As I told you last year in a series of three presentations (https://www.simonhanmer52.ca/solar-system--origins.html; https://www.simonhanmer52.ca/solar-system--chondrules.html; https://www.simonhanmer52.ca/solar-system--migration.html) the planetary science of our Solar System is evolving - and it’s evolving rapidly. However, in recent months the American administration has been giving direct instructions to NASA regarding its planetary exploration and science program, and in particular with regard to the Moon. With the emphasis placed on sending humans to Mars, the Moon as an essential staging post for such a mission has become something of a political football, with the President of the United States telling NASA to stop talking about the Moon – and to just concentrate on Mars, budget cuts specific to the lunar program, and resignations of recently hired lunar program senior management.
So, it occurs to me that the time is ripe for us as amateur astronomers to take a look at where planetary scientists are in terms of lunar scientific endeavour in 2019. What are the current “hot topics” in lunar science, what kinds of key questions are scientists asking, and what kind of answers are they finding ? As in all science fields, there will be differences of opinion about what’s “hot” and what’s not ! So I make no apologies : what I will present here today is my own personal choice – a linked series of choices, rather than a shot-gun selection.
Before we start : let’s be clear. The most important new scientific endeavour related to the Moon would be re-evaluation of the absolute time history of the major lunar impact basins. I discussed this in 2016 and explained how the timing of lunar impacts is the basis for traditional understanding of Solar System history (https://www.simonhanmer52.ca/lunar-cataclysm.html) and how the traditional conception of lunar impact history may be wrong. Here, however, I want to highlight four principal categories where on-going lunar science has been active - and is currently publishing new results : (1) The age of the Moon; (2) The Moon’s early history - with emphasis on how it formed, the evolution of a global magma ocean, and the origin of the stark difference in crustal thickness and general appearance between the near- and far-sides of the Moon; (3) Recent, possibly modern, geological events on the Moon, and (4) what controls the Earth-Moon separation distance ? So let’s start at the beginning …
So, it occurs to me that the time is ripe for us as amateur astronomers to take a look at where planetary scientists are in terms of lunar scientific endeavour in 2019. What are the current “hot topics” in lunar science, what kinds of key questions are scientists asking, and what kind of answers are they finding ? As in all science fields, there will be differences of opinion about what’s “hot” and what’s not ! So I make no apologies : what I will present here today is my own personal choice – a linked series of choices, rather than a shot-gun selection.
Before we start : let’s be clear. The most important new scientific endeavour related to the Moon would be re-evaluation of the absolute time history of the major lunar impact basins. I discussed this in 2016 and explained how the timing of lunar impacts is the basis for traditional understanding of Solar System history (https://www.simonhanmer52.ca/lunar-cataclysm.html) and how the traditional conception of lunar impact history may be wrong. Here, however, I want to highlight four principal categories where on-going lunar science has been active - and is currently publishing new results : (1) The age of the Moon; (2) The Moon’s early history - with emphasis on how it formed, the evolution of a global magma ocean, and the origin of the stark difference in crustal thickness and general appearance between the near- and far-sides of the Moon; (3) Recent, possibly modern, geological events on the Moon, and (4) what controls the Earth-Moon separation distance ? So let’s start at the beginning …
Determining the age of the major impact that was likely responsible for the formation of the Moon is an inherently complicated business involving comparing and contrasting the measured ratios of various isotopes of various radioactive elements and their daughter products. The work is fascinating, but rather a slog for the non-expert, so I’ve decided to highlight a recent study from 2016 that gives you an idea of how planetary scientists address the subject. Models of the origins of the Earth-Moon system are now well enough refined that they require increasingly precise age data to test them. Absolute dating is achieved in the lab by very precise measurement of such isotopic systems as U/Pb - but then you have to know what do with that precise data ! Remember, we’re talking here about isotopes in a once very dynamic environment : a giant impact ! U and Pb behave very differently in the thermodynamic environment of a major impact scenario. Pb is volatile and U is not (it’s “refractory”). Hence Pb will be lost to open space while U will be retained, with major consequences for the evolution of the U/Pb ratio that will eventually be measured in the lab billions of years later. The 2016 study proposed a two-stage model wherein the first step was the loss of 98% of the Earth’s Pb to open space as a result of the Moon-forming impact. The second stage was calculating how the resulting post-impact U/Pb ratio would evolve over the following billions of years. The result is a calculated age of ~4.42 Ga, ~100 Myr younger than previously thought … and that is significant for scientists trying to understand the Earth-Moon system.
