The asteroid that slammed
into Earth 66 million years ago left behind more than a legacy of mass
destruction. That impact also sent superheated seawater swirling through the
crust below for more than a million years, chemically overhauling the rocks. Similar transformative hydrothermal systems, left in the wake of powerful impacts much earlier in
Earth’s history, may have been a crucible for early microbial life on Earth,
researchers report May 29 in Science Advances.

The massive Chicxulub crater
on Mexico’s Yucatán peninsula is the fingerprint of a killer, probably responsible for the destruction of more
than 75 percent of life on Earth, including all nonbird dinosaurs (SN: 1/25/17).
In 2016, a team of scientists made a historic trek to the partially submerged crater,
drilling deep into the rock to study the crime scene from numerous angles.

One of those researchers was
planetary scientist David Kring of the Lunar and Planetary Institute in
Houston. A dozen years earlier, Kring had found evidence at Chicxulub that the
layers of rock bearing the signs of impact — telltale features such as shocked
quartz and melted spherules — were subsequently cut through by veins of newer minerals
such as quartz and anhydrite. Such veins, Kring thought, suggest that hot
hydrothermal fluids had been circulating beneath Chicxulub some time after the

Hydrothermal systems can
occur where Earth is tectonically active, such as where tectonic plates pull the
seafloor apart, or where mantle plumes like the one beneath Yellowstone rise up
into the crust. The molten rock rising through the crust in these regions superheats
water already circulating within the crust.

But the Yucatán peninsula is
tectonically quiescent, and has been for 66 million years, Kring says. So, as
part of the International Ocean Discovery Program’s Expedition 364 to
Chicxulub, he and colleagues drilled 1,335 meters below the ring of the crater,
retrieving long cores of sediment and rock.

The team then analyzed the
minerals found in the cores. “It was immediately obvious that they had been
hydrothermally altered. It was pervasive and apparent,” Kring says. The intense
heat of the circulating seawater caused chemical reactions within the rock,
transforming some minerals into others. By identifying the different types of
minerals, the team determined that the initial temperature of the fluids was
more than 300° Celsius,
later cooling to about 90°

The chemically altered rocks
beneath the crater extended down about four or five kilometers below the crater’s
peak ring, a circular, mountainous region within the vast crater. The
hydrothermally altered zone covers a volume more than nine times that of the Yellowstone
Caldera system, Kring says. Paleomagnetic data suggest that the hydrothermal system
lasted for more than a million years.

Rock core
A core of rock and sediment extracted from within the Chicxulub impact crater revealed centimeter-sized cavities within the rocks containing hydrothermally altered minerals. Here, tiny cavities within impact breccia — a type of rock formed of broken fragments cemented together by fine-grained sediment — contain analcime (transparent crystals), which forms at temperatures around 200° Celsius and dachiardite (red crystals), which forms at temperatures around 250° C.D. Kring

Those conditions, the
researchers say, may have also been capable of fostering life akin to the
extremophiles that thrive in Yellowstone’s boiling pools. In addition to the
metal-rich fluids that could provide an energy source for microbes, the
Chicxulub cores revealed that the rocks were both porous and permeable — in
other words, filled with interconnected nooks and crannies that could have been
cozy shelters for microbes.

“It looks like a perfect
habitat,” Kring says.

Kring has previously
suggested that the very same destructive impacts that annihilate life may also
create appealing habitats — not just on Earth, but potentially on other
planetary bodies such as Mars. Even more tantalizing is the possibility that
hydrothermal systems, engendered beneath ancient impacts, may have been where life on Earth began (SN: 3/1/13).

Evidence from lunar craters
suggests that Earth was heavily bombarded by asteroids about 3.9 billion years ago (SN: 10/18/04). Most of those more ancient craters on Earth have long since vanished or been altered by
the constant tectonic recycling of Earth’s surface (SN: 12/18/18). So
the hydrothermal system beneath Chicxulub offers a window into what such
systems might have actually looked like much deeper in the past, says
geophysicist Norman Sleep of Stanford University, who was not involved in the
study. “It shows the reality of the process,” Sleep says.

The new study may set the
stage for the possibility of life thriving beneath an impact. But whether a
microbial cast of characters was actually present beneath Chicxulub is a
question for future studies, Kring says.

“Let me be clear: This paper
has no evidence of microbial life,” Kring says. “We just have all the
properties of hydrothermal systems that do support life elsewhere on Earth.”

Ancient environments that
provided water, chemical building blocks and energy “are very promising
candidates for hosting [life’s] origins and early evolution,” says NASA
astrobiologist David Des Marais, who was not involved in the study.
Impact-generated hydrothermal systems aren’t the only such environments;
researchers have also made a compelling case for hot springs, Des Marais says.

That’s an ongoing debate, he
notes, adding “I consider hydrothermal systems to be highly promising
exploration targets for astrobiology.”

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