Thursday, May 22, 2008

Impact gap in Moon's Southern Highlands?

A small sampling of one hundred 'official' impacts, mostly believed to be of cometary origin, recorded by specialists at Marshall Space Flight Center between 2005 and 2006

A quick glance at the Moon's nearside, tidally locked facing Earth, dramatically shows the Southern Highlands hardly devoid of impacts. Like most of the lunar farside, the impacts there over 4.5 billion years are well preserved and seem to saturate every available location. Close examination of the relatively smooth basaltic lunar "seas" of course, show a similar, newer and smaller saturation not as easy to recognize with the naked eye from Earth.

In the short term, however, and during a comparative 'blink of an eye' in lunar history, in a period of little more than a couple of years, NASA observations appear to show a familiar pattern.

Are some places on Earth's Moon safer from impact than others?

The possibility of lowering the probability of a mission being "impacted" in increasingly permanent presence makes this possibility worth study.

All meteor observers on Earth know the likelihood of seeing that sporadic flash in the sky, and not necessarily associated with an annual shower, increases after midnight. The reason is also well-known. At dawn, more or less overhead, observers are under the forward direction of Earth's orbit, perched on the front bumper, and at sunset on the back.

Driving through a swamp, bugs are impacted on the front windshield. To hit the back window, bugs would need to catching up and moving faster than the car. That comparison breaks down quickly when dealing with meteoroids and the Moon.

From Earth, the image above shows "100 impacts on the moon" of what were larger chunks of debris sufficient to release kinetic energy quickly turned into visible energy on impact, enough to to be indisputably register 400,000 kilometers away.

Apparently seen in this small sample is a similar pattern from what is observed on Earth. The patters seems to show a slightly higher probability of an impact on the Moon's leading limb in its orbit around Earth. And the sample might also show a "Meteor-graph" of the far wider distribution of cometary orbits above and below the ecliptic.

Accounting for the Moon always traveling along with Earth around the Sun also, from Full Moon through New, the lunar farside faces Earth's forward but, as always, invisible from Earth.

The apparent low incidence of recorded impacts toward the poles may reflect the Oort Cloud's distant belt, and it's lower number of comets above and below the primal proto-planetary disk, but it also might only show a lower likelihood of an impact's visibility at high latitudes, and as seen from Earth.

And there are seasonal visibilities and the inclination of the lunar orbit, prejudices of distance, obscuring abyssal crater walls and mountains, etc. But no recorded impacts on the Moon, at least during the sample period, upon the area of the Moon "closest" to Earth. So it might mean nothing at all.

So what about this apparent gap in sightings in the familiar highlands?

Along with being "closest" to Earth, which is always situated directly overhead (and also most incident to a crowded solar ecliptic where all the planets and most, but certainly not all the debris resides) the "Highlands" also appear to be short on impacts, at least in this sample.)

The moon shows recent and ancient, both large and microscopic, impact history in that area. So, given enough time, the impacts do come, but perhaps less frequently. An impact of the kind seen in this sample, coming from overhead, would almost always have had to pass through the Earth first. That would block a fragment of the ecliptic's debris field, of course, and make impacts ordinarily visible in a sample such as this trend toward those of higher, more oblique angles.

Does this mean there are far more impacts everywhere on the moon, with visibility trending toward those closest to being in line-of-sight with Earth? That would make the apparent gap a sign of something far more troublesome.

For whatever reason, this last possibility is a victim of Occam's Razor. The brief sample does correlate nicely with one predicted pattern. If so perhaps it does show an unlikely, but clearly seen nevertheless, an apparent lower likelihood of being "impacted" by a meteor the higher Earth is over your head. As with everything else, it may be worth further study.

From NASA's Science News, 100 Explosions on the Moon, May 21, 2008:

"They're explosions caused by meteoroids hitting the Moon," explains Bill Cooke, head of NASA's Meteoroid Environment Office at the Marshall Space Flight Center (MSFC). "A typical blast is about as powerful as a few hundred pounds of TNT and can be photographed easily using a backyard telescope."

As an example, he offers this video of an impact near crater Gauss on January 4, number 86 on the list of 100 impacts recorded by the MEO team since their survey began in 2005. Larger movies: 0.8 MB gif, 5.9 MB avi."