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Was Mars ever habitable? A new study sheds light on the planet's past

Despite ongoing efforts to find ancient life on Mars, Martian meteorites tell a different story.

The search for life beyond Earth may be at stake. A new study suggests that Mars may not have been habitable after all.

The reason? Its small size.

Despite previous evidence that the Red Planet once hosted lakes, rivers, and other possible bodies of water, analysis of Martian meteorites shows that Mars may have had a much drier past than scientists believed.

As NASA’s Perseverance rover scours the Martian terrain to look for clues of ancient microbial life, the recent research could mean that the rover’s search will turn up empty and our hunt for life beyond Earth may suffer a major setback.

“Mars is likely ‘drier’ than we previously thought,” Zhen Tian, a Ph.D. candidate at Washington University and author on the paper, tells Inverse. “Therefore, the likelihood for a habitable Mars is limited.”

The study was published Monday in the journal Proceedings of the National Academies of Sciences.

WHAT’S NEW — The team of astronomers behind the new study examined 20 Martian meteorites that traveled 33.9 million miles before crash landing on Earth. The meteorites ranged between 200 million years to 4 billion years old.

This illustration shows what Mars may have looked like if it once had liquid water similar to Earth’s on its surface.NASA Earth Observatory/Joshua Stevens; NOAA National Environmental Satellite, Data, and Information Service; NASA/JPL-Caltech/USGS; Graphic design by Sean Garcia/Washington University

The team measured the amount of potassium isotopes in the meteorites. Potassium is a volatile chemical, or one that vaporizes easily rather than melts, leaving telltale chemical clues behind. Volatile chemicals include nitrogen, carbon dioxide, hydrogen, methane, and others, and are considered to be essential to life on Earth.

The levels of volatile elements in the meteorites suggest that Mars lost more potassium and other volatiles than Earth during its formation, but still retained more volatiles than Earth’s moon and asteroid Vesta (one of the largest asteroids in the asteroid belt).

“We found a well-defined correlation between (potassium) isotopic composition and body sizes among four bodies in the inner Solar System,” Tian says

This suggests that size comes into play when it comes to maintaining volatiles such as water during a planet’s formation. Mars is only slightly larger than half the size of Earth, while the Moon is a bit more than one-fourth of Earth’s size.

Smaller planets may lose more water during their early history, as well as their global magnetic field — both of which happened on Mars. Earth’s global magnetic field protects its atmosphere from solar radiation, which also helps preserve its water.

WHY IT MATTERS — There have been around 50 missions sent to the Red Planet so far, many hunting for signs of past or present life. These missions have delivered some promising evidence that Mars may have once been warm and wet, and potentially habitable.

The first piece of evidence came in 1976 when NASA’s Viking 1 Lander touched down on Mars. Viking found ancient river beds and vast flooding, as well as all of the elements that are essential to life on Earth, such as carbon, nitrogen, hydrogen, oxygen, and phosphorus, but the exact amounts are not well understood.

The Viking lander captured this image of the Martian surface right before sunset, giving humanity its first look at the Red Planet.NASA

On February 18, 2020, the Perseverance rover landed on Mars with the unique task of collecting Martian rock and stowing them away for a future return mission to Earth where they will be examined in a lab.

The mission could finally provide scientists with the answers they are looking for in terms of Mars’ habitability and finding life outside of our planet.

The study does not dispute that Mars ever had water on its surface. But it does put into question just how much water the Red Planet did have, and if it was enough for life to thrive.

“Mars may be just too small to hold onto large amounts of water in the first place. Its fate had been decided from the beginning,” Kun Wang, a planetary science professor at Washington University and author on the study, tells Inverse.

WHAT’S NEXT — Although this study may put a damper on finding Martian life, it does assist scientists in finding life on other planets.

The new findings now put size as a matter of consideration when questioning the habitability of a planet as it defines an average size for water to exist on a planet. And being that size is one of the easiest factors to determine about a planet, the study could find life beyond Earth and even Mars.

“Although the search for life on Mars might turn up empty, our new findings would potentially help us constrain the ‘Goldilocks’ zone for life among exoplanets based on body size along with the other parameters that play important roles in developing a habitable environment,” Tian says. “Planetary body size ultimately controls the retention of volatiles, including life-sustaining elements and compounds such as water.”

Abstract: The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent “Perseverance” and “Tianwen-1” missions, “follow the water,” and, more broadly, “volatiles,” has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average “bulk silicate” δ41K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na “volatility” ratio, and most notably, bulk planet H2O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo) planets to retain enough H2O to enable habitability and plate tectonics, with mass exceeding that of Mars.
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