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This image was captured while NASA’s Perseverance rover first drove to Mars on March 4, 2021. Credit – NASA / JPL-Caltech
Mars is the world close to the solar system. Earth may have understood it all when it comes to sustaining life – the atmosphere, the water, the proximity to the sun. Mercury, Venus, and the outer planets, with their extreme temperatures and inhospitable chemistry, may have it all wrong. Mars, on the other hand, got so close, but was unsuccessful.
Thanks to data from rovers and other spacecraft, we know that the Red Planet was once quite laden with water, with dry deltas, riverbeds and sea basins etched on its surface. But 4 billion years ago, the Martian core cooled, stopping the dynamo that fed its magnetic field. This made the planet vulnerable to the solar wind, which carried the atmosphere and allowed Martian water to evaporate into space. In a short time, in geological terms, the planet was a desert.
At least that’s been the thought for a long time. But a new article published on September 20 in the Proceedings of the National Academy of Sciences suggests otherwise. According to the new research, Mars was doomed from the start. Its small size – about half the diameter of Earth and less than a ninth the mass – simply never produced the gravitational muscle that allows the planet to hold back its air or water. With or without a magnetic field, Mars was doomed to death.
The study, led by planetologist Zhen Tian of Washington University in St. Louis, was based on the analysis of 20 meteorites whose chemical composition establishes that they came from pieces of Mars. The age of the rocks varies from a few hundred million to 4 billion years. Such a sample comfortably covers Mars’ transition from a humid to a dry world, but even in the oldest rocks there would be no trace of moisture. The passage of time – not to mention the long journey meteorites made into space – would have caused the complete evaporation of a compound as volatile as water.
Instead of water, the researchers looked for isotopic potassium, which was present in abundance on early Mars and can be used as a kind of chemical substitute for water. “Potassium is a moderately volatile element (mimicking the behavior of very volatile elements), but it is not too volatile to be completely lost,” wrote Kun Wang, a planetologist at the University of Washington and one of the study authors, in an email to TIME. “It’s easier to measure with great precision. The more isotopic potassium the researchers found in the rocks, the more water there would have been.
Turns out there wouldn’t have been much. The age of the rocks and the level of potassium they contained indicated that not only did the water not wait for the magnetic field to stop and the atmosphere to flow before also disappearing; it actually began to disappear even before the once-melted planet had cooled completely.
“The total budget for water and other volatile substances was set during the formation of Mars,” says Wang. “The loss of water and volatiles to the planet has occurred at its hot and violent stage.” This step, he adds, was “much earlier” than stopping the magnetic field.
The researchers did not limit their study to Mars only. By comparing the amount of potassium in meteorites to the known water on the moon and to the 525 km (326 mi) wide asteroid Vesta, they found a kind of staircase in the amount of water that directly follows gravity and mass: Mars is drier than Earth, but wetter than the smaller moon, which in turn is wetter than the tiny Vesta.
This has implications for the possibility of finding life not only on Mars but on exoplanets, worlds orbiting other stars. Astronomers generally look for small, rocky planets like Earth as the most likely places where biology could emerge, especially if these planets are in the so-called “Goldilocks zone” around their stars – a place where temperatures are no longer. neither too hot nor too cold for liquid water to exist. But a planet too small, it now seems, would be dry regardless of the local temperature.
“There is likely a threshold on the size requirements of rocky planets to hold enough water to allow habitability,” Wang said in a statement accompanying the document.
The new study does not entirely rule out the possibility of a Martian biology. The once humid world has not lost all of its water in space. Some are drawn into permafrost at the poles, and others may have retreated underground, into aquifers where the water would be liquid at least during parts of the Martian year. Microbial life forms that emerged in the earlier, wetter era of the planet may have taken refuge there since. Mars was never going to be a garden world like Earth, but that doesn’t mean it can’t move yet.
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