Thread by @TheEarthStory

On Friday, NASA announced 3 companies have been contracted to land payloads at 3 sites on the Moon over the next few years: Mare Imbrium, Lacus Mortis, and Oceanus Procellarum. But, as far as I can tell, no one wrote anything about the geology of those sites, so let's do it.

It looks like all 3 of these sites were picked based on mission requirements; safe landing and so forth. However, all 3 of these sites fit into the story of the Moon that was originally figured out based on rocks returned from Apollo spacecraft.

Here is Lacus Mortis, on the lunar near side. This is the proposed landing site for missions by @astrobotic

Images from Wikipedia. commons.wikimedia.org/wiki/File:Loca…

@astrobotic Here is Oceanus Procellarum, also on the lunar near side, landing site for @Int_Machines of Houston. Image from NASA/ASU, edited by: slate.com/technology/201…

@astrobotic @Int_Machines Finally, here is Mare Imbrium, the target landing site for @OrbitBeyond . This area is one of the most prominent features on the lunar near side, you can see it any night you have a good moon view. Edited image from earthsky.org/space/protopla…

@astrobotic @Int_Machines @OrbitBeyond All 3 of these landing sites have some stuff in common. When you look at the moon, they are all "dark". That means they are all "Maria", the features originally named because they looked low and flat, as though they were lunar seas like our oceans.

@astrobotic @Int_Machines @OrbitBeyond The first Apollo spacecraft landed on Lunar Maria also, Mare Tranquillitatis - the Sea of Tranquility, was the landing site for the Apollo 11 mission) These sites were chosen because they were relatively safe, flat places to land - same reason these sites were likely picked.

@astrobotic @Int_Machines @OrbitBeyond Ok, so why are there bright and dark regions on the Moon? Why are the dark regions lowlands, flat, and easier to land on? What is interesting about that?

Welcome to the geology part of the story. The answer to that question stretches back 4.5 billion years.

@astrobotic @Int_Machines @OrbitBeyond Scientists are still working out all the details, but our moon, with 1% of Earth's mass but a far smaller core, is thought to have formed when a giant protoplanet struck the proto-Earth about 75 million years after the start of our solar system.

@astrobotic @Int_Machines @OrbitBeyond Here's one simulation of that style of impact, by @MikiNakajima1. In this case, the object that hits the Earth is the size of Mars. It has so much energy that it superheats the whole earth and shoots out Debris, that debris is our moon

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 Here is a recent simulation by @SarahTStewart where the shape after the impact is as "Synestia", a rapidly spinning, donut-shaped disk, where the Moon pops out of the debris at the edge of the spinning disk

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Anyway, the even that formed the moon is violent.y. If you were standing on the Earth, there is nothing left of you.

Colliding 2 things that big is a huge amount of kinetic energy, all converted to thermal energy (heat).

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart For like 1000 years after this giant impact, the Earth's surface was literally as hot as the surface of the sun. Something greater than 5000 K. The Earth was likely molten to nearly 1000 kilometers (500 miles) deep. That's like molten rock at least as deep as Florida is long.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart When all the debris came together to form the Moon, it was equally hot. Collide all that debris in Earth's orbit and you have a huge amount of energy. The Moon's radius is 1079 km, imagine the moon molten to 800-1000 kilometers deep.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart The entire mantle of the moon was molten. That heat has to leave, it is gradually going to escape out to space. Removing heat from a liquid at the top means the liquid is going to convect. So, the moon's mantle churned and mixed until it got cold enough for crystals to form.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart The first mineral to form from the cooling lunar mantle is the most common mineral in Earth's upper mantle, and a large chunk of Earth's mantle: olivine. When you find a chunk of Earth's mantle, ~60% of it is olivine, a pale green mineral. flic.kr/p/2bFwdpf

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Olivine is denser than magma. As magma cools, the olivine it forms is denser than the magma. That means the olivine crystals will sink once they get big enough.

This would create a lower mantle of the moon that is a giant, few hundred km pile of olivine crystals.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Eventually other minerals, pyroxenes, start crystallizing also, but these are also denser than the lunar magma ocean, so they also sink. This concept is the lunar magma ocean model - created based on Apollo samples. Most of the Moon's mantle settled out of this magma ocean.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Imagine that you take a glass of seawater and evaporate it. For a while, nothing happens. Eventually, it gets so concentrated that salts start forming - gypsum, halite, and sylvite. This process is similar to minerals forming from the Moon's magma ocean. One mineral then the next

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart When the moon is about 80-90% solidified, something weird happens. It forms plagioclase, the most common mineral in Earth's crust. All the white stuff in this image is plagioclase flickr.com/photos/violetp…

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart That image is a thin section, taken of a thin slice of rock under a microscope.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Plagioclase is interesting. Both olivine and pyroxene minerals have iron in them. They look green because of iron. The iron is a heavy element, that makes those minerals dense.

