Today, Mars is colloquially known as the “Red Planet” on a count of how its dry, dusty landscape is rich in iron oxide (aka. “rust”). In addition, the atmosphere is extremely thin and cold, and no water can exist on the surface in any form other than ice. But as the Martian landscape and other lines of evidence attest, Mars was once a very different place, with a warmer, denser atmosphere and flowing water on its surface. For years, scientists have attempted to determine how long natural bodies existed on Mars and whether or not they were intermittent or persistent.
Another important question is how much water Mars once had and whether or not this was enough to support life. According to a new study by an international team of planetary scientists, Mars may have had enough water 4.5 billion years ago to cover it in a global ocean up to 300 meters (almost 1,000 feet) deep. Along with organic molecules and other elements distributed throughout the Solar System by asteroids and comets at this time, they argue, these conditions indicate that Mars may have been the first planet in the Solar System to support life.
The study was conducted by researchers from the University of Paris’ Institut de Physique du Globe de Paris (IPGP), the University of Copenhagen’s Centre for Star and Planet Formation (StarPlan), the Institute of Geochemistry and Petrology (GeoPetro) at ETH Zürich, and the University of Bern Physics Institute. The paper that describes their research and findings recently appeared in Science Advances. As they indicate in their paper, the terrestrial planets endured a period of significant asteroid impacts (the Late Heavy Bombardment) following their formation over 4.5 billion years ago.
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These impacts are believed to be how water and the building blocks for life (organic molecules) were distributed throughout the Solar System. However, the role of this period in the evolution of rocky planets in the inner Solar System – particularly where the distribution of volatile elements like water is concerned – is still debated. For the sake of their study, the international team reported on the variability of a single chromium isotope (54Cr) in Martian meteorites dated to this early period. These meteorites were part of Mars’ crust at the time and were ejected due to asteroid impacts that sent them off into space.
In other words, the composition of these meteorites represents Mars’ original crust before asteroids deposited water and various elements on the surface. Since Mars does not have active plate tectonics like Earth, the surface is not subject to constant convection and recycling. Therefore, meteorites ejected from Mars billions of years ago offer a unique insight into what Mars was like shortly after the planets of the solar system formed. As co-author Professor Bizzarro from the StarPlan Center said in a UCPH faculty press release:
“Plate tectonics on Earth erased all evidence of what happened in the first 500 million years of our planet’s history. The plates constantly move and are recycled back and destroyed into the interior of our planet. In contrast, Mars does not have plate tectonics such that planet’s surface preserves a record of the earliest history of the planet.”
By measuring the variability of 54Cr in these meteorites, the team estimated the impact rate for Mars ca. 4.5 billion years ago and how much water they delivered. According to their results, there would have been enough water to cover the entire planet in an ocean at least 300 meters in depth (~1000 feet) and up to 1 km (0.62 mi) deep in some areas. In comparison, there was very little water on Earth at this time because a Mars-sized object had collided with Earth, leading to the formation of the Moon (i.e., the Grand Impact Hypothesis).
In addition to water, asteroids also distributed organic molecules like amino acids (the building blocks of DNA, RNA, and protein cells) to Mars during the Late Heavy Bombardment. As Bizarro explained, this means that life could have existed on Mars when Earth was sterile:
“This happened within Mars’s first 100 million years. After this period, something catastrophic happened for potential life on Earth. It is believed that there was a gigantic collision between the Earth and another Mars-sized planet. It was an energetic collision that formed the Earth-Moon system and, at the same time, wiped out all potential life on Earth.”
This study is similar to recent research that used the deuterium-to-hydrogen ratios of Martian meteorites to create models of atmospheric evolution. Their findings showed that Mars may have been covered in oceans when Earth was still a molten ball of rock. These and other questions related to Mars’ geological and environmental evolution will be investigated further by robotic missions destined for Mars in this decade (followed by crewed missions in the 2030s).
Further Reading: University of Copenhagen