Astronomy Now - January 2021

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Bill Leatherbarrow

Moonwatch: Noisy neighbour

imagine that we all hope for considerate, quiet and non-intrusive neighbours, but not all of us are
lucky enough to get them. It’s the same on the Moon. Cyrillus, a large complex crater some 98
kilometres in diameter, had long settled into a comfortable middle age, perched between two rings
of the multi-ringed Nectaris impact basin, when a new unruly neighbour moved in to the north-east
and changed things forever.

It is difficult to be precise about when Cyrillus was formed, since its structure is now much degraded,
but it can probably be dated back to the late-Nectarian or early-Imbrian era between 3.9 and 3.8
billion years ago. It clearly formed after the giant impact that created the Nectaris Basin, since it sits
on top of that basin’s rim. It is also ‘young’ enough, in geological terms, to retain reasonably well-
dened traces of its central peak complex and interior terracing – features no longer discernible in
Catharina, its older neighbour to the south.

eophilus, the unruly newcomer to the northeast of Cyrillus, is one of the Moon’s most spectacular
and well-preserved craters. Around 100 kilometres in diameter, it displays to perfection the major
morphological characteristics of a large complex crater. Its rim is sharply dened, rising
approximately 1,200 metres above the surrounding terrain and dropping in precipitous cliffs and
elaborate terracing to the crater oor some 4,000 metres below.

Its central mountains are particularly impressive – a complex cluster of individual peaks rising to a
height of around 2.8 kilometres above the lowest parts of the crater oor. ese peaks show
extraordinary detail in telescopes of moderate aperture and reward careful observation. e oor to
the south-west of the central peak complex appears rougher and higher than that to the north-east.
is may be a result of wall collapse where eophilus has intruded into Cyrillus. e north-eastern
oor of eophilus is lower and smoothened by ponding of impact melt.

It is likely that eophilus formed during the Eratosthenian era, which would place its age
somewhere between 3.2 and 1.1 billion years – but it is probably towards the younger end of that
scale. Its arrival had a devastating effect on the older Cyrillus. Not only did it destroy the north-
eastern wall of Cyrillus, pushing the remains onto its oor, but it also draped the older crater with a
thick layer of rubbly impact ejecta, smoothing out its terraces and central peaks and softening its
entire appearance. A spectacular view from Lunar Orbiter 3 shows this particularly well.

e appearance can be replicated using the 3D-modelling tools in NASA’s QuickMap data set.
Increasing the vertical exaggeration to ×2 helps to bring out the extent to which material from the
eophilus impact has lled Cyrillus and spoiled its good looks.

e QuickMap suite of tools also allows us to generate cross-sectional-elevation proles, and doing
this from south-west to north-east across Cyrillus and eophilus tells us a great deal. It shows the
great depth excavated by the eophilus impact (to the right of the plot) and the relative height of
Cyrillus (to the left), now inlled with impact ejecta. It also, intriguingly, demonstrates that the
north-eastern rim of eophilus is considerably lower (by some 1.2 kilometres) than that in the
south-east, where it joins Cyrillus. is might suggest that eophilus was the result of an oblique
impact from the north-east that pushed material downrange in the direction of Cyrillus. It might
also account for the pools of smooth impact melt that can be seen in a moderate-aperture telescope
outside the northern rim of eophilus.

It has been suggested that inner-wall collapse at the point where eophilus joins Cyrillus might
have sent a tsunami of impact melt northwards, where it sloshed over the lower parts of eophilus’
rim as well as veneering the lower parts of the northern oor.

Finally, take a look at the strange ridges and rilles that cross the oor of Cyrillus in a general south-
westerly direction, one of them even traversing the southern rim of the crater. What are these and
how did they form? Floor-fractured craters are common enough on the Moon, where the fractures
are caused by underlying magma lifting and cracking the crater oor. But the rilles in Cyrillus appear
to lie on top of the ejecta layer from eophilus, suggesting they are relatively young. Chuck Wood,
an American lunar expert, has tentatively suggested that they might be ‘the result of a southward
shove from the formation of eophilus’, or are they perhaps huge ejecta scars caused by blocks of
material thrown out during that impact. As with so much in this intriguing area of the Moon, there
are more questions than answers!

eophilus and Cyrillus should be well seen on 18 and 19 January 2021.

A 3D model of Theophilus, viewed from the south-west. A vertical exaggeration of 2× has been applied using the 3D-modelling
tools in NASA’s QuickMap data set.

Crater Theophilus’ subsequent formation wreaked havoc on neighbouring crater Cyrillus’

erstwhile peaceful existence, writes Bill Leatherbarrow.

Theophilus, one of the Moon’s best-preserved formations, and older Cyrillus, the latter having been devastated by the impact that
formed Theophilus. This image was shot on 26 June 2020.

A cross-sectional elevation prole from south-west to north-east across craters Cyrillus and Theophilus generated by the QuickMap
suite of tools.

View

Moonwatch: Noisy neighbour

January 2021

Astronomy Now