Plagioclase has calcium, aluminum, silicon, and oxygen in it. These are all less dense than iron.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart All the other minerals are dense and sink. Plagioclase, however, is less dense than the magma. It floats, like the ice on top of my water glass. This is the lunar magma ocean model: a mantle of crystals that sink and a crust of plagioclase that floats psrd.hawaii.edu/Aug11/LMO-crys…

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart ALL of the light areas on the moon's surface? They are dominated by plagioclase, which looks white when you see it. Those are the old areas of the moon's crust, the plagioclase that floated to the moon's surface as it crystallized en.wikipedia.org/wiki/File:Full…

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart The dark areas did not form until after the moon fully formed and crystallized. They are related to the magma ocean, but there is another step or a few in-between.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Remember my metaphor for evaporating the ocean? If I take ocean water and evaporate it, the salt doesn't evaporate. It would form salts that are left over. When the Lunar Magma Ocean crystallizes, the last bit of it is a few layers that are left over.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart These layers have all the elements that don't fit into the main minerals that form.

Olivine takes iron, magnesium, and silicon into it. Plagioclase takes Calcium, aluminum, silicon, and sodium. What is abundant and left over?

Titanium. Ti is a big part of the story of the moon

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart As the moon finishes crystallizing, it forms 2 other interesting rock types.

One is rich in Titanium.

The other is rich in "Everything else". Everything that is left over includes elements like potassium and the "Rare Earth Elements" at the bottom of the periodic table.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Some of the Apollo rocks were recognized to be rich in potassium (K), rare earth elements (REE), and phosphorus (P). This was likely the last layer to form as the lunar magma ocean solidified.

Rocks with elevated levels of these 3 elements are called "KREEP" rocks.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart KREEP = high in Potassium (K), Rare Earth Elements ( REE) and Phosphorus (P).

KREEP forms as a natural part of crystallization of a magma ocean. It is the very last bit of molten rock, left over after everything else solidifies.

Our moon has this.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart So, after the moon finishes crystallizing, we have:

A plagioclase rich crust
A KREEP layer
A Titanium Rich Layer
A mantle of olivine and pyroxenes

2 of these are important later, the Ti layer and the KREEP layer.

The Ti rich layer acts first.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Titanium is a dense element. Titanium rich minerals are dense compared to the entire lunar mantle. They crystallize at the top of the mantle, but as soon as they form they want to sink.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart This Ti-rich layer will probably sink as blobs or diapirs moving through the lunar mantle. (Image: psrd.hawaii.edu/Mar06/mars_mag…).

Other parts of the mantle must upwell to take the place of the Ti-rich layer.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart At the surface, any time you wanna melt something, you add heat to it. You wanna melt ice? You add heat, warm it up.

Planets work differently. Most planets, if you wanna melt something, what do you do? You drop the pressure. Any time anything moves up, it might melt.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart Stuff moving down in the mantle of the moon? It's going up in pressure. It WONT MELT. Pressure is going up, it won't melt.

Stuff that flows up? It might melt. Anything that melts early out of the Mg-rich lunar mantle could be the Mg-suite.

Talk to @OMGsuite he asked for it.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart @OMGsuite So early in the moon's history we have stuff that melts as the mantle overturns.

Then, 2 other things happen.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart @OMGsuite The Ti-rich stuff sank down near the moon's core. The moon's core is the hottest place on the moon. It is going to heat up over time from all the heat that surrounds it. This will make new Ti-rich magmas.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart @OMGsuite The KREEP layer contains all the radioactive elements, including potassium, uranium, and thorium. Wait a billion years and this layer will heat up due to radioactive decay.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart @OMGsuite Given enough time for heat to build up, radioactive decay can warm rocks up to the point that they are even hotter than the moon's core. Both the Ti-rich-layer and the KREEP-layer will get so hot that they will melt.

@astrobotic @Int_Machines @OrbitBeyond @MikiNakajima1 @SarahTStewart @OMGsuite So first: The moon forms a plagioclase rich crust. You can see this layer from your house.

Then, the Moon's mantle overturns, and radioactive decay happens. This will lead to much later melting of the other layers in the mantle.

Now, remember how I said in this thread that in a planet "Dropping the pressure" makes things melt?

Start with a lunar crust. What is going to make something come up through the crust? A gigantic crater. Blast away the crust, something has to fill the space.

