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note: @Mithron's proposed duplicate Why can't rust form without water? does not have anything about the conditions on the Moon, so no, it's not a duplicate.

Several popular news articles mention that "rust" has been found on the Moon, and this is surprising because rust requires the presence of both oxygen and liquid water.

When I open a plastic-wrapped cake or muffin there is often an "O-buster" packet with finely powdered iron. When I open the packet (chemistry is more interesting than cake) and watch the powder, indeed it rusts. Of course air has plenty of both oxygen and water, so this is no surprise.

Question: But why and how is rust forming on moon? Why does it need both oxygen and water there, and more importantly, how is it getting them?

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    $\begingroup$ Hmm,but rust is not the same as hematite, that can be reportedly formed without water, e.g. as result of volcanic activity. $\endgroup$ – Poutnik Sep 9 '20 at 3:56
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    $\begingroup$ @Mithoron no it doesn't, because it doesn't seem to address what's happening on the Moon. Luckily though the answer posted here does! btw it's certainly to simply ask "Does this answer your question" in a comment without simultaneously voting to close, which has the primary effect of preventing other users from posting Moon-based answers. $\endgroup$ – uhoh Sep 9 '20 at 4:19
  • $\begingroup$ @Poutnik I have not followed it closely, but on Mars the presence of hematite, or at least Martian blueberries is often cited as evidence of the previous presence of bodies of water, but I guess it's the shape that's key in this case, not simply the presence of mineral itself. $\endgroup$ – uhoh Sep 9 '20 at 4:24
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    $\begingroup$ @ToddMinehardt can you double check the first sentence, right there at the top, that begins note:... Thanks! $\endgroup$ – uhoh Sep 14 '20 at 23:07
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    $\begingroup$ @uhoh - Done... $\endgroup$ – Todd Minehardt Sep 14 '20 at 23:56
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UPDATE: Today I got a chance to read the abstract the original paper which made news everywhere that moon has rust on the side which faces the Earth.

The authors propose that upper atmosphere oxygen from the Earth reaches the moon. In their words

"Oxygen delivered from Earth’s upper atmosphere could be the major oxidant that forms lunar hematite. Hematite at craters of different ages may have preserved the oxygen isotopes of Earth’s atmosphere in the past billions of years. Future oxygen isotope measurements can test our hypothesis and may help reveal the evolution of Earth’s atmosphere."

Widespread hematite at high latitudes of the Moon in Science Advances (open access

Let us split your query into three parts:

a) Will iron oxidize without water and form oxides?

Iron would happily burn in oxygen to form iron oxides. But we cannot call it iron rust. So iron can oxidize without moisture. In fact, iron powder may spontaneously burn in air.

b) Will iron form rust without water?

Rust is more of a semantic issue. Iron rust may not have constant composition. As per the dictionary definition (OED) "

A red, orange, or yellowish-brown substance which forms progressively as a flaking, permeable coating on the surface of iron and its alloys as a result of oxidation, esp. through exposure to air and moisture.

Therefore, rust is an product of ambient environmental factors that affect iron at (approx.) typical room temperature. Rusting in sea water might different than rusting on a mountain. Acidic gases such as carbon dioxide, sulfur dioxide, nitrogen oxides certainly accelerate this process. Iron rust can be green and it can be common brown version. Since, chemically rust is a hydrated oxide of iron plus its hydroxide, water's presence is a must. More importantly water is a necessary ingredient which works as mediator of the electrochemical cell that forms between oxygen and iron surface. If you search Google Scholar you would find tons of articles on the electrochemical mechanism of iron rusting.

Coming to the more interesting question of rust on the moon:

nasaspaceflight.com: Rust on the Moon. How is that possible without oxygen and liquid water? "But how can rust form far from water ice deposits on a barren oasis devoid of oxygen?" Axios: Researchers find rust on the Moon "Scientists were surprised by the findings because rust requires oxygen and water to form on Earth."

Since you would know more astronomy than I do, wasn't moon a part of the Earth? I recall this from a documentary. I do remember seeing layers of "rust" in moutainous paths, so the oceans had a lot of iron which settled as a layer in mountains. If moon were a part of Earth, why is that surprising?

a) The question is what was the age of this rust?

Secondly, Mars is also "rusty", its surface is highly oxidizing with a huge amount of perchlorate in the soil (there is a long story how that was discovered by the folly of analytical chemists). Who knows what is the moon soil like. What about the shower of radiation on the moon. That may promote oxidation as well.

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    $\begingroup$ Just a small correction: there is almost surely no evidence of life on the Moon. Its creation has been geologically dated to well before any kind of life on Earth, much less multicellular life - Earth was still a molten ball of rock. Furthermore, its genesis was so cataclysmic that any evidence of life would have been destroyed in the first place (as well as resterilising Earth...). The impactor was roughly the size of Mars! That's no small pebble! $\endgroup$ – Nicolau Saker Neto Sep 9 '20 at 10:11
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    $\begingroup$ Good point. I do recall some mention moon fossils from some documentary that must be wrong. $\endgroup$ – M. Farooq Sep 9 '20 at 12:37
  • $\begingroup$ @uhoh, You might have seen the original paper. $\endgroup$ – M. Farooq Sep 10 '20 at 0:26
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    $\begingroup$ @M.Farooq oh, Widespread hematite at high latitudes of the Moon I haven't seen it yet, I need to be in certain places to access paywalled articles, but I'll make a point of getting it today! I've had my morning coffee but it hasn't really kicked in yet, so I'm a little slow right now :-) update: Oh, this one is not paywalled! I can see it right now! $\endgroup$ – uhoh Sep 10 '20 at 0:37
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What is rust?

According to Wikipedia, rust is an iron oxide, a usually reddish brown oxide formed by the reaction of iron and oxygen in the presence of water or air moisture.

So, how does rust is forming on Moon although it has no Earth-like atmosphere, devoid of oxygen/moisture?

According to nasa.gov: moon has trace amounts of oxygen. The source of that oxygen is speculated to be Earth. 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 traveling 239,000 miles (385,00 kilometers) to the Moon.

More on space.com: Earth's magnetotail can reach all the way to the near side of the moon, where most of the hematite was found. Moreover, it was found out that at every full moon, the magnetotail blocks 99% of solar wind** from blasting the moon, drawing a temporary curtain over the lunar surface, allowing periods of time for rust to form. It is also possible that more rust were formed when earth and moon were close together in the ancient past

Where does water comes from?

Water ice has been found in shadowed lunar craters on Moon's surface. But those ice deposits are far from where rust is detected. So, scientist proposed that fast-moving dust particles that regularly pelt on the Moon could release water molecules from ice-water deposits. These dust particles carried the water molecules, gets implanted onto the lunar soil which contains iron, mixes with it and formed rust. Heat from these impacts could also increase the oxidation rate.

**What's with the solar wind?

The solar wind is a 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 since, it's a reducer and while the Earth has a magnetic field and an ozone layer like atmosphere, shielding from hydrogen is possible, but not for the Moon.

Why it hasn't been detected earlier?

This rust formation is not sudden and took years to form, also taking into account the inhibition due to hydrogen from solar wind, the process was very, very slow. Also, the rust was detected on the far-side of the moon, where it is difficult for probes to reach, or proper satellite image was difficult to take, this is a mystery and it will take quite time to solve.

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