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Satellite keeps its eye on ice

Cryosat 2 is a European satellite designed to look down on Earth and monitor ice and snow. It’s part of an overall programme to better understand Earth’s climate.

And interesting aspect of this mission is that the launch vehicle was a converted Dnepr ICBM – Intercontinental Ballistic Missile – which explains why it launched from a silo beneath the ground.

Adapted from information issued by ESA.

Comet crash caused climate change?

Artist's impression of an asteroid or comet striking the Earth.

Artist's impression of a comet striking the Earth.

Earth was struck by thousands of cometary fragments over the course of an hour 13,000 years ago, leading to a dramatic cooling of the planet, according to astronomer Professor Bill Napier of the Cardiff University Astrobiology Centre.

The cooling, by as much as 8 degrees C, interrupted the warming which was occurring at the end of the last ice age and caused glaciers to readvance.

Evidence has been found that this catastrophic change was associated with some extraordinary extraterrestrial event.

The change is marked by the occurrence of a “black mat” layer a few centimetres thick found in rock layers at many sites throughout the United States containing high levels of soot indicative of continental-scale wildfires.

There are also microscopic hexagonal diamonds (nano-diamonds), which are produced by high-pressure shock events and are found only in meteorites or impact craters.

These findings led to the suggestion that the catastrophic changes of that time were 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.

The cooling lasted over a thousand years, and its onset coincides with the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture.

The chief objection to the idea is that the odds against the Earth being struck by an asteroid this large only 13,000 years ago are a thousand to one against. And the heat generated by the rising fireball would have been limited by the curvature of the horizon and could not explain the continent-wide occurrence of wildfires.

Multiple comet crashes the cause?

Professor Napier has now come up with an model that accounts for the major features of the catastrophe without involving such an improbable event. According to his concept, the Earth ran into a dense trail of material from a large disintegrating comet.

Fragments of comet 73/P Schwassman-Wachmann 3

Comets sometimes break into many pieces, such as 73/P Schwassman-Wachmann 3, seen in this Hubble Space Telescope image from 2005.

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.

In the course of the giant comet’s disintegration, the environment of the interplanetary system would have been hazardous and the Earth would probably have run through at least one dense swarm of cometary material.

The new model indicates that such an encounter would last for about an hour during which thousands of impacts would take place over continental areas, each releasing the energy of a megaton-class nuclear bomb, generating the extensive wildfires which took place at that time. The nano-diamonds at the extinction boundary would then be explained as having come in with the comet swarm.

One recent meteorite is known which may have come from this giant comet progenitor—the Tagish Lake meteorite, which fell over Yukon Territory in January 2000. It has the highest abundance of nano-diamonds of any meteorite so far analysed.

Professor Napier sums up his model: “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.”

Adapted from information issued by the RAS. Image credits: NASA / ESA / H. Weaver (JHU/APL) / M. Mutchler / Z. Levay (STScI) / G. Rhemann and M. Jager.

Antarctic glacier retreats

Image of Crane Glacier on the Larsen B Ice Shelf on April 6, 2002

This image of Crane Glacier on the Larsen B Ice Shelf on the Antarctic Peninsula was captured on April 6, 2002. Compare with the image below.

In late Southern Hemisphere summer of 2002, the Larsen B Ice Shelf on the Antarctic Peninsula disintegrated into thousands of pieces.

The collapse appears to have been due to a series of warm summers on the Antarctic Peninsula, which culminated with an exceptionally warm summer in 2002. On the surface of the shelf, rows of melt ponds settled into natural crevasses, driving the cracks all the way through the ice shelf.

This pair of images from NASA’s Landsat 7 satellite shows the dramatic impact the collapse had on many of the glaciers that fed the Larsen B Ice Shelf. The loss of the shelf caused the flow of most of the glaciers around the bay to accelerate significantly. More rapid flow and calving of icebergs caused the margins to retreat inland.

The image above was captured on April 6, 2002, about two months after the dramatic collapse. The bay (image right) is filled with slush and icebergs from the collapsed shelf.

Autumn snows have probably already dusted the surface of the mélange of ice; snowfall and seasonal sea ice kept much of the debris frozen in place the first winter after the collapse. The terminus of the Crane Glacier extends into the bay like a fan.

