Oxygen and water showcase Mars, Europa mysteries and struggles

by Chris Gebhardt

Follow the water and the oxygen.  Just two of the key tenets of humanity’s early and ongoing exploration drives within our solar system.  That research has received huge boons in the last two decades with in-situ exploration of Titan and Enceladus by Cassini.

But in the last three years, it has been NASA’s Curiosity rover in Gale Crater on Mars and a powerful telescope in Hawai’i that have given NASA scientists new insights – and questions – into the role of oxygen and water on Mars and Jupiter’s moon Europa, respectively.

That’s not how oxygen is supposed to behave:

One of the significant benefits to the Curiosity rover’s multi Mars-year tenure has been its ability to monitor seasonal atmospheric changes on the Red Planet — something its SAM (Sample Analysis at Mars) chemistry lab was – in part – designed to do.

Over the last three Martian years (six Earth years), SAM has inhaled samples of the local Martian ground atmosphere, resulting in the first-ever measurement of seasonal changes in the gases directly above the surface of Gale Crater.

In short, SAM returned results that the local Gale Crater atmosphere is, by volume, comprised of: 

  • 95% carbon dioxide (CO2), 
  • 2.6% molecular nitrogen, 
  • 1.9% argon, 
  • 0.16% molecular oxygen, and 
  • 0.06% carbon monoxide.

Seasonal oxygen levels observed by Curiosity. (Credit: NASA)

These measurements have also revealed how the seasonal freezing of carbon dioxide at the poles in winter lowers the overall air pressure around the planet – air pressure that then rises in the Martian polar springs when the CO2 evaporates and redistributes in the atmosphere.

The same results also show that the nitrogen and argon present in Mars’s atmosphere follow predictable and understood seasonal patterns.

In this manner, scientists expected the oxygen present in the atmosphere to do the same, but it didn’t. 

Instead, the amount of oxygen rose unpredictably throughout spring and summer by as much as 30% before dropping back to levels predicted by known chemistry in fall. 

This pattern repeated each spring, though the amount of oxygen added to and taken from the atmosphere varied.

“The first time we saw that, it was just mind boggling,” said Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor – co-author of a paper on this topic published on 12 November in the Journal of Geophysical Research: Planets.

When the strange results were first observed, NASA scientists and engineers repeatedly checked to make sure SAM was functioning properly and not returning inaccurate readings.  SAM was fine.

Scientists then considered if CO2 and H2O could be releasing the oxygen – but there would have to be five times more water on Mars’ surface and in its atmosphere than is present to account for the oxygen level rises.

They also considered if solar radiation could account for the oxygen’s rapid disappearance.  But scientists found that it would take 10 years of solar radiation exposure to dissipate that much oxygen.

“We’re struggling to explain this,” said Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland who led this research.  “The fact that the oxygen behavior isn’t perfectly repeatable every season makes us think that it’s not an issue that has to do with atmospheric dynamics. It has to be some chemical source and sink that we can’t yet account for.”

However, scientists have noticed a seeming correlation between the oxygen rise and the seasonally observed rises of methane at Gale Crater.

Methane is constantly in the air in extremely small quantities (0.00000004% on average).  But SAM’s Tunable Laser Spectrometer revealed that while methane rises and falls seasonally, it increases in abundance by about 60% in summer months for inexplicable reasons.

Could the spikes in methane and oxygen be related?

“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year,” Atreya said.  “I think there’s something to it.”

The observed correlation of methane and oxygen levels at Gale Crater over three Martian years of observation by Curiosity. (Credit: NASA)

Methane and oxygen rises certainly bring up the question of whether biologic or geologic processes are responsible for these seasonal variations.

While biologic sources cannot be completely ruled out because Curiosity is not designed to investigate the current biological conditions at Gale Crater, scientists are working through the non-biologic potential answer to this quandary.

“I just don’t have the answers yet,” said Atreya.  “Nobody does.”

Water water everywhere – Europa edition:

Europa has, for several decades, been a tantalizing target in the search for potential life elsewhere in the solar system other than Earth. 

From the first direct images of the moon returned by NASA’s Voyager probes in the 1970s to present day observations, Europa has captivated scientific interest due to its icy exterior. 

Recent evidence has mounted for a large subterranean ocean underneath Europa’s icy surface, but direct confirmation of this has proven elusive – as has direct detection of water or water vapor in, on, or around Europa.

Previous in-situ observations of the moon by NASA’s Galileo spacecraft from 1995 to 2003 and via the Hubble Space Telescope have returned evidence of hydrogen and oxygen in and around Europa, but no direct evidence of water. 

That is, until now.

A team of NASA scientists led by Lucas Paganini of the Goddard Space Flight Center in Greenbelt, Maryland, have announced the first concrete detection of water vapor spewing in volcanic eruptions from Europa’s large vents. 

The observations and confirmation came from the team’s use of the W. M. Keck Observatory atop the Mauna Kea volcano in Hawai’i.

Paganini and his team’s findings are detailed in the 18 November publication of the journal Nature Astronomy.

The observations, performed over 17 nights throughout 2016 and 2017, returned an immense amount of water vapor erupting from Europa’s surface at a rate of 2,360 kg per second – enough to fill an Olympic-size swimming pool within minutes.

Despite this tremendous release, the team also found that water vapor appears quite infrequently. 

“For me, the interesting thing about this work is not only the first direct detection of water above Europa, but also the lack thereof within the limits of our detection method,” said Mr. Paganini.

What’s more, the team was only able to detect water vapor along Europa’s leading hemisphere, defined as the side of the moon that always faces the direction of Europa’s orbit of Jupiter.

Like Earth’s Moon, Europa is tidally locked with its host planet, so the same hemisphere always faces Jupiter.

Europa Clipper. (Credit: NASA)

“This first direct identification of water vapor on Europa is a critical confirmation of our original detections of atomic species, and it highlights the apparent sparsity of large plumes on this icy world” said Lorenz Roth, an astronomer and physicist from KTH Royal Institute of Technology in Stockholm who led the 2013 Hubble study and was a co-author of this recent investigation.

This ultimately highlights the challenge in trying to study Europa from Earth.  The amount of observation and detailed scientific equipment needed to further unlock Europa’s mysteries are either extremely difficult or not possible to do from Earth.

Enter Europa Clipper, a flagship NASA science mission set to launch No Earlier Than 2025 that will perform direct, detailed observation and study of Jupiter’s icy moon.

Europa Clipper’s three main scientific objectives are to:

  1. confirm the existence and characterize the nature of water within or beneath the ice and the processes of surface-ice-ocean exchange,
  2. identify the distribution and chemistry of key compounds and the links to ocean composition., and 
  3. study the characteristics and formation of surface features, including sites of recent or current activity.

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