Internal workings: Mars at the beginning could have boasted of a big ocean and a cool climate


When Mariner 4 buzzed to Mars in 1965, it revealed a dry, parched world in stark contrast to the habitable planet dreamed of by decades of science fiction writers. Subsequent sightings revealed the apparent scars of rivers and deltas, and even potential sea shores. The revelations brought hope that the planet had already been wetter.

For years, researchers have suggested that Mars once had an ocean, as this conceptual illustration shows. But the details of the formation, disappearance and impact of this ocean on the Martian landscape remain hotly debated. Image credit: NASA / GSFC.

But the implications of these findings have remained unclear, with theories of potential climates falling into two camps. In the “hot and humid” camp, precipitation carved out the river-like features of Mars and a large ocean spread over its northern hemisphere. The “cold and frozen” camp attributes the melting of the ice to the deepening of the valleys.

Now, new research (1) suggests that a large ocean was indeed needed to form the features across the planet’s surface. But rather than hot and humid, the planet would have been cool and semi-arid. This intriguing third hypothesis has started to gain attention as researchers continue to debate the peculiarities of the early climate of the Red Planet.

Oceans of evidence

The first clue of an ocean on Mars came in the late 1980s, when researchers identified an apparent shore from satellite images (2). Then, as the snapshots of the regions became more detailed, they quickly realized that the supposed shoreline was at different elevations. In contrast, the water in an ocean is at the same level, sea level, which has led some researchers to suggest that the features may have a volcanic origin rather than indicating a shore. Others argue that the difference could come from changes in the planet’s pole of rotation (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) located at the same elevation. Fan-shaped structures that form when a river expands as it enters an ocean, deltas form on relatively short time scales of thousands of years. Such deltas also argue for an ocean, although it may not have lasted long.

On Earth, the continents are much higher than the ocean floors, and the Red Planet appears to have a similar configuration. “On Mars, from the start, we could see that the northern hemisphere is lower than the southern hemisphere,” says Kirsten Siebach, a Martian geologist at Rice University in Houston, Texas, 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 boulder crashing into the planet, then filled with water later (5).

The strongest evidence of liquid water in the Martian past comes from the valley systems in the southern highlands, which stretch for hundreds to thousands of kilometers in length and can reach tens to hundreds of meters. depth. These valleys lie on the edge of the region identified as a potential northern ocean.

Whether these valleys were carved out by precipitation or snowmelt is the subject of bitter debate. Climate modelers have struggled to heat the planet enough to allow precipitation for the tens to hundreds of millions of years needed to erode the landscape. According to the icy highlands hypothesis, the planet never reached these temperatures but remained cold, collecting ice on its mountains, where a lower temperature and pressure allows the water to be solid. Explosions of volcanic action or meteorite impacts could have created temporary warming events that melted this ice, causing it to sink briefly across the planet. Several studies have suggested that melting ice could cut valley networks. “If the ice melts, it will descend, creating networks of valleys, lakes and maybe even an ocean,” says James Head, a researcher at Brown University in Providence, RI. In 2015 (6), Head calculated how much water could come from the ice trapped on top of the Martian mountains and found that this was enough to create the water-sculpted features we see today.

But not everyone is convinced that this theory can explain the Martian valleys. Some of the valley systems are the size of the Mississippi River, Hynek explains. “Forming the Mississippi River from tiny pulses of water over millions of years just won’t work. “

Air of uncertainty

Researchers have long struggled to explain how liquid water could have risen to the surface of Mars. Today, its atmosphere is thin, with pressures too low to prevent liquid water from boiling, even at the low temperatures typical of the planet. In the past, a denser atmosphere could have increased the pressure to prevent liquid water from becoming a gas. During the 4.5 billion years since the formation of the solar system, this gas could have been gradually lost in space, the gravitational pull of the small planet being too weak to hold its atmosphere. The loss of an atmosphere caused Mars to switch from a potentially habitable environment to a barren wasteland. “This is the biggest environmental disaster we’ve ever seen,” says Edwin Kite, who studies habitability at the University of Chicago, IL.

“Right now, carbon dioxide and hydrogen are the most promising greenhouse combination for warming the start of March.”

