Huge Spiral Troughs on Mars are Young, Formed by Erosion

Spiral troughs found on the surface of Mars are in situ ice-excavated canyons with a total volume 10 times that of the Grand Canyon, making them one of the largest and youngest geologic mega-structures in the Solar System, a new paper by Planetary Science Institute Senior Scientist Alexis Palmero Rodriguez says.
“Erosion formed a huge ice canyon system, and that erosion is a source of the long-known mid-latitude mantles on Mars,” said Rodriguez, lead author of “North polar trough formation due to in?situ erosion as a source of young ice in mid?latitudinal mantles on Mars” (www.nature.com/articles/s41598-021-83329-3) that appears in Nature Scientific Reports.

The troughs are arranged in a vast spiral pattern covering an area the size of Texas. We find that their growth lateral to katabatic wind (wind that carries high-density colder air from a higher elevation down a slope) directions produced widespread simple intersections, from which the highly complex spiral arrangement emerged, Rodriguez said.

“The spiral pattern seen in the troughs is basically an erosional byproduct,” he said. “As the pits grow and intersect over a pre-existing dome-shaped polar cap, the spiral pattern emerges.

“It has long been proposed that sublimation of water ice from the north polar cap during high-obliquity cycles was an essential source of the planet’s mid-latitude icy plains. Our finding identifies the troughs as direct evidence of those sublimation phases,” Rodriguez said.

These spiral trough features formed very recently, in geologic terms: between a few million and 50,000 years ago, Rodriguez said.

The presence of northern plains ice has long been known and has been recently suggested as a potential human resource. However, that ice could include frozen brines with compounds toxic to humans – for example, residues of ancient lakes, seas, and oceans.

“We estimated that about 25% of the northern plains icy substrates were derived from trough eroded ice. The ice was never liquid, so it never dissolved toxic compounds. We are looking at enormous volumes – equivalent to approximately 16 billion Olympic-sized swimming pools – of potentially pure clean water,” Rodriguez said. “On the other hand, these icy layers, including those sampled by NASA’s Phoenix Mars Lander, might not be the best places to look for life, for precisely the same reasons.”

The age of the polar cap remains a subject of controversy, and numerous estimates exist. Based on detailed geologic mapping, a view is that most of its deposition happened during the last billion years. Consequently, while the troughs might be geologically young, the ice they exposed could be extremely ancient.

To provide perspective, the oldest ice cores on Earth are approximately 2.7 million years old, which has been very useful to reconstruct our planet’s recent paleoclimatic evolution. The layers of Martian ice exposed within the troughs could enable in-depth paleoclimatic investigations potentially spanning several hundred million years into the past. An equivalent record on Earth would allow us to sample ice from when the dinosaurs roamed Earth and much more precisely understand the paleoenvironments under which they lived.

Co-author Kenneth Tanaka, an affiliate scientist with PSI who has studied the surface of Mars for more than 40 years, said, “For hundreds of years, astronomers have been intrigued by the presence of the north polar ice cap on Mars. Now with modern spacecraft observations, we can decipher how that ice cap has responded to a climate history that reflects changes in the atmosphere and resulting geologic activity evidenced elsewhere on the planet.

“Similarly, the study of water stored in the polar regions on Earth tells much of the story of our planet’s climate history. On the other hand, where the melting of ice caps on Earth leads mainly to increases to the volume of ocean water, on Mars water ice is commonly thought to be redistributed mainly as ice deposits forming mantles at mid-latitudes and even mountain glaciers at low latitudes,” Tanaka said. “Our study helps demonstrate that the water ice that is removed from the north polar region indeed is redistributed to these lower latitudes rather than in a more localized recycling process that also has been suggested.”

The project was funded by a grant to PSI from NASA’s Mars Data Analysis program.