Interplanetary storm

A meteoric storm raged over the Earth 13,000 years ago as thousands of pieces of rock each the size of the Tunguska comet rained down over the course of an hour. The end result was a dramatic cooling of the planet, according to astronomer Bill Napier of the Cardiff University Astrobiology Centre.

Writing in the journal Monthly Notices of the Royal Astronomical Society, Napier suggests that the temperature drop was as high as 8 Celsius and interrupted global warming at the end of the last ice age, causing glaciers to re-advance.

Scientists have puzzled over a boundary layer marked by the occurrence of a “black mat” tens of millimetres thick present at sites throughout the United States, which contains high levels of soot from continental-scale wildfires and nanoscopic hexagonal diamonds found only in meteorites or impact craters. The evidence hinted at a catastrophic change at that time caused by the impact of an asteroid or comet 4 km across on the Laurentide ice sheet, which at that time covered what would become Canada and the northern part of the United States.

Napier points out that the cooling lasted a more than a millennium and led to the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture. However, the chances of an asteroid impacting the Earth during that period were extremely low. Moreover, the heat generated by the rising fireball would be limited by the curvature of the horizon and could not have led to the continent-wide occurrence of wildfires.

Napier has now devised a model that can account for the evidence.
According to Napier’s model, the Earth ran into a dense trail of material from a large disintegrating comet. He points out that there is compelling evidence that such a comet entered the inner planetary system between 20,000 and 30,000 years ago and has been fragmenting ever since, giving rise to a number of closely related meteor streams and asteroids known as the Taurid Complex.

As the comet disintegrated, the Earth would have ploughed through at least one dense swarm of cometary fragments over an hour-long period.
Thousands of individual impacts would have occurred across what is now continental America, each releasing the energy of a megaton atomic bomb and triggering extensive wildfires.

2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3).
2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3). Credit: NASA / ESA / H.Weaver (JHU/APL) / M. Mutchler / Z.Levay (STScI)

“A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision. It gives a convincing match to the major geophysical features at this boundary,” says Napier. Indeed, a recent meteorite which may have come from this giant comet progenitor fell on Yukon Territory in January 2000 and has the highest abundance of nanodiamonds of any meteorite so far analysed.

Links.
Monthly Notices Royal Astronom Soc, 2010, in press Preprint link
Cardiff staff

Meteoric rise of life on Earth

Seemingly endless meteor storms that bombarded the Earth four billion years ago helped to create the right growing conditions from which life could first emerge. The same meteoric bombardment may also have had a similar effect on our planetary neighbour Mars.

Richard Court and Mark Sephton of the Department of Earth Science and Engineering, at Imperial College London and their colleagues publish details of a theory that could change our understanding of natural terraforming of the primordial Earth this month in the journal Geochimica et Cosmochima Acta.

Professor Mark A. Sephton

Professor Mark A. Sephton

The researchers have analysed the remaining mineral and organic content of fifteen fragments of ancient meteorites to see how much water vapour and carbon dioxide they would release when subjected to very high temperatures. The experiments aimed to replicate the conditions experienced by the meteoric material as it entered the Earth’s atmosphere billions of years ago.

When a meteor enters a planet’s atmosphere, extreme heat is produced because of the retarding compression of the air due to the meteor travelling at supersonic speeds. This heat causes some of the minerals and organic matter on the meteor’s outer crust to vaporise releasing water and carbon dioxide before it breaks up or hits the ground.

A fragment of the Murchison meteorite was analysed by the IC team

A fragment of the Murchison meteorite was analysed by the IC team

This water source could have added large quantities of moisture to the atmospheres of both Earth and Mars billions of years ago. Moreover, the addition of the greenhouse gas carbon dioxide to the atmosphere would have helped trapped solar energy and so make the primordial planets warm enough for liquid oceans.

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Court and Sephton used a novel analytic technique known as pyrolysis-FTIR spectroscopy to test the meteorites. The pyrolysis process involved blasting a meteorite fragment with electricity to heat it from room temperature at a rate of 20,000 degrees Celsius per second to 250 Celsius or 1000 Celsius to cause the material to break apart and vaporize. FTIR, or Fourier-transform infra-red spectroscopy, then provides a chemical fingerprint of any small molecules, such as water and carbon dioxide, produced.