Let’s turn now to the physical conditions of the Moon’s formation. In 2017, I discussed the contradictions between lunar origin models based on Physics and those models based on Chemistry (https://www.simonhanmer52.ca/lunar-origins.html). The theoretical physical models were having a lot of trouble explaining the observed and measured chemical data. To put it simply, the physics models predict that the Earth and Moon should be chemically different in terms of isotopic composition … and the chemistry data demonstrate that this is simply not the case ! In addition, the physics models have trouble getting Fe-Ni out of the Earth’s core and into the Moon. There have been numerous attempts to account for this, including a 2018 physics-based theoretical model of a collision between Earth and an impactor that gave rise to a giant doughnut-like bubble (synestia) wherein Earth and impactor materials got thoroughly mixed … with the Earth condensing from the centre of the bubble - and the Moon condensing from the periphery. Very smart - but quite complex.
Even more recently (2019), another group tackled the issue from a different perspective : they asked what would happen if the Earth still had a global magma ocean at the time of the Moon-forming giant impact ? They proposed that energy from the impact would heat and expand terrestrial magma faster than it would heat and expand the solid rocks of the smaller impactor. Hence, superheated terrestrial magma would expand rapidly into space and contribute hugely to the composition of the Moon, thereby accounting for the chemical similarity of the Earth and the Moon. Their model also involved ejection of terrestrial core material, thereby accounting for the Moon’s Fe-Ni core. It sounds good on paper, and would appear to remove the need for other, complex (unconventional) physics-based models formulated to account for the observed chemistry. However, this model assumes that the Moon-forming collision occurred while Earth was very young – only ~50 Ma after formation of the Sun at ~4.55 Ga. But, linking back to the beginning of this presentation, we have just seen that recent isotope dating models suggest that the Moon-forming collision occurred at ~4.42 Ga … over 100 Ma later ! I guess things are still in play and up for debate on this one !
After the Moon had coalesced from whatever was ejected into space during the Moon-forming impact – however that occurred - it too was covered by a deep, global magma ocean. A current “hot” topic is modelling how this magma ocean cooled and crystallised in order to see what kinds of rocks formed from it. From basic geological principles, it was thought that relatively dense Fe-Mg bearing mineral phases would have crystallised first and sunk to the bottom of the magma ocean, while less dense CaAl-rich plagioclase (a feldspar mineral) would have crystallised later and floated toward the top like pond scum (and formed the “white” anorthosite rock of the lunar highlands). It’s not a bad first pass, but geological reality is more complicated than that.
For a number of years now, planetary scientists have performed crystallisation experiments in the lab using artificial magmas thought to represent what was going on in the primordial Lunar Magma Ocean. However, there’s much debate about exactly what these conditions were. For example, how deep was the magma ocean; what was the crystallisation sequence of minerals; was plagioclase really free to float, or was it tangled with and weighed down by other denser crystals; and what was the effect of water … if any ? Here, for simplicity, I just want to focus on one, double-barrelled – but tangible - aspect of these otherwise somewhat complicated considerations of the Lunar Magma Ocean : Can we account for how thin the anorthosite crust is on the Moon – and why it is thicker on the far-side ? It turns out that standard application of laboratory experiments to the primordial Moon produces a much thicker plagioclase (anorthosite) crust than is observed. In 2017 it was suggested that water in the Lunar Magma Ocean might be responsible for reducing the amount of plagioclase that could crystallise from the magma ocean. However, work published in 2018 suggests that things are far more complicated than this and that various factors can each play a role in limiting plagioclase crystallisation. Clearly, this is an ongoing field of study.