—Ramsès Ramirez

In the decades since the first missions placed Mars at their sites, researchers have struggled to determine the details of the planet’s early atmosphere. An atmosphere hot enough to hold liquid water, or even frozen snow, requires just the right cocktail of gases, and trying to build the environment in simulations continues to confuse researchers.

“Right now, carbon dioxide and hydrogen are the most promising greenhouse combination for warming early Mars,” says Ramses Ramirez, of the Tokyo Institute of Technology, Japan. Ramirez and his colleagues used a two-dimensional model with these gases to create a planet with typical temperatures above the freezing point of water.

The model also estimates rainfall runoff at different latitudes around the world. They tested it on three oceans of different sizes, with larger oceans producing more precipitation. The runoff rates from the largest ocean were just sufficient to produce the observed characteristics. This ocean would have covered up to a third of the planet’s surface.

Importantly, the researchers determined that even such a large ocean does not give Mars a humid and hot climate similar to that of Earth. Instead, this results in cool, semi-arid conditions.

Rain shadow

Despite these new works, navigation is not smooth on the Martian Ocean. The debate between Mars was hot or cold became so heated that an editorial (7) in Geosciences of nature called it a “war” – although Head and his colleagues responded with a letter (8) saying that there is no war, just “healthy debate”.

Conventional thought held that Tharsis Montes, a trio of volcanoes near the equator of Mars, had formed before the valley networks were carved. This would have moved precipitation away from the valley’s networks, according to a study by climate modeler Robin Wordsworth, a researcher at Harvard University in Cambridge, MA (9). As humid air enters the mountains, it rises and condenses into rain, squeezing moisture from the air and leaving the leeward side of the mountain dry – a well-known phenomenon known as the shadow of the mountain. rain. This would mean that the precipitation could not have deepened the valleys. Wordsworth concludes that the Tharsis snowmelt was responsible instead.

But Ramirez says if Tharsis had formed this early, the area would be littered with more impact craters. Instead, in his climate model, he assumes that Tharsis formed after the river valleys.

One concern with the Ramirez model is that it uses a simplified, two-dimensional approach rather than the more traditional three-dimensional perspective. “If you want to understand how the hydrologic cycle is going to behave, you need sophisticated models,” says Wordsworth. “It’s a level of detail needed for atmospheric dynamics to be properly captured in three dimensions so that you can look at it with the topography present. However, another new climate model from early March (10), from a team led by Arihiro Kamada at Tohoku University in Miyagi, Japan, is three-dimensional and comes to conclusions similar to Ramirez’s. , favoring a flat region of Tharsis and a cool climate, including a northern ocean.

In another twist, a study published in August (11) suggests that much of the valley network could have been formed by the movement of glaciers rather than requiring a lot of running water. The researchers, led by Anna Grau Galofre of Arizona State University in Tempe, found similarities between Martian valleys and those found on the Canadian island of Devon, which were sculpted by glaciers. At some sites, they found traces of water flowing upstream, which could be subglacial streams driven by the pressure of the ice above.

Ramirez is not convinced, pointing to the absence of other evidence of surface glaciation. And he says the apparent upflow of water might instead be attributable to how different rocks weather over time.

Grau Galofre argues that the lack of water on Mars today makes it difficult to design an ocean in the past. Oceans need much more water to form than glaciers in the highlands. So where could the water have gone? He could have been buried more than a few meters, below the depth that the instruments have been able to reach so far. Or he could have returned to the atmosphere, only to be lost in space. This would require a brutal climate transition, which Grau Galofre thinks unlikely. But NASA’s MAVEN spacecraft found evidence, reported in recent years, that the composition of the Red Planet’s atmosphere disappeared early in the solar system’s life (12??–14), as charged particles from the sun knocked out the gases, although the speed with which this happened remains a matter of debate. According to Ramirez, MAVEN’s results suggest that the atmosphere could have vanished quickly enough to provide a plausible escape route for water from a pristine ocean.

Sitting between the “hot and humid” and “cold and dry” models of Mars, the new scenario offers common ground for understanding the planet’s climate. This is a photo that will be tested soon. When NASA’s Perseverance rover lands in February 2021, it should be able to probe how weather conditions change in Jezero Crater over time. “Having a rover there and having more information from different places on Mars is going to help us,” Ramirez said. “This will be a game-changer and hopefully provide a boost for later [human] exploration.”

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