The sky is not the limit for this month’s blog, as our Community Geologist, Dr Ian Kille, takes us on an exploration of the geology of the moon and Mars to reveal more about Mystery Rock 13 from Our March newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
Bedtimes were strict when I was 11, so there was a going-on-holiday sense of excitement when we were woken in the early hours of the morning. Still drowsy, my older brother Steve and I were brought down to sit in front of the black and white television to watch the first man step out onto the moon’s surface. We had followed the whole mission on the BBC from the launch 5 days earlier and watched as the Lunar Excursion Module had separated from the Command Module on the evening of Sunday 20th July 1969. I can remember little of the BBC programme other than the air of excitement and the strange other worldly images that unfolded as Neil Armstrong made his famous small step speech as he bounced onto the moon’s surface. For an eleven-year-old me, the extraordinary achievement was merely what was happening in front of me, my normal. Now, I look back on 
the levels of danger involved, the near total vacuum and absolute zero emptiness of space, the solar radiation, the primitive computing technology, the nail-bitingly fine margin of fuel and the absolute hostility to life of the moon’s greyscale airless surface and I am amazed.
The rock samples brought back have been examined and analysed and we now know a great deal more about the ancient igneous surface of the moon and how it has been scarred by impacts. The favoured theory for its formation has the earth colliding with another smaller, Mars-sized planet named Theia about 4.5 billion years ago. Both 
planets had already accreted enough mass and heat to have differentiated into a metallic core and rocky mantle. The glancing impact resulted in a significant chunk of Theia’s core being dumped inside the earth with the remaining fragmented core and mantle material ejected to form a disc of material rotating around the earth. This fragments then accreted to form the moon with the multiple impacts generating so much heat that the surface of the moon down to a depth of maybe 500km became a magma ocean. The subsequent history of the moon is dominated by two processes. The first is a consequence of asteroid and comet impact, the results of which are an obvious feature of the cratered surface of the moon. The order in which impacts happen can be worked out from the way that ejecta from impacts overlap allowing for a timeline to be calculated for both the impacts and the features they interact with. The size and frequency of impacts 
has reduced significantly over time after a particularly intense phase known as the late heavy bombardment between 4 and 3.85 billion years ago. The second process is the progressive cooling of the moon with its magma ocean consequentially crystallising, with the heavier minerals (olivine, pyroxene) sinking and the lighter ones (plagioclase feldspar) floating, a process called fractionation. The paler grey uplands of the moon owe their colour to these lighter, feldspar rich fractionates. Volcanism is part of this process and vast amounts of magma (and some explosive pyroclastic material) flowed into lowlands on the nearside of the moon to form its “seas” cooling to form the dark basaltic plains of the moon’s Maria. Most of the Maria were formed between about 3 to 3.5 billion years ago but some volcanism may have carried on until about 1 billion years ago, 500 million years or so before multicellular life started to evolve on earth.
The moon is very different from the earth, it has no atmosphere, no magnetic field, no tectonics sufficient to rejuvenate its surface, very small amounts of water and no life. Yet there is a great deal we can learn from the moon’s geology to help understand the Earth’s pre-history. Not least is that early collision with Theia which tells us part of the story of the internal structure and geochemistry of our earth. The Moon’s collision-scarring tells another hidden part of the Earth’s past. The Earth’s tectonic activity means that the surface of our planet continues to be refreshed and reworked. The bombardment the moon suffered would have been equally inflicted on the Earth, however on Earth the scarring has since largely been erased. On a broader scale, our knowledge of the moon’s geology confirms and enrichens our understanding of the way that planets accrete and differentiate. The moon’s early history as a hot tectonically active planet before it cooled and died confirms and informs our knowledge of igneous processes.
50 years on from the Apollo moon landings, there have been no repeat performances. In the intervening time, humans have only ventured as far as the earth’s orbit. This underlines the achievement of the Apollo program not only technologically but also politically. It is only now with a much more robust baseline of technology, a firmer “foothold” in the Earth’s orbit and with the growing involvement of private enterprise that the prospect of manned exploration is in sight, this time to Mars.
Part of the reason for writing this blog now is the current attention on Mars as Perseverance, the Mars rover and its sidekick the Insight helicopter send back yet more astonishing images of the planet. Mars has had quite a lot of attention from unmanned spacecraft in the years since Apollo, with orbiters and several landings with robotic exploring devices.
