From Harvard Gazette (US) : “How asteroid and comet strikes may have delayed evolution of atmosphere”

From Harvard Gazette (US)


Harvard University (US)

October 22, 2021
Juan Siliezar

Artist rendering of an asteroid entering Earth’s atmosphere. Photo illustration by Dzika Mowka/iStock/The National Aeronautics and Space Agency (US).

Study by Harvard professor’s team finds collisions that stalled oxygen growth on planet more common than thought.

Between 2.5 and 4 billion years ago, a time known as the Archean eon, Earth’s weather could often be described as cloudy with a chance of asteroid.

Back then, it was not uncommon for asteroids or comets to hit Earth. In fact, the largest ones, more than six miles wide, altered the chemistry of the planet’s earliest atmosphere. While this has all been generally accepted by the geologists, what hasn’t been as well understood is how often these large asteroids would hit and how exactly the fallout from the impacts affected the atmosphere, specifically oxygen levels. A team of researchers now believe they have some of the answers.

In a new study, Nadja Drabon, a Harvard assistant professor of Earth and planetary sciences, was part of a team that analyzed remnants of ancient asteroids and modeled the effects of their collisions to show that the strikes took place more often than previously thought and may have delayed when oxygen started accumulating on the planet. The new models can help scientists understand more precisely when the planet started its path toward becoming the Earth we know today.

“Free oxygen in the atmosphere is critical for any living being that uses respiration to produce energy,” Drabon said. “Without the accumulation of oxygen in the atmosphere we would probably not exist.”

The work is described in Nature Geoscience and was led by Simone Marchi, a senior research scientist at The Southwest Research Institute (US).

The researchers found existing planetary bombardment models underestimate how frequently asteroids and comets would hit Earth. The new, higher collision rate suggest impactors hit the planet roughly every 15 million years, about 10 times higher than current models.

The scientists realized this after analyzing records of what appear to be ordinary bits of rock. They are actually ancient evidence, known as impact spherules, that formed in the fiery collisions each time large asteroids or comets struck the planet. The energy from each impact melted and vaporized the rocky materials in the Earth’s crust, shooting them up in a giant plume. Small droplets of molten rock in that cloud would then condense and solidify, falling back to Earth as sand-sized particles that would settle back onto the Earth’s crust. These ancient markers are hard to find since they form layers in the rock that are usually only about an inch or so.

“You basically just go on long hikes and you look at all the rocks you can find because the impact particles are so tiny,” Drabon said. “They’re really easily missed.”

Scientists, however, have caught breaks. “Over the last couple of years, evidence for a number of additional impacts have been found that hadn’t been recognized before,” Drabon said.

These new spherule layers increased the total number of known impact events during the early Earth. This allowed the Southwest Research Institute team to update their bombardment models to find the collision rate had been underestimated.

Researchers then modeled how all these impacts would have influenced the atmosphere. They essentially found that the accumulated effects of meteorite impacts by objects larger than six miles probably created an oxygen sink that sucked most of the oxygen out of the atmosphere.

The findings align with the geological record, which shows that oxygen levels in the atmosphere varied but stayed relatively low in the early Archean eon. This was the case until around 2.4 billion years ago, during the tail end of the time period when the bombardment slowed down. The Earth then went through a major shift in surface chemistry triggered by the rise of oxygen levels known as the Great Oxidation Event.

“As time went on, collisions become progressively less frequent and too small to be able to significantly alter post-GOE oxygen levels,” Marchi said in a statement. “The Earth was on its course to become the current planet.”

Drabon said next steps in the project include putting their modeling work to the test to see what they can model in the rocks themselves.

“Can we actually trace in the rock record how the oxygen was sucked out of the atmosphere?” Drabon wondered.

See the full article here .

See also from SWRI here.


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Harvard University campus

Harvard University (US) is the oldest institution of higher education in the United States, established in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. It was named after the College’s first benefactor, the young minister John Harvard of Charlestown, who upon his death in 1638 left his library and half his estate to the institution. A statue of John Harvard stands today in front of University Hall in Harvard Yard, and is perhaps the University’s bestknown landmark.

Harvard University (US) has 12 degree-granting Schools in addition to the Radcliffe Institute for Advanced Study. The University has grown from nine students with a single master to an enrollment of more than 20,000 degree candidates including undergraduate, graduate, and professional students. There are more than 360,000 living alumni in the U.S. and over 190 other countries.

The Massachusetts colonial legislature, the General Court, authorized Harvard University (US)’s founding. In its early years, Harvard College primarily trained Congregational and Unitarian clergy, although it has never been formally affiliated with any denomination. Its curriculum and student body were gradually secularized during the 18th century, and by the 19th century, Harvard University (US) had emerged as the central cultural establishment among the Boston elite. Following the American Civil War, President Charles William Eliot’s long tenure (1869–1909) transformed the college and affiliated professional schools into a modern research university; Harvard became a founding member of the Association of American Universities in 1900. James B. Conant led the university through the Great Depression and World War II; he liberalized admissions after the war.

