The arrival of the first lunar samples

The first samples from the Moon arrived at the Lunar Receiving Laboratory of the Manned Spacecraft Center (now Johnson Space Center), Houston Texas on July 25th 1969.  For the first time in human history scientists would be able to analyse another world at close range – in effect doing astronomy with a microscope. A new science was born – lunar science. In total, after Apollo we have 381.7 kg (841.5 lbs) of rocks and soil from the Moon plus an additional 321grams of soil from the unmanned Russian Luna sample return spacecraft on earth, together with 15 or so lunarites – meteorites that have come from the Moon and landed on Earth.

The Lunar Surface

The lunar surface is made up of what is called the regolith. It is the part we see from Earth, and all of the remote sensing is a measure of the regolith. The regolith is made up of the rocks and soils of the Moon. It covers the surface to a depth of 5 to 10 meters for the lunar maria, and to a depth of 10 to 15 meters for the highlands. The regolith is formed from the constant rain of meteoroids that strike the lunar surface at cosmic velocities of many kilometres per second. They have been raining down on the surface from the beginning of the formation of the Moon, 4.6 billion years ago. The meteoroids "garden" the regolith to this depth and cause erosion of the rocks and boulders, and cause the smooth rounded shapes of the lunar mountains. Before this erosion was recognized it was thought that the mountains were jagged as their shadows fooled us into thinking. The regolith is made of rocks (counted as anything greater than 1cm) and lunar soil or "fines" are material that is less than 1 cm in diameter.

Lunar rock types

There are several different types or classes of lunar rocks and they can be grouped as follows:

Basaltic volcanic rocks.

This includes lava flows and volcanic ash (pyroclastic) rocks. These form deep in the lunar mantle (~ 100 - 400 km down) and rose to the surface through the fractures caused by the giant impact that formed the lunar basins. These then filled with lava forming the maria we see from Earth as the darker parts of the Moon, and is mainly made up of the rock basalt. The maria fills the maria basins, the basin formed first by the impact of an asteroid and maria formed later by lava flows. The basins formed many millions of years before the lava flooded them.

The lunar basalts are very rich in iron and when melted to over 1200° C they flow very fluidly (consistency of motor oil), and can cover very large distances. This also prevents them forming large volcanic cones. We do see structures called domes on the Moon, which are probably caused when the lava viscosity changed, causing it to solidify more rapidly. The lava also cooled at different rates depending on the thickness of the flow, and this resulted in different rock textures. Fine-grained rocks for fast cooling, coarse grained for slow cooling. The course grained rocks could have formed underground, where they are called plutonic rocks, and been raised to the surface during impacts. Basalt lava is composed of the mineral pyroxene and is quite dark.
 
 

Pristine rocks from the highlands.

These are rocks that have their original compositions and have not been contaminated by impact mixing. It is hard to tell them apart from rocks that have been completely melted and reformed later in lunar history as their "clocks" get reset by the impact melting. A common rock found there is the ferroan anorthosite, an iron rich rock that is composed of the mineral plagioclase feldspar, a white mineral containing aluminium.
 

Polymict breccias.

These rocks are made of pieces of previously shattered rocks, re-melted into one consolidated rock by the energy of the impact. Some form impact melts, which are rocks formed from the liquid rock that has totally melted during the impact and then cooled. This forms a new rock made out of the mixture of the melted component rocks’ chemistry.

The breccias tell a tale of great violence during the early history of the Moon. There is even recent evidence from dating the ages of the lunar spherules, which shows that the impact rate has increased in the last 400 million years, after having declined for the past 3 billion years. The craters range in size to the largest in the solar system at 2400km of the South Pole-Aitken (SPA) basin, down to the size of the microcrater.
 

The pyroclastic deposits or volcanic ash.

This is the result of fire fountains on the Moon that sprayed a mist of volcanic glass into the vacuum of space, from deep within the Moon. It cooled in flight and landed as colored glass beads. The orange soil (sample 74220) found at Shorty crater in Taurus-Littrow on Apollo 17, and the green glass (15426) of Apollo 15, are representative of these deposits. The spherules as they are termed, can also be caused during impacts that spray out molten rock that solidifies in flight, forming spherical glass beads.

Lunar Soil or Fines

The soils are the finer material that makes up the Moon’s surface. Soil or "fines" are caused by the erosion of the rocks from micrometeoroids impacts over billions of years. This erosion is about 1mm per million years. The impact of micrometeoroids also make up larger particles called agglutinates, so it is not just all destruction on the Moon. These are welded soil grains, fused by the heat of impact and bonded together by glass. They are irregular in shape. They also contain droplets of fairly pure iron.
Agglutinates cause the light that strikes the Moon from the sun to be mainly reflected back along the path it came, a bit like the reflectorized road signs you see at night. This leads to an interesting effect of the Moon as seen from Earth. You may have noticed that when the Moon is full, that it is evenly illuminated across the entire lunar disk, limb to limb. Now, if you held a ball up to the light and looked at it with the light behind you (as in full Moon), you would see that the brightness of the disk falls off toward the edges. This is because the light is reflected away (at right angles at the limb or edge of the disk), and not returned to you at its full strength. The Moon doesn’t show this effect. Why? Because the light is trapped inside the "fluffy" nature of the lunar soil, and the light is primarily returned back along the incident path. The darkness (low albedo) of the Moon rocks doesn’t reflect a great deal of light back (7% – 24%) but it is enough to make the Moon quite bright in the sky. Also, in the days just before and up to full Moon there is a rapid brightening of the Moon, also caused by this effect. The astronauts also noticed this effect when looking along their zero phase line, which is a direction directly down-sun with the sun behind the astronaut. It caused a bright spot on the lunar surface, in which not much could be seen. The details being all washed out.
 

