While the present-day Martian surface is generally dry and cold, its sedimentary rocks contain compelling evidence for the former presence of liquid water. According to a new analysis of orbital images of 3.7-billion-year-old sedimentary layers at Izola mensa, an outcrop in the northwestern rim of the Hellas impact crater on Mars, deep rivers were active in this region for over 100,000 years.
This false-color map, produced by the Mars Orbiter Laser Altimeter (MOLA), depicts the topography of the Martian surface. Hellas basin, the large, dark blue region below the center, has a diameter of 1,430 miles (2,300 km), and is one of the largest identified impact craters both on Mars and within the Solar System. It is thought to have formed some 4 billion years ago. Image credit: MOLA Science Team.
Hellas Planitia is the largest well-preserved impact structure on Mars and the third or fourth largest in the Solar System.
It spans 2,300 km (1,430 miles) across in the Martian southern hemisphere, a region that is much more heavily cratered and higher in average elevation than the northern hemisphere.
The depth of Hellas from its bottom to its inner rim is more than 4 km (2.5 miles). To put this in perspective, the depth of the Grand Canyon in the United States is roughly 1.6 km (1 mile).
It contains a variety of 3.7-billion-year-old sedimentary plains, overlain by 3.3-billion-year-old lava flows.
Landforms preserved on its surface provide evidence of an incredibly large lake and a network of ancient rivers, deltas and outflow channels.
“The extremely high resolution imagery allowed us to ‘read’ the rocks as if you are standing very close to the cliff,” said Dr. Francesco Salese, a geologist at Utrecht University and senior scientist in the International Research School of Planetary Sciences.
“Unfortunately, we don’t have the ability to climb, to look at the finer-scale details, but the striking similarities to sedimentary rocks on Earth leaves very little to the imagination.”
Channel-forms preserved in sedimentary layers in Hellas Planitia on Mars. Image credit: NASA / JPL-Caltech / University of Arizona / Matt Balme.
In the study, Dr. Salese and colleagues examined sedimentary-stratigraphic architecture of a 1,500-m (4,921-foot) wide, 190-m (623-foot) thick sedimentary succession at Izola mensa.
They used images captured by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.
“Here on Earth, the stratigraphy (i.e. the order and position) of sedimentary rocks has been used by geologists for generations to place constraints on what conditions were like on our planet millions or even billions of years ago,” said Dr. William McMahon, a geologist at Utrecht University.
“Now we have the technology to extend this methodology to another terrestrial planet, Mars, which hosts an ancient sedimentary rock record which extends even further back in time than our own.”
The team’s observations and analysis favor steady water discharges that are most consistent with a precipitation-driven hydrological cycle.
“Our study demonstrates sustained river deposition on Mars 3.7 billion years ago,” Dr. Salese said.
“Such perennially flowing rivers would require an environment capable of maintaining large volumes of water for extensive time-periods, and almost certainly necessitated a precipitation-driven hydrological cycle.”
“More in line with slower climatic change, and less in line with catastrophic hydrologic events. This kind of evidence, of a long-lived watery landscape, is crucial in our search for ancient life on the planet.”
“For the first time, orbital data has allowed us to examine, through detailed high-resolution architectural analysis, a large outcrop, and draw reliable paleoenvironmental interpretations based on sedimentary-stratigraphic evidence.”
The findings were published in the journal Nature Communications.
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F. Salese et al. 2020. Sustained fluvial deposition recorded in Mars’ Noachian stratigraphic record. Nat Commun 11, 2067; doi: 10.1038/s41467-020-15622-0
