This week I’m concentrating on the Aristarchus region. Aristarchus crater was named after the Greek astronomer Aristarchus of Samos by an Italian mapmaker called Giovanni Riccioli.
The crater is relatively young, being formed approximately 450 million years ago and is one of the brightest craters on the nearside with an albedo almost double that of other similar features. It has a diameter of between 40 and 45km and a depth of 3.7km.
The following image of the Moon was taken by Jules and shows Aristarchus as the bright spot in the top left quadrant. It can be seen by the naked eye.
The Aristarchus area of the Moon is one of the most interesting and diverse regions on the nearside. It consists of a rectangular shaped plateau about 200km across which is located in the middle of the vast Oceanus Procellarum mare. This plateau was probably uplifted and tilted when the Imbrium basin was formed and has experienced much volcanic activity. The largest sinuous rille known, Vallis Schroteri, is found here and is about 160km long and up to 11km wide. The second article at the end of this post has a very interesting description of the head of the Vallis Schroteri rille and some great images.
The Aristarchus region has had many transient lunar phenomena (TLP) reported. When the Apollo 15 lander passed over this region in 1971 it recorded a significant rise in alpha particles which was believed to be caused by the decay of Radon-222. This radioactive gas has a half-life of only 3.8 days and is thought to be released through tidal stresses.
The following image shows the Aristarchus region. Aristarchus crater is on the left with Herodotus crater on the right and the Vallis Schroteri rille starting below it.
Nasa image from the Apollo 15 Mapping Camera
Some images of the Aristarchus area posted by users of the Moon Zoo forum:
Part of the collapsed walls of Aristarchus. Geoff
Latitude: 24.199° / Longitude: 312.434°
Impact melt in Aristarchus crater. Tom128
# ID: AMZ10036sv
# Latitude: 24.3744° / # Longitude: 312.405°
Interesting terrain in Aristarchus area. DJ_59
Latitude: 23.9853° / Longitude: 312.908°
More interesting terrain. Thornius
# ID: AMZ2000985
# Latitude: 23.5672° / # Longitude: 312.451°
An article from LROC about the geology of the central peak of Aristarchus crater and how the different rock types exposed by the impact help geologists to see what the interior of the Moon is made from.
An article from the Space Fellowship site discussing the Cobra Head feature which is thought to be the source vent of a tremendous outflowing of lava that formed the Vallis Schroteri rille.
In Anthony’s Looking For Change blog he says that we should be looking out for any interesting surface features that may have been freshly disturbed or contain a vent from which outgassing has occurred. “TLP” projects have been set up in the forum for each of the features mentioned, except for Deep Seated Fractures and another feature, “Atmospheric” phenomena (which was not mentioned by Anthony).
Deep seated fractures can be found on some images from LROC but “Atmospheric” phenomena will probably not be found. Some images taken during the Apollo project had fuzziness along the horizon when it was expected that the horizon would be sharp-edged. This may have been caused by dust particles suspended above the lunar surface by electrostatic charging.
Deep Seated Fractures
The one feature which Anthony mentioned and which is not covered in the other TLP Project threads is deep seated fractures and he uses an example from the floor of Tycho crater:
Tycho Crater floor, from LROC image M114031031LE
One of the Moon Zoo users, ElisabethB (Els), found something similar near Proclus crater:
Sun Angle: -76.23°
Scale: 0.50 meters / pixel
Zoom Level: 3
Apollo 17 astronauts saw and sketched what they called “bands”, “streamers” and “twilight rays” which were visible just before lunar sunset or sunrise. These rays were also seen by astronauts on the Apollo 8, 10 and 15 missions.
from Nasa Media Library
On the side of the Moon in daylight, the solar radiation knocks electrons out of atoms and molecules in the regolith. A positive charge builds up which is sufficient to loft particles 1 micron and smaller in size above the surface. These particles can go up more than a kilometre.
The Moon seems to have a tenuous atmosphere of moving dust particles. We use the word ‘fountain’ to evoke the idea of a drinking fountain: the arc of water coming out of the spout looks static, but we know the water molecules are in motion. In the same way, individual bits of moondust are constantly leaping up from and falling back to the Moon’s surface, giving rise to a “dust atmosphere” that looks static but is composed of dust particles in constant motion.
