An Atom's-Eye View of the Planet

Chelyabinsk asteroid – crowdsourced science?

Simon Redfern November 7, 2013

Croudsourced data from dash-cams, videos and photos reveal the secrets of the Chelyabinsk asteroid. Credit: Alexeya

The asteroid impact that burst over Chelyabinsk, Russia, on the morning of February 15 has provided a huge collection of new data that scientists have been analysing since. This week, three papers, two in Nature and one in Science, describe new aspects of the meteorite’s airburst, building the most-detailed forensic picture of the events of that morning.

First reports of the Chelyabinsk airburst came from a plethora of dash-cams that caught the event. For the first time, a meteorite impact was recorded widely on camera, a consequence of technological advance and (presumably) increasingly litigious or bad Russian drivers. Alongside the dash-cam recordings, the fireball and the transient shadow that it cast was recorded across the region by fixed CCTV cameras. And looking back at Earth from space, the trajectory of the fireball was observed in satellite imagery.

The brightness of the fireball has provided an estimate of the energy of the airburst, equivalent to an explosion of more than 500,000 tonnes of TNT, a couple of hundred times greater than the Hiroshima atomic bomb. Similar estimates of the size of the explosion were obtained earlier this year from the array of infrasound detectors operated by the Comprehensive Nuclear Test Ban Treaty Organization, which maintains an array of nuclear bomb monitoring equipment.

The new papers exploit an even wider array of data. Much of the information is, effectively, a superb example of crowdsourced science: damage reports, surveys of damage, injury reports, camera recordings and other data have provided an unprecedented set of measurements of the event, as reported in Science by Olga Popova and colleagues.

Alongside the data from Earth is information from astronomy, planetary science, geophysics, meteoritics and cosmology. The meteorite that fell to Earth has now been classified as an LL chondrite. It formed early in the history of the Solar System, as asteroids and eventually planets condensed from the nebula.

Fragments of the meteorite recovered from near Chelyabinsk, including an enormous rock dredged from the bottom of Lake Chebarkul, have revealed its early history. This, despite that fact that less than one thousandth of the asteroid has been retrieved, and more than three quarters is estimated to have evaporated.

Fragments of the meteorite recovered from the impact site. Credit: Popova et al.

Measurements of the radioactive decay products from traces of uranium in the meteorite minerals show that it must have itself suffered a harsh collision during the maelstrom in which asteroids condensed, which occurred at around 115 million years after the birth of the Solar System. Its existence as a discrete asteroid ended almost four and a half billion years later when it struck Russia.

The eyewitness reports of the airburst, as well as the damage it caused, give an idea of the sorts of effects caused by such “near miss” events. Entering the atmosphere almost 100km above the surface, at speeds of around 20km/second, the 20-metre wide asteroid set up a shockwave at 90 km altitude. By 83 km it had started to fall apart. By the time it got to around 35 km above Russia it was shining as a bright shooting star, emitting light that burnt the retinas of any watching it, causing sunburn for many, and sending out a shockwave sideways from its path that blew others off their feet.

The shockwave broke phone networks, upset the electric grid, and interrupted the gas supply in some districts of Emanzhelinka as the valves closed in response to the vibration. No bones were broken, but some residents were hurt by flying debris and glass, while others suffered concussion.

Similar descriptions of the trajectory, determined from video data, are reported by in the first Nature paper. Risk estimates for asteroid fireball damage have, up to now, been based on data from nuclear bomb airburst tests. In a second Nature paper, researchers compare damage caused by the Chelyabinsk airburst with previous models for asteroid damage showing that the risks have been underestimated. The latest data suggest that the potential danger of impacts from asteroids tens of metres across is far greater than previously thought.

These results demonstrate the forensic value of the asteroid that fell to Earth in February this year, both for assessing how such bodies come into existence, and interact with our planetary home, but also how we might assess the risk of such events into the future.

This article was originally published at The Conversation. Follow me on twitter @sim0nredfern.

How plankton record climate

Simon Redfern October 27, 2013

Synchrotron X-ray CT scans reveal the structure of plankton shells. Credit: O Branson, University of Cambridge

Climate changes from millions of years ago are recorded at daily rates in ancient sea shells, new research shows. A synchrotron X-ray microscope has revealed growth bands in plankton shells that show how shell chemistry records the sea temperature.

The results could allow scientists to chart short timescale changes in ocean temperatures hundreds of millions of years ago. Plankton shells show features like tree rings, but representing daily growth bands, recording historical climate.

It’s important to understand current climate change in the light of how climate has varied in the geological past. One way to do this, for the last few thousand years, is to analyse ice from the poles. The planet’s temperature and atmosphere are recorded by bubbles of ancient air trapped in polar ice cores. The oldest Antarctic ice core records date back to around 800,000 years ago.

