Lunar Ejecta Rays

Super-moon 2015. The impact crater Tycho with bright ejecta rays radiating from the bottom left of this image are easily visible with low power binoculars.

When we look at the full moon through a low power eyepiece on a telescope, the ray systems associated with the very many impact craters are blatantly apparent. The most easly observed is associated with the crater Tycho.  Although there is plenty of evidence for volcanism on the moon, the majority of craters easily visible are related to massive impacts with asteroids which occurred aeons ago. The bright rays emanating from craters were created by the explosive nature of the impact and the ejection of excavated incandescent rock.

I am grateful to the excellent magazine 'Astronomy Now' for focussing my attention on a not so obvious issue:
" Have you ever wondered why the overwhelming majority of lunar impact craters are circular or nearly circular when, statistically, most incoming projectiles would presumably not have arrived from directly above?" Bill Leatherbarrow- Director of the British Astronomical Association's Lunar Section.

It turns out that unless the angle of incidence is very low indeed the impact crater remains roughly circular.  Once the angle drops below 15 degrees, strange things start to happen to the crater's shape and more dramatically to the pattern of the ejecta rays. In such circumstances a butterfly shape of ejecta is formed - with ejecta being thrown out down range with a zone of avoidance up range from the point of impact and 'butterfly wings' extending out at 90 degrees to the line of travel of the impacting projectile.

The craters: Proclus, Stevinus A and Furnerius A, and Messier and Messier A, show the affects of impacts incident at low angles. In the case of the Messier twin craters they probably show the affects of a glancing blow from one projectile!

The  large crater Copernicus with ejecta rays radiating out more or less equally in all directions from the centre of the crater.  The enormous impactor that created Copernicus would have had an incident angle greater than 15 degrees.

Craters Stevinus A and Furnerius A with their butterfly ejecta ray patterns associated with low incidence impacts

Impacting asteroids coming in from the west at less than 15 degrees

Small craters Messier and Messier A - thought to have been created by one impactor coming in at a very low glancing angle from the east. The 'comet like tail' is the down range ejecta from the double impact.

Credits: Astronomy Now and Bill Leatherbarrow
All images taken from the Jodrell Plank Observatory.

" Each and every day, I like to discover something new and fascinating about the universe outside my own backyard!" Kurt Thrust Director the Jodrell Plank Observatory.