A protoplanet is covered by magma oceans and surrounded by a field of planetesimals.
Frequent impacts contributed to the formation of magma oceans on growing protoplanets. Credit: Sarah A. Brands

Noble gases and their isotopes tell the tale of rocky planet formation.

Noble gases and their isotopes tell the tale of rocky planet formation. Because these gases do not react with other species, they store the earliest atmospheric records of terrestrial planets like Mars. To date, the only direct detection of the noble gas neon in Mars’s atmosphere came from the Viking landers in the late 1970s. Martian meteorites, rocks flung from the planet by giant impacts and subsequently found on Earth, also contain traces of different neon isotopes.

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State-of-the-art modeling of atmospheric processes has revealed that neon should escape the Martian atmosphere in less than a hundred million years. However, the large amount of neon that Viking landers found in the Martian atmosphere suggested that the gas is being replenished over time. Comets, asteroids, and interplanetary dust particles could deliver neon to Mars. Energetic cosmic rays battering the Martian surface also create neon in the process. But the amount of neon produced by all of these processes falls short of the measured value.

The only other option is that the neon came from inside Mars itself. The presence of neon, therefore, suggests recent or ongoing volcanic activity on Mars, according to a new study published in Icarus. The result agreed with the recent claim that volcanic eruptions on Mars could have taken place within the past 50,000 years.

According to Hiroyuki Kurokawa, an assistant professor at the Tokyo Institute of Technology and lead author of the new study, Mars’s mantle possibly contains the largest reservoir of neon on the planet. The team estimated that the abundance of neon in the mantle is 5–80 times higher than in Earth’s mantle. And because only a small portion of neon deposited on Mars would stay trapped in the mantle over a long period, the actual source of the neon had to be very rich. Where did all this neon come from?

A Cry for On-Site Measurements

If Mars had a dense ancient atmosphere, neon could have been deposited from the atmosphere into the mantle before magma oceans solidified. However, given the planet’s small size, this scenario works only if Mars grew to its former size and accreted its primordial atmosphere before solar radiation swept the atmosphere away.

“It seems that Mars grew very fast. But to understand the whole picture, you need to consider all the planets, then figure out the differences and try to figure out why [there is] a difference.”

“It seems that Mars grew very fast,” said Helmut Lammer, a research group leader on planetary atmospheres at the Space Research Institute of the Austrian Academy of Sciences who was not involved in the research. The finding, if true, will help to reconstruct the bigger picture of terrestrial planet formation. “But to understand the whole picture, you need to consider all the planets, then figure out the differences and try to figure out why [there is] a difference,” said Lammer.

Additional neon sources could be dust, asteroids, or comets. The measured ratios of neon isotope abundances disfavor asteroids and comets; however, the large uncertainties prevented the authors from reaching firm conclusions and called for new in situ measurements. “NASA and ESA [European Space Agency] are planning to return some Martian soil samples” to Earth, by 2031 if all goes according to plan, said Kurokawa. “Hopefully, if such a sample contains some trapped gas of Martian atmosphere, we may be able to obtain the neon from the atmosphere.” But success is not guaranteed.

Researchers should also give attention to other volatile gases that must have been abundant in the Martian mantle. Solidification of the magma oceans and later volcanic activity released the gases into the atmosphere. Knowing more about the mantle’s content in Martian history could shed light on, among other things, conditions leading to the emergence of liquid water on the planet’s surface.

—Jure Japelj (@JureJapelj), Science Writer

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Citation: Japelj, J. (2021), Noble gas hints at Mars’s rapid formation, Eos, 102, https://doi.org/10.1029/2021EO210557. Published on 18 October 2021.
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