Mars was half covered by an ocean

The existence of water on Mars is a central topic in planetary research. Previous studies have already provided evidence of oceans and rivers on Mars, indicating a once humid and possibly habitable environment. Evidence of former water and a possible ocean have also been discovered for the Valles Marineris – the largest canyon system on Mars, which stretches along its equator. These come, among other things, from discoveries of minerals that have been altered by water.

A research team from the University of Bern, in collaboration with the INAF – Osservatorio Astronomico di Padova, has now gained new insights into the geological past of Valles Marineris: Using high-resolution images from various Mars cameras, the researchers have found geomorphologic structures near the canyon system that resemble river deltas on Earth. These structures represent the mouth of a river into an ocean. The new study thus provides clear evidence of a coastline and consequently of an earlier ocean on Mars. The study was recently published in the journal npj space exploration.

Bernese Mars camera delivers high-resolution images

To investigate the geomorphological structures of Mars, the researchers used high-resolution images from various cameras on Mars probes, including the Bernese Mars camera CaSSIS (Colour and Stereo Surface Imaging System) on board the European Space Agency's ExoMars Trace Gas Orbiter, as well as data from the ESA space probes Mars Express and Mars Reconnaissance Orbiter. The investigation focused on the geomorphologic structures of the southeast Coprates Chasma, part of the Valles Marineris.

"The unique high-resolution satellite images of Mars have enabled us to study the Martian landscape in great detail by surveying and mapping," says Ignatius Argadestya, PhD student at the Institute of Geological Sciences and the Physics Institute of the University of Bern. Ignatius Argadestya is conducting research as part of the "Swiss Government Excellence Scholarship for Foreign Scholars (ESKAS)" at the University of Bern and is the first author of the study, which was produced as part of his dissertation. This was financially supported by the Swiss National Science Foundation and ESKAS.

"CaSSIS has been providing high-resolution color images of the surface of Mars since April 2018. The images are regularly used in scientific studies. I am personally very pleased that the images have now also been used in a geomorphological study by the Institute of Geological Sciences," says Nicolas Thomas, Professor at the Department of Space Research & Planetary Sciences (WP) at the University of Bern, under whose leadership the CaSSIS camera system was developed and built by an international team and which is funded by SERI's Swiss Space Office through ESA's PRODEX program (see info box).

River deltas as evidence of an earlier ocean on Mars

"When measuring and mapping the Martian images, I was able to recognize mountains and valleys that resemble a mountainous landscape on Earth. However, I was particularly impressed by the deltas that I discovered at the edge of one of the mountains," says Argadestya. At the lower end of the canyon system, so-called "scarp-fronted deposits" were discovered, which are interpreted as "fan deltas". Fan deltas form where a fan-shaped cone of debris and sand grows directly into a standing body of water. The researchers found that the structures mapped on Mars are very similar to classic deltas on Earth.

"Delta structures develop where rivers debouch into oceans, as we know from numerous examples on Earth," explains Fritz Schlunegger, Professor of Exogenous Geology at the Institute of Geological Sciences at the University of Bern. "The structures that we were able to identify in the images are clearly the mouth of a river into an ocean," Schlunegger continues. The study thus confirms and proves earlier scientific evidence that there was an ocean on Mars around three billion years ago.

An ocean as large as the Arctic Ocean

The results also show that the ocean found was at least as large as the Arctic Ocean on Earth. Schlunegger says: "We are not the first to postulate the existence and size of the ocean. However, earlier claims were based on less precise data and partly on indirect arguments. Our reconstruction of the sea level, on the other hand, is based on clear evidence for such a coastline, as we were able to use high-resolution images."

"With our study, we were able to provide evidence for the deepest and largest former ocean on Mars to date – an ocean that stretched across the northern hemisphere of the planet," says Argadestya. Today, wind-sculpted dunes cover the former delta structures. However, their original shape is still clearly recognizable.

Conditions on Mars could once have been favorable to life

The discovery of deltas and the confirmation of evidence of a former ocean suggest that conditions once prevailed on Mars that could have favored the emergence and development of life. "We know Mars as a dry, red planet. However, our results show that it was a blue planet in the past, similar to Earth. This finding also shows that water is precious on a planet and could possibly disappear at some point," emphasizes Argadestya.

"This project is particularly exciting for us researchers from the field of geology because it allows us to transfer concepts that we have developed from studies on Earth to other planets such as Mars," adds Schlunegger. "We used a sedimentological approach for this study, meaning we compared depositional environments between Earth and Mars." In a next step, the research group will now investigate the mineralogical composition of the ancient Martian soils. "Now that we know that Mars was a blue planet, we also want to know what kind of weathering took place there," concludes Argadestya.