Our results just published in the journal Earth and Planetary Sciences Letters reveal how ancient climate change, pushing back hundreds of millions of years ago into deep time, is recorded in the shells of oceanic plankton.

As microbial plankton grow in ocean waters, their shells, made of the mineral calcite, trap trace amounts of chemical impurities, maybe only a few atoms in a million getting replaced by impurity atoms. Scientists have noticed that plankton growing in warmer waters contain more impurities, but it has not been clear how and why this “proxy” for temperature works.

When the plankton die, they fall to the muddy ocean floor, and can be recovered today from that muddy ocean floor sediments, which preserve the shells as they are buried. The amount of impurity, measured in fossil plankton shells, provides a record of past ocean temperature, dating back more than 100 million years ago.

Now, alongside co-workers from the Department of Earth Sciences at the University of Cambridge, we have measured traces of magnesium in the shells of plankton using an X-ray microscope in Berkeley, California, at the “Advanced Light Source” synchrotron – a huge electron accelerator that generates X-rays to study matter in minuscule detail.

Magnesium bands in the foram shell demonstrate incorporation into the mineral structure. Credit” O Branson, University of Cambridge

The powerful X-ray microscope has revealed narrow nanoscale bands in the plankton shell where the amount of magnesium is very slightly higher, at length scales as small as one hundredth that of a human hair. They are growth bands, rather like tree rings, but in plankton the bands occur daily or so, rather than yearly.

These growth bands in plankton show the day by day variations in magnesium in the shell at a 30 nanometre length scale. For slow-growing plankton it opens the way to seeing seasonal variations in ocean temperatures or plankton growth in samples dating back hundreds of millions of years.

The X-ray data show that the trace magnesium sits inside the crystalline mineral structure of the plankton shell. That’s important because it validates previous assumptions about using magnesium contents as a measure of past ocean temperature.

The chemical environment of the trace elements in the plankton shell, revealed in the new measurements, shows that the magnesium sits in calcite crystal replacing calcium, rather than in microbial membranes in their impurities in the shell. This helps explain why temperature affects the chemistry of plankton shells – warmer waters favour increased magnesium in calcite.

Our group are now using the UK’s “Diamond Light Source” synchrotron X-ray facility to measure how plankton shells grow and whether they change at all in the ocean floor sediments. Their latest results could allow scientists to establish climate variability in Earth’s far distant past, as well as providing new routes to measure ocean acidification and salinity in past oceans.

Hear me talk more on this here: From ABC Radio National “The Science Show”

X-ray vision gives new view of the core

Simon Redfern October 11, 2013

creative commons licence

Our planet’s interior is complex and has many layers. Their formation and structure contain many unsolved mysteries. But new research is providing some clues about how Earth’s internal structure may have evolved.

If you were to take a journey to the centre of the Earth you would find most stuff there is made of just three elements, at least until you’re about around 3000 km below the surface. These elements – oxygen, silicon and magnesium (plus a little bit of iron) – make up more than 90% of Earth’s “ceramic” mantle. Electrically and thermally insulating, the minerals of the mantle are the stony part of the planet.

But as you go deeper, things suddenly change. About midway to the centre, you cross a boundary from the stony mantle into the metallic core, initially liquid in its upper stretches, and then solid right in the centre of the Earth. The chemistry changes too, with almost all of the core being composed of iron.

The boundary between the metallic core and rocky mantle is a place of extremes. In physical characteristics, Earth’s metallic liquid outer core is as different to the rocky mantle as the seas are from the ocean floor. One might imagine an inverted world that has storms and currents of flowing red-hot metal in the molten outer core. It is this flow of metal in the core that gives Earth its magnetic field, protects us from the solar storms that constantly bombards us, and has allowed life to thrive.

How did such distinct layers of material end up next to each other? In a paper published in the journal Nature Geoscience, a group of scientists led by Wendy Mao of Stanford University have shown how metallic iron may be squeezed out of rocky silicates at depths of around 1000km beneath the crust.

Filaments of iron join together at depth in the Earth to allow molten metal to flow and form a core. Nature Geoscience

Experiments on mixtures of silicate minerals and iron cooked up in the lab show that iron sits in tiny isolated lumps within the rock, remaining trapped and pinned at the junctions between the mineral grains. This observation has led to the view that iron only segregates in the early stages of planetary formation, when the upper part of the silicate mantle was fully molten. It is thought that droplets of iron rained down through the upper mantle and pooled at its base, then sank as large “diapirs” driven by gravity. These fell through the deeper solid mantle to eventually form a core.

