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Early Mars May Have Boasted a Large Ocean and Cool Climate Nola Taylor Redd, Science Writer

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Early Mars May Have Boasted a Large Ocean and Cool Climate Nola Taylor Redd, Science Writer INNER WORKINGS INNER WORKINGS Early Mars may have boasted a large ocean and cool climate Nola Taylor Redd, Science Writer When Mariner 4 buzzed Mars in 1965, it revealed a Now, new research (1) suggests that a large ocean dry, desiccated world that stood in stark contrast to was indeed required to form the features across the the habitable planet dreamed of by decades of science planet’s surface. But rather than warm and wet, the fiction writers. Subsequent observations revealed the planet would have been cool and semi-arid. This in- apparent scars of rivers and deltas, and even potential triguing third hypothesis has started to garner atten- sea shorelines. The revelations brought hope that the tion as researchers continue to debate the particulars ’ planet had once been wetter. of the red planet s early climate. But the implications of these findings have remained Oceans of Evidence unclear, with theories of potential climates falling into The first hint of an ocean on Mars came in the late 1980s, two camps. In the “warm and wet” camp, rainfall exca- when researchers identified an apparent shoreline vated Mars’ river-like features, and a large ocean from satellite imagery (2). Then, as snapshots of the “ stretched across its northern hemisphere. The cold regions became more detailed, they quickly realized ” and frozen camp credits melting ice with scooping that the supposed shoreline sat at various different out the valleys. elevations. In contrast, water within an ocean sits at the same level—sea level—leading some researchers to suggest that the features may have a volcanic origin rather than being indicative of a shoreline. Others ar- gue that the difference could have come from changes in the planet’s rotational pole (3). But more recent evidence suggests that a third of the planet was covered by a northern ocean. Brian Hynek, a geologist at the University of Colorado, Boulder, and his colleagues identified 52 deltas (4) sitting at the same elevation. Fan-like structures that form as a river spreads out when it enters an ocean, deltas form over relatively short timescales of thou- sands of years. Such deltas also make a good case for an ocean, although it might not have been long-lived. On Earth, continents are much higher than the ocean floors, and the red planet seems to have a similar setup. “On Mars, from the get-go, we could see that the northern hemisphere is lower than the southern hemisphere,” says Kirsten Siebach, a Martian geologist at Rice University in Houston, TX, who was not part of the study. “Essentially the overall picture of the shape of the planet makes it look like there could have been an ocean on the northern half.” The northern basin could have been carved out by a massive rock smashing into the planet and then filled with water later (5). The strongest evidence for liquid water in the For years, researchers have suggested that Mars once had an ocean, as depicted Martian past comes from the valley networks in the in this concept illustration. But the details about that ocean’s formation, disappearance, and impact on the Martian landscape remain hotly debated. southern highlands, which extend hundreds to thou- Image credit: NASA/GSFC. sands of kilometers in length and can be tens to Published under the PNAS license. First published December 2, 2020. 31558–31560 | PNAS | December 15, 2020 | vol. 117 | no. 50 www.pnas.org/cgi/doi/10.1073/pnas.2022986117 Downloaded by guest on September 30, 2021 hundreds of meters deep. These valleys lie on the three different-sized oceans, with larger oceans pro- fringe of the region identified as a potential northern ducing more precipitation. The runoff rates for the ocean. largest ocean were just enough to produce the ob- Whether these valleys were carved by rainfall or served features. This ocean would have covered up to melting snow is fiercely debated. Climate modelers a third of the planet’s surface. have struggled to heat the planet enough to allow Crucially, the researchers determined that even rainfall for the tens to hundreds of millions of years such a large ocean does not give Mars a moist, warm needed to erode the landscape. According to the icy Earth-like climate. Instead, it results in cool, semi-arid highlands hypothesis, the planet never reached those conditions. temperatures but instead stayed cold, collecting ice on its mountains, where lower temperature and pres- Rain Shadow sure allows water to be solid. Bursts of volcanic action Despite this new work, it’s not smooth sailing on the or meteorite impacts could have created temporary Martian ocean. The debate between whether Mars warming events that melted this ice, causing it to was