Oceans of Evidence

The first hint of an ocean on Mars came in the late 1980s, when researchers identified an apparent shoreline from satellite imagery (2). Then, as snapshots of the regions became more detailed, they quickly realized that the supposed shoreline sat at various different 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 argue 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 thousands 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 Martian past comes from the valley networks in the southern highlands, which extend hundreds to thousands of kilometers in length and can be tens to hundreds of meters deep. These valleys lie on the fringe of the region identified as a potential northern ocean.

Whether these valleys were carved by rainfall or melting snow is fiercely debated. Climate modelers have struggled to heat the planet enough to allow rainfall for the tens to hundreds of millions of years needed to erode the landscape. According to the icy highlands hypothesis, the planet never reached those temperatures but instead stayed cold, collecting ice on its mountains, where lower temperature and pressure allows water to be solid. Bursts of volcanic action or meteorite impacts could have created temporary warming events that melted this ice, causing it to trickle briefly across the planet. Several studies have suggested that melting ice could cut out the valley networks. “If the ice melts, it’s going to flow downhill, creating valley networks, 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 at the top of Martian mountains and found that it was enough to create the water-carved features seen today.

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

Air of Uncertainty

Researchers have long struggled to explain how liquid water could have surged across the surface of Mars. Today, its atmosphere is thin, with pressures too low to keep liquid water from boiling away, even at the planet’s typical low temperatures. In the past, a denser atmosphere could have increased the pressure to keep liquid water from becoming a gas. Over the 4.5 billion years since the solar system formed, that gas 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 from a potentially habitable environment to a barren wasteland. “It’s the greatest environmental disaster we know of,” says Edwin Kite, who studies habitability, at the University of Chicago, IL.

“Right now, carbon dioxide and hydrogen is the most promising greenhouse combination to warm early Mars.”

—Ramses Ramirez

In the decades since the earliest missions set Mars in their sites, researchers have struggled to nail down the details of planet’s early atmosphere. An atmosphere warm enough to hold onto liquid water, or even frozen snow, requires just the right cocktail of gases, and trying to build the environment in simulations continues to puzzle researchers.

“Right now, carbon dioxide and hydrogen is the most promising greenhouse combination to warm 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 across different latitudes of the planet. They tested it across three different-sized oceans, with larger oceans producing more precipitation. The runoff rates for the largest ocean were just enough to produce the observed features. This ocean would have covered up to a third of the planet's surface.

Crucially, the researchers determined that even such a large ocean does not give Mars a moist, warm Earth-like climate. Instead, it results in cool, semi-arid conditions.

Rain Shadow

Despite this new work, it's not smooth sailing on the Martian ocean. The debate between whether Mars was hot or cold has become so heated that an editorial (7) in Nature Geoscience referred to it as a “war”—although Head and his colleagues responded with a letter (8) saying that there is no war, just “healthy debate.”

Conventional thinking held that Tharsis Montes, a trio of volcanoes near the equator of Mars, formed before the valley networks were carved. This would have kept rainfall away from the valley networks, according to a study by climate modeler Robin Wordsworth, a researcher at Harvard University in Cambridge, MA (9). As moist air moves into mountains, it rises and condenses into rain, wringing the moisture out of the air and leaving the lee side of the mountain dry—a well-known phenomenon called a rain shadow. That would mean rainfall could not have carved the valleys. Wordsworth concludes that snowmelt from Tharsis was responsible instead.

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

A 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 hydrological cycle is going to behave, you need sophisticated models,” says Wordsworth. “It's a necessary level of detail to have the atmospheric dynamics captured properly in three dimensions so you can look at it with the topography present.” However, another new climate model of early Mars (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 Tharsis region 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 motion of glaciers rather than requiring lots of running water. Researchers, led by Anna Grau Galofre of Arizona State University in Tempe, found similarities between the Martian valleys and those found on Canada’s Devon Island, which were carved by glaciers. At some sites, they found evidence of water running uphill, which could be subglacial streams driven by the pressure of ice above.

Ramirez is not convinced, pointing to a lack of other evidence for glaciation on the surface. And he says that the apparent uphill flow of water could instead be attributable to the way different rocks weather over time.

Grau Galofre contends that the lack of water on Mars today makes it hard to conceive of an ocean in the past. Oceans require far more water to form than do glaciers on the highlands. So where could the water have gone? It might have been buried deeper than a few meters, below the depth that instruments have been able to reach so far. Or it could have returned to the atmosphere, eventually to be lost to space. That would require a sharp climate transition, which Grau Galofre thinks unlikely. But NASA’s MAVEN spacecraft has found evidence, reported in the last few years, that the composition of the red planet’s atmosphere disappeared early in the life of the solar system (12⇓–14), as charged particles from the sun stripped away gases—although how quickly this happened remains a matter of some debate. According to Ramirez, MAVEN’s results suggest that the atmosphere could have disappeared fast enough to provide a plausible escape route for the water in an early ocean.

Falling between the “warm and wet” and “cool and dry” models of Mars, the new scenario provides a middle ground when it comes to understanding the planet's climate. It’s a picture that will be tested soon. When NASA’s Perseverance rover lands in February 2021, it should be able to probe how climate conditions changed at Jezero crater over time. “Having a rover there and having more information from different locations on Mars is going to help us,” Ramirez says. “It's going to be a game changer and will hopefully provide the impetus for later [human] exploration.”