After the moon formed, it was plastered by craters. We're not sure exactly the timing of all them, but the biggest is on the lunar far side. It's diameter is 1/4 of the Moon, I believe the largest known crater in the solar system, the South Pole-Aitken Basin crater.

The South-Pole Aitken Basin crater is 1500 miles in diameter. That's the distance from Los Angeles to Nebraska.

The Chinese Chang'e 4 spacecraft is currently inside this crater. It can't be seen from Earth, it's only visible on the lunar far side.

On the lunar near side, the side facing Earth, there were big impacts that punctured through the plagioclase rich lunar crust.

When a big asteroid blasted a hole in the crust, it created a void. The moon's mantle flowed upwards to fill these voids.

Here's the Moon's gravity map from @GRAIL_101 . Areas in red are extra density. These are giant craters that formed early enough for the mantle to rise up and fill the giant gap. They were called "Mascons" in the Apollo days because they were concentrations of Mass, extra gravity

@GRAIL_101 Ok, so start off with a big, plagioclase-rich crust. Occasionally things blast giant holes in this crust.

These giant holes are great places for new molten rocks to erupt.

New molten rocks - those are the ways you make the dark spots on the Moon.

@GRAIL_101 These dark areas are low, they filled in with rocks after the lunar highlands formed, early telescopes saw them as flat. These are the lunar "Maria", the seas.

These seas are the targets of the next landing missions on the moon.

The rocks in the lunar Maria are a consequence of melting after these craters formed. As the lunar mantle shifted, it re-melted as it decompressed, and the magma created by these processes concentrated in the low spots, the craters.

What rock types are we going to have to deal with?

We have Ti-rich basalts. The Ti-rich layer that sank to near the Moon's core heated up, started melting, and created magma.

The KREEP rich layer heated up from radioactive decay, that heat also generated magma.

These 2 rock types dominate the igneous rocks that humans will land on if the next batch of missions fly.

Let's start with Mare Imbrium. It's a fascinating place. The Chinese Change'e 3 probe landed on this area and measured the rock compositions in situ.

Several spectrometers, including the Clementine mission, measured the composition and mineralogy of the Moon's surface from orbit. The red areas in this image are high iron and mostly are recent lava flows. lpi.usra.edu/lunar/missions…

All 3 of the proposed landing sites show up as red in this image.

All 3 of the proposed landing site are on Mare basalts. These are therefore lava flows that formed 100s of millions of years after the moon finished cooling. The crust filled, then new lava emerged from giant volcanic systems to fill these 3 craters.

Mare Imbrium contains several different rock types, most notably a portion of the high-titanium basalts. That means as the Imbrium basalts were forming, the early-formed Ti-rich layer was melting to contribute to the Imbrium basin.

The @LRO_NASA LROC instrument can detect Ti bearing minerals and the Apollo 11 landing site in the sea of tranquility is loaded with them, the reddish things in this shot are the lower-Ti rocks outside the mare basin itself. Blue is high-Ti phys.org/news/2011-10-s…

@LRO_NASA The Chinese Chang'E 3 rover landed in this basin and measured the composition of the rocks it was sitting on, confirming orbital measurements. The rocks in Imbrium are moderate to high-Ti basalts. As the Ti-rich layer melted, it flooded low spots, like the Imbrium crater

@LRO_NASA Imbrium is an enormous impact basin. Estimates say that it probably formed about 3.8 billion years ago, at the end of the period of lots of meteorite impacts throughout the solar system.

@LRO_NASA The Imbrian period is the first time period of the moon officially defined based on a certain area, as is done on Earth en.wikipedia.org/wiki/Lunar_geo…

@LRO_NASA So Mare Imbrium, the target landing site for the @OrbitBeyond landers, will land on basalts that erupted after the Imbrium basin formed, including high-Ti basalts erupted from the high-Ti layer that sank to the deep part of the lunar mantle after it formed.

@LRO_NASA @OrbitBeyond The main research targets will include the rocks, but will also include how the rocks are interacting with the space environment, including how hydrogen from the solar wind is implanted in the soil theverge.com/2019/5/31/1864…

@LRO_NASA @OrbitBeyond The rock composition may not be the main point of this lander, but it will still be interesting no matter what it is, as long as it is measured. How high in Ti is it? can we measure an age?

@LRO_NASA @OrbitBeyond Characterizing rocks across Imbrim is also hugely important as Imbrium is likely also a contaminant in every rock we have sampled from the moon. the Imbrium impact fired debris across the entire lunar near side, so basically ALL the Apollo sites contain ejecta from Imbrium

@LRO_NASA @OrbitBeyond Imbrium and the ejecta from it are therefore defined as the earliest geologic period on the Moon.