hot or cold has become so heated that an editorial trickle briefly across the planet. Several studies have (7) in Nature Geoscience referred to it as a “war”— suggested that melting ice could cut out the valley although Head and his colleagues responded with a “ ’ networks. If the ice melts, it s going to flow downhill, letter (8) saying that there is no war, just “healthy creating valley networks, lakes, and maybe even an debate.” ” ocean, says James Head, a researcher at Brown Conventional thinking held that Tharsis Montes, a University in Providence, RI. In 2015 (6), Head calcu- trio of volcanoes near the equator of Mars, formed lated how much water could come from the ice trap- before the valley networks were carved. This would ped at the top of Martian mountains and found that it have kept rainfall away from the valley networks, was enough to create the water-carved features according to a study by climate modeler Robin seen today. Wordsworth, a researcher at Harvard University in But not everyone is convinced this theory can ex- Cambridge, MA (9). As moist air moves into moun- plain the Martian valleys. Some of the valley systems tains, it rises and condenses into rain, wringing the are the size of the Mississippi River, says Hynek. moisture out of the air and leaving the lee side of the “Forming the Mississippi River from tiny little pulses of mountain dry—a well-known phenomenon called a water over millions of years just isn’t going to work.” rain shadow. That would mean rainfall could not have Air of Uncertainty carved the valleys. Wordsworth concludes that snow- melt from Tharsis was responsible instead. Researchers have long struggled to explain how liquid But Ramirez says that if Tharsis had formed this water could have surged across the surface of Mars. early, the region would be littered with more impact Today, its atmosphere is thin, with pressures too low craters. Instead, in his climate model he assumes that to keep liquid water from boiling away, even at the Tharsis formed after the river valleys. planet’s typical low temperatures. In the past, a denser A concern with the Ramirez model is that it uses a atmosphere could have increased the pressure to keep liquid water from becoming a gas. Over the 4.5 simplified, two-dimensional approach rather than the “ billion years since the solar system formed, that gas more traditional three-dimensional perspective. If could have gradually been lost to space, the small ’ planet s gravitational pull too weak to hold onto its “ atmosphere. The loss of an atmosphere swung Mars Right now, carbon dioxide and hydrogen is the most from a potentially habitable environment to a barren promising greenhouse combination to warm early wasteland. “It’s the greatest environmental disaster Mars.” ” we know of, says Edwin Kite, who studies habitability, —Ramses Ramirez at the University of Chicago, IL. In the decades since the earliest missions set Mars in their sites, researchers have struggled to nail down you want to understand how the hydrological cycle is ’ the details of planet s early atmosphere. An atmo- going to behave, you need sophisticated models,” sphere warm enough to hold onto liquid water, or says Wordsworth. “It’s a necessary level of detail to even frozen snow, requires just the right cocktail of have the atmospheric dynamics captured properly in gases, and trying to build the environment in simula- three dimensions so you can look at it with the to- tions continues to puzzle researchers. pography present.” However, another new climate “Right now, carbon dioxide and hydrogen is the most promising greenhouse combination to warm model of early Mars (10), from a team led by Arihiro early Mars,” says Ramses Ramirez, of the Tokyo In- Kamada at Tohoku University in Miyagi, Japan, is stitute of Technology, Japan. Ramirez and his col- three-dimensional and comes to conclusions similar to leagues used a two-dimensional model with these Ramirez’s—favoring a flat Tharsis region and a cool gases to create a planet with typical temperatures climate, including a northern ocean. above the freezing point of water. In another twist, a study published in August (11) The model also estimates rainfall runoff across suggests that much of the valley network could have different latitudes of the planet. They tested it across been formed by the motion of glaciers rather than Taylor Redd PNAS | December 15, 2020 | vol. 117 | no. 50 | 31559 Downloaded by guest on September 30, 2021 requiring lots of running water. Researchers, led by MAVEN spacecraft has found evidence, reported in Anna Grau Galofre of Arizona State University in the last few years, that the composition of the red Tempe, found similarities between the Martian valleys planet’s atmosphere disappeared early in the life of and those found on Canada’s Devon Island, which the solar system (12–14), as charged particles from the were carved by glaciers.
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