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planetary science

Meteorite impact turns silica into stishovite in a billionth of a second

Meteorite impact turns silica into stishovite in a billionth of a second

The Barringer meteor crater is an iconic Arizona landmark, more than 1km wide and 170 metres deep, left behind by a massive 300,000 tonne meteorite that hit Earth 50,000 years ago with a force equivalent to a ten megaton nuclear bomb. The forces unleashed by such an impact are hard to comprehend, but a team of Stanford scientists has recreated the conditions experienced during the first billionths of a second as the meteor struck in order to reveal the effects it had on the rock underneath.

The sandstone rocks of Arizona were, on that day of impact 50,000 years ago, pushed beyond their limits and momentarily – for the first few trillionths and billionths of a second – transformed into a new state. The Stanford scientists, in a study published in the journal Nature Materials, recreated the conditions as the impact shockwave passed through the ground through computer models of half a million atoms of silica. Blasted by fragments of an asteroid that fell to Earth at tens of kilometres a second, the silica quartz crystals in the sandstone rocks would have experienced pressures of hundreds of thousands of atmospheres, and temperatures of thousands of degrees Celsius.

A meteroite impact event would generate shock waves through the Earth.
NASA

What the model reveals is that atoms form an immensely dense structure almost instantaneously as the shock wave hits at more than 7km/s. Within ten trillionths of a second the silica has reached temperatures of around 3,000℃ and pressures of more than half a million atmospheres. Then, within the next billionth of a second, the dense silica crystallises into a very rare mineral called stishovite.

The results are particularly exciting because stishovite is exactly the mineral found in shocked rocks at the Barringer Crater and similar sites across the globe. Indeed, stishovite (named after a Russian high-pressure physics researcher) was first found at the Barringer Crater in 1962. The latest simulations give an insight into the birth of mineral grains in the first moments of meteorite impact.

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Simulations show how crystals form in billionths of a second.

The size of the crystals that form in the impact event appears to be indicative of the size and nature of the impact. The simulations arrive at crystals of stishovite very similar to the range of sizes actually observed in geological samples of asteroid impacts.

Studying transformations of minerals such as quartz, the commonest mineral of Earth’s continental crust, under such extreme conditions of temperature and pressure is challenging. To measure what happens on such short timescales adds another degree of complexity to the problem.

These computer models point the way forward, and will guide experimentalists in the studies of shock events in the future. In the next few years we can expect to see these computer simulations backed up with further laboratory studies of impact events using the next generation of X-ray instruments, called X-ray free electron lasers, which have the potential to “see” materials transform under the same conditions and on the same sorts of timescales.

The Conversation

This article was originally published on The Conversation. Read the original article.

Titanic lakes revealed in Cassini’s extraterrestrial bathymetry

NASA/ESA's map of Titan's northern lakes

NASA/ESA’s map of Titan’s northern lakes

The joint NASA-ESA Cassini space probe, exploring Saturn and her moons, has revealed extraordinary lakes and seas of liquid methane around the north pole of Titan. Scientists associated with the Cassini mission described a strange rectangular area of large seas, picked out by imaging instruments aboard the probe. I heard all about it at the recent American Geophysical Union Fall Meeting last month.

Elongated lakes and seas connected by long skinny peninsulas characterise the two seas picked out in the new image. Reminiscent of the topographic depressions in the basin and range province of USA, shaped by the movements of tectonic plates on America’s western fringe, there are suggestions that the large lakes seen on Titan may be tectonically shaped-seas.

“Scientists have been wondering why Titan’s lakes are where they are. These images show us that the bedrock and geology must be creating a particularly inviting environment for lakes,” said Randolph Kirk, a Cassini RADAR team member at the US Geological Survey. “We think it may be something like the formation of the prehistoric lake called Lake Lahontan near Lake Tahoe in Nevada and California, where deformation of the crust created fissures that could be filled up with liquid.”

Scientists described the observations of huge polar lakes called Ligeia and Kraken on Titan, at the meeting of the American Geophysical Union in San Francisco, the world’s largest gathering of Earth scientists.

Alongside the two large liquid bodies picked out so clearly, there is a myriad of smaller lakes that are seen scattered around the pole of Titan. Their origins are unclear, with speculations ranging from volcanic crater lakes to giant sinkholes formed in dissolved Titan crust.

Marco Mastrogiuseppe from Sapienza University, Rome, described the results from RADAR imaging of the fluid bodies at Titan’s surface. “For the first time we were able to observe the topography of the subsurface of an extraterrestrial sea”, he explained.

Cassini’s RADAR has charted the areas of the lakes and seas near the pole, but has also bounced signals off the lake beds in the first depth soundings of an extraterrestrial sea.

“Ligeia Mare turned out to be just the right depth for radar to detect a signal back from the sea floor, which is a signal we didn’t think we’d be able to get,” said Mastrogiuseppe. A maximum depth of around 170 meters, similar to Lake Michigan, was found, and the lake was crystal clear to RADAR eyes.

The total volume of Ligeia is put at 9000 cubic kilometres and it is filled not with water, but with hydrocarbon fluids. The total volume of the hydrocarbon Titanic seas corresponds to around 300 times that of Earth’s oil reserves, in a celestial body smaller than Earth.

The RADAR reflectivity suggests that the lakes are mainly filled with methane alongside a few other heavier hydrocarbon fluids. These include ethane and nitrogen. Alongside Ligiea sits another sea, Kraken. Comparable in size to the Caspian Sea here on Earth, Kraken is four or more times the area of Ligeia. Cassini will return to carry out bathometry of it in August 2014.

Jeffrey Kargel, from the University of Arizona Tucson, pointed out that the presence of extensive methane seas and lakes at Titan’s north pole makes worse a long acknowledged deficiency of heavier hydrocarbons expected from models of Titan’s chemistry. Among them are ethane, ethylene, propylene, acetylene and benzene – heavy hydrocarbons generated as sunlight causes chemical reactions in Titan’s soup of natural gas. Using visual imaging instruments Cassini has revealed that Titan has a northern polar cap larger than Greenland.

Bright deposits around the lakes show the nature of the solid surface. In a world that is difficult to imagine, crystallised heavy hydrocarbons form Titan’s crust, with suggestions of huge dune fields of solid hydrocarbon sand around the equator. While these equatorial “rocks” are saturated in ethane the polar regions appear to be made of methane.

We are now close to summer solstice on Saturn, and Titan has weather that changes with the seasons. Giant storms arise on Saturn, with jets of gas seen shooting from the south pole of cousin moon, Enceladus. A fly-by is planned in 2015 in which Cassini will fly through these plumes and take a closer look at Enceladus’ north pole.

Cassini is now in a set of intricate complicated orbits. Only 4% of its propulsion is left, and future fly-bys are largely powered by the gravitational fields of Saturn and its moons. The probe’s final journey, planned for September 2017 will skirt Saturn’s innermost ring and touch her atmosphere before finally succumbing to the giant planet’s grasp.

This article was originally published at The Conversation.
Read the original article.

Chelyabinsk asteroid – crowdsourced science?

 

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.

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

Eyeing up the weather on distant super-Earths

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.

GJ 1214 b in transit

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