Mao’s work suggests that this model needs revising. The team used intense X-rays to probe samples held at extreme pressure and temperature squeezed between the tips of diamond crystals. They found that when pressure increases deep into the mantle, iron liquid begins to wet the surfaces of the silicate mineral grains. This means that threads of molten iron can join up and begin to flow in rivulets through the solid mantle, a process called percolation. More importantly, this process can occur even when the mantle is not hot enough to form a magma ocean.

The percolation of iron deep in the Earth provides a multi-stage route to forming a core, early in the planet’s history. Nature Geoscience

“In order for percolation to be efficient, the molten iron needs to be able to form continuous channels through the solid,” Mao explained. “Scientists had said this theory wasn’t possible, but now we’re saying, under certain conditions that we know exist in the planet, it could happen. So this brings back another possibility for how the core might have formed.”

Commenting on the results, Geoffrey Bromiley of the University of Edinburgh said “This new data suggests that we cannot assume that core formation is a simple, single stage event. Core formation was a complex, multi-stage process which must have had an equally complex influence on the subsequent chemistry of the Earth.”

Mao’s data raises important questions about how we start the formation of cores in planets. The prevailing idea in earth sciences is that studying the cores of meteorites and asteroids may help reveal insights about our own planet. But, Bromiley said, “their deep percolation model implies that early core formation can only be initiated in large planets. As a result, the chemistry of the Earth maybe have been ‘reset’ by core formation in a markedly different way from smaller planets and asteroids.”

He added, “The challenge now lies in finding a way to model the numerous processes of core formation to understand their timing and subsequent influence on the chemistry of not just the Earth, but also the other rocky bodies of the inner solar system.”

Bromiley and his colleagues are now investigating whether other factors might influence structure formation, like the deformation that asteroids and other bodies might have experienced on their chaotic pathways through the early Solar System. His work is adding other interesting questions. “We are increasingly observing metallic cores in bodies much smaller than the Earth. What process might have aided core formation in bodies which were never large enough to permit percolation of core forming melts at great depths?”

This article was originally published at The Conversation.
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Eyeing up the weather on distant super-Earths

Simon Redfern September 4, 2013

The_Keck_Subaru_and_Infrared_obervatories

The Subaru telescope sits to the left of the Keck and Infrared observatories, at Mauna Kea’s summit. (Source: Wikimedia Commons)

At the summit of Mauna Kea, Hawaii, the National Astronomical Observatory of Japan’s (NAOJ) Subaru Telescope has been turning its attention to distant worlds. Latest reports of blue-light observations from the telescope indicate that a super-Earth called Gilese 1214b (GJ 1214 b) appears to have a water-rich atmosphere. GJ 1214 b sits forty light years away in the constellation Ophiuchus, northwest of the centre of the Milky Way.

Artist’s rendition of a transit of GJ 1214 b in blue light. The blue sphere represents the host star GJ 1214, and the black ball in front of it on the right is GJ 1214 b. (Credit: NAOJ)

As a “super-Earth”, GJ 1214 b is one of a number of planets that are larger than Earth, but smaller than the Solar System’s gas giants, like Uranus and Neptune. The reported results were obtained by looking at the influence of GJ 1214 b’s atmosphere on scattered light as it passed in front of its star. Changes in the spectrum of light received from the star as the planet passed in transit can be explained in terms of light scattering in its atmosphere, which in turn provides clues as to its formation and birthplace within its star system – hydrogen-rich atmospheres are typical of distant planets, beyond the star system’s “snow line”, for example.

Artist’s rendition of the relationship between the composition of the atmosphere and transmitted colors of light.

Top: If the sky has a clear, upward-extended, hydrogen-dominated atmosphere, Rayleigh scattering disperses a large portion of the blue light from the atmosphere of the host while it scatters less of the red light. As a result, a transit in blue light becomes deeper than the one in red light.
Middle: If the sky has a less extended, water-rich atmosphere, the effect of the Rayleigh scattering is much weaker than in a hydrogen-dominated atmosphere. In this case, transits in all colors have almost the same transit depths.
Bottom: If the sky has extensive clouds, most of the light cannot be transmitted through the atmosphere, even though hydrogen dominates it. As a result, transits in all colors have almost the same transit depths. (Credit: NAOJ)

Two cameras on the Subaru Telescope were fitted with a blue transmission filter, and found no evidence for strong Rayleigh scattering as GJ 1214 b passed before its star. This implies that it has a water-rich or a hydrogen-dominated atmosphere. Combining the results with earlier data the team concluded that the atmosphere was most likely water-rich, with extensive clouds.

These are the first steps in discovering what sorts of planets these large objects are – are they “super-Earths” or “sub-Neptunes”. Watch out for more news of exo-geology as the results from telescopes like Subaru begin to accumulate.

For more on this story go to http://www.naoj.org/Pressrelease/2013/09/03/index.html