Harry examines Moretus a crater way down south on the Moon

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A sketch of the lunar crater Moretus with marked features that are described in the text below. Sketch and copyright Harry Roberts ©, all rights reserved

Moretus is a spectacular lunar crater – and one of the more ‘mysterious’. Sited right on the 70ºS line of latitude it is much affected by libration, particularly libration of latitude, and is not always seen well. On 2012 Nov 24 the libration in latitude was a favourable -5º and despite constant cloud I got a quick sketch (Fig).

We see a deep crater 114km wide, forty percent larger than nearby Tycho, with a strikingly tall central peak and steep crater walls: a wonderful sight.

Moretus is said to be an ‘Eratosthenian’ impact, i.e. over 1 By old. However it looks no older than Copernicus (200 My) and hasn’t been much modified: we can say it’s a ‘pristine formation’. The full moon view (Fig, LPOD) shows Moretus, Clavius and Tycho – with a Tycho ray across Moretus’ floor.

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The region of the Moon near Moretus includes the large craters Clavius and Tycho. This is a detail from an image by Dzmitry Kananovich on the lunar photo of the day website for 19 November 2012

In fact Moretus is very like Tycho but is deeper with a taller central peak. Because of its high south latitude we look down into Moretus as if in lunar orbit – and I have oriented my sketch to show an ‘astronaut’s-eye’ view of an iconic impact crater!

Like most of the limb craters, Moretus has had little in the way of lava flooding – compared to a crater like Copernicus, say, where more of the central peak formation is covered. Orbiter imagery of Moretus shows only shallow ponds of lava on the crater floor.

Moretus’ central peak is quite spectacular: tall white and pyramidal – in fact almost 2.7km high, amongst the tallest. This is (or was, really) the most ‘mysterious’ feature of Moretus.

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A section across Moretus showing the depth of the crater and the central peak. Note that the top version exaggerates the vertical scale. Compiled by Harry Roberts from Lunar Reconnaissance Orbiter Camera data

The volcano: For a long time astronomers (particularly amateurs) were convinced they could see a distinct crater atop the peak – and this convinced generations of Moon watchers that the whole formation was a large volcanic cone. If you are lucky enough to own a copy of Harold Hill’s “Portfolio of Lunar Drawings” (the hardcover version is perhaps available online. Beware of the very poor quality soft cover version.) you can follow his attempts, over decades, to resolve the reality of the “vent” in the central peak. At times he was quite sure of its reality, as his sketches show. However, as he laments (p122), the Lunar Orbiter images did not confirm this – and it now seems to be an optical illusion due to small-scale topographic details.

On the night, I too saw the mountaintop ‘vent’ – as the sketch shows. As we all know, volcanoes are positive relief features: i.e. they stick up above their surroundings (e.g. Hawaii). Yet the craters that had the central ‘volcanoes’ were all deep holes in the Moon’s surface. This ‘slight’ problem with the volcanic theory did not deter its proponents.

The Moon has many small volcano-like formations but nothing akin to a terrestrial volcano – the products of complex geology on Earth.

However, Moretus does have a range of landslip formations on its inner walls, some are captioned on the sketch. Y-YI is a large scree-slope where the original wall has collapsed into the crater, creating a ‘rubble’ slope with ‘toes’ where material has invaded the crater floor (captioned X-XI) almost to the central peak.

Z-ZI are perhaps rotational slippage blocks that have, it seems’ formed terraces in the crater’s western wall, and have also eroded much of the crater’s original rim: a process called scalloping. In fact all Moretus’ SW rim has been remodelled this way.

By contrast, the eastern wall of Moretus (left hand side) is almost ‘pristine’– there we see the steep inner wall created by the impact event, with many narrow terraces, at the top of which is the original rim still quite intact: all in partial shadow at the time – a spectacular sight!

Detail on the crater floor eluded me despite the high magnification (416X)- but there are some small ridges and one crater that I should have seen (site marked +). I did log a small mountain near the central peak, marked ‘m’.

The are few named formations close to Moretus: secondary crater Moretus A is dotted to the right (west) and an apparently unnamed crater on the SE limb is perhaps an earlier impact than Moretus.

The bar scale (25km) gives an idea of the size of this magnificent formation: one that preserves near pristine landforms and can be viewed in the ‘scope as if from orbit above the site: A rare and breathtaking ‘telescopic treat’.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers. He is the recipient of the Astronomical Society of New South Wales’ McNiven Medal for 2012, which is the highest award from the society.

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