Small comets hitting the Moon?

[The Warthog]

A few eyars ago, I read in Discovery about the idea that millions of house-sized comets every year hit the earth, contributing enormous amounts of water over billions of years. I thought the article might have been one of their spoofs, but I Googled it later, and again tonight, and found that it is a real theory, detailed here: http://smallcomets.physics.uiowa.edu/lecture/

This looks like solid science to me. However, I was reading Sir PM's 1964 book, Survey of the Moon in which he mentions in his chapter on the 'Lunar Atmosphere' (there isn't one) several observations by competent astronomers, of obscurations of the lunar surface under what appeared to be mists, and occasional bright features that appeard and then quickly went away. It seems to me that these rare events could be consistent with a 40-tonne 'soft snowball' hitting the surface, then vaporizing to produce a local atmosphere which then dissipates into space in the sunlight.

Sounds reasonable to me. The small comet theorists raise the possibility of small craters, but don't raise the possibility of observable effects, so I wrote them to ask about it.

I kinda like the idea of those millions of fluffy comets raining down on us...

[Talitha]

Comets leave vapor trails though, don't they? Seems logical that we'd know a comet was coming long before it got into our immediate neighborhood, and half the scopes on the planet would be aimed at Luna when the impact occurred.

I think Sir Moore was referring to LTPs. I haven't done too much research on them, but have read that the periodic 'mists' and bright features occur in certain areas.. maybe they're product of outgassing.

[The Warthog]

I'm glad you replied! I was beginning to think that everyone was either astounded by my scientific deductive ability, or had decided I was playing this here banjo with only three strings, and were just being politely silent.

The 'comets' in question are very small, about the size of a suburban bungalow or split-level. They contain little dust, and not enough water to cause a significant amount of vapour. When they hit the high atmosphere, they begin to break up immediately at about 800 miles, so by the time they reach the low earth orbital altitude, they are diffuse clouds which fall slowly upon the earth. On the moon, they would be invisible until they impacted as a cloud of vapour, and wouldn't produce enough heat for a flash of light, although they may produce a crater in the surface layer of dust.

I think I got that right. They are talking on the order of 10 million house sized comets a year, all small enough to escape detection. I did some math, though, and realized this rate of fall would put one of these comets in the atmosphere over Niagara about once every five days. An object the size of a house is impossible to see in space with a telescope.

Lunar Transient Phenomena are certainly what Sir Patrick (Not 'Sir Moore,' eh? ) was talking about, but I think at least two of the ones mentioned in the article you cite are possible candidates for small comets, those being obscurations and abnormal albedo. If you put forty tonnes of snow down in a warm crater, it will produce a fair amount of mist, which will disperse fairly quickly. The daytime temperature of the surface of the Moon is about 250 degrees F. I saw snow falling on my barbecue once, which was about that hot, and it dispersed fairly quickly. Maybe I should have thrown a snowball at it. That would have been neat!

Sir Patrick mentions the possibility of outgassing, as his preferred candidate (at least in 1963,) but that begs the question of what is there to outgas. Was there ever any amount of water on the Moon, and as it probably never had a substantial atmosphere, and the Moon started out as a ball of very hot rock, would any water have survived the formation of the Moon?

[The Warthog]

And, actually, I just came across this sentence on rereading the material. I'd missed it before: "The small comets that strike the Moon will not make impact craters;they probably kick up some lunar dust and produce strange glows, and indeed these kinds of anomalous events have been reported by lunar observers for centuries."

[Talitha]

Well there you go, maybe that's what's causing it!

Here are some articles written about the behavior of moondust which you might find interesting:

Magnetic Moondust

Mesmerized by Moondust

Moon Fountains