@LRO_NASA @OrbitBeyond The landing site targeted by @astrobotic is also a moderate to high-Ti basalt field like imbrium, a smaller crater called Lacus Mortis. This is a flat, dark area, but with a later impact crater imposed on top of it en.wikipedia.org/wiki/Lacus_Mor…

@LRO_NASA @OrbitBeyond @astrobotic The crater at the center is mostly uninteresting to this mission. Instead, they are taking advantage of the fact that this crater is filled by a huge lava flow to target what they think is a lava tube, a giant cave on the Moon.

@LRO_NASA @OrbitBeyond @astrobotic Lava tubes form when the surface of a lava flow solidifies and cools rapidly, while the inside is still hot and flowing. Once eruption stops, lava flows away, leaving a gap beneath the solidified crust

@LRO_NASA @OrbitBeyond @astrobotic Similar lava tubes are observed on Earth. The Thurston lava in the @hawaiivolcNP is several hundred feet long with a solid shell above. commons.wikimedia.org/wiki/File:2013…

If you're concerned about the radiation environment for living on the moon, these lava tubes are a natural safe spot

@LRO_NASA @OrbitBeyond @astrobotic @hawaiivolcNP The @astrobotic mission to this site will hopefully include a small rover. If landing is precise enough this rover may be able to explore a lava tube, a site like humans have never visited on the moon, but a HUGELY important site for human habitation of the moon.

@LRO_NASA @OrbitBeyond @astrobotic @hawaiivolcNP Finally, Oceanus Proellarum is a site humans have never visited on the Moon. It is perhaps the largest flat area on the moon, implying huge lava flows after the moon's crust formed. But, it is not in an obvious impact crater, it just covers an area of the surface

@LRO_NASA @OrbitBeyond @astrobotic @hawaiivolcNP Here is a map of concentrations of the element Thorium on the lunar near side. Thorium should be elevated in the KREEP terrain as it behaves like a rare earth element. It undergoes radioactive decay so it can be detected by neutron spectroscopy. en.wikipedia.org/wiki/Lunar_ter…

@LRO_NASA @OrbitBeyond @astrobotic @hawaiivolcNP The Procellarum terrane is rich in Thorium, meaning that this landing site is the best KREEP target of the upcoming mission sites, richest in potassium, thorium, rare earth elements, and phosphorus

@LRO_NASA @OrbitBeyond @astrobotic @hawaiivolcNP KREEP is never 100% of a rock, it is usually a small fraction of the rock. Whether it is 1 or 5 or 10% of a rock is hugely important for how the magma actually formed, so measuring those elements compared to the rest of the rock is a lot of information about how this area formed

Measuring rock compositions in this area will therefore tell us how the KREEP component mixed with other components in the lunar mantle.

Moreso, the KREEP terrain is a fascinating area in th e lunar crust. Why is it only on the lunar nearside? When this compositional measurement was collected it was proposed that all the KREEP rich areas were a gigantic impact basin, a giant crater

Instead, the GRAIL mission measured lunar gravity and found that this area might instead be a huge rift system, fractures that allowed the KREEP component to move up through the curst ntrs.nasa.gov/archive/nasa/c…

While most geologists do not believe the Lunar nearside pattern is a single impact basin, the gravity data does support the hypothesis that the rocks in all these basins are related by a major process in the moon's history

As the moon responded to cooling and magmatic activity after the crust and impact basins formed, a series of fractures developed that allowed magma to well up to the surface, filling the Procellarum basin.

Therefore, to sum up, the 3 mission targets are all basalts that formed after the period of heavy asteroid bombardment. 2 of them filled huge impact crater basins on the lunar nearside. All 3 had something to do with this fracture system.

The rocks that erupted were melted from rocks that formed during the magma ocean. At least 2 of them are high titanium basalts, formed from remelting the high titanium layer formed from magma ocean crystallization.

All 3 of these sites have variable amounts of the KREEP contaminant, the last dregs of lunar magma ocean crystallization.

So, the rocks may not be the main target of these missions, but each of them is in a geologically interesting area. The closest we have come to sampling them is measurements by the short-lived Chang'e 3 rover in Mare Imbrium.

Anything that we measure about the rock compositions at these sites will tell us a part of the story of the moon. How much Ti at each spot? How much KREEP? On Earth, you just sample a unit and see how it changes as you walk along it. On the moon?

Every single rock you measure is a new part of the story. Measure more rocks and you improve your story of the whole body. Every rock measurement is part of the story of the Earth-Moon System.

That's the end for tonight. I'm gonna go watch a movie. Live long and prosper.