Our understanding of Mars has come a long way since the Viking missions touched down in the 1970s. Since then, more than 18 spacecraft have operated in Mars’ orbit, seven of which are still active. Numerous rovers have crisscrossed the planet’s barren landscape, revealing invaluable discoveries.

With all these eyes above and on the ground, you’d think something as massive as a mountain would have been mapped out by now. But Mars is always full of surprises.

Scientists have identified a new volcano on Mars, named provisionally Noctis Mons (or Noctiv Volcano), situated south of the planet’s equator in the Eastern Noctis Labyrinthus region. This thing is absolutely enormous, towering at 9,022 meters (30,000 feet) and spanning 450 kilometers (about 280 miles).

“We were examining the geology of an area where we had found the remains of a glacier last year when we realized we were inside a huge and deeply eroded volcano,” said Dr. Pascal Lee, a planetary scientist with the SETI Institute and the Mars Institute based at NASA Ames Research Center.

An elusive giant hiding in plain sight

While it’s now dead quiet, Mars used to have a rich history of volcanism. The assumption is that, since it’s smaller than Earth, the planet would have cooled faster than ours after it formed. As its thermal activity decreased, so did its ability to generate new volcanoes and so-called “marsquakes”. More recently, scientists have found that there is still some volcanic activity that makes some regions prone to mild tremors. Still, these are nothing as intense as what it used to experience billions of years ago.

Given Noctis Mons’ immense size and deeply eroded features, it must be very old, a landmark of a bygone era. But when it was active, it must have caused quite the stir.

Noctis Mons is located on the eastern part fractured maze-like grander geological formation known as Noctis Labyrinthus. The reason why it wasn’t identified sooner despite countless flybys by satellites is due to the deep erosion it has suffered over millions of years.

The eastern section of Noctis Labyrinthus is characterized by a chaotic mix of layered flat-topped hills (mesas) and valleys — important clues that first set scientists on this path of investigating its volcanic origin. At the heart of this area, a group of elevated mesas forms a semicircle, creating the highest point in the region. These mesas and valleys stretch outward for up to 225 kilometers (140 miles), sloping away in various directions. Near the center, there’s a caldera remnant, the remains of a volcanic crater that collapsed after hosting a lava lake. Throughout this area, one can find traces of past volcanic activity, including lava flows, layers of volcanic debris (like ash, cinders, pumice, and tephra), and spots where minerals formed in the presence of water.

“This area of Mars is known to have a wide variety of hydrated minerals spanning a long stretch of Martian history. A volcanic setting for these minerals had long been suspected. So, it may not be too surprising to find a volcano here,” explained Sourabh Shubham, a graduate student at the University of Maryland’s Department of Geology and the study’s co-author.

“In some sense, this large volcano is a long-sought ‘smoking gun’”.

Volcanoes and glaciers

That’s not all. Within the bounds of Noctis Mons lies a vast area filled with volcanic deposits, spanning 5,000 square kilometers (1,930 square miles). This region is dotted with numerous low, rounded, and elongated mounds resembling blisters, which the scientists call “rootless cones.” Scientist speculate that these formations would have been created by bursts of steam or the expansion of steam when a thin layer of hot volcanic material settled on a surface rich in water or ice.

Previously, Lee had found the well-preserved remains of a glacier, known as a “relict glacier,” within the same volcanic area. This glacier was marked by a deposit of light-colored sulfate salts. These likely formed from the chemical reaction between volcanic material and ice. Breaches in rootless cones also contained polyhydrated sulfates, suggesting the volcanic layer may conceal a significant glacier ice sheet beneath it.

“It’s really a combination of things that makes the Noctis volcano site exceptionally exciting. It’s an ancient and long-lived volcano so deeply eroded that you could hike, drive, or fly through it to examine, sample, and date different parts of its interior to study Mars’ evolution through time. It has also had a long history of heat interacting with water and ice, which makes it a prime location for astrobiology and our search for signs of life. Finally, with glacier ice likely still preserved near the surface in a relatively warm equatorial region on Mars, the place is looking very attractive for robotic and human exploration,” said Lee.

The rich interaction of heat with water and ice over time raises questions about the potential for life at this site. As such, the discovery of the Noctis volcano offers a new and exciting location for studying Mars’ geological evolution, searching for life, and planning future exploration, particularly because of the possible presence of glacier ice in a relatively warm equatorial region. Perhaps this site could be suitable for human settlements if its water resources are abundant, confirmed and easily extractable.

“We now have to determine if, and how much, water ice might actually be present in this relict glacier, and whether other light-toned deposits might also have, or have had, ice-rich substrates,” Lee said in a statement.