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Mars’s lost water may be buried beneath the planet’s crust

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Mars’s lost water may be buried beneath the planet's crust


Current estimates suggest Mars may have had between 100 and 1,500 meters global equivalent layer (m GEL) of water on its surface. (m GEL refers to a layer of 1 meter of water that would cover an even surface of the planet—Scheller says 1,000 m GEL is equivalent to roughly half the water of the Atlantic Ocean.) Even the lower end of this estimate is still plenty of water that potential life could have used to make a home for itself. 

So learning how it disappeared is critical. If we know what happened, we could have a better understanding of what locations on Mars could have preserved evidence of any life that evolved during that time—and how current and future Mars missions could look for that evidence.

In most water loss models that assume atmospheric loss, the idea has been that UV radiation causes water high in the air to dissociate into hydrogen and oxygen. Both elements—but especially the lighter hydrogen molecules— escape the atmosphere and head into space. Scientists measure this hydrogen loss (using neutron detectors like the FREND instrument on ESA and Russia’s Trace Gas Orbiter) as a proxy for determining the rate of water loss on Mars over time. 

However, there are two problems with this theory. For one, it doesn’t explain why TGO or other missions still detect so much water in the Martian crust. Second, the rate of hydrogen loss measured so far is too small to account for how much water we think Mars originally had. “It could only really account for the lower end of what most geologists think,” says Scheller.

At the same time, we now have a better understanding of how much water is buried within the Martian crust. Much of this is thanks in great part to rover missions like Curiosity that have studied Martian rocks directly, as well as lab analysis of meteorites from Mars that have landed on Earth. And all of that data has slowly led scientists to take more seriously the idea that the crust played a more significant part in the loss of water on Mars. 

Now Scheller and her colleagues have come up with a new model that uses current data to examine whether the water could have gone underground instead.

This water would not have been sucked down into huge subterranean oceans. Instead, water molecules became incorporated into mineral structures like clays as a result of processes like weathering. The same happens here on Earth. 

This process could account for anywhere between 30% and 99% of the total water loss in the planet’s first 1 to 2 billion years, according to the model. Atmospheric loss could make up the rest.

“It’s an extremely intriguing model,” says Joe Levy, a geologist at Colgate University, who wasn’t involved with the study. “Hydrated minerals and vein-forming minerals are almost everywhere we look on Mars. Runaway chemical weathering is a really provocative hypothesis to explain what happened to Mars’s water.”

A range of 30% to 99% is, of course, huge. That’s because we simply don’t know enough about the water content in the crust (least of all on a global scale), or what the ancient atmosphere of Mars looked like and to what extent it encouraged or limited atmospheric water loss. The model also attempts to take into account how geological activity in the ancient past (such as volcanism) could have affected these water loss mechanisms.

The model gives us new clues when it comes to Martian habitability. “The findings don’t just answer how Mars might have lost its water, but also when it lost its water,” says Scheller. The authors are certain the hydrated minerals in the crust are over 3 billion years old, which means Mars was potentially most habitable before that. Any search for evidence of ancient life would be best geared toward rocks that have been preserved from this earlier period.

Scheller suggests that both the Curiosity and Perseverance rovers may be able to look for samples within this time range. Perseverance in particular, whose mission is mainly dedicated to looking for evidence of Martian life, will explore a former lake bed that’s 3.8 billion years old. “It will be right there to investigate what might have been the mechanisms that caused water sequestration in these minerals in the crust,” says Scheller. Even if it cannot do the job on its own, it will capture samples that scientists could study for themselves in the lab. 

Earth and Mars started out as very similar wet worlds but ended up taking drastically different paths. The loss of water to hydrated minerals in the crust isn’t unique to Mars; this happens on Earth all the time. But Earth benefits from the fact that its tectonic plates actively recycle its crustal rocks in a process that would release this water. Plus, it retained a thick atmosphere that kept the planet at the perfect temperature for life to evolve and thrive. Mars has no tectonic plates, and it hemorrhaged its atmosphere once its magnetic field shut down 4 billion years ago. 

“Ultimately, this is the thing to keep in mind about habitability on terrestrial planets,” says Scheller. “It’s very fragile.”

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The Download: Introducing our TR35 list, and the death of the smart city

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JA22 cover


Spoiler alert: our annual Innovators Under 35 list isn’t actually about what a small group of smart young people have been up to (although that’s certainly part of it.) It’s really about where the world of technology is headed next.

