Has Earth's oxygen rusted the Moon for billions of years?

Date:
September 2, 2020
Source:
University of Hawaii at Manoa
Summary:
To the surprise of many planetary scientists, the oxidized iron
mineral hematite has been discovered at high latitudes on the Moon.



FULL STORY ==========================================================================
To the surprise of many planetary scientists, the oxidized iron mineral hematite has been discovered at high latitudes on the Moon, according to
a study published today in Science Advances led by Shuai Li, assistant researcher at the Hawai'i Institute of Geophysics and Planetology (HIGP)
in the UH Manoa School of Ocean and Earth Science and Technology (SOEST).


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Iron is highly reactive with oxygen -- forming reddish rust commonly
seen on Earth. The lunar surface and interior, however, are virtually
devoid of oxygen, so pristine metallic iron is prevalent on the Moon and
highly oxidized iron has not been confirmed in samples returned from
the Apollo missions. In addition, hydrogen in solar wind blasts the
lunar surface, which acts in opposition to oxidation. So, the presence
of highly oxidized iron-bearing minerals, such as hematite, on the Moon
is an unexpected discovery.

"Our hypothesis is that lunar hematite is formed through oxidation of
lunar surface iron by the oxygen from the Earth's upper atmosphere that
has been continuously blown to the lunar surface by solar wind when the
Moon is in Earth's magnetotail during the past several billion years,"
said Li.

To make this discovery, Li, HIGP professor Paul Lucey and co-authors
from NASA's Jet Propulsion Laboratory (JPL) and elsewhere analyzed the hyperspectral reflectance data acquired by the Moon Mineralogy Mapper
(M3) designed by NASA JPL onboard India's Chandrayaan-1 mission.

This new research was inspired by Li's previous discovery of water ice
in the Moon's polar regions in 2018.

"When I examined the M3 data at the polar regions, I found some spectral features and patterns are different from those we see at the lower
latitudes or the Apollo samples," said Li. "I was curious whether it is possible that there are water-rock reactions on the Moon. After months investigation, I figured out I was seeing the signature of hematite."
The team found the locations where hematite is present are strongly
correlated with water content at high latitude Li and others found
previously and are more concentrated on the nearside, which always faces
the Earth.

"More hematite on the lunar nearside suggested that it may be related to Earth," said Li. "This reminded me a discovery by the Japanese Kaguya
mission that oxygen from the Earth's upper atmosphere can be blown
to the lunar surface by solar wind when the Moon is in the Earth's
magnetotail. So, Earth's atmospheric oxygen could be the major oxidant
to produce hematite. Water and interplanetary dust impact may also have
played critical roles" "Interestingly, hematite is not absolutely absent
from the far-side of the Moon where Earth's oxygen may have never reached, although much fewer exposures were seen," said Li. "The tiny amount
of water (< ~0.1 wt.%) observed at lunar high latitudes may have been substantially involved in the hematite formation process on the lunar
far-side, which has important implications for interpreting the observed hematite on some water poor S-type asteroids." "This discovery will
reshape our knowledge about the Moon's polar regions," said Li. "Earth may
have played an important role on the evolution of the Moon's surface."
The research team hopes the NASA's ARTEMIS missions can return hematite
samples from the polar regions. The chemical signatures of those samples
can confirm their hypothesis whether the lunar hematite is oxidized by
Earth's oxygen and may help reveal the evolution of the Earth's atmosphere
in the past billions of years.


========================================================================== Story Source: Materials provided by University_of_Hawaii_at_Manoa. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shuai Li, Paul G. Lucey, Abigail A. Fraeman, Andrew R. Poppe,
Vivian Z.

Sun, Dana M. Hurley and Peter H. Schultz. Widespread hematite
at high latitudes of the Moon. Science Advances, 2020 DOI:
10.1126/sciadv.aba1940 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200902152152.htm

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