AVIATION WEEK AEROSPACE DAILY & DEFENSE REPORT

March 19, 2021

Vol. 275 Issue 53

13.  Research Effort Suggests Mars Water Not Lost To Space

Mark Carreau, mark.carreau@gmail.com

Though the Martian surface is now cold and dry, imagery dating back to the 1960s reveals a planet where large amounts of water once flowed and pooled, perhaps contributing to an environment amenable to life.

A new study of data gathered from a steady sequence of NASA Mars orbiters, landers and rovers focused on a "Follow the Water" theme has emerged with a surprise.

While some of the Martian water--estimated to have once been enough to cover the entire surface to a depth of between 100 and 1,500 m (330 to 4,920 ft.)--rose into the atmosphere and escaped thanks to the solar wind, most of it remains on the surface, locked in the soil and rock.

And while there is also evidence for water ice at the Martian poles, vast subsurface water ice deposits and possibly a pond of liquid water at the south pole, the water chemically bound in the soil and rock would be recovered if exposed to temperatures in the range of 300 to 400 deg. centigrade (572 to 752F).

The surprising data that vast amounts of Martian water remain locked in Martian crust was presented at a March 16 news briefing hosted by the virtual 52nd Lunar and Planetary Science Conference (LPSC) and published as "Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust" in the journal Science.

While Mars formed along with the rest of the Solar System's planets about 4.6 billion years ago, the once-vast water supply disappeared 3 to 4 billion years ago. This was partially by rising into space but primarily by merging into the soil and rock, according to Eva Scheller, a doctoral candidate in geological and planetary science studies at Caltech who led the study. She spoke to the LPSC briefing.

That did not happen on the Earth, which is much more massive than Mars, because of plate tectonics. Plate tectonics, a geological process evident on Earth but not Mars, features distinct layers of crust and upper mantle that move over one another. As Earth's older crust melts into the mantle over time, new crustal plates form and recycle water and other molecular structures back into the atmosphere through volcanism.

"We have always recycled our crust on the Earth through plate tectonics. It's kind of like our crust is continuously getting melted," Sheller said. "The water is locked in the minerals and then escapes back into the atmosphere. At Mars, it's the opposite. Mars did not have the geological processes that enable recycling. Instead, we have water locked in the minerals and they are still there from 4 billion years ago."

The processes for water loss on Mars were determined by Sheller and her colleagues from Caltech, NASA's Jet Propulsion Laboratory and Michael Meyers, NASA's Mars Exploration Program lead scientist, by comparisons of two types of hydrogen atom, which along with oxygen comprises the water molecule.

Most hydrogen atoms have a nucleus with one proton, but a very small number, about 0.02%, have both a proton and neutron in the nucleus and are categorized as deuterium, or "heavy hydrogen." Hydrogen, because its lighter, escapes the Martian gravity into space more easily than deuterium.

The study determined that between 40% and 95% of Martian surface water was lost between 3.7 and 4.1 billion years ago and that between 30% and 99% of the loss was incorporated into minerals buried in the Martian crust and evidenced as veins in observable clay, sulfate and other water mineral formations in a range of existing land forms.

The researchers determined the remainder escaped into space by comparing the ratio of deuterium to hydrogen currently observed in the Martian crust and thin atmosphere.

NASA's Perseverance Mars 2020 Rover, which touched down Feb. 18 at Jezero Crater on Mars, site of an early crater lake and stream delta, is to gather and cache samples of soil and rock over a two-year primary mission. The materials are to be returned to Earth with a joint NASA/European Space Agency Mars Sample Return mission in 2031, and will help to validate and further explain the ancient Martian environment and its water history, the researchers said.

But the primary objective of the sample return is to seek out evidence of possible biological activity in the samples using tools and techniques too advanced for spaceflight.

Among the missions that contributed to the surprise findings are NASA's still-active Curiosity rover; the Maven, Mars Reconnaissance and Mars Odyssey orbiters and the European Space Agency's Mars Express orbiter.