June 10, 2023
Mars could have been hot and humid while Earth was still a glowing ball of molten rock

Mars could have been hot and humid while Earth was still a glowing ball of molten rock

Since the 1970s, continued exploration of Mars has revealed that the planet has had a more interesting history. While conditions there are inhospitable to life today, scientists know that Mars was once a much warmer, wetter place, with running water on its surface. According to new research led by the University of Arizona (UoA), Mars may have been a “pale blue dot” covered in oceans while Earth was still a slowly cooling ball of molten rock. This discovery could enable new research into a previously overlooked period in the geological history of Mars and the formation and evolution of the Solar System.

The team was led by Kaveh Pahlevan, a researcher at ASU’s School of Earth & Space Exploration (SESE) and the Carl Sagan Center SETI Institute. He was joined by Laura Schaefer, assistant professor of Geological Sciences at Stanford University. Linda T. Elkins-Tanton, professor of planetary sciences and Director of ASU’s SESE; SESE Professor of Astrophysics Steven J.Desch and ASU-SESE. and Peter R. Buseck, Regents Professor at SESE and the ASU School of Molecular Sciences (SMS).

This low-angle self-portrait of NASA’s Curiosity Mars rover shows the vehicle at the point from which it reached down to drill into a rock target called “Buckskin” on lower Mount Sharp. Credits: NASA/JPL-Caltech/MSSS

The paper describing their findings, titled “A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars,” appeared in the Oct. 1 issue of Earth and Planetary Science Lettersmall. Based on multiple lines of evidence obtained from robotic orbiters, landers and rovers, scientists have determined that around 4.2 to 3.7 billion years ago, Mars began to transition from a warmer, wetter planet to the extremely cold and dry environment that see you there today. However, unanswered questions remain about how long liquid water has flowed on the Martian surface and whether it was intermittent or consistent.

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Mars Primordial Atmosphere

To answer this question, astronomers have been trying to reconstruct what the Martian atmosphere was like billions of years ago. A popular method used on Mars missions involves collecting samples and analyzing them for deuterium to hydrogen ratios (D/H or 2H/1H), or the number of deuterium atoms in a sample divided by the number of normal hydrogen atoms. This method allows scientists to measure the prevalence of molecular hydrogen (H) in the Martian atmosphere over time, which is a powerful greenhouse gas. As Professor Desch said in an ASU news release:

“It is paradoxical that so many observations point to liquid water on early Mars, even though water freezes on present-day Mars and the ancient sun was 30% dimmer than today. Traditionally considered greenhouse gases such as CO2 would freeze on an early Mars. Hydrogen in the atmosphere is an unexpected way to stabilize liquid water.”

For their study, the team developed the first model of early atmospheric evolution on Mars that included high-temperature processes associated with different geological periods. This included the formation of Mars, the time when its surface was covered by a magma ocean, and the formation of the first oceans and atmosphere. These models showed that the main gases emerging from the molten rock were a mixture of molecular hydrogen and water vapor, and that the early Martian atmosphere was much denser than it is today.

Simple life may have thrived on early Mars because of a dense, hydrogen-rich atmosphere. Credit: ESO/M. Kornmesser

Their model also showed that water vapor in the Martian atmosphere behaves similarly to the way it does in Earth’s atmosphere today. Essentially, it would condense in the lower atmosphere as clouds, while very little would be retained in the upper atmosphere. Meanwhile, molecular hydrogen (the main component of the atmosphere) did not condense and was slowly lost to space. They further calculated that the molecular hydrogen content of the atmosphere would have a significant greenhouse effect, to the point that Mars could have warm (or even warm) water oceans.

These oceans were stable and would have remained on the Martian surface for many centuries before atmospheric hydrogen was gradually lost to space. As Dr. Pahlevan explained:

“This basic idea—that water vapor condenses and is held on early Mars, while molecular hydrogen does not condense and can escape—allows the model to be directly linked to measurements made by spacecraft, namely the Mars Science Laboratory’s Curiosity rover. This is the first model to physically reproduce these observations, giving us some confidence that the evolutionary scenario we have described corresponds to the earliest events on Mars.”

Implications for life

The results are consistent with clay samples received by NASA Rover curiosity was revealed for the Hesperian Epoch (about 3.7 – 2.9 billion years ago) and reinforced what previous studies of Martian meteorites had shown. Martian meteorites are largely composed of igneous (ie, volcanic) rocks that formed inside Mars and were ejected from magma rising to the surface. These meteorites contain water dissolved in the interior and are found to have D/H ratios similar to Earth’s oceans. This indicates that Earth and Mars got their water from the same source during the early Solar System.

The Curiosity rover takes an image of the location where it took a drill sample. Credits: NASA/Jet Propulsion Laboratory, Caltech

In addition, research conducted by Dr. Pahlevan and colleagues showed that if the primordial atmosphere of Mars was dense and rich in hydrogen, surface waters would naturally be enriched in deuterium by a factor of two to three compared to the interior. With this the clay samples of Hesperia were obtained Curiosity showed, which was a D/H value about three times that of Earth’s oceans. The only explanation is that molecular hydrogen was lost in the time between when Mars was still forming (about 4.5 billion years ago) and the Hesperian Age.

As the heaviest element, deuterium was lost at a slower rate, leading to the observed levels of enrichment in surface waters. These findings could also have implications for the ongoing search for evidence of past life on Mars (which may still exist underground today). These include the Stanley-Miller experiments dating back to the mid-20th century, which showed that prebiotic molecules formed more readily in hydrogen-rich “reducing” atmospheres than in “oxidizing” atmospheres – like those on Earth and Mars today.

In recent years, planetary scientists have also shown that atmospheric hydrogen can play a critical role in habitability and expand a planet’s habitable zone. These findings suggest that ancient Mars had an environment that was just as amenable to early life as Earth. Perhaps even more so, since the Earth did not fully form until after the massive impact that formed the Moon (Theia) 4.5 billion years ago. While the Earth-Moon system was still covered in molten magma, Mars had a thick atmosphere, warm temperatures, and a surface covered in blue oceans.

Further reading: ASU

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