Unusual Martian mounds could help solve one of the red planet’s biggest mysteries

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Thousands of mysterious mounds on Mars preserve layers of evidence pointing to ancient water on the red planet that likely sculpted the towering formations.

More than 15,000 mounds jut skyward from the lowlands of Mars’ Chryse Planitia, located near a natural dividing line between the planet’s southern and northern hemispheres. The geological features have long intrigued scientists, who up until now weren’t sure what created the mounds.

The formations, which span an area as large as Texas, can be seen in images taken by NASA and European Space Agency orbiters that whirl around the red planet. The unusual features resemble the famed buttes and mesas of Monument Valley along the Arizona-Utah border.

A new analysis of imagery and data collected by the orbiters, published January 20 in the journal Nature Geoscience, has shed light on the history of the mounds. The findings suggest why there could be such a stark difference between the planet’s northern and southern hemispheres: erosion caused by water 4 billion to 3.8 billion years ago.

The mounds act like time capsules that could help astronomers unlock the secrets of Mars’ past.

“Each mound is made up of a series of layers, each of which is a record of a past event,” said lead study author Dr. Joe McNeil, a planetary scientist and postdoctoral researcher at London’s Natural History Museum, via email. “The oldest are at the bottom and are made up of rock that is around four billion years old. For a geologist, looking at these layers is like looking at pages of a book — each one tells a story!”

The mounds are near the future landing site of the ESA’s ExoMars Rosalind Franklin rover, expected to launch in 2028. There is a chance the mounds could be investigated by the rover, which may reveal more about the history of water on the red planet and pinpoint resources for future human exploration.

Investigating Mars from space

McNeil and his team used images collected by the orbiters circling around Mars to gain insights about the geology of the mounds. The orbiters carry sensors, cameras and other scientific instruments that collect a range of data.

The mounds appeared similar to one another from a distance, but the high-resolution imagery captured by the orbiters allowed McNeil to zoom in, revealing “incredible variety in their structure.”

While Monument Valley’s buttes and mesas can reach up to 1,000 feet (300 meters) above the valley floor and cover an area of 145 square miles, the Martian mounds reach 1,800 feet (550 meters) and sprawl across an area 2,000 times larger, McNeil said. The Martian mounds are also much older, having existed for billions of years, rather than millions.

But McNeil’s analysis shows that the Martian mounds are likely the last remnants of ancient highlands that were washed away and worn down, formed in much the same way as Monument Valley’s features through erosion.

“Our findings reveal that water was present both at the surface and in the subsurface of this region over geological timescales, between 4.0 (billion) and 3.8 billion years ago,” McNeil said. “The mounds were originally part of the highlands, composed of hundreds of meters of clay-rich rock that formed in the presence of liquid water. Their erosion over hundreds of kilometers shows that the highlands extended much (farther) north than they do today, providing new insights into the ancient geography and hydrology of Mars.”

The clay-rich rock layers within the mounds suggest that an abundance of water was present on the Martian surface, forming chemical reactions with the rocks. Water, either in liquid or ice form, could have caused the erosion by infiltrating fractures in the rocks.

While there is evidence to show that raging rivers and lakes once existed on Mars, it’s difficult to say what kind of water feature caused the erosion, McNeil said.

“Most of the evidence has been eroded away, and what’s left — the mounds — are so old that they’ve also had an additional 3.8 billion years of wind erosion on top of whatever made them mounds in the first place,” McNeil said.

“It’s like trying to understand the plot of a book where most of the pages are missing, and the remaining ones are torn and faded. We can piece together some of the story, but much of it is left to interpretation.”

One question McNeil and his colleagues explore in the study is whether an ancient northern ocean on Mars may have caused the erosion, but the idea is a controversial one debated by scientists.

However, robotic exploration of this region could determine whether the ocean was ever present — and if life could have existed within it.

Exploring a Martian mystery

The mounds are just north of what’s known as the Martian dichotomy, a natural boundary between the shallow plains of the northern hemisphere and the soaring highlands of the southern hemisphere. The southern highlands are heavily pockmarked by craters and stand as much as 5 miles (8 kilometers) higher on average than the smooth, rolling plains of the lowlands above them.

But researchers have struggled to identify what created the boundary, which stretches around the entire planet, resulting in one of Mars’ biggest mysteries, McNeil said. The appearance of this boundary varies around the planet, with some areas seeming to link the lowlands directly to the highlands, while others contain sharp cliffs.

Scientists have two main theories about the boundary’s origins.

“Plate tectonics give the Earth its continents and ocean basins, but Mars has no plate tectonics of its own,” McNeil said. “This means it’s difficult to explain why the southern highlands of Mars have a very thick and ancient crust, while the northern lowlands are very young and thin.

“Some models have suggested that Mars’ north was struck by one or more giant impacts early in its history, effectively creating the northern lowlands as a large depression,” he explained. “Alternatively, it could be the result of mantle-driven processes similar to plate tectonics, but which ended quite early on in Mars’ history.”

There is evidence for and against both theories, and the debate rages on, McNeil said.

A study published in the January 16 issue of the journal Geophysical Research Letters suggests that marsquakes, or tremors from within Mars, detected by the now-retired NASA InSight mission point to convection within the Martian subsurface as a driver of the dichotomy.

Convection, or the transfer of heat from one spot to another, likely took place in the mantle, or inner layer, of Mars billions of years ago, the study authors said. The mantle is located between the planet’s crust and core.

Getting up close with the mounds

McNeil began investigating the mounds of the Chryse Planitia region near the dichotomy as part of his doctoral program while at the Open University in England because he believes they are “a prime location for future Mars missions.”

Areas close to the mounds, such as Oxia Planum and Mawrth Vallis, are of interest to scientists because they may preserve past signs of life on Mars. The mounds span the 310 miles (500 kilometers) between Mawrth Vallis and Oxia Planum, but the majority of the mounds in the study are closer to Mawrth Vallis, located in the lowlands north and west of the area.

Both Mawrth Vallis and Oxia Planum were candidates for the future landing site of the Rosalind Franklin rover. But the mission team has determined Oxia Planum will provide a safer site for landing and traversing the terrain, according to the ESA.

The mounds investigated in the study are hundreds of kilometers away from Oxia Planum, but smaller mounds with the same composition at the landing site will be accessible to the rover to observe up close and photograph.

“This is super exciting because this means that by exploring these plains, the rover will tell us a lot about the larger region, and possibly about the environments that were present there and whether they were habitable,” McNeil said. “(The mounds) were the key to understanding how this area of Mars all links together.”

François Poulet, an astronomer at the French Institute of Space Astrophysics who has studied the Mawrth Vallis region, said he believes that McNeil’s study provides more detail and evidence for how the mounds were formed. Poulet was not involved in the new study.

“This study reinforces my feeling that the Mawrth Vallis is truly a unique region and one of the best regions to be explored by a future robotic mission,” Poulet said.

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