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

Tubes in space

Carbon nanotubes form in space but use a metal-free chemistry until now unavailable to chemists on Earth. The discovery is a surprising outcome of laboratory experiments designed by Joseph Nuth at NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, and his colleagues. They were hoping to understand how carbon atoms are recycled in stellar nurseries, the regions of space where stars and planets are born, but the finding could have applications in nanotechnology, as well as help explain some characteristics of supernovae.

Writing in the journal Astrophys J Lett, Nuth and colleagues explain how astrochemistry makes carbon nanotubes without requiring a metal catalyst. Nanotubes are produced, they say, when graphite dust particles are exposed to a mixture of carbon monoxide and hydrogen gases, conditions that exist in interstellar space.

The finding corroborates the discovery of graphite whiskers, bigger than nano nanotubes, in three meteorites. The meteoric discovery hinted at why some supernovae appear dimmer and farther away than they ought to be based on calculations using current models. Nuth’s approach is a variation of a well-established way to produce gasoline or other liquid fuels from coal. It’s known as Fischer-Tropsch synthesis, and researchers suspect that it could have produced at least some of the simple carbon-based compounds in the early solar system. Nuth proposes that the nanotubes yielded by such reactions could be the key to the recycling of the carbon that gets released when carbon-rich grains are destroyed by supernova explosions.

Stellar Nursery
A stellar nursery could be home to carbon nanotube factories (Credit: NASA, http://apod.nasa.gov/apod/ap021102.html)

The structure of the carbon nanotubes produced by Nuth and colleagues was determined by materials scientist Yuki Kimura, of Tohoku University, Japan, using transmission electron microscopy. He observed particles on which the original smooth graphite gradually morphed into an unstructured region and finally to an area rich in tangled hair-like masses. A closer look with an even more powerful microscope showed that these tendrils were in fact cup-stacked carbon nanotubes, resembling a stack of disposable drinking cups with the bottoms removed. If further testing indicates that the new method is suitable for materials-science applications, it could supplement, or even replace, the familiar way of making nanotubes, explains Kimura.

Researchers might also now evaluate whether graphite whiskers absorb light. A positive result would lend credence to the proposition that the presence of these molecules in space affects the observations of some supernovae.

LINKS

Astrophys J Lett, 2010, 710, L98-L101

http://dx.doi.org/10.1088/2041-8205/710/1/L98

The Martian Lake District

Three billion years ago, the red planet, Mars, was warm enough to sustain lakes of liquid water, according to satellite images just published in the journal Geology. Previously, astronomers had assumed that this period was simply too cold and arid for surface water.

Researchers at Imperial College London and University College London now suggest that during the Hesperian Epoch, the Martian surface around the equator was spotted with lakes, each approximately 20 kilometres across, formed from melted ice. Earlier studies had hinted at the warm and wet early history of Mars during the period 4 billion to 3.8 billion years ago, well before the Hesperian Epoch. Detailed images from NASA’s Mars Reconnaissance Orbiter, which is currently circling the planet, suggest that there were later warm and wet periods; age is determined by meteorite crater count. The evidence lies in several flat-floored depressions located above Ares Vallis, a giant gorge that runs 2000 km across the equator of Mars.

Martian lakes
Martian lakes

According to Nicholas Warner, of IC’s Department of Earth Science and Engineering, “Most of the research on Mars has focused on its early history and the recent past. Scientists had largely overlooked the Hesperian Epoch as it was thought that Mars was then a frozen wasteland. Excitingly, our study now shows that this middle period in Mars’ history was much more dynamic than we previously thought.”

Warner and colleagues, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin, and Jan-Peter Muller, claim that there may have been increased volcanic activity, meteorite impacts or shifts in Mars’ orbit during this period, which could have warmed its atmosphere enough to melt ice. This in turn would have bolstered the greenhouse effect temporarily, trapping more heat from the sun and making the planet warm enough for liquid water to exist on its surface.

Until now, the Ares Vallis depressions have remained a mystery to scientists, although they suspected that their formation was due to sublimation of ice directly to water vapour. The loss of ice would have created cavities between the soil particles, which would have caused the ground beneath to subside.

Martian channels
Martian channels

The researchers have now discovered small, sinuous channels that connect the depressions, which they say could only have been formed by running water, essentially making the sublimation theory redundant. The team also compared the Mars images with images of thermokarst landscapes on Earth in places such as Siberia and Alaska. Thermokarst landscapes are areas where permafrost is melting, creating lakes that are interconnected by the same type of channels the team says exist on Mars. The team says that the melting ice created lakes that may have burst their banks allowing water to carve pathways through the frozen ground from higher lakes into lower-lying lakes.

UCL’s Muller who works at the Mullard Space Science Laboratory who carried out the 3D mapping of the Martian surface, explains how modelling with sub-metre resolution allowed the team to test their hypotheses much more rigorously than ever before.

Topographic image
Topographic image

One thing that the scientists do not yet know is how long the warm and wet period lasted during the Hesperian epoch or how long the lakes remained liquid. Nevertheless, the study may have implications for so-called “astrobiologists” looking for evidence of life on Mars. The team say these lake beds indicate regions on the planet that may have once been suitable for some form of microbial Martian life. As such, they represent good targets for future robotic missions seeking out ancient life on Mars.

LINKS

Geology, 2010, 38, 71-74
http://dx.doi.org/10.1130/G30579.1
Video flypast