Mars haunts us as a vision of a planet gone wrong. It was once warm and wet, with rivers flowing across its surface and (potentially) simple life residing in its water bodies. Now it’s dry and freezing.
Could Earth suffer this fate? Are there innumerable other worlds throughout the Universe that were habitable for a period of time before becoming uninhabitable?
To answer those questions, we have to answer one of the big questions in space science: What drove the changes on Mars? New research shows that hydrogen played a critical role in keeping ancient Mars warm for periods of time, as the planet’s temperature oscillated between warm and cold.
The research is “Episodic warm climates on early Mars primed by crustal hydration.” It’s published in Nature Geoscience, and the lead author is Danica Adams, a postdoctoral fellow in the Department of Earth and Planetary Sciences at Harvard University.
“Early Mars is a lost world, but it can be reconstructed in great detail if we ask the right questions.”
Robin Wordsworth, Harvard University.
There’s ample evidence of flowing surface water on ancient Mars. NASA’s Perseverance rover is exploring Jezero Crater, an ancient paleolake with deep sediment deposits carried there by flowing water. Satellite views show numerous ancient river channels. There’s also clear evidence of ancient lakes.
For a long time, the dominant scientific thought was that Mars was once warm and then became cold. In recent years, more thorough evidence suggests that Mars oscillated between being a warm and a cold planet.
If that’s true, what drove those oscillations?
The first difficulty in explaining early warm periods on Mars is the faint young Sun paradox. Astrophysicists calculate that the young Sun emitted only 70% of the energy it does now. How could Mars have had liquid surface water with so little solar output?
“It’s been such a puzzle that there was liquid water on Mars, because Mars is further from the sun, and also, the sun was fainter early on,” said lead author Danica Adams in a press release.
Evidence suggests that Mars once had enough water for an equivalent global ocean from 100 m to 1,500 m deep during the planet’s late Noachian period. Scientists have found hundreds of lakebeds from the Noachian, some as large as the Caspian Sea. However, the planet is suspected to have been too cold to host this much liquid water without a more efficient heat-trapping atmosphere. CO2 alone couldn’t do it, but researchers think that a more hydrogen-rich atmosphere could.
The problem is that hydrogen doesn’t tend to persist in atmospheres.
“Greenhouse gases such as H2 in a CO2-rich atmosphere could have contributed to warming through collision-induced absorption, but whether sufficient H2 was available to sustain warming remains unclear,” the authors write in their paper. Collision-induced absorption (CIA) is when molecules in a gas collide, and interactions from the collision allow molecules to absorb light. CIA could amplify the atmospheric CO2’s warming effect.
If there was a hydrogen source that allowed the atmosphere to replenish itself, that could explain how Mars oscillated between cold and dry and warm and wet. The researchers used a combined photochemical and climate model to understand how the atmosphere responded to climate variations and reactions between H2O and rock.
“Early Mars is a lost world, but it can be reconstructed in great detail if we ask the right questions,” said study co-author Robin Wordsworth from Harvard. “This study synthesizes atmospheric chemistry and climate for the first time to make some striking new predictions – which are testable once we bring Mars rocks back to Earth.”
The team’s research showed that early Mars had two distinct climate states that persisted for long timescales. The warm climate sustained surface liquid water and lasted between 100,000 and 10 million years. These periods were created and sustained by H2 from crustal hydration with some help from volcanic activity. During crustal hydration, water is lost to the ground, and H2 is released into the atmosphere. The cool climate lasted about 10 million years and featured a CO-dominated atmosphere caused by oxidant sinks in the planet’s surface.
“We find that H2
The team’s models showed that Mars’ climate oscillated like this for about 40 million years during the Noachian and Hesperian periods. Each warm period lasted at least 100,000 years. According to the researchers, these timescales are in agreement with the length of time it took to carve Mars’ river valleys.
The planet’s atmospheric chemistry fluctuated during these periods. As sunlight struck CO2, it was converted to CO. During warm periods, the CO cycled back into CO2, and CO2 and H2 were dominant.
During cold periods, the CO recycling slowed down, CO built up in the atmosphere, and it triggered a more oxygen-reduced state. In this way, the redox state of the atmosphere oscillated dramatically over time.
“We’ve identified time scales for all of these alternations,” Adams said. “And we’ve described all the pieces in the same photochemical model.”
Mars’s modern-day surface supports the researchers’ alternating atmospheric redox hypothesis. The surface shows a “paucity of carbonates,” the researchers explain in their paper. These should form in an atmosphere dominated by CO2 where neutral pH water is present, as long as there is abundant open-system crustal alteration at the planet’s surface. Adams and her co-researchers say their hypothesis can explain the lack of carbonates.
Carbonates were first detected on Mars in 2008, and scientists expected to find large deposits of them. However, those large deposits were never found. If early Mars had abundant water for a long time, there would be abundant carbonates.
Mars’ surface rocks also contain both oxidized and reduced species of minerals. The authors say that is evidence the surface is far out of equilibrium, which their hypothesis supports. “While both oxidized and reduced species may form under one climate, the deposition rate of different species is sensitive to the climate. For example, warm climates preferentially deposit nitrate while cool climates preferentially deposit nitrite,” the authors write.
In any case, Mars is an extremely interesting puzzle. Without plate tectonics, its surface is largely unchanged from ancient times. Unlike Earth, which recycles its surface and erases evidence, evidence of Mars’ warm, wet periods is easy to see. “It makes a really great case study for how planets can evolve over time,” lead author Adams said.
Much of what scientists hypothesize about Mars can only be confirmed by in-situ measurements. The NASA rovers MSL Curiosity and Perseverance both have onboard labs to study rocks. Perseverance, however, is also caching rock samples for eventual return to Earth. Those samples, if they make it to Earth labs, will be critical in answering our questions about Mars.
“Hence, full interpretation of the redox paradox will require careful comparison of our alternating atmospheric redox hypothesis with chemical and isotopic datasets collected in situ and with igneous and water-altered rocks from the first 1–2 billion years of Mars’s history that comprise the samples presently being collected by the Perseverance rover,” the authors conclude.
This hypothesis raises questions about Mars’s habitability in the past. According to our understanding, oscillations between warm and wet and cold and dry pose a significant barrier to life starting and evolving. But that’s beyond the scope of this paper.
