Hanmer articles JAXA

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JAXA's Moon ... As Though you Were There! (March 2010)

by: Simon Hanmer

During the past year or so, the European and Japanese Space Agencies (ESA and JAXA) have successfully pushed the edge of the envelope in terms of very high resolution imaging of our planetary neighbours; Mars and the Moon, respectively.
Here, I present some of the spectacular high resolution images of Earth’s Moon, captured by a JAXA satellite. Some of these images are so detailed that they are the next best thing to being there. Although they contain many details that you will likely not see with an amateur telescope, I think that these images will nonetheless help us to better understand and appreciate what we can see through our favourite scope or binos.
The JAXA satellite, variously known as SELENE or Kaguya, was placed into polar orbit around the Moon, It was equipped with various cameras, including forward and backward looking HDTV. What we’ll examine here is a selection of stills taken from the HDTV movies that JAXA has successfully processed in exciting and imaginative ways. All of these images will be accompanied by an inset of the Moon with a small white box to keep us oriented geographically.
So let’s start looking South over the ever familiar crater Plato, on the north side of the Imbrium impact basin. Notice the very obvious slumping of the original crater walls, and the flat floor that represents the lavas that flooded the crater and almost filled it to the brim – but not quite. To the South, in the distance, you can see the floor of the Mare Imbrium. This smooth floor represents the lavas that filled the gigantic hole left by the Imbrium impact; lavas that left an alignment of relict mountain peaks. These are interpreted by lunar geologists as the relics of the inner ring of the original Imbrium multi-ring basin that formed from a massive impact, 4.0 billion or so years ago.
Now look at the detail of these high-resolution images. Here we see the slumped cliffs of the south rim of the crater Plato. We can clearly see the scars on the cliff faces made by material sliding down the cliffs into the original crater cavity. In mid-field we see Mons Pico and Mons Beta poking up through the Mare Imbrium lavas, but we can also see some remarkable details. Note the small straight linear feature in the left Imbrium foreground; it looks like a very narrow valley with one wall in shadow and the opposite wall illuminated by the Sun. Notice how few impact craters there are in the Imbrium lavas, and the two chains of small pits just north of Mons Pico. Straight features and chains of pits will be a recurring theme here.
Here we are zeroed in on Mons Pico. Notice how the JAXA scientists have manipulated their satellite images to bring the viewer’s sight-line pretty much down to the Moon’s surface, without any appreciable distortion. While this in itself is amazing, take a good look at the resolved texture of the surface around these impact craters. I was expecting to see rock, but this surface is covered with a thick layer of remarkably uniform dust. This is the result of billions of years of “gardening”. That's the technical term for incessant micrometeorites bombardment that, over time, pummelled the rock surface to a powder.
Moving to the SE side of the Imbrium Basin, we see the classical flooded crater Archimedes, with the Spitsbergen mountains in the foreground. The simple impact craters in the basalt lavas that flooded the Imbrium Basin are obvious, but notice the closely-spaced, linear features in the lower left corner that appear to be alignments of what are best described as "pockmarks". These things do not look like aligned impacts produced by meteors that had broken up in flight; there’s too many of them, they're too systematic, and they’re all identical in size. Planetary geologists interpret this kind of feature as a swarm of parallel fractures that guided lava and gas from within the ancient Moon - billions of years ago – making these aligned pockmarks volcanic in origin
Moving a bit further South, and gaining some height, we can clearly see the slumping along the inner wall of the crater Archimedes, with the slumped blocks drowned by basalt lava that flooded the impact crater. Note also the apron of ejecta that was thrown up and out of the crater cavity. Because of the image resolution, we can also make out the variety of different size craters that pepper the mare basalts that flood the Imbrium Basin. Notice how, no matter how small they get, they do not form linear alignments here; so they are probably impact related.
Why is the SE edge of the Imbrium Basin knife-edge sharp at this location? Because this is a large fault, rather like the Eardley Escarpment on the Quebec side of the Ottawa Valley. Notice also the second fault (Rima Bradley) out in the Imbrium lavas. These faults were the mechanism that allowed the walls of the original Imbrium Basin cavity to slump into the hole created by the massive impact that formed the Basin.
Here’s a very unusual perspective on the crater Tycho. The central peak in the middle of the crater is impressive as it sticks up through the rubble of the crater floor. Note the minimal impact cratering within the crater, which is just what we would expect, given that this is the youngest large crater known on the near-side of the Moon. This high resolution image really brings out the fact that the rocks of the crater floor are actually covered with a uniform coat of dust, thin enough here that it can’t quite hide the underlying rocky texture, unlike what we saw at Mons Pico in the Imbrium Basin. In the distance we see the terraced crater wall, as though we were only a few kilometres above the Moon’s surface; and without apparent distortion. Let’s get an even better look.
Here we are looking along the Tycho crater rim, where we can clearly see the steep walls and the down-slope scarring made by the landslides and slumping that gave rise to the terraces that rim the crater floor. But now look for the fine details. Notice the apparently round, illuminated objects in the foreground; they are huge boulders!
Here’s an even closer, more detailed image of the same features. Notice the swath of boulders in the foreground; did they roll down-slope into the crater, or were they thrown out of the crater by the impact? Look carefully and you can see subtle layering in the rocks of the crater terraces, oriented parallel to the terrace walls.. This is the Moon, so it can't be sedimentary layering because there’s no water and no air, and never has been. While it’s not impossible that this represents layers of volcanic ash, it's far more likely that this is layering due to the settling of crystals as the Global Magma Ocean, that formed the early Moon’s uppermost 500 kilometres (at least), cooled and slowly froze shortly after the Moon had formed, Again, in this detailed image we can see just how dusty all these rock surfaces are.
