful series of questions that populations should be educated to ask rather than the standard “when” question, the first being; does my area have a history of earthquakes. If you happen to live in central Italy the answer is obviously and demonstrably “yes”.

The next question that should be asked is; do I live in an earthquake zone? If you live near to an active fault, the short answer is “yes, you do.” But how active is it? How can that be measured? The question to ask here is “how much has the fault shifted, and how quickly?” This is measurable, believe it or not, with the help of supernovae – burnt out and blown up stars that send out high energy particles that react when they hit calcium, for example in limestone around L’Aquila to produce new 36Cl atoms. 

Professor Roberts stressed that although earthquakes cannot be predicted, in areas containing active faults they are inevitable and this needs to be communicated to populations.

The more time that elapses between each earthquake the more tension builds up and therefore the bigger the quake will be when it does happen. Which it will. They are inevitable, but not predictable.

Given that it is the building that kills you, not the earthquake itself, the best way to prepare for a quake is to make sure your buildings will take the strain. Buildings made out of cubes are weak and no match for the ferocity of  nature. But if you reinforce the buildings with struts to make triangles in corners, you will improve the integrity straight away.

There is, therefore, a need to educate populations about the right questions to ask and about the significance of small tremors. These questions should be asked many years in advance to ascertain whether earthquakes are inevitable in the area they inhabit and how they can undertake actions to prepare buildings to withstand the seismic shaking.

Just because there has not been an earthquake for a long time does not mean that you are safe. In fact, quite the opposite. All this needs to be conveyed, but without being alarmist.

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