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Fire, dust and acid rain: Researchers have proposed a variety of climate effects
that might accompany a massive impact such as the one at the Cretaceous/Tertiary
(K/T) boundary, which created the Chicxulub crater.
“Several-kilometer-sized objects create a rain of shooting stars —
10,000 per centimeter squared,” says Owen Brian Toon, an atmospheric modeler
at the University of Colorado at Boulder. “It’s like turning the entire
Earth into a broiler oven.”
Last year, based on an object that could have created the Chicxulub crater,
David Kring and Dan Durda of the Lunar and Planetary Lab at the University of
Arizona modeled pillars of fire and ejecta. The massive amount of material catapulted
into the air could have taken from six months to several years to come down.
The fires set by the material would have spread over a world-encircling belt
in several hours.
Some researchers have calculated that the soot layer found at the K/T boundary
around the world requires burning the entire terrestrial biomass. The disappearance
of vegetation would change Earth’s surface reflectance of the sun’s
light and heat, says Michael Rampino, of New York University. Depending on the
type of vegetation that survived or evolved later, this change could result
in longer-term cooling.
An atmospheric curtain of dust would also block heat from the sun, stopping
photosynthesis: The ensuing effects could be as much as 5 to 10 degrees Celsius
cooler over at least several months, Rampino says. Although the amount of dust
expelled at Chicxulub is controversial, he says, the Chicxulub object, if it
hit an evaporite deposit made of sulfur-heavy minerals such as gypsum, also
would have launched sulfur aerosols into the atmosphere. Such sulfuric acid
fallout might last for years, decades or centuries, and those aerosols could
quickly cool the climate by about 3 degrees Celsius, Rampino says.
If fire, sulfur particles and dust aren’t enough, the energy from a large
object’s entry alone could be devastating. The force would volatilize nitrogen
and oxygen in the atmosphere, sending nitric acid rain pouring down, says Ken
Caldeira of Lawrence Livermore National Laboratory in Livermore, Calif. That
rain would be sufficient to dissolve the carbonate-shelled creatures in the
oceans’ top layers, he says. Their disappearance would be offset by the
increased erosion on land, adding carbonate runoff to the seas. The resulting
increase in alkalinity of the oceans would trap more carbon dioxide from the
atmosphere, and the shift in that greenhouse gas could decrease global temperatures.
But even those conditions, Caldeira says, would only be temporary. Even in the
case of Chicxulub, Rampino says, “eventually the climate does bounce back
almost to what it was in the late Cretaceous.”
Although climate effects probably also would not be long-lasting after a similar
impact today, Toon says, the short-term effects still would be drastic. “Imagine
what this is like — fire, no infrastructure services, it’s below freezing…
there’s no fuel for tractors, and nothing to make agricultural products
with,” he says. “We’d be back to primitive techniques, where
the carrying capacity of Earth is only a few hundred million people.”
Naomi Lubick
Geologists have widely accepted that an extraterrestrial impact event contributed
to the Cretaceous/Tertiary extinction 65 million years ago. They have also generally
accepted that volcanism probably also played a role: At roughly the same time
that the Yucatan Peninsula was being pulverized, 2 million cubic kilometers
of basalt were rapidly flooding India to form the Deccan Traps.
The proximity of such catastrophic events, either temporal or spatial, has led
many researchers to link impacts to volcanism, on scales ranging from local
to global. The debate over whether impact events can trigger volcanism —
and what role such a process has played in shaping Earth since the heavy meteor
bombardment period ended 3.8 billion years ago — has been ongoing since
the idea was first suggested in the early 1960s.
Now, Boris Ivanov of the Institute of the Dynamics of the Geospheres in Moscow
and Jay Melosh of the Lunar and Planetary Lab at the University of Arizona say
they have shown conclusively through numerical modeling that most impacts do
not initiate volcanism — at least not at the impact site.
Using a hydrocode, a complex computer code that describes the behavior of solids
that act as fluids at extreme pressures and temperatures, the researchers modeled
the impact of a 20-kilometer-wide asteroid hitting both cold and hot crust.
Reported in the October 2003 Geology, the results suggest that a crater
250 to 300 kilometers wide (larger than all but two of Earth’s craters,
according to some estimates) would produce only 10,000 cubic kilometers of melt
from the target rocks — orders of magnitude less than the Deccan Traps.
“What Melosh and Ivanov have demonstrated quite clearly is that under normal
circumstances, impacts do not penetrate deep enough into the crust, and certainly
not into the mantle, to release any magma,” says Christian Koeberl, a geochemist
at the University of Vienna, Austria, who specializes in meteoritic impact craters.
The researchers do say, however, that an impact four to five times as large
could probably produce sufficient melt to form a flood basalt. But an impact
of that size occurring after the heavy-bombardment period is so unlikely, they
state, that impact volcanism could not be considered a significant or usual
geologic process. And the only other way to produce a flood basalt using an
asteroid, they write, would be to directly hit a nascent hot spot, which is
highly unlikely.
Adrian Jones, a petrologist at University College London (UCL) who also does
hydrocode modeling of impacts, agrees with the improbability of the hot spot
scenario. He says, however, that an impactor need not be as large as Ivanov
and Melosh predict in order to produce significant volcanism, as long as the
impactor hits hot crust.
“They may not have tried a hot-enough scenario,” Jones says. “If
you have a hotter gradient, you need a smaller impactor to make the same amount
of melting, or the same size impactor will make a lot more melt.”
Jones, along with geophysicist David Price at UCL, say some flood basalts, though
not all, could have been formed by an impact near the mid-ocean ridge system,
where crust even millions of years from the ridge is still thin and hot. “The
chances of hitting a hot spot must be very, very low,” Jones says. “But
the chances of hitting a mid-oceanic ridge are not zero,” he adds. “The
odds are certainly much higher.”
In any impact, some of the target rock melts and fuses. The most commonly suggested
mechanism for how an impact could set off larger, longer-lasting episodes of
volcanism is the same one that explains how mantle rocks melt as they near Earth’s
surface. In this process, called decompression melting, pressure on rock underlying
the crater rapidly decreases when an asteroid blasts away the overburden. This
pressure decrease in turn decreases the rocks’ melting point, allowing
them to liquefy.
Whether any given impact will result in decompression melting depends on a number
of variables, including the size, speed and composition of the impactor, as
well as the temperature and composition of the target rock. But in recent years
the myriad scenarios have become easier to model due to advances in computing
capacity.
“The contribution by Melosh and Ivanov is very important because it puts
actual numbers to things that have been speculated about for years,” Koeberl
says. “Nobody has used as realistic a numerical model of impact cratering,
with realistic parameters, to actually check the speculation.”
Sara Pratt
Geotimes contributing writer
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