As you read about the problems this year’s winners have set out to solve, you’ll also glimpse the near future of AI, biotech, materials, computing, and the fight against climate change.

To connect the dots, we asked five experts—all judges or former winners—to write short essays about where they see the most promise, and the biggest potential roadblocks, in their respective fields. We hope the list inspires you and gives you a sense of what to expect in the years ahead.

Read the full list here.

The Urbanism issue

The modern city is a surveillance device. It can track your movements via your license plate, your cell phone, and your face. But go to any city or suburb in the United States and there’s a different type of monitoring happening, one powered by networks of privately owned doorbell cameras, wildlife cameras, and even garden-variety security cameras. 

The latest print issue of MIT Technology Review examines why, independently of local governments, we have built our neighborhoods into panopticons: everyone watching everything, all the time. Here is a selection of some of the new stories in the edition, guaranteed to make you wonder whether smart cities really are so smart after all:

– How groups of online neighborhood watchmen are taking the law into their own hands.

– Why Toronto wants you to forget everything you know about smart cities.

– Bike theft is a huge problem. Specialized parking pods could be the answer.

– Public transport wants to kill off cash—but it won’t be as disruptive as you think.

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Toronto wants to kill the smart city forever

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Toronto wants to kill the smart city forever


Most Quayside watchers have a hard time believing that covid was the real reason for ending the project. Sidewalk Labs never really painted a compelling picture of the place it hoped to build. 

Quayside 2.0

The new Waterfront Toronto project has clearly learned from the past. Renderings of the new plans for Quayside—call it Quayside 2.0—released earlier this year show trees and greenery sprouting from every possible balcony and outcropping, with nary an autonomous vehicle or drone in site. The project’s highly accomplished design team—led by Alison Brooks, a Canadian architect based in London; the renowned Ghanaian-British architect David Adjaye; Matthew Hickey, a Mohawk architect from the Six Nations First Nation; and the Danish firm Henning Larsen—all speak of this new corner of Canada’s largest city not as a techno-utopia but as a bucolic retreat. 

In every way, Quayside 2.0 promotes the notion that an urban neighborhood can be a hybrid of the natural and the manmade. The project boldly suggests that we now want our cities to be green, both metaphorically and literally—the renderings are so loaded with trees that they suggest foliage is a new form of architectural ornament. In the promotional video for the project, Adjaye, known for his design of the Smithsonian Museum of African American History, cites the “importance of human life, plant life, and the natural world.” The pendulum has swung back toward Howard’s garden city: Quayside 2022 is a conspicuous disavowal not only of the 2017 proposal but of the smart city concept itself.

To some extent, this retreat to nature reflects the changing times, as society has gone from a place of techno-optimism (think: Steve Jobs introducing the iPhone) to a place of skepticism, scarred by data collection scandals, misinformation, online harassment, and outright techno-fraud. Sure, the tech industry has made life more productive over the past two decades, but has it made it better? Sidewalk never had an answer to this. 

 “To me it’s a wonderful ending because we didn’t end up with a big mistake,” says Jennifer Keesmaat, former chief planner for Toronto, who advised the Ministry of Infrastructure on how to set this next iteration up for success. She’s enthusiastic about the rethought plan for the area: “If you look at what we’re doing now on that site, it’s classic city building with a 21st-century twist, which means it’s a carbon-neutral community. It’s a totally electrified community. It’s a community that prioritizes affordable housing, because we have an affordable-housing crisis in our city. It’s a community that has a strong emphasis on green space and urban agriculture and urban farming. Are those things that are derived from Sidewalk’s proposal? Not really.”

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Rewriting what we thought was possible in biotech

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Rewriting what we thought was possible in biotech


What ML and AI in biotech broadly need to engage with are the holes that are unique to the study of health. Success stories like neural nets that learned to identify dogs in images were built with the help of high-quality image labeling that people were in a good position to provide. Even attempts to generate or translate human language are easily verified and audited by experts who speak a particular language. 

Instead, much of biology, health, and medicine is very much in the stage of fundamental discovery. How do neurodegenerative diseases work? What environmental factors really matter? What role does nutrition play in overall human health? We don’t know yet. In health and biotech, machine learning is taking on a different, more challenging, task—one that will require less engineering and more science.

Marzyeh Ghassemi is an assistant professor at MIT and a faculty member at the Vector Institute (and a 35 Innovators honoree in 2018).

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