By Will Dunham
WASHINGTON, Nov 4 (Reuters) - An orbiting atmospheric research spacecraft has detected the pungent gas hydrogen chloride above surface fissures along a massive ancient volcano on Mars in a finding that suggests Earth's planetary neighbor is still volcanically active.
In a discovery that has implications for whether Mars might presently have subsurface conditions conducive to microbial life, researchers said the ExoMars Trace Gas Orbiter has documented the gas, a common constituent of
volcanic emissions on Earth, in the low atmosphere in the region of Alba Mons.
The largest Martian volcano by surface area, Alba Mons is a broad and gently sloping structure, called a shield volcano because it resembles a warrior's shield. It is located in the planet's northern hemisphere.
"We found that the most likely origin for the hydrogen chloride is volcanic seepage," said planetary scientist Daniel Mège of the Space Research Center of the Polish Academy of Sciences and the University of Bern in Switzerland, co-lead author of the research published in the journal Science Advances.
The hydrogen chloride seepage may be volcanic outgassing from the swarms of surface fissures in the Alba Mons region, perhaps connected to a subsurface magma chamber.
"It is released from chlorine-bearing compounds dissolved in magma as magma rises and pressure decreases. Its presence can, therefore, be consistent with magmatic degassing," said planetary scientist and study co-lead author Alexander Trokhimovskiy of the Space Research Institute of the Russian Academy of Sciences.
That would indicate Mars is not geologically dead inside but rather in a stage of relative dormancy.
"It suggests that the planet's interior may still retain sufficient heat to sustain magmatic processes, and that atmospheric trace gases can provide a way to detect and investigate geological activity that is otherwise difficult to observe directly," Trokhimovskiy said.
AN ELITE CLUB
Confirmed volcanically active bodies are an elite club in our solar system — the planets Earth and Venus along with Jupiter's moon Io.
The Martian surface was heavily shaped by ancient volcanism. In fact, Mars boasts the solar system's tallest volcano in Olympus Mons. But whether Mars has ongoing volcanism has been less clear.
"Evidence of volcanism from a few million years ago on Mars has been reported but no evidence of current magmatic activity had been reported," Mège said.
Marsquakes detected by NASA's InSight seismometer also suggested volcanic activity.
On Earth, sulfur dioxide typically is more abundant than hydrogen chloride in volcanic emissions. But hydrogen chloride may be the more abundant gas on Mars because of differences in the compositions of the two planets.
"It is pungent, and I would discourage anyone from sniffing it because it is corrosive and can irritate or burn your nasal tissues," Mège said.
The orbiter, a collaboration between the European Space Agency and Russian space agency Roscosmos, first detected hydrogen chloride in the Martian atmosphere and has documented annual spikes in the gas along the northern flank of Alba Mons in the early summer.
The researchers suspect the fissures may be blocked by ice during other times of the year, but that thawing caused by rising air temperatures in the early summer enables volcanic gases trapped underground to escape into the atmosphere.
The existence of subsurface supplies of magma has interesting implications for the question of whether Mars currently harbors living organisms. Active volcanism would mean that pockets of heat may persist in the planet's crust, its outermost layer.
Mars is already known to have water trapped beneath the surface, and life on Earth is thought to have first emerged billions of years ago in hydrothermal systems.
"As rising magma moves through the crust, it can encounter, melt and heat underground water, creating environments shielded from the harsh surface conditions," Trokhimovskiy said. "Such active hydrothermal systems may provide the energy sources and favorable conditions needed to support microbial life. This makes warm subsurface environments one of the most promising targets in the search for life on Mars."
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
