Astronomical Society of Coonabarabran

The Biggest Crater of them all: Mare Imbrium

by Harry Roberts

"What's the biggest crater on the Moon?" - is often asked by children who are viewing the Moon for the first time. It's great to watch as they approach the eyepiece, catch their first glimpse of the real Moon, and "erupt" with delight.

So which is the biggest lunar crater? From here on Earth we can't see giant South Pole Aitken, only some big peaks on its rim, and Mare Orientale hugs the lunar west limb; a difficult target for moonwatchers. So Mare Imbrium, I think, must claim the title of "the biggest crater".

And what a structure it is! At 1300 km diameter it occupies most of the moon's northwest quadrant. Like many lunar craters Imbrium has been flooded with volcanic lava that hides most of its floor features except some protruding peaks. Because Imbrium is so fresh, a mere 3,500 million years old, it postdates much of the moon's terrain, and Imbrium debris can be traced across vast areas of the lunar surface.

Historically large craters were called "basins", but both are now known to be impact formations, the only difference being the size of the impactor and the diameter of the resulting "divot". Impact events on the moon involve extreme velocities, typically 10 to 20 km per second between impactor and target, resulting in a high temperature explosion. Such events are still being modelled using the "Ames High Velocity Vertical Gun" and model targets. While the vertical gun is large, the projectiles it fires are small, usually tiny beads, propelled into targets at 7 km per second!

Real lunar craters are quite shallow; about the same as the saucer under your cup, that is13 times wider than its depth and looks very shallow. A fresh crater like Aristarchus is ten times wider than its depth (10:1). However, lava flooded Mare Imbrium is only 200:1, showing that bigger craters (or basins if you prefer) are shallower than smaller ones.

About half of Imbrium's rim is still intact, the western part having been mostly demolished at the time of the impact. On the north side Sinus Iridum takes a bite out of the crater's rim, a secondary impact that seems to have occurred about the same time as the main event. On the northeast side the force of the impact shattered much of the crater rim leaving a huge strewn field of debris beyond the lunar Alps, with boulders 4 to 5 km in diameter spread over a wide area, "like a flock of grazing sheep" as Don Whiteman aptly describes them. This region is a fantastic sight under low angle lighting. By contrast, there is an open gap in the east rim of Imbrium where the impact overlapped the pre-existing Mare Serenitatis crater, and so no rim formed there (block arrow in fig. 1). The Carpathian Mountains, low and scattered peaks, partly define Imbrium's southern rim.

Now let's look at what is really the most fantastic impact feature on the whole moon, Mare Imbrium's southeast rim, the Apennine Mountains. When craters form much lunar crust is either vaporized or ejected from the site, and the surviving crater rim is compressed and tilted upwards. Since supporting crust is now gone the steep faces partly collapse. The ejected material leaves the impact site as a circular wave of debris, partly molten, partly solid, that covers the area around the site. And these features are what we see in the eyepiece when we look at the Apennines. Consult Rükl's "Atlas" maps 22 and 33. Map 22 shows the curved Apennines well. We see the steep inner (western) faces rising to peaks ~4 to 5 km high. From the chain of peaks outwards we see gently sloping debris fields that blanket the older terrain.

In the centre of map 33 is a giant plume of such debris, a discrete structure that is apparently unnamed. This is material that has been swept out of the Imbrium site and draped over the landscape for a distance of about 400 km and it seems several km thick. It's shown on fig. 1 by arrow B. Another bigger but more diffuse plume further north is labelled A. This plume (B) is the best example of an ejecta landform on the moon, but attracts little attention, and is seldom imaged. The plume is over 100 km wide and 400 km in length. It's well worth close study, containing several volcanic features including nice examples of vents and lava channels. Such features probably result from impact melt flows following the massive cratering event. Much later, quieter lava flows partly drowned the big plume.

Take a close look around gibbous Moon at the biggest crater of them all, Mare Imbrium, and its extensive strewn fields; a real cosmic bombsite!

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