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FEATURE
Spotlight: A 'Smoking Gun' for Dinosaur Extinction
It is hard to imagine that one of the largest impact craters on
Earth, 180-kilometers (112-mile) wide and 900-meters
(3,000-feet) deep, could all but disappear from sight, but it did.
Chicxulub, located on Mexico's Yucatan peninsula, eluded
detection for decades because it was hidden (and at the same
time preserved) beneath a kilometer of younger rocks and
sediments. Size isn't the only thing that makes Chicxulub special.
Most scientists now agree it's the "smoking gun" -- evidence that
a huge asteroid or comet indeed crashed into Earth's surface 65
million years ago causing the extinction of more than 70 percent
of the living species on the planet, including the dinosaurs. This
idea was first proposed by the father and son team of Luis and
Walter Alvarez in 1980.
Though the buried giant can't be seen, the impact crater has left
subtle clues of its existence on the surface. "When I talk to
school children, I describe it like this," says Dr. Gary Kinsland, a
geology professor at the University of Louisiana at Lafayette
who has been doing research on Chicxulub since 1994. "Put a
bowl on your bed, then throw the sheets and blankets over it. All
you'll probably see of the bowl now is a subtle depression."
FEATURE
Spotlight: A 'Smoking Gun' for Dinosaur Extinction
It is hard to imagine that one of the largest impact craters on
Earth, 180-kilometers (112-mile) wide and 900-meters
(3,000-feet) deep, could all but disappear from sight, but it did.
Chicxulub, located on Mexico's Yucatan peninsula, eluded
detection for decades because it was hidden (and at the same
time preserved) beneath a kilometer of younger rocks and
sediments. Size isn't the only thing that makes Chicxulub special.
Most scientists now agree it's the "smoking gun" -- evidence that
a huge asteroid or comet indeed crashed into Earth's surface 65
million years ago causing the extinction of more than 70 percent
of the living species on the planet, including the dinosaurs. This
idea was first proposed by the father and son team of Luis and
Walter Alvarez in 1980.
Though the buried giant can't be seen, the impact crater has left
subtle clues of its existence on the surface. "When I talk to
school children, I describe it like this," says Dr. Gary Kinsland, a
geology professor at the University of Louisiana at Lafayette
who has been doing research on Chicxulub since 1994. "Put a
bowl on your bed, then throw the sheets and blankets over it. All
you'll probably see of the bowl now is a subtle depression."
| Side-by-side of Shuttle Radar Topography Mission data (top) and Landsat data (bottom), Yucatan Peninsula, Mexico |
| "There is not a big hole anymore," he continues, "but if you look at the rim of the depression on your bed, you'll see that it is still in the same position as the rim of the bowl beneath. That's how surface expression allows us to interpret something about the buried structure." The view from space lets scientists see some of Chicxulub's surface features that are not nearly so obvious from the ground. Satellite images showing a necklace of sink holes, called cenotes, across the Yucatan's northern tip are what first caught |
| Chicxulub impact crater trough and sinkholes |
| the attention of NASA researchers Drs. Kevin Pope, Adriana Ocampo and Charles Duller in 1990. They were among the first to propose Chicxulub as the impact site linked to the mass extinctions that occurred at the end of the Cretaceous and beginning of the Tertiary geological ages, called the K/T boundary. "We were ignorant of the existence of a crater," says Pope, now an independent geologist, "We were working on a project on surface water and Mayan archaeology when we saw this perfect semi-circular structure in images from the Landsat Thematic Mapper. "We were fascinated and got the magnetic and gravity data from the area collected earlier by the Mexican petroleum company, who had been looking for oil. Their data showed a large, remarkably circular structure that they had identified as an impact crater." Pope and his colleagues reasoned that the cenotes resulted from fractures in the buried crater's rim and that the area within the cenote ring corresponded with the crater's floor. Further studies by other researchers of the magnetic and gravity data plus analysis of rocks and ocean sediments published in 1991 helped convince the scientific world that Chicxulub was the site of the impact that sent life on Earth in a new direction, from the age of dinosaurs to the age of mammals. Scientists continue to comb through the clues the impact has left behind, some of |
which show up best from space. "The classic spaceborne synoptic view," says
Pope, "is what you need to see a large structure like this." Maps of the region's
wetlands, produced by the spaceborne imaging radar-C (SIR-C) mission in 1994,
identified zones of groundwater discharge that correlate with the crater's structure.
Now researchers are getting their first look at detailed, three-dimensional
topographical data from the Shuttle Radar Topography Mission. "This new image gives
us both corroboration of what we expected and also shows up things we haven't
seen before," says Kinsland. "We'll be working to get as much out of the data as
possible. Anything we learn at the surface tells us more about the buried crater."
Much about Chicxulub remains mysterious. "We don't know exactly how the impact
caused the mass extinctions," says Pope. "We believe it did, but we don't know what
the "kill mechanism" was. One theory is that the impact threw up so much dust into the
atmosphere than it obscured the Sun and stopped plants from growing. Another is
that the sulfur released by the impact lead to global sulfuric acid clouds that also
blocked the Sun and fell as acid rain. Global wild fires triggered by the atmospheric
reentry of red-hot debris from the impact are another possibility.
Pope, "is what you need to see a large structure like this." Maps of the region's
wetlands, produced by the spaceborne imaging radar-C (SIR-C) mission in 1994,
identified zones of groundwater discharge that correlate with the crater's structure.
Now researchers are getting their first look at detailed, three-dimensional
topographical data from the Shuttle Radar Topography Mission. "This new image gives
us both corroboration of what we expected and also shows up things we haven't
seen before," says Kinsland. "We'll be working to get as much out of the data as
possible. Anything we learn at the surface tells us more about the buried crater."
Much about Chicxulub remains mysterious. "We don't know exactly how the impact
caused the mass extinctions," says Pope. "We believe it did, but we don't know what
the "kill mechanism" was. One theory is that the impact threw up so much dust into the
atmosphere than it obscured the Sun and stopped plants from growing. Another is
that the sulfur released by the impact lead to global sulfuric acid clouds that also
blocked the Sun and fell as acid rain. Global wild fires triggered by the atmospheric
reentry of red-hot debris from the impact are another possibility.
| SIR-C data of Yucatan impact crater site |
Also unresolved is whether Chicxulub is the result of a collision with a comet or an asteroid. "There are
arguments on both sides," says JPL's Dr. Don Yeomans, who manages NASA's Near-Earth Object
Program Office. "There are far more asteroids in Earth's orbital vicinity than comets," says Yeomans, "but
most are much smaller than the 10-kilometer size (6.2-mile) of the one that hit Chicxulub. When you get to
that size range, then comets are about as prevalent. Most people say it was an asteroid, but it is still not
altogether clear. Whatever it was, once you're hit with something that large, it's horrendous.
JPL/Rosemary Sullivant
arguments on both sides," says JPL's Dr. Don Yeomans, who manages NASA's Near-Earth Object
Program Office. "There are far more asteroids in Earth's orbital vicinity than comets," says Yeomans, "but
most are much smaller than the 10-kilometer size (6.2-mile) of the one that hit Chicxulub. When you get to
that size range, then comets are about as prevalent. Most people say it was an asteroid, but it is still not
altogether clear. Whatever it was, once you're hit with something that large, it's horrendous.
JPL/Rosemary Sullivant
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