Let’s jump to the North Polar region, where the craters are spectacularly imaged due to the relief brought out by the very low sun-angle. However, the key feature to notice here are the multiple, straight, spaced and parallel fractures that cut left to right across the centre of the image. Remember the similar array of linear features just north of Archimedes? This appears to be the same phenomenon, but without the volcanic pockmarks.
On the subject of fractures, this is one of the most famous faults on the Moon’s near-side. The shadows here tell us that the Straight Wall is a west-facing slope, but look closely and we can see that it's not continuous. It's made up of several, slightly off-set, overlapping segments; the typical pattern of fracture systems. This is a fault formed by the weight of the lavas that fill the Nubium Impact Basin, whose centre is to the left of this image. But what’s the curvilinear feature to the West of the crater Burt? It’s "anchored" at either end by two small craters, and it's made up of a string of tiny pits, rather like a string of pearls. It’s not straight, but - like the Straight Wall - it is discontinuous. This appears to be a curved version of the alignment of pockmarks we saw north of Archimedes; in other words a fracture-controlled volcanic feature. I’ll explain why I think it’s a collapse feature in the next slide.
On the east side of the Mare Humorum is one if the most extraordinary images in this JAXA set. The lavas that filled the mare and flooded, or nearly drowned, the ancient crater Hippalus are themselves cut by three, superbly resolved, parallel rift valleys. As rifts go they are long, skinny and shallow. They also vary in character along their lengths, such that the central one seems to fade out in its mid-section. In fact, if you look very carefully toward the far end, you can see that it's made of at least two segments, which are staggered; just like the feature we saw in the Straight Wall. Skinny, shallow, staggered rifts like these on Venus and Mars have been interpreted by planetary geologists as fractures which guided the rise of narrow sheets of lava from deep down (what geologists call dykes), lava that got close to, but did not reach the surface. When the lava cooled and crystallised, it shrank, leaving a gap just beneath the surface into which the surface rocks dropped like a keystone, thereby creating the appearance of a very shallow, skinny rift valley. There seem to be as many impact craters affecting the rifts as there are affecting the mare basalts, telling us that these dyke-related rifts are also ancient.
Many amateur astronomers are familiar with the amazing Schroter’s Valley near the NW limb of the Moon. It’s well imaged here as an eroded squiggly valley that was carved by basalt lava flowing from the high ground adjacent to the craters Herodotus and Aristarchus, down to the floor of Oceanus Procellarum. Now shift your attention to the foreground of this image. Notice the multiple, wiggly alignments of small pits. Let’s zoom in and change angle.
Several of these squiggly alignments are clearly made up of chains of small pits, identical to each other like peas in a pod, or like pearls on a necklace. To the right we can see a similarly squiggly valley, except that this one is clearly a skinny rift whose northern part has a serrated wall. This appears to be a set of fracture-controlled, volcanic collapse pits that progressively coalesced with one another to form a collapsed rift above a series of squiggly dykes; a variation on the theme of dyke-related rifts that we just looked at in the Mare Humorum.
At the end of Kaguya's mission, JAXA had determined that it would crash into the Moon’s surface near the South Pole, just on the Far Side. The close approach of the satellite has given us unprecedented high resolution images of the lunar surface. Here we can really appreciate the dusty texture of the lunar surface, the result of billions of years of “gardening” by micrometeorite bombardment. This dusty lunar “soil” is technically referred to as “regolith” and is peppered with small impact craters, so it’s not that young. The next image approximates to the red box.
As we drop closer to the surface, the dusty nature of the surface is even more apparent - and you can now see large numbers of pockmarks all over the place. Again, the next image approximates to the red box.
The foreground here looks like a sand dune, but in fact it’s rock coated with a thick, uniform layer of dust. One final red box.
Finally, just before impact, dust is the dominant feature in this image. However, look carefully and you can see that impact-related pockmarks in the dust in the lower left corner are partly covered by dust associated with the large impact crater that dominates the image; a crater that is impact-free. In short, the crater is younger than the pockmarks. This indicates that the meteor that formed the big crater impacted an already thoroughly “gardened” dusty lunar surface with a regolith that was at least as thick as the crater’s cavity is deep.
Many of the features illustrated by these high resolution images will not be apparent even through powerful amateur telescopes, though many will be. Nonetheless, I think that they will stimulate many amateur astronomers to see the Moon through different eyes. Remember the following next time you observe the Moon through a telescope:

- The rocks you are looking at are likely covered by a thick, uniform layer of dust.

- There’s more to the lunar surface than impact craters. Keep an eye out for arrays of parallel fracture systems, and don’t be surprised to see two sets at different angles in the same place. This is actually standard for fracture systems.

- In some cases, these fracture systems appear to control volcanic processes, especially the emplacement of vertical sheets of lava (dykes) and collapse structures above them, which can even lead to the formation of skinny – and observable – rift valleys.

If you want to see these and more Kaguya images in high definition, go to the JAXA web site; the catalogue is extensive and excellent. If you’re hooked on HDTV, go to the JAXA channel on YouTube where you can follow the HD movies that will make you feel as though you are really orbiting the Moon yourself!


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