Among the most remarkable geological formations on Mars is the vast shield volcano Olympus Mons that towers up to 26 kilometres above the surrounding plains. It is situated on the Tharsis plateau, a volcanic province the size of Europe. Volcanism is a widespread phenomenon on Mars. A second large geological structure is found close to the eastern edge of Tharsis – the canyon system of Valles Marineris, which stretches almost 4000 kilometres along the equator, with its tectonic graben reaching up to 10 kilometres deep into its surface. Scientists have wondered for decades whether there has been volcanic activity within Valles Marineris. Now four researchers led by Petr Brož from the Institute of Geophysics of the Czech Academy of Science have discovered a region with 130 small volcanoes at the base of Valles Marineris.

Published in the journal Earth and Planetary Science Letters, the study by Mars researchers reveals that the Coprates Chasma canyon, one of the lowest points in Valles Marineris, is home to a large number of volcanic cones and solidified lava flows. Their findings are based on the analysis of high-resolution images acquired by the NASA Mars Reconnaissance Orbiter (MRO), which reveal the cones of scoria and volcanic ash. On Earth’s continents, this type of volcano is the most common conveyance system for lava. Nevertheless, the authors were initially uncertain in how to interpret these new volcanoes in terms of their type. Brož, the lead author of the study, says: “The 400-metre high cones have also been interpreted as mud volcanoes. But we observed morphological details such as bulging of solidified lava caused by the injection of more recent lava beneath the hardened crust, as well as characteristic surface patterns identical to lava fields on Earth. This reinforces our assumption that we are looking at magmatic rock volcanism, and not liquid mud.”

Young volcanoes at the base of Coprates Chasma on Mars (image obtained by Mars Reconnaissance Orbiter, NASA/JPL/University of Arizona).

“The spatial distribution of the cones also suggests their volcanic origin,” adds planetary geologist Ernst Hauber from the German Aerospace Center (DLR). “They appear to occur more frequently along tectonic fractures that formed the trough in the surface and whose fracture interfaces continue into the subsurface, creating pathways for the magma to ascend.” Most importantly, the morphological similarities between these cone volcanoes in Coprates Chasma and volcanic cones elsewhere on Mars where mud volcanism is not possible –as well as similarities with Earth analogues – is a strong argument that these formations are genuine volcanoes.

Young volcanic cones

Along with the existence of volcanoes on the floor of the eastern Valles Marineris, their young age is the second surprise of this study. “In geological terms, the volcanic cones are very young, just 200 to 400 million years in age,” explains Gregory Michael from the Freie Universität Berlin, one of the study’s four authors. Therefore, the volcanoes in Coprates Chasma are unrelated to the original formation of the Valles Marineris, as they are significantly younger. The scientists can use the prevalence of impact craters in the surrounding plains to estimate the maximum age of geological units. This method is based on the fact that the number of craters on a surface will increase the longer it is exposed to bombardment by asteroids and meteorites. A near-absence of impact craters is therefore indicative of a very young age.

However, the main period of volcanic activity on Mars took place around 3.5 billion years ago, in the early phase of the planet’s existence. While the images acquired by DLR’s High Resolution Stereo Camera aboard Mars Express have repeatedly shown indications for Martian volcanism younger than half a billion years, this volcanism tends to be the exception and is usually located in one of the known volcanic provinces. The discovery of more recent volcanic activity from the Amazonian period of Mars in Coprates Chasma in the southeast of Valles Marineris –several thousand kilometres from the centre of the volcanic province Tharsis – now demonstrates that volcanoes formed late in Martian history, far from the main volcanic areas of Tharsis and Elysium.

A potential discovery site for fossilised life

The four scientists also investigated the Coprates Chasma region using the imaging spectrometer CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) on board the Mars spacecraft MRO to learn more about the composition of the lavas and the volcanic cones. “In doing so, we encountered minerals with high silica content, and even opaline-like substances at one of the peaks,” adds co-author James Wray from the Georgia Institute of Technology. Opals are amorphous, water-bearing silicate materials that have not formed a crystalline structure. Such mineralogical compositions can be produced by so called hydrothermal processes, i.e., when minerals precipitate from supersaturated, hot solutions and mineralise. This also renders Coprates Chasma interesting for astrobiological studies, as microorganisms on Earth find such warm, energy-rich and mineral-rich environments an ideal abode.

The valleys of Coprates Chasma in the east of Valles Marineris. This perspective view was created using stereo image data from DLR’s HRSC (High Resolution Stereo Camera) camera on board ESA’s Mars Express spacecraft (ESA/DLR/FU Berlin, CC-BY-SA 3.0 IGO).

This observation makes the volcanic field of Coprates Chasma an interesting area for a future investigation of whether life may have emerged in these warm and aqueous environments on Mars. Investigations conducted on Earth have shown that potential traces of earlier life are well preserved in opaline mineral compounds. Therefore, these opals might also be potential sites for the discovery of bio-signatures on Mars. “But Coprates Chasma is not just interesting with regard to the question of previous life on Mars. The region would also be an excellent landing site for future Mars Rovers,” says Hauber. “Here we could investigate many scientifically important and interesting topics. Analysing samples for their elemental isotopic fractions would allow us to determine with far greater precision when the volcanoes were actually active. On the towering, steep walls, the geologic evolution of the Valles Marineris is presented to us almost like a history book – gypsum strata and layers of old, crustal rocks can be observed, as well as indications for liquid water trickling down the slopes even today during the warm season. That is as much Mars geology as you can get!”

Valles Marineris in the east of the Tharsis volcanic region. Higher regions are marked red in this topographic map, while yellow and green indicate moderate elevations, and the lowest points are shown in blue (MOLA, NASA/JPL/University of Arizona).

Prepared by: Institute Geophysics of the Czech Academy of Science
Photo: Introductory image shows the group of terrestrial scoria cones in the Harrat Lunayyir area, Saudi Arabia (photographed by John Pallister, U.S. Geological Survey).