Mars has more affinity with Earth than the moon albeit doesn’t have the rather exotic collision and core-sharing co-history. Mars in common with the other rocky planets (Earth, Venus and Mercury) has a metallic core a rocky mantle and a crust. As it is larger than the moon its tectonic history stretches almost to the present albeit in its last throes. It still has an atmosphere, and this leads to the interaction between its atmosphere and Mars’ considerable topography. With lower gravity than the Earth, Mars has been unable to hang onto much of its atmosphere which has progressively spun-off into space, leaving a carbon dioxide remnant to kick up dust-devils. There was a time, however, when Mars’ atmosphere was more like that on Earth and there was free water and ice. What this means is that on Mars, we not only see the volcanic activity and the impact cratering but also an impressive range of sedimentary features.
One of the images for this month’s mystery rock for the WallCAP project is an example of this. We know from looking at terrestrial examples that when water or air transport sedimentary material, that they deposit that material in layers. Sometimes, when ripples or dunes are created these layers will be in the form of cross-lamination. The image on Mars was taken by the Curiosity rover and is 
of a rock outcrop named Mont Mercou which is located within Gale Crater on the slopes of Aeolis Mons (also called Mount Sharp). Aeolis Mons is thought to be the remnants of a large stack of layered sedimentary material. In the image of Mont Mercou you can see that it has two parts. The upper portion has cross-bedding in it, which is similar in form to that seen in the other image for this month’s WallCAP mystery rock. This is a picture of wind-blown, aeolian dune deposits from the Yellow Sands Formation at Tynemouth. The lower portion of Mont Mercou is a stack of horizontally laminated sediments which remind me of the layering which can be found in the sediments at Ladycross 
Quarry. The Martian deposits have been interpreted as lake deposits which progressively infilled Gale Crater after its formation between 3.8 and 3.5 billion years ago. To make these sorts of lake deposits would require the repeated, rhythmic influx of flowing water to bring the layers of coarser sediment into the lake. These coarser laminations are separated by thin bands of finer material which imply periods of limited water movement in which these finer grained sediments could settle out. There is an active debate about exactly what processes are reflected in the images being sent back from Mars and I am looking forward to following this story as more information becomes available and the debate continues.
The astonishing range of images and data which Curiosity, Ingenuity, Perseverance, Insight and their ilk are bringing us seems to have become my new normal. There is so much more that could and may well be explored not only within our nearest rocky neighbours but also out to the distant gas giants with their extraordinary moons as well as to the remote ice planets. Yet the long intermission of lockdown has also meant that my other new normal in the physical tangible world, the one that I can touch and see and smell and hear, has been focused down to what may be easily reached from North Northumberland. What is clear, is that I am not going to run out of interest between the local, tangible and touchable and the huge variety of what may be accessed via the low carbon format of the internet.
Acknowledgements and further information
You can follow Curiosity Rover on this interactive map here: https://mars.nasa.gov/msl/mission/where-is-the-rover/
Moon image by Gregory H. Revera – Own work, CC BY-SA 3.0, httpscommons.wikimedia.orgwindex.phpcurid=11901243
Vandelinus Crater: By James Stuby based on NASA image – Reprocessed Lunar Orbiter 4 image cropped in Adobe Photoshop to show Vandelinus crater and surrounding terrain. The original image should be public domain as it is a work of the U.S. Government (NASA). Immediate source: Lunar and Planetary Institute, Lunar Orbiter Photo GalleryLunar Orbiter 4, image 184, h1 [1], Public Domain, https://commons.wikimedia.org/w/index.php?curid=30497934
Anorthosite: a coarsely-crystalline anorthosite from Labrador from the Ten Mile Bay Quarry, near the town of Nain along the Labrador coast, eastern Canada. The quarry exploits the Nain Anorthosite (Nain Plutonic Suite), a mid-Mesoproterozoic intrusion (1.29 to 1.35 billion years) emplaced along the Abloviak Shear Zone. https://commons.wikimedia.org/wiki/File:Blue_Eyes_Granite_(anorthosite)_Labrador.jpg
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Barringer Crater Arizona: By National Map Seamless Server – NASA Earth Observatory, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7549781
Mont Mercou image edited by Kevin Gill: https://twitter.com/kevinmgill
Mars Image: By ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, CC BY-SA IGO 3.0, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=56489423