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In 1997, the Pathfinder Mission of NASA unknowingly landed near the shores of an old Mars Sea

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

Nearly 25 years ago, the Pathfinder spacecraft explored a suspicious flood plain on Mars. Without the knowledge of NASA at that time, the water that long ago cut the landing site of Pathfinder from a previously undocumented inland sea in the area has shown new research.

A new article published in Scientific Reports finally confirms the landing location of the Pathfinder as a marine spillover. Now dry, this spill was once covered with water and no lava or debris as some scientists had speculated. It is important that the new research also shows that the water came from a nearby inland sea, the presence of which was previously unknown. It is even more proof that liquid water once existed on the surface of Mars – a possible sign that the Red Planet was once able to cherish life. The landing site of the Pathfinder, as the new research suggests, was more intriguing than NASA could have foreseen.

This story begins in the 70s with the launch of NASA & # 39; s Mariner 9 spaceship. The spotted surface of the Mars orbital is different from anything that has been observed before, seemingly spill-over effect landscapes known as Mars outflow channels. These vast floodplains formed about 3.4 billion years ago, when abundant amounts of running water, flowing from below the surface, formed the Mars landscape. These old outflow channels are larger than what you see on earth. The channel in Kasei Vallis, for example, is approximately 2200 km long (3500 km) and more than 250 km wide (400 km). So yes, they are huge.

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

The current theory is that the outflow channels were formed by bursts of water from subterranean aquifers, but scientists have been unable to exclude other options, namely lava or debris flows (such as highly porous mud).

To closely examine these outflow channels, NASA sent the Mars Pathfinder $ 280 million spacecraft down to the lower reaches of Tiu and Ares Valles, which investigated the probe in 1997 with its companion, the Sojourner rover. This specific site has been chosen due to intriguing surface features detected by the Mariner orbiter. Observations during the Pathfinder mission confirmed the landscape as the location of a previous flood, but a flood at a considerably shallower level than estimates derived from space-based observations – up to a 10-fold shallower level. This surprising result meant that scientists were still unable to exclude other mechanisms from the eroded surface characteristics, namely the assumed lava and debris flows.

Now, 22 years later, new research led by Alexis Rodriguez, a senior scientist from the Planetary Science Institute, is finally presenting the evidence needed to show that the region was indeed sculpted by flowing liquid water. This evidence included data collected by Pathfinder, as well as new geological maps and numerical modeling.

Diagrams with the spill mechanism (left) and the assumed inland sea and the Northern Ocean. The area marked with A corresponds to the diagram on the left. The location of Pathfinder is also displayed.
Image: J. A. P. Rodriguez et al., 2019 / Scientific Reports

The key to the discovery was the detection and characterization of the Simud Interior Basin – a large basin about 250 kilometers south and upstream of the Pathfinder landing site. This basin is about the size of California, and at an altitude slightly higher than the assumed northern ocean, a former body of water just north and downstream of the Pathfinder landing site. According to the new research, this basin is not connected to other giant Martian channels.

"The basin was dried around 3,400 million years ago and has therefore undergone extensive changes due to asteroid effects and wind," Rodriguez told Gizmodo. "The shape, somewhat hidden in the existing orbital image and topographic datasets of Mars, has not been identified for years."

In simulations, the researchers looked at how high water in the river basin filled the large inland sea. This created a land barrier between the Northern Ocean and the new sea – an area that Pathfinder visited in the 1990s. Eventually, the waters in the inland sea raided, causing the land barrier to flood and creating the overflow areas observed by Pathfinder.

"The Mars scout has identified countless geological features that are thought to have been produced by shallow floods. Our article shows that these floods were marine overflow discharges from a previously unknown sea," said Rodriguez. "When the sea disappeared, it left extensive marine sedimentary materials behind. These deposits are particularly interesting because they probably represent some of the easiest marine sediments on Mars."

A layer of ice eventually covered the inland sea and disappeared entirely, due to the rapid evaporation and sublimation, according to the new research.

It is important that the simulations showed that the inland sea acted as a kind of dam, which weakens or limits the amount of water flowing out of it – an observation that nicely reconciles Pathfinder's observations. Also, debris or lava flows could not have formed the features seen on the Pathfinder site, the researchers claim, because such currents would have completely destroyed any sign of the basin; as a flowing liquid, lava or fluidized debris flows would have behaved just like water, filling the Simud inner basin before passing into the land barrier.

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

Rodriguez and her colleagues are now investigating the astrobiological potential of marine sediments. To this end, it is working with a new Ames initiative led by NASA that could result in the evaluation of the remaining marine sediments as a potential landing site.

"Because the water that formed the sea was driven out of the subsoil, the sediments may have a record of habitability," she said, nothing that would not have known this had it not been for the Mars Pathfinder mission.

"That is a legacy of enormous importance," said Rodriguez.

[Scientific Reports]

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