Almost as an aside, this same 2017 study suggests that - theoretically - the differences in crustal thickness between the near- and far-sides of the Moon could also be explained by the intricacies of Lunar Magma Ocean crystallisation … if it was coupled with asymmetrical tidal heating. For this to occur, we have to assume that the Moon was already tidally locked such that the current near-side was already facing the Earth billions of years ago. This would allow for tidal heating of the Moon - due to the non-circularity of its orbit around the Earth - to preferentially heat the near-side as opposed to the far-side. Potentially, this would delay cooling of the Lunar Magma Ocean on the near side, and thereby delay plagioclase crystallisation. Since most of the Ca and Al required for crystallising plagioclase would have been extracted from the magma ocean by crystallisation on the cooler far-side, the hotter near-side crust would end up thinner, as is observed. It’s a neat idea. However, as always in these matters, there is another school of thought !
You may remember the GRAIL experiment (Gravity Recovery & Interior Laboratory) that I discussed in 2013 (https://www.simonhanmer52.ca/grail.html). This was a satellite-based experiment orbiting the Moon that measured the lunar gravity field in great detail. From that new data it was possible to calculate a more precise model of lunar crustal thickness on both the near- and far-sides.
A new paper (2019) suggests that it can match the new GRAIL results by invoking the impact of a body about the size of the asteroid (or dwarf planet) Ceres early in the Moon’s history. Blue colours in this slide represent thin crust, while green, yellow and red colours represent thicker crust on the far-side, with red being the thickest. The star represents the approximate modelled location of impact on the lunar near-side, and the contours describe the resulting giant impact basin cavity.
This diagram shows the effects of the collision, according to the model. The left side of each of the 4 time-stages represents the geological materials involved : a thin crust in yellow, the lunar mantle in light blue and the lunar core in orange. The right side of each time-stage shows the evolving temperature distribution as the impact progresses. Now, the important features as far as we are concerned are : the size and depth of the impact basin, the shape of the ejected material, and its trajectory from the near-side towards the far-side - where it was deposited. It turns out that the amount of lunar material subtracted from the near-side and added to the far-side fits the crustal thickness estimates derived from GRAIL. Which then leaves us with at least two new, potentially viable explanations for the two-faced nature of the Moon. Both studies are still on-going - so the game is still afoot !
Let’s now change perspective and take a look at the very outside of the Moon : the lunar regolith, sometimes referred to as the lunar soil. The image on the left needs no introduction (it nicely illustrates what fine grained regolith looks like), but the colour image does. A recent paper (2018) undertook to model what happened to the early (solid) lunar crust when it was pummelled by large impactors. The image shows a time sequence – starting at the top – from essentially time zero to 1/10th of a second following the strike of an impactor 10m across (this is really fast !). Each time-slice shows a vertical cross-section through the hemispherical zone beneath the point of impact that expands downward as the pressure wave from the impact sweeps through the solid bedrock, reaching about ¾ km down in the lowest time slice (1/10th sec). The colours on the left of each time-slice indicate pressure. The bold red line at the very edge of the downward expanding pressure zone represents the leading high pressure front, while the pale blue that fills the rest of the hemisphere represents a trailing zone of negative pressure. Don’t confuse this hemisphere with the crater cavity : this is all within bedrock. The red on the right side of each time-slice represents total “explosive” destruction of the initially solid bedrock (dark blue) in response to the initial high pressurisation followed by a catastrophic depressurisation (rebound)as the pressure wave moves through the rock. Notice how the damage is 100% by the second-time slice : it doesn’t continue to follow the still-propagating pressure wave downward. At depth, the pressure represented by the weight of the overlying rock is enough to prevent the rock from expanding explosively and thereby limits the development of lunar regolith at depth in this “small impactor” scenario.
