Asteroids boiled young Earth’s oceans

It didn’t take long for early life on Earth to get into hot water. Very hot water.

Evidence from ancient rocks suggests that asteroids slammed into our planet around 3.3 billion years ago. The impacts would have released huge amounts of energy. So much energy that this heat baked the skies and boiled the oceans.

These dramatic events would have shaped the evolution of early life, says Donald Lowe. A geologist, he works at Stanford University in Palo Alto, Calif. His team detailed the evidence for this hellish scenario in Geology. It appeared early online May 7.

Life had been around for only about 500 million years when these asteroids hit. At once, things got tough. Following the impacts, air temperatures for weeks would have exceeded 500° Celsius (932° Fahrenheit). The ocean surface would have boiled for more than a year. Global sea levels might have plummeted by as much as 100 meters (333 feet).

Such impacts “would have a profound influence on any life trying to evolve into more complex, low-temperature organisms,” says Lowe. “They’d keep getting whacked by these giant [asteroids] and driven to extinction or near extinction.”

Early Earth was hostile

Asteroids and other space debris left over from the solar system’s creation regularly slammed into Earth early in its history. These massive impacts initially were quite common. Over time, they became smaller and less frequent.

Around 3.3 billion years ago, several large asteroids hit Earth. They had been traveling at about 36,000 kilometers per hour (22,000 miles per hour). Lowe estimates that the massive rocks measured 50 to 100 kilometers (up to 62 miles) across. Each would have dwarfed the roughly 10-kilometer rock blamed for killing off the dinosaurs.

The force of these collisions heated the atmosphere. It vaporized rock. And it sent debris flying. Geologists had speculated that these impacts had global effects. However, until now geologic evidence of any environmental changes had been lacking.

The newfound evidence

Lowe teamed up with geologist Gary Byerly of Louisiana State University in Baton Rouge. In South Africa, they went looking for evidence of those massive collisions, focusing on rock layers that had formed about 3.3 billion years. In them, they found spherical bits of rock about the size of BB pellets. Solidifying rock vapor and debris had sent this big grit raining down following the impacts.

Lowe and Byerly found sediments below and above each layer. This suggests that the location had once been deep underwater. The impact layers, however, show evidence of having eroded in shallower water. That suggests sea levels dropped dramatically after the asteroid impacts. Eventually, the sea levels returned to normal.

The researchers also found the mineral silica coating the impact layer. That indicates that the sea surface would have boiled away. As the liquid turned to gas, any dissolved silica was left behind. The ocean surface had turned into a global hot spring beneath a scorching sky, Lowe now concludes.

The findings are exciting but speculative, says geologist James Day. He works at the Scripps Institution of Oceanography in La Jolla, Calif. No trace of the craters created by the impacts remains. So it’s hard to pinpoint how large the asteroids were, he says.

Understanding the destructive effects of such giant impacts is important. It can help experts understand the environment in which early life lived and adapted, Lowe says. Primitive microbes that conducted photosynthesis probably lived near the ocean surface. These organisms could not have survived temperatures above about 73 °C (163 °F). The only life that could have survived in this boiling world must have been underground, been living deep in the ocean or been capable of coping with high temperatures, Lowe says.

These mass die-offs could have shaped early evolution, says planetary scientist Kevin Zahnle. He works at NASA’s Ames Research Center in Moffett Field, Calif. A common ancestor to all current life is thought to have thrived at hot temperatures. The reason could be that all life forms that preferred cooler conditions died off, he now says. They couldn’t survive the hot aftermath of asteroid impacts. He says: “You can picture these asteroids as a crazed tree trimmer showing up and chopping branches off the tree of life.”

Power Words

(for more about Power Words, click here)

asteroid  A rocky object in orbit around the sun. Most today orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

BB pellets   The pellets used as ammunition in BB guns. The term “BB” comes from the name for the caliber (size) of the pellets used as shot in some 19^th century shotguns. Today, the term has been expanded to include round shot used in a number of pellet guns.

crater  A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body.

dinosaur  A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.

evolution  A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

extinct  An adjective that describes a species for which there are no living members.

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.

microbe  Short formicroorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

mineral The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns). 

National Aeronautics and Space Administration  Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.

oceanography  The branch of science that deals with the physical and biological properties and phenomena of the oceans. People who work in this field are known as oceanographers.

organism  Any living thing, from elephants and plants to bacteria and other types of single-celled life.

photosynthesis  (verb: photosynthesize)The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

sea level  The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

sediment  Material (such as stones and sand) deposited by water, wind or glaciers.

silica  A mineral, also known as silicon dioxide, containing silicon and oxygen atoms. It is a basic building block of much of the rocky material on Earth and of some construction materials, including glass.

solar system  The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

tree of life    A diagram that uses a branched, treelike structure to show how organisms relate to one another. Outer, twiglike, branches represent species alive today. Ancestors of today’s species will lie on thicker limbs, ones closer to the trunk.