The Martian Paradox

How one of the biggest wonders of Mars has remained unresolved

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A camera on the Mars Reconnaissance Orbiter captured this image of slope lineae. The dark streaks change throughout the seasons, possibly indicating salty liquid water on the surface of Mars. False color image by NASA/JPL-Caltech/Univ. of Arizona.

In looking at pictures of Mars, I am reminded of its versatility. Sand dunes and charming little spherical pebbles on its surface create the same reassuring patterns we find here on Earth. The slopes of the rosy sand are highlighted beautifully at their peaks, almost resembling waves but so expansive that they became a purgatory for some of the rovers which were stuck in the Meridiani Plains and eventually killed by unrelenting sandstorms. The Martian sunset is cool and pale compared to ours, filling the alien skies with a color so composed and so milky blue. And while much of this planet may remind one of Earth, Mars is also our idea of the future. It’s exciting, brimming with the potential to make humankind true explorers of the solar system. Its red iron oxide body offers itself as a home away from home. The planet has also prompted one of the most fascinating questions of science: did Mars once support life?

On Earth, life needs three basic things to survive. The first is energy, such as sunlight for our crops and our machines, or food for our bodies. The second is water and, lastly, organics (things like hydrogen, carbon and oxygen). Sunlight provided energy for the rovers to run, allowing them to later go on and discover organics in the sedimentary rock at Gale Crater. Samples also confirmed that the soil was alkaline and does in fact have the right nutrients to support plant and microbial life. But the most stunning aspect of the planet has to be its abundance of water.

Immense accumulations of ice blanket both the poles and persist underground, with some ultra-pure glaciers measuring as much as 330 feet (100 meters) thick. These deposits just beneath the surface are beautifully clean, having hardly any debris like rocks or dust mixed in. They point to ancient snowstorms similar to the sandstorms of modern day. The rovers journey through lake beds and deltas, examining rocks which point to a water cycle responsible for deposits of sediment and mud. That same sediment — along with boulders — provides evidence of tsunami impacts and even a tentative ocean at the north. There’s even geological evidence that rain once fell here, too. And while most water today is frozen or permeating the atmosphere, liquid water is present on Mars as well.

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Martian spherules, sometimes known as “blueberries”, contain a lot of grey hematite. Red hematite is responsible for the planet’s iconic color but grey hematite appears where liquid water is found. Image by NASA/JPL/Cornell/USGS.

Besides the recurring slope lineae, in 2018 it was announced that an underground lake was buried beneath sheets of ice at the planet’s south pole. While the lake is liquid and spans 12 miles (20 km), it’s more of a brine or even a sludge than the typical clear, summer lakes that we’re used to. The European Space Agency’s MARSIS instrument was able to detect the water using radar. It received signals similar to those given off on Earth when liquid water is sloshing around beneath ice. But MARSIS’s resolution is limited and because of this, scientists estimate there are many more bodies of water that we can’t yet detect. The lake would have remained liquid thanks to the presence of perchlorates, a class of salts that lower the freezing point of water.

Liquid water on Mars is different, perhaps more substantial, than frozen deposits because it leads to sustainability. Evidence like the slope lineae and lakes point to possible aquifers and wells. After filtering, they could provide drinking water, breathable air, and rocket fuel. This is possible because of the hydrogen and oxygen composition of water — molecules which can be split. Not to mention that microbes could be thriving in the cold salty waters. They do survive in even harsher conditions here on Earth, after all.

There is, however, what NASA calls the paradox of ancient Mars.

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Strips of turquoise material lie no more than 6 feet beneath eroded banks. Scientists believe this material to be water ice. Image by NASA/JPL/University of Arizona.

All this evidence of former water systems has no confirmed explanation and leading theories as to how liquid water could have existed conflict with findings from surface samples. While its first billion years or so were likely to be the warmest and wettest, the problem is that the sun was only about 60% as hot as it is today. And while isotopic ratios do suggest Mars’s atmosphere was thicker and allowed it to trap more heat, it still wouldn’t have been enough to allow liquid water for very long. Those two factors — atmosphere and sunlight — were the main characteristics scientists relied on to account for the ancient watery environment. What we do know from observations is that any oceans would have frozen over in just 4,000 years and if an ocean did exist during Mars’s Hesperian period (about 2–3 billion years ago), the thin atmosphere would have led the body of water to evaporate. A paper by researchers at the French National Center for Scientific Research uses these facts to dispute events like the tsunami impacts. If the ocean never existed to begin with, deposits may have resulted from groundwater outflows and sudden geological changes. An even more unlikely solution involves repeated asteroid impacts: the first to thaw the frozen ocean, the second to cause the actual tsunami.

Neither did the young Mars have enough carbon dioxide to produce a greenhouse effect. Minerals from rocks maintain that even if there was 100 times more carbon dioxide in the atmosphere, it still wouldn’t be enough to have kept the water liquid. The bedrock samples contain far less carbonate than they would if there was a thick CO2 atmosphere overhead. Even after two decades of searching, carbonate deposits are far lower than anticipated. Successful models mix carbon dioxide with hydrogen but scientists aren’t sure how such a composition would form, much less remain constant. There were also no ice wedges or any other evidence of freezing found at Gale Crater, meaning that the lake there was liquid and not liquid beneath ice.

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A depiction of what Mars’s surface may have looked like once upon a time. Image by Jon Wade and James Moore.

For now, scientists are looking into methane to help solve the paradox. Methane is a greenhouse gas just like carbon dioxide but it’s 28–100 times more powerful, depending on the timescale. If the early environment of Mars had the right mix of methane, CO2, and hydrogen, it could account for the water-bearing temperatures. Research shows that this mixture results in better absorption of radiation, meaning that up until now, methane and hydrogen could have been highly underestimated. But while methane can be converted into hydrogen over time, where the methane originated is a mystery in and of itself. Methane here on Earth is largely caused by living organisms — microbes, humans, and animals. This means microbes could have been responsible for the more tropical Martian temperatures billions of years ago.

Models show global warming could also have come from ice thaw, set in motion by a tilt in Mars’s axis that allowed it to receive more sunlight. Methane formerly trapped in the ice would then have permeated the air for hundreds of thousands of years until the sun dissolved the methane molecules and given us the colder climates we see today.

With the recent death of Opportunity, the only rover left on Mars is Curiosity. It might be the most apt name for a craft exploring the planet; Mars has as complex a terrain and a history as that of Earth. Both planets only look plain in their pictures from far away. One a pale blue dot and the other a sultry red one. But being on their surface one can begin to understand that there is a past there, a tumultuous one of change and storms and perhaps life — a curious unfolding of events. And on both planets, too, a future for humanity.

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