The university is composed of ten academic faculties plus the Radcliffe Institute for Advanced Study. Arts and Sciences offers study in a wide range of academic disciplines for undergraduates and for graduates, while the other faculties offer only graduate degrees, mostly professional. Harvard has three main campuses: the 209-acre (85 ha) Cambridge campus centered on Harvard Yard; an adjoining campus immediately across the Charles River in the Allston neighborhood of Boston; and the medical campus in Boston’s Longwood Medical Area. Harvard University (US)’s endowment is valued at $41.9 billion, making it the largest of any academic institution. Endowment income helps enable the undergraduate college to admit students regardless of financial need and provide generous financial aid with no loans The Harvard Library is the world’s largest academic library system, comprising 79 individual libraries holding about 20.4 million items.

Harvard University (US) has more alumni, faculty, and researchers who have won Nobel Prizes (161) and Fields Medals (18) than any other university in the world and more alumni who have been members of the U.S. Congress, MacArthur Fellows, Rhodes Scholars (375), and Marshall Scholars (255) than any other university in the United States. Its alumni also include eight U.S. presidents and 188 living billionaires, the most of any university. Fourteen Turing Award laureates have been Harvard affiliates. Students and alumni have also won 10 Academy Awards, 48 Pulitzer Prizes, and 108 Olympic medals (46 gold), and they have founded many notable companies.


Harvard University (US) was established in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. In 1638, it acquired British North America’s first known printing press. In 1639, it was named Harvard College after deceased clergyman John Harvard, an alumnus of the University of Cambridge(UK) who had left the school £779 and his library of some 400 volumes. The charter creating the Harvard Corporation was granted in 1650.

A 1643 publication gave the school’s purpose as “to advance learning and perpetuate it to posterity, dreading to leave an illiterate ministry to the churches when our present ministers shall lie in the dust.” It trained many Puritan ministers in its early years and offered a classic curriculum based on the English university model—many leaders in the colony had attended the University of Cambridge—but conformed to the tenets of Puritanism. Harvard University (US) has never affiliated with any particular denomination, though many of its earliest graduates went on to become clergymen in Congregational and Unitarian churches.

Increase Mather served as president from 1681 to 1701. In 1708, John Leverett became the first president who was not also a clergyman, marking a turning of the college away from Puritanism and toward intellectual independence.

19th century

In the 19th century, Enlightenment ideas of reason and free will were widespread among Congregational ministers, putting those ministers and their congregations in tension with more traditionalist, Calvinist parties. When Hollis Professor of Divinity David Tappan died in 1803 and President Joseph Willard died a year later, a struggle broke out over their replacements. Henry Ware was elected to the Hollis chair in 1805, and the liberal Samuel Webber was appointed to the presidency two years later, signaling the shift from the dominance of traditional ideas at Harvard to the dominance of liberal, Arminian ideas.

Charles William Eliot, president 1869–1909, eliminated the favored position of Christianity from the curriculum while opening it to student self-direction. Though Eliot was the crucial figure in the secularization of American higher education, he was motivated not by a desire to secularize education but by Transcendentalist Unitarian convictions influenced by William Ellery Channing and Ralph Waldo Emerson.

20th century

In the 20th century, Harvard University (US)’s reputation grew as a burgeoning endowment and prominent professors expanded the university’s scope. Rapid enrollment growth continued as new graduate schools were begun and the undergraduate college expanded. Radcliffe College, established in 1879 as the female counterpart of Harvard College, became one of the most prominent schools for women in the United States. Harvard University (US) became a founding member of the Association of American Universities in 1900.

The student body in the early decades of the century was predominantly “old-stock, high-status Protestants, especially Episcopalians, Congregationalists, and Presbyterians.” A 1923 proposal by President A. Lawrence Lowell that Jews be limited to 15% of undergraduates was rejected, but Lowell did ban blacks from freshman dormitories.

President James B. Conant reinvigorated creative scholarship to guarantee Harvard University (US)’s preeminence among research institutions. He saw higher education as a vehicle of opportunity for the talented rather than an entitlement for the wealthy, so Conant devised programs to identify, recruit, and support talented youth. In 1943, he asked the faculty to make a definitive statement about what general education ought to be, at the secondary as well as at the college level. The resulting Report, published in 1945, was one of the most influential manifestos in 20th century American education.

Between 1945 and 1960, admissions were opened up to bring in a more diverse group of students. No longer drawing mostly from select New England prep schools, the undergraduate college became accessible to striving middle class students from public schools; many more Jews and Catholics were admitted, but few blacks, Hispanics, or Asians. Throughout the rest of the 20th century, Harvard became more diverse.

Harvard University (US)’s graduate schools began admitting women in small numbers in the late 19th century. During World War II, students at Radcliffe College (which since 1879 had been paying Harvard University (US) professors to repeat their lectures for women) began attending Harvard University (US) classes alongside men. Women were first admitted to the medical school in 1945. Since 1971, Harvard University (US) has controlled essentially all aspects of undergraduate admission, instruction, and housing for Radcliffe women. In 1999, Radcliffe was formally merged into Harvard University (US).

21st century

Drew Gilpin Faust, previously the dean of the Radcliffe Institute for Advanced Study, became Harvard University (US)’s first woman president on July 1, 2007. She was succeeded by Lawrence Bacow on July 1, 2018.