Visual aspects of the lunar rocks

Above: Author examining lunar samples 12022,92 (Apollo 12) and 76015,143 (Apollo 17) at Johnson Space Center – photo by Lesley Collins

What do Moon rocks look like? They look like very pretty rocks! But it is where they have come from that makes them so special. The first time I saw a Moon rock (77035,99) was when one came on tour to the town where I went to school, way back in 1978, and I was in awe of it. Others said it just looked like a piece of concrete, but I knew it came all the way from the Moon!

Lunar rocks are in some ways very similar to terrestrial rocks; the same categories we use for classifying rocks here can be applied to the Moon, and in other ways they very different. They have zap pits (microcraters), clasts (breccias), vesicles (basalt) or fine grained textures (basalts) that are smooth to the touch. They are hard, and soft (friable) - some broke in the sample return bags on the way back to Earth.

They contain similar Earth minerals like pyroxenes and ilmenite (basalts) and anorthite, olivine and plagioclase feldspar (highlands). Also, some new minerals that are a solid-solution of common minerals on Earth, but in different proportions. Armalcolite is one, tranquillityite is another.

There is a whole new class of rocks called KREEP rocks. They are enriched in potassium (K) and rare Earth elements (REE) and phosphorus (P). They are more radioactive that the other Moon rocks, they are high in thorium and their distribution can and has been mapped from orbit, first with Apollo, then globally with lunar prospector.

Lunar rocks are generally gray to black in color; some, (anorthosites) are white, and even green. Buzz Aldrin said he saw a purple rock, but did not bring it back, so only he knows if it really was purple; and brown ones. Some are glassy, some are friable while others are very hard. This makes cutting sections of them very difficult and is done slowly, as water cannot be used as a lubricant for the saw. They are cut with a stainless steel bandsaw with diamonds in the cutting edge. There are some rocks that are coated by a patina or desert varnish, caused by "space weathering" by the relentless and unfiltered solar wind over the many hundreds of millions of years that the rock as been exposed. One of the most interesting aspects of seeing a lunar rock up close is seeing the zap pits formed from these micrometeoroids, no Earth rock has them due to our shielding atmosphere. Not all the lunar samples have zap pits, it depends on how they were sitting on, or under the regolith, and whether they were sheltered from direct exposure or not by another rock. Also it depends how long a rock has been on the surface. Gardening of the regolith covers and uncovers rocks as craters excavate them. There is a lot of vertical mixing, but less lateral mixing than would be supposed, as rocks tend to be dug up, but not thrown all that far when a crater forms – a small crater that is.

In the highlands, sampled by Apollo 16 (and also parts of the traverse of Apollo’s 14, 15 and 17) the rocks are almost entirely breccias. The lunar maria are mainly basalts or regolith breccias, that is soil that has been fused into rock by an impact.

Some Moon rocks are vesicular, and have large holes caused by some gas, as yet unknown, it could have been carbon monoxide, that has caused the molten lava to have frothed on exposure to the hard vacuum of the Moon.

Lunar Origin

Theories of lunar formation still require refining. We now believe we know how the Moon formed – a big whack of a Mars size planetesimal that knocked part of the Earth’s mantle into space that then coalesced into the Moon we see today. This was first proposed publicly in 1984 at a conference in Kona, on the Big Island of Hawaii, on the origin of the Moon. Before then none of the theories seemed to explain what was found in the lunar samples.

The other major theory is that the Moon was captured after having formed elsewhere in the solar system, most likely from an orbit near to the Earth.

The ages of Moon rocks are between 3.2 and 4.6 billion years old, which is the age of the Moon itself. The youngest Moon rocks are as old as the oldest Earth rocks, so they can tell us a bit about what the early Earth must have been like – impacted like the Moon we see preserved in the highlands of the Moon today.

The chemistry of the Moon is different than the Earth. The Moon has been heated to high temperatures during formation, which has driven off the volatile elements (low melting points) and retained the refractory (high melting point elements) and formed from these. The Earth as a balance of both, and the lava from the Moon has a higher content of Iron, Magnesium and Titanium (5-7%) (more mafic) than does the Earths basaltic lavas (more felsic). There are no sedimentary rocks on the Moon, and the highlands are composed mainly (as far as is know from the limited sampling sites) to be mainly breccias and anorthosite. This anorthosite tells something very important about the early Moon. It seems to have been molten to a depth of maybe 400km and the lighter weight rocks (plagioclase) floated to the surface forming "rockbergs" and the heavier iron and magnesium rocks sank. These later erupted to the surface as lava after the major impact basins formed after the "Magma Ocean" solidified. So the Moon is a differentiated planet with a unique history of its own, just like the Earth. This was a surprise to some investigators who thought prior to Apollo that the Moon was a cold undifferentiated body – but before the return of samples, it was anybody’s guess as to what would be found – Apollo closed the bets and the dice was rolled! We now know that the Moon is an evolved planet, just like the Earth.

Research is Continuing

The Moon rocks continue to be kept at the Johnson Space Center, in building 31N which was built in 1979. They are protected by more than 18 inches of concrete and steel, and a sophisticated motion detector system. Inside this, they are protected in stainless steel vaults and the rocks are stored in gaseous nitrogen as it is fairly unreactive. If they were stored in oxygen they would rust. A representative sample collection is stored at Brookes Air Force base in San Antonio as backup is something happened to the Johnson Space Center.

Now we know the basics of the Moon, and lunar science research is continuing with about 1000 samples being sent out to scientists each year.

Perhaps in the not too distant future we will have new samples from the Moon from areas not yet visited. There is much more to learn from our mysterious Moon.