Throughout the summer of 2003, remaining fragments of the shelf broke away, and the mélange of icebergs and smaller ice pieces from the previous summer’s collapse began to drift away.

Without the stabilizing presence of the ice shelf, the Crane Glacier retreated dramatically. Its fan-shaped terminus became C-shaped as the glacier’s centre crumbled more rapidly than the edges pressed against the mountain walls.

By 2003, Crane Glacier had retreated dramatically

By 2003, Crane Glacier had retreated dramatically as fragments of the ice shelf broke away.

The unusually bright blue tinge of the ice debris in the February 20 image (above) is the reflection from the pure ice on the underside of the ice shelf fragments. Many of the icebergs that crumbled from the edge of the shelf were too tall and narrow to float upright, and they toppled over.

The surface of an ice shelf gets covered by snow, but the underside is very pure ice. Pure, thick ice absorbs a small amount of red light. Photo-like satellite images such as these are made by combining the satellite’s observations of red, green, and blue wavelengths of light reflected from the Earth’s surface. When all these visible wavelengths are strongly reflected, the surface looks white; when the reddest light is absorbed, the reflection takes on a bluish tinge.

NASA images by Robert Simmon based on Landsat-7 data. Text adapted from information issued by Rebecca Lindsey.

Earth was wet in its youth

Earth seen from space

Extreme greenhouse concentrations weren't needed to keep Earth's oceans from freezing billions of years ago.

Four billion years ago, our then stripling Sun radiated only 70 to 75 percent as much energy as it does today. Other things on Earth being equal, with so little energy reaching the planet’s surface, all water on the planet should been have frozen.

But ancient rocks hold ample evidence that the early Earth was awash in liquid water – a planetary ocean of it. So something must have compensated for the reduced solar output and kept Earth’s water wet.

To explain this apparent paradox, a popular theory holds there must have been higher concentrations of greenhouse gases in the atmosphere, most likely carbon dioxide, which would have helped retain a greater proportion of the solar energy that arrived.

But a team of scientists including researchers from Stanford have analysed the mineral content of 3.8-billion-year-old marine rocks from Greenland and concluded otherwise.

Swirls of cloud over the ocean

Swirls of cloud over the ocean

“There is no geologic evidence in these rocks for really high concentrations of a greenhouse gas like carbon dioxide,” said Dennis Bird, professor of geological and environmental sciences.

Instead, the team proposes that the vast global ocean of early Earth absorbed a greater percentage of the incoming solar energy than today’s oceans, enough to ward off a frozen planet.

Earth was a water world

Because the first landmasses that formed on Earth were small – mere islands in the planetary sea – a far greater proportion of the surface was covered with water than today.

The crux of the theory is that because oceans are darker than continents, particularly before plants and soils covered landmasses, seas absorb more sunlight.

“It’s the same phenomenon you will experience if you drive to Wal-Mart on a hot day and step out of your car onto the asphalt,” Bird said. “It’s really hot walking across the blacktop until you get onto the white concrete sidewalk.”

Another key component of the theory is in the clouds. “Not all clouds are the same,” Bird said.

Clouds reflect sunlight back into space to a degree, cooling Earth, but how effective they are depends on the number of tiny particles available to serve as nuclei around which the water droplets can condense. An abundance of nuclei means more droplets of a smaller size, which makes for a denser cloud and a greater reflectivity, or albedo, on the part of the cloud.

The edge of Earth's atmosphere

The edge of Earth's atmosphere

Most nuclei today are generated by plants or algae and promote the formation of numerous small droplets. But plants and algae didn’t flourish until much later in Earth’s history, so their contribution of potential nuclei to the early atmosphere circa 4 billion years ago would have been minimal. The few nuclei that might have been available would likely have come from erosion of rock on the small, rare landmasses of the day and would have caused larger droplets that were essentially transparent to the solar energy that came in to Earth, according to Bird.

“We put together some models that demonstrate, with the slow continental growth and with a limited amount of clouds, you could keep water above freezing throughout geologic history,” Bird said.

Adapted from information issued by Stanford University.