For a long time, scientists have been trying to understand why Earth is so water rich compared to other planets in our solar system, Sephton explains. The team found that on average, each meteorite fragment could release 12% of its weight as water vapour and 6% as carbon dioxide gas. These figures, the researchers suggest, are not enough that a few small meteorites could have made a significant contribution to the atmosphere’s water and carbon dioxide levels.

However, the team also analysed data from an ancient meteorite shower called the Late Heavy Bombardment (LHB), which occurred 4 billion years ago, where millions of rocks crashed to Earth and Mars over a period of 20 million years. They calculated that the LHB could have added 10 billion tonnes of carbon dioxide and 10 billion tonnes of water vapour to the planets’ atmospheres every year. This rate of addition is certainly adequate to make both planets warmer and wetter enough to sustain life.

The LHB provides a missing clue. This may have been a pivotal moment in our early history where Earth’s gaseous envelope finally had enough of the right ingredients to nurture life on our planet, adds Sephton.

Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water, says Court, Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet. Of course, the existence of life on Earth is obvious, but habitable conditions on Mars apparently did not last. Unlike Earth, Mars has no magnetic field to shield it from the Sun’s lethal solar wind, its atmosphere was eventually mostly stripped away, and warming volcanic activity subsided. As such any liquid water retreated to the frozen poles leaving behind the barren red planet with which we are almost familiar.

Further reading

Geochim. Cosmochim. Acta, 73 (11), 1 June 2009, 3512-3521

Professor Mark A. Sephton
http://www3.imperial.ac.uk/people/m.a.sephton

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It’s all in the marine mix

Mixing of surface waters in the Atlantic Ocean seems to have reverted in the winter of 2007/2008 to normal levels for the first time in almost a decade, according to an analysis of the data published in Nature Geoscience.

One of the most worrying effects of climate change was that oceanic currents would be disrupted. With that disruption would come far-reaching changes to weather patterns and temperate northern zones plunged into a mini-ice age. At least that was the warning. Scientists have for the best part of a decade seen evidence that the mixing of surface waters to depths greater than 1 km in the northern North Atlantic Ocean may have been reduced to the levels at which currents might be retarded.

Atlantic bathymetry (Credit: U.S. National Oceanic and Atmospheric Administration)

Atlantic bathymetry (Credit: U.S. National Oceanic and Atmospheric Administration)

Kjetil Våge and Robert Pickart of the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts and colleagues in Canada, France, and the US, explain that over the last few years, deep mixing has been almost absent in the Labrador Sea and was linked to global warming. The researchers explain how this deep mixing is an essential part of the Atlantic Ocean circulation and as such is a key regulator of both carbon dioxide uptake by the oceans and heat transfer between the ocean and the atmosphere.

They have now used data from a network of measuring floats (from the Argo program) to detect the deep mixing of surface waters and evaluated local observations. The team also carried out computer simulations of past climate and mashed this with satellite data to help them understand the mechanisms that lead to this deep convection.

They conclude that it is a combination of air temperatures in the northern hemisphere, storm pathways, the flow of freshwater into the Labrador Sea and the distribution of pack ice that cools the ocean surface, and allows convection to mix surface waters to much greater depths than would otherwise occur. The complexity of this system suggests that modelling the convective system of the North Atlantic Ocean is no simple matter and that predicting outcomes given specific climate change parameters is going to be far more difficult than scientists hoped.

Kjetil Våge, Robert S. Pickart, Virginie Thierry, Gilles Reverdin, Craig M. Lee, Brian Petrie, Tom A. Agnew, Amy Wong, Mads H. Ribergaard (2008). Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008 Nature Geoscience DOI: 10.1038/ngeo382

Further reading

Kjetil Vaage profile
http://www.whoi.edu/profile.do?id=kjetil

Suggested searches

climate change
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ocean currents