But what happens if we increase the size of the impactor ? Here we see the model results for 3 impactors of different sizes. Note the different depth scales. Again, the red (this time on the left) represents total “explosive” destruction of bedrock by depressurisation after the pressure front has passed through. However, now the regolith development extends to 4 and 20 km as the impactor size goes from 100m to 1 km. The increased impact energy with larger impactor size is better able to overcome the resistance offered by the overburden pressure due to the weight of the overlying rocks. Now look at what happens when the impactor reaches 10 km in diameter. Although the resulting crater cavity extends to greater depths, the explosive damage zone is depth-limited by the overburden pressure due to the weight of the overlying rocks - and has spread out sideways instead, up to 300 km out from the impact site. This is a really neat piece of modelling that clearly shows how effective these impacts would have been in generating the initial lunar regolith. In fact, this evaluation of initial regolith generation gives us a window into just how effective major impacts could be in wiping out the early history of the Moon, hence explaining why planetary scientists are having so much trouble establishing the early history of lunar impacts. By the way, the right side of these diagrams shows the fragment size of the resulting regolith. Pale blue is 1mm to 1 m. That’s a lot of fine grained regolith - and at great depth too !
So far, we’ve been looking at what I consider to be good, solid science based on careful observation, measurement and experiment. Now I want to take a few minutes to look at one of my pet peeves in current lunar science. In 2011, I discussed a suggestion that short, low-relief lobate scarps on the lunar surface are related to ongoing thermally-driven global shrinkage of the Moon- in the very recent geological past (https://www.simonhanmer52.ca/shrinking-moon.html). I pointed out that if the Moon was going to shrink globally, due to cooling, it would have done it early in its history - not late ! This is the kind of thing proponents of this idea are talking about, which looks quite impressive until you check out the scale. In addition, I’d like you to note the location of this small, very short scarp on low ground at the foot of a steep crater wall : we’ll come back to this point in just a minute.
Well, that was 2011, but in 2019 this idea is back with the suggestion that at least some of these scarps represent faults that are responsible for some of the seismic tremors that were identified by equipment left on the Moon by astronauts in the 1970s. This map of Mare Frigoris, north of Imbrium, shows the distribution of what are known as “wrinkle ridges” (red) that lie within the mare basalts, and lobate scarps – the subject of the study – shown in dark blue. Again, note the locations of the lobate scarps - mostly on low ground adjacent to high ground.
Here’s a close-up, with a scale bar, of the kind of structure that’s been studied : the scarp is interpreted as the surface expression of a fault plane that slopes very shallowly underground to the left. The left side of the image would have been tectonically pushed over the right side, interpreted as meaning that the ground here has contracted between the left and right sides of the image. Again, notice how small the scarp really is.
Press releases for this study include this detailed image of the Lee-Lincoln scarp, located at the Apollo 17 landing site (see inset). By the way, this is the only lunar lobate scarp that has actually been visited on the ground. Note how it sits on the low ground adjacent to nearby high ground. The NASA press releases also includes a very short fly-thru video : https://www.youtube.com/watch?v=xbW-MeXR464. The video clearly shows that the lobate Lee-Lincoln scarp runs across a flat-bottomed valley. Iit came from the left but it clearly runs upslope and onto the steep sides of the flanking hills. If lunar lobate scarps are due to global thermal shrinkage of the Moon then they should be present in bedrock. However, NASA’s web-site that hosts the original video makes it very clear : lobate scarps occur in lunar regolith - not in lunar bedrock ! Take another look at the video : I think you’ll see the Lee-Lincoln scarp as the toe of a landslide that fell from high ground onto low ground.
Where else do we see landslides climbing up slopes ? Here’s a first example from Hebes Chasma on the north side of the Marineris Valley on Mars that I discussed in 2010 (https://www.simonhanmer52.ca/valles-marineris-i.html). The black material on the left of each image has slid down from major cliffs and climbed up the steep slopes of the central mesa.
Here’s a second example : the Frank Slide. This was a major landslide that occurred in 1903 in the Crowsnest Pass of B.C. above the town of Frank. I’m sure you learned about this in geography in high school. Notice on the B&W image, taken in 1911 before the highway was built, how the landslide fell from Turtle Mountain on the left and ran upslope on the right. I strongly suggest that lobate scarps on the Moon are major landslides formed as fine grained lunar regolith slid from high ground to low ground, which would explain why they are not seen in bedrock. Perhaps it is still going on today - but it has nothing to do with global thermal shrinkage of the Moon !
