In the mid-1980s, a group of American archaeologists pored over satellite images trying to understand what had become of the Mayan civilization that had once ruled over Mexico’s Yucatan Peninsula, discovered a pattern: a near-perfect ring of sinkholes -cenotes- about 200 kilometers across, encircling the Yucatecan capital, Merida, and port towns of Sisal and Progreso. A pattern created by an ancient asteroid explosion that one young NASA scientist thought may yield clues to the lost ocean and atmosphere of Mars.
Defining Event in the History of Planet Earth
When the researchers presented their findings to fellow satellite specialists at a scientific conference in Acapulco, Mexico, in 1988, one scientist in the audience, Adriana Ocampo, then a young planetary geologist at NASA, saw not just a huge ring, but a bullseye –the impact crater of an asteroid that hit with the force of 10 billion Hiroshima nuclear bombs that scarred the planet in ways still being revealed 66 million years on. An impact that ended the reign of dinosaurs and opened a pathway for the emergence of the human species. “It gave us a leg up to be able to compete, to be able to flourish, as we eventually did,” she said.
“It was perhaps the defining event in the history of planet Earth,” University of Washington planetary scientist Mark Richards wrote in an email to The Daily Galaxy.
The ‘Aha!’ Moment
Today, the center of the bullseye is buried a kilometer below a tiny town called Chicxulub Puerto. “As soon as I saw the slides that was my ‘Aha!’ moment. ‘This is something amazing, ‘This could be it’,” Ocampo, later director of NASA’s Lucy mission, told the BBC. “I was really excited but I kept cool because obviously you don’t know until you have more evidence. They didn’t even know what I was talking about!” she laughed, three decades later.
The key to her ‘Aha! moment’ had been an intuition she’d picked up after working with a legendary figure in space science, Eugene Shoemaker. Shoemaker is a pioneering American geologist who helped found the field of planetary science and remains the only person whose ashes are buried on the Moon. Shoemaker instructed Ocampo that near perfect circles were unlikely to have been caused by other terrestrial forces, and could provide clues to Earth’s geological development.
Mexico’s Chicxulub Crater is “a natural laboratory because of its similarities to what we can find on other planets like Mars where humans can’t go,” Ocampo said of debris discovered from asteroid impacts on Mars compared with ejecta from the Chicxulub Crater.
Death of Mars?
The satellite images revealed similarities that indicate that Mars must once have had a much thicker atmosphere than it does now – one closer to the atmosphere that supports life on Earth. “It’s important for us to know what happened in the past to be prepared for the future,” Ocampo said. “It provides a really good insight into what has happened in the geological evolution of Mars.”
Today, Mars is a frigid desert world with a carbon dioxide atmosphere 100 times thinner than Earth’s. But evidence suggests that in the early history of our solar system, Mars’ surface likely hosted an ocean as deep as the Mediterranean Sea. As the planet’s atmosphere thinned, however, most of the ocean was lost to space. The remainder of the water is locked in the Martian ice caps.
Mystery of the Two-faced Nature of Mars
Astronomers from UC Santa Cruz, Caltech and MIT proposed that a giant asteroid or comet the size of Pluto, more than 1,200 miles in diameter – sped toward ancient Mars at up to 21,600 miles an hour, crashed at a steep slant into the planet about 3.9 billion years ago, and blasted out the huge elliptical scar measuring 5,300 miles across that now forms all of the planet’s northern lowlands, while leaving the southern highlands relatively undamaged. An impact so big that it has left half the red planet at a lower elevation.
If the theories are right, it blasted out the biggest crater that any planet has ever survived. It was a convulsion far bigger than the one that drove the dinosaurs to extinction on Earth. One region of the surface is the huge oval-shaped scar of the impact itself, covering more than a third of the Martian surface and including all the vast low-lying lands of the planet’s far north. The other is the even larger highland region to the south, marked by deep canyons, high mountains and the remains of giant volcanoes.
NASA’s Mars Reconnaissance Orbiter and Mars Global Surveyor have provided detailed information about the elevations and gravity of the Red Planet’s northern and southern hemispheres. The mystery of the two-faced nature of Mars has perplexed scientists since the first comprehensive images of the surface were beamed home by NASA spacecraft in the 1970s. A giant northern basin that covers about 40 percent of Mars’ surface, sometimes called the Borealis basin, is the remains of a colossal impact early in the solar system’s formation, the new analysis suggests. At 8,500 kilometers (5,300 miles) across, it is about four times wider than the next-biggest impact basin known, the Hellas basin on southern Mars.
An accompanying report calculated that the impacting object that produced the Borealis basin must have been about 2,000 kilometers (1,200 miles) across. That’s larger than Pluto. Researchers speculate that Borealis basin — which spans a size on Mars comparable to the combined areas of the continents of Asia, Europe and Australia– could have once held an Ocean. This would have been in the planets infancy, before Mars lost so much of its atmosphere and the Ocean either sublimated away or froze beneath the surface.
Mars bears the scars of five giant impacts shown in image above, including the ancient giant Borealis basin (top of globe), Hellas (bottom right), and Argyre (bottom left). A NASA-funded team at SwRI discovered that Mars experienced a 400-million-year lull in impacts between the formation of Borealis and the younger basins .(University of Arizona/LPL/Southwest Research Institute).
Scientists have detected frozen water on the surface of the red planet. Martian seas could have disappeared when the planet was bombarded by smaller meteors that changed its atmosphere and dried it out, said Ocampo.
New 2021 findings by NASA’s Mars Curiosity rover, which continues to explore the base of Mount Sharp (officially Aeolis Mons), a mountain several kilometers high at the center of the Gale crater, using the telescope on the ChemCam instrument, has discovered that the Martian climate alternated between dry and wetter periods, before drying up completely about 3 billion years ago, according to a French-US team headed by William Rapin, CNRS researcher at the Institut de Recherche en Astrophysique et Planétologie.
Editor’s Note: this article has been edited with new content and updated on May 15, 2021.
Image top of page: This view of Mars is a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris. NASA/JPL/Malin Space Science Systems
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