[quote byTimothy J. Stubbs, of the Laboratory for Extraterrestrial Physics at NASA’s Goddard Space Flight Center]
It is believed that the dust on the night side of the Moon is negatively charged due to electrons from the solar wind flowing around the Moon onto the night side. So at the Moon’s terminator between the positively charged dust of the daylight side and the negatively charged dust of the night side, there could be flows of dust which may resemble auroras.
The four links below from NASA discuss various aspects of this phenomena:
For centuries Earth-based astronomers have reported seeing transient changes on the Moon’s surface, known as Transient Lunar Phenomena, or TLP. These have taken the form of short term brightness changes, coloured and non-coloured glows, obscuration of detail, shadow anomalies and flashes of light. The duration of TLP can be from a fraction of a second up to several tens of minutes; then everything returns to normal as far as we can tell from Earth. Unfortunately many of these so called TLP are from inexperienced observers, or made under poor observing conditions, however some TLPs are more intriguing as they are confirmed by more than one observer.
Although the Moon is widely believed by planetary scientists to be effectively geologically dead, there are four currently accepted theories that could explain some of these mysterious TLP reports: (1) Flashes of light seen on the night side are simply due to energy released at optical wavelengths from meteorite impacts – these are being studied by project ALaMO at NASA’s Marshall Space Flight Center and also by amateur astronomers at the Association of Lunar and Planetary Observers (ALPO). (2) Eruptive out-gassing from residual radiogenic gases such as Radon and Argon that accumulate in deep seated cracks and are released during Moon Quakes – we would effectively see the dust kicked up from this out-gassing. A robotic telescope at Cerro Tololo, Chile, is being used to look for white light changes on the Moon every few tens of seconds, and amateur astronomers at ALPO and the British Astronomical Association (BAA) are searching for coloured events, and TLP in Earthshine or more shaded areas not easily accessible by the Cerro Tololo scope. (3) Electrostatic levitation of dust particles forming dust clouds in electric fields that build up around sunrise in the form of Moon dust fountains. (4) Internal reflectance off volcanic glass beads – analogous to rainbows. However the only attempt to prove a TLP was caused in this way, in Torricelli B crater, was unsuccessful at reproducing this effect.
Figure 1. (Left) part of an LROC image (M116282876RC) of the Ina formation with an enlarged inset (right) showing craters on the chaotic terrain
Here at Abewrystwyth University we have been analysing some of the statistics of past TLP reports. A preliminary report was given at the Lunar and Planetary Science Conference in Houston in 2009 and we will provide an update in September 2010 at the European Planetary Science Congress in Rome. However we are also very interested to hear from Moon Zoo users, via the on-line forum (e.g. Picture of the Week) of any interesting surface features that they think might have been freshly disturbed or perhaps a vent from which out-gassing has come from. You may find such features as you go through “Boulder Wars” or the “Crater Survey”. However for those really keen to find additional interesting objects, then they can dip into the Apollo Metric and Panoramic Camera or higher resolution LROC image archives, though the latter has a bit of a learning curve. Things to look out for include small craters which are completely shadow filled, whereas their neighbours are not – these could be ceiling vents into underground lava tubes. Also unusual disturbed surfaces that do not look like anything elsewhere in the vicinity such as Figures 1-4. We do not propose that any of these are examples of where out-gassing may have occurred, but they are the kinds of unique features that we would like to know about, just in case.
Figure 2 (Left) M104476560LE LROC image of a small part of the floor of Hyginus Crater containing some geologically young sunken areas. (Right) M111972680LE LROC image of bright ray crater (east of Reiner Gamma) with a remarkably featureless central floor patch.
Figure 3(Left) Unusual dark shaded depressions, or dark textured material, to the north west of Vallis Alpes from LROC image M122218486RE. (Right) deep seated fractures on the floor of Tycho from LROC image M114031031LE.
So as Jules says in an earlier Moon Zoo blog, please get out there, looking at the Moon close up and report your findings to the Forum. Just maybe you will be the first to identify a true TLP site, and if not then I am sure that your findings will interest the Moon Zoo geologists anyway!