Other authors (2019) suggest that “wrinkle ridges” (similar to lobate scarps, but different, according to some) are the result of modern, ongoing thermal evolution (cooling and contraction) of the basalt lavas of the major lunar mare, as opposed to an on-going global lunar contraction. They think they are between 10 Ma and a few 10’s Ma in age cos of the relationships with very young impact craters : you can see here how wrinkle ridges cut across, and are therefore younger than, small, very recent impact craters. Interestingly, these wrinkle ridges are most common in the inner regions of the major mare basins, so they may have a point. However, the waters get further muddied when the same scientists that proposed a globally shrinking Moon are quoted in a 2019 press release saying that their purported modern lunar faults are the result of the Moon being tidally flexed as it orbits the Earth : what happened to global cooling and shrinkage ? Such flexing involves a few tens of centimetres at the global scale of the Moon, and is elastic. Elasticity means the deformation is recovered at each cycle of flexing : it is not a permanent deformation. It might generate heat, but it is not going to generate a global pattern of faults. As I said, this global lunar shrinkage issue is my pet lunar peeve !
OK, let’s end on a positive note. We all know that the Moon has been slowly migrating away from the Earth since the formation of the Earth-Moon system, and you probably know that this is due to an exchange of tidal energy between the two bodies, coupled with the requirement to conserve total angular momentum of the system. However, that’s not the whole story - as I only recently learned from a 2019 media communication.
The Earth-Moon separation is currently taking place at ~4 cm/yr. However, at that speed it would require 13 Ga for the Moon to drift from adjacent to the Earth at the time of its formation to where it is today : way too long ! It turns out that the exchange of tidal energy between the Earth and Moon is affected by the friction between the ocean water and the ocean floor. However, over geological time plate tectonics has changed the ocean floor configuration, thereby changing the tidal parameters too - and we cannot calculate those changes cos we don’t know enough about plate tectonic history. It also turns out that the rate of separation depends on the actual separation distance and hence slows with time (George Darwin, 1880). However, Darwin’s calculation placed the Moon and the Earth adjacent to each other at ~1.5 Ga : way too early ! To correct for this requires a modern recession rate that is only 25% of what we can measure today - implying that tidal interactions in the Earth-Moon system are abnormally large today : perhaps as a function of the current configuration of the North Atlantic ocean, but because of plate tectonics, this is not going to last ! So the bottom line is that we really cannot trace out the history of Earth-Moon separation after all !
So, what’s my take-home message here ? Difficult to say. Although there’s much excellent on-going lunar science right now, NASA doesn’t seem to be doing a very good job of communicating this to the US administration who foots the bill. In 2016 I discussed a 2009 Community White Paper written by a collective of planetary scientists that laid out the scientific rationale for returning to the Moon if we really want to understand how our Solar System formed and evolved (https://www.simonhanmer52.ca/lunar-cataclysm.html). They were right in 2009 and they are still right today : NASA should look to those scientists for inspiration for their lunar program communications strategy. However, see also my comment on Advancing Science of the Moon : Report of the Lunar Exploration Analysis Group Special Action Team (2017) : https://www.simonhanmer52.ca/planetary-geology.html.
So, what’s my take-home message here ? Difficult to say. Although there’s much excellent on-going lunar science right now, NASA doesn’t seem to be doing a very good job of communicating this to the US administration who foots the bill. In 2016 I discussed a 2009 Community White Paper written by a collective of planetary scientists that laid out the scientific rationale for returning to the Moon if we really want to understand how our Solar System formed and evolved (https://www.simonhanmer52.ca/lunar-cataclysm.html). They were right in 2009 and they are still right today : NASA should look to those scientists for inspiration for their lunar program communications strategy. However, see also my comment on Advancing Science of the Moon : Report of the Lunar Exploration Analysis Group Special Action Team (2017) : https://www.simonhanmer52.ca/planetary-geology.html.