While our Moon is airless, research indicates the presence of hematite, a form of rust that normally requires oxygen and water. That has scientists puzzled.
Mars has long been known for its rust. Iron on its
surface, combined with water and oxygen from the ancient past, give the Red
Planet its hue. But scientists were recently surprised to find evidence that our
airless Moon has rust on it as well.
A new paper in Science Advances reviews data from the
Indian Space Research Organization’s Chandrayaan-1 orbiter, which discovered
water ice and mapped out a variety of minerals while surveying the Moon’s
surface in 2008. Lead author Shuai Li of the University of Hawaii has studied
that water extensively in data from Chandrayaan-1’s Moon Mineralogy Mapper
instrument, or M3, which was built by NASA’s Jet Propulsion
Laboratory in Southern California. Water interacts with rock to produce a
diversity of minerals, and M3 detected spectra – or light reflected
off surfaces – that revealed the Moon’s poles had a very different composition
than the rest of it.
Intrigued, Li homed in on these polar spectra. While the
Moon’s surface is littered with iron-rich rocks, he nevertheless was surprised
to find a close match with the spectral signature of hematite. The mineral is a form of iron oxide, or rust, produced when
iron is exposed to oxygen and water. But the Moon isn’t supposed to have oxygen
or liquid water, so how can it be rusting?
The mystery starts with the solar wind, a stream of
charged particles that flows out from the Sun, bombarding Earth and the Moon
with hydrogen. Hydrogen makes it harder for hematite to form. It’s what is
known as a reducer, meaning it adds electrons to the materials it interacts with.
That’s the opposite of what is needed to make hematite: For iron to rust, it
requires an oxidizer, which removes electrons. And while the Earth has a
magnetic field shielding it from this hydrogen, the Moon does not.
“It’s very puzzling,” Li said. “The Moon is
a terrible environment for hematite to form in.” So he turned to JPL
scientists Abigail Fraeman and Vivian Sun to help poke at M3‘s data
and confirm his discovery of hematite.
“At first, I totally didn’t believe it. It shouldn’t
exist based on the conditions present on the Moon,” Fraeman said.
“But since we discovered water on the Moon, people have been speculating
that there could be a greater variety of minerals than we realize if that water
had reacted with rocks.”
After taking a close look, Fraeman and Sun became convinced
M3‘s data does indeed indicate the presence of hematite at the lunar
poles. “In the end, the spectra were convincingly hematite-bearing, and
there needed to be an explanation for why it’s on the Moon,” Sun said.
Three Key Ingredients
Their paper offers a three-pronged model to explain how rust
might form in such an environment. For starters, while the Moon lacks an atmosphere,
it is in fact home to trace amounts of oxygen. The source of that oxygen: our
planet. Earth’s magnetic field trails
behind the planet like a windsock. In 2007, Japan’s Kaguya orbiter
discovered that oxygen from Earth’s upper atmosphere can hitch a ride on this
trailing magnetotail, as it’s officially known, traveling the 239,000 miles
(385,00 kilometers) to the Moon.
That discovery fits with data from M3, which
found more hematite on the Moon’s Earth-facing near side than on its far side. “This
suggested that Earth’s oxygen could be driving the formation of hematite,”
Li said. The Moon has been inching away from Earth for billions of years, so
it’s also possible that more oxygen hopped across this rift when the two were
closer in the ancient past.
Then there’s the matter of all that hydrogen being delivered
by the solar wind. As a reducer, hydrogen should prevent oxidation from
occurring. But Earth’s magnetotail has a mediating effect. Besides ferrying
oxygen to the Moon from our home planet, it also blocks over 99% of the solar
wind during certain periods of the Moon’s orbit (specifically, whenever it’s in
the full Moon phase). That opens occasional windows during the lunar cycle when
rust can form.
The third piece of the puzzle is
water. While most of the Moon is bone dry, water ice can be found in shadowed
lunar craters on the Moon’s far side. But the hematite was detected far from
that ice. The paper instead focuses on water molecules found in the lunar
surface. Li proposes that fast-moving dust particles that regularly pelt the Moon could release these surface-borne water
molecules, mixing them with iron in the lunar soil. Heat from these impacts
could increase the oxidation rate; the dust particles themselves may also be
carrying water molecules, implanting them into the surface so that they mix
with iron. During just the right moments – namely, when the Moon is shielded
from the solar wind and oxygen is present – a rust-inducing chemical reaction
More data is needed to determine exactly how the water is
interacting with rock. That data could also help explain another mystery: why
smaller quantities of hematite are also forming on the far side of the Moon,
where the Earth’s oxygen shouldn’t be able to reach it.
More Science to Come
Fraeman said this model may also explain hematite found on other
airless bodies like asteroids. “It could be that little bits of water and
the impact of dust particles are allowing iron in these bodies to rust,”
Li noted that it’s an exciting time for lunar science. Almost
50 years since the last Apollo landing, the Moon is a major destination again. NASA
plans to send dozens of new instruments and technology experiments to study the
Moon beginning next year, followed by human missions beginning in 2024 all as
part of the Artemis
JPL is also building
a new version of M3 for an orbiter called Lunar Trailblazer. One of
its instruments, the High-resolution Volatiles and Minerals Moon Mapper (HVM3),
will be mapping water ice in
permanently shadowed craters on the Moon, and may be able to reveal new
details about hematite as well.
“I think these results indicate that there are more
complex chemical processes happening in our solar system than have been
previously recognized,” Sun said. “We can understand them better by
sending future missions to the Moon to test these hypotheses.”
News Media Contact
Jet Propulsion Laboratory, Pasadena, Calif.
Alana Johnson / Grey Hautaluoma
NASA Headquarters, Washington
202-672-4780 / 202-358-0668
email@example.com / firstname.lastname@example.org
Source: Jet Propulsion Laboratory