The Pathfinder spacecraft and Sojourner rover, which can be seen investigating a large rock.
Image: NASA

Nearly 25 years ago, the Pathfinder spacecraft explored a suspected floodplain on Mars. Unbeknownst to NASA at the time, the waters that long ago carved Pathfinder’s landing site spilled out from a previously undocumented inland sea nearby, new research shows.

A new paper published in Scientific Reports finally confirms the Pathfinder landing site as a marine spillover plain. Now dry, this spillway was once covered in water, and not lava or debris as some scientists had speculated. Importantly, the new research also shows that the floodwaters originated from a nearby inland sea, the presence of which wasn’t known until now. It’s yet further evidence that liquid water once existed at the Martian surface—a possible sign that the Red Planet was once capable of fostering life. The Pathfinder landing site, as the new research suggests, was more intriguing than NASA could have ever anticipated.

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This story starts in the 1970s with the launching of NASA’s Mariner 9 spacecraft. The Martian orbiter spotted surface features unlike anything seen before—apparent spillover landscapes known as Martian outflow channels. These massive floodplains formed around 3.4 billion years ago as copious amounts of rushing water, pouring out from beneath the surface, carved the Martian landscape. These ancient outflow channels are larger than anything seen on Earth. The channel in Kasei Vallis, for example, is around 2,200 miles long (3,500 km) and more than 250 miles wide (400 km). So yeah, they’re enormous.

Martian outflow channels at Aurorae Chaos.
Image: ESA/DLR/FU Berlin/J. Cowart, CC BY-SA 3.0 IGO

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The going theory is that the outflow channels were formed by water bursts from subterranean aquifers, but scientists haven’t been able to rule out other possibilities, namely lava or debris flows (like highly porous mud).

To investigate these outflow channels up close, NASA sent the $280 million Mars Pathfinder spacecraft to the lower reaches of Tiu and Ares Valles, which the probe investigated in 1997 with its companion, the Sojourner rover. This particular site was chosen on account of intriguing surface features detected by the Mariner orbiter. Observations made during the Pathfinder mission affirmed the landscape as being the site of a former flood, but a flood at considerably shallower levels than estimates derived from the space-based observations—to the tune of 10 times shallower. This surprising result meant scientists were still unable to rule out other mechanisms for the eroded surface features, namely the purported lava and debris flows.

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Now, 22 years later, new research led by Alexis Rodriguez, a senior scientist at Planetary Science Institute, finally presents the evidence needed to show that the region was indeed carved by rushing liquid water. This evidence included data gathered by Pathfinder, but also new geological mapping and numerical modeling.

Diagrams depicting the spillage mechanism (left) and the hypothesized inland sea and Northern Ocean. The region marked A corresponds to the diagram at left. The location of Pathfinder is also shown.
Image: J. A. P. Rodriguez et al., 2019/Scientific Reports

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The key to the discovery was the detection and characterization of the Simud Interior Basin—a large basin about 155 miles (250 km) south and upstream of the Pathfinder landing site. This basin is roughly the size of California, and at an elevation slightly higher than the hypothesized Northern Ocean, a former body of water just north and downstream of the Pathfinder landing site. This basin is not connected to other gigantic Martian channels, according to the new research.

“The basin has been dried for around 3,400 million years, and thus it has experienced extensive modifications due to asteroid impacts and wind,” Rodriguez told Gizmodo. “Its shape, somewhat hidden in the existing orbital image and topographic datasets of Mars, has eluded identification for years.”

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In simulations, the researchers watched as floodwaters filled the basin, forming a large inland sea. This created a land barrier between the Northern Ocean and the new sea—an expanse visited by Pathfinder in the 1990s. Eventually, the waters in the inland sea spilled over, flooding the land barrier and creating the spillover surface features observed by Pathfinder.

“The Mars Pathfinder identified numerous geologic features thought to have been produced by shallow floods. Our article shows that these floods were marine spillover discharges from a previously unknown sea,” said Rodriguez. “When the sea disappeared, it left behind extensive marine sedimentary materials. These deposits are particularly interesting because they likely represent some of the most easily accessible marine sediments on Mars.”

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A layer of ice eventually covered the inland sea, and it disappeared altogether on account of rapid evaporation and sublimation, according to the new research.

Importantly, the simulations showed that the inland sea acted as a kind of dam, attenuating, or limiting, the amount of water spilling out from it—an observation that nicely reconciles the observations made by Pathfinder. Also, debris or lava flows couldn’t have formed the features seen at the Pathfinder site, the researchers argue, as such flows would have completely obliterated any sign of the basin; as a flowing liquid, lava or fluidized debris flows would have behaved just like water, filling up the Simud Interior Basin prior to spilling over into land barrier.

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“Our paper ends the controversy,” Rodriguez told Gizmodo. “This is simply because the basin still exists, indicating that it was only temporarily filled up. Lava or debris flows would have permanently buried it.”

Rodriguez and her colleagues are now exploring the astrobiological potential of the marine sediments. To that end, she’s teaming up with a new NASA Ames-led initiative, which could result in the evaluation of the leftover marine sediments as a potential landing site.

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“Because the water that formed the sea was expelled from the subsurface, its sediments might contain a record of habitability,” she said, nothing that this wouldn’t have been known had it not been for the Mars Pathfinder mission.

“That is a legacy of tremendous importance,” said Rodriguez.

[Scientific Reports]

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