Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA (Dave Platt) (01/11/86)
I'm no expert on the subject, but the discussions I've heard have pointed out that it doesn't actually take all that high a comet density to cause extinctions on Earth... given that the effect of even one 5-mile-diameter solid comet nucleus hitting Earth would be rather atrocious. An impact that would leave a crater of moderate size on Luna would probably have a far severer effect here on Earth... most of the effects which might lead to a mass extinction on Earth would not apply on Luna. Consider: firestorms, dust diffusing through the atmosphere, feedback effects resulting from decreased insolation -> greater snowfall -> higher albedo -> colder weather, possible changes in seawater chemistry, triggering of volcanic activity with massive releases of sulphate and acid rain, etc. When the island of Tambora (I think it was) in Indonesia blew up back in the late 1800's (?), it triggered a "year without a summer" across much of the Northern hemisphere... snow in June, massive crop failures and famine, and so forth. Tambora's erruption was a good deal larger than Mt. St. Helens or Krakatao, but it was probably much smaller than the ruckus raised by (for example) the massive erruptions that occurred in what is now Yellowstone (ash carried all the way out into the Atlantic). I suspect that the impact of a 5-mile-diameter comet would have been at least as impressive... especially if it happened to strike in an area with a substantial body of magma at a shallow depth (on a mid-ocean ridge, for example). Also, Earth is a substantially larger target than Luna... both because of its larger diameter and because its gravity well is deeper. Although it's true that Luna's surface has been mapped quite well by telescope and orbiting satellite, very few of the craters have actually been physically sampled in a way that permits an accurate on-the-spot measurement of their age. The currently-popular theories do indicate that the majority of the cratering on Luna occurred billions of years ago... but it's quite conceivable that the large number of old craters conceal a smaller, but still very significant number of younger ones. I saw a mention somewhere within the past couple of years that there is historical evidence that people actually saw the impact of a good-sized object on Luna. I believe that an Arab writer (historian, natural philosopher, or something of the sort) wrote that he, and quite a few other people, saw a very bright light spring out on the surface of the Moon, in the dark area between the "horns" of the new Moon; it faded to a dimmer glow over a period of several minutes. If I recall correctly, some scientists believe that this was probably the impact of an asteroid: a member of the class known as Apollo objects, which have orbits that cross Earth's; they even suggested one specific crater as being the probable point of impact (somewhere near Tycho, I think???). There's been another interesting speculation recently, concerning Apollo objects of the sort that may have hit Luna. Quite a few of these object have been detected... a pretty good-sized one passed within the bounds of Luna's orbit within the past five years or so, and something not previously plotted cut a swath through the outer layers of Earth's atmosphere during the '70s, and plowed its way back out again (a VERY impressive fireball and trail, according to those who saw it... photograph appeared in Astronomy magazine during '84 or '85, I recall). It turns out that the orbits of Apollo objects aren't all that stable, in terms of geologic time; they're subject to substantial alteration by Jupiter and the inner planets (including Earth). It seems that there must be some mechanism for refreshing the supply of Apollo objects in order to account for their current numbers. Some fairly extensive simulations of the gravitational dynamics of the asteroid belt indicates that there are several "forbidden zones" within the belt. If an asteroid is ever perturbed into an orbit with an orbital period which resonates with Jupiter's, then Jupiter's gravity will fairly quickly perturb the asteroid out of that orbit... and one fairly common result, it seems, is for the asteroid to drop into an orbit that crosses Earth's. There may be some subtle second- or third-order interactions between Jupiter, Saturn, the asteroids, Earth, and maybe the other planets which would lead to a periodic change in the number of asteriods perturbed into Earth-crossing orbits. So... Nemesis may exist, and be sitting right in front of our faces: it's Jupiter! This hypothesis seems to meet a number of the requirements: the asteroids are primitive in composition and would probably account for the heavy-metal traces that have been detected in the clay interlayers associated with some extinctions, and there's a good supply of them. Jupiter has a 12-year orbit rather than a 30-million-year one, and so it's not likely to be yanked away by a passing star (or, if that happens, the question of further comet showers is probably going to be the least of our worries!). Perhaps the biggest question that remains is: how accurate and meaningful are the calculations and statistics that indicate a strong periodicy in the extinction (and/or impact) rate... and is there a mechanism that could cause the Apollo-object-insertion rate to vary in that way? Other note of interest... CCD imaging of the skyglow in Earth's atmosphere has indicated something unexpected. Apparently, something is punching "holes" in the skyglow; they start out small, spread to a diameter of several miles, and slowly fade. They're more common on the dawn side of the planet... which seems to indicate that they're probably associated with infalling meteors. If the researchers who reported this are correct, this data may indicate that meteoroids may contain far more volatile material than had been previously believed; the "pebbles" that we see at ground level may be only the solid residue of a much larger dirty-snowball-like clump that hits atmosphere. If this is true, it would indicate that meteors start out life as something very similar to a miniature comet nucleus, and that the amount of matter entering Earth's atmosphere may be much larger than previously believed.
henry@utzoo.UUCP (Henry Spencer) (01/12/86)
> ...When the island of Tambora (I think it was) in Indonesia blew up back > in the late 1800's (?), it triggered a "year without a summer" across > much of the Northern hemisphere... snow in June, massive crop failures > and famine, and so forth... In fairness, it should be pointed out that these effects weren't quite as universal as they are often presented. Many areas of the world did not have particularly unusual weather at that time (I believe it was something like 1816, although I don't have my references handy). Unfortunately there are few good quantitative weather records dating from that time -- even thermometers were rare and crude then -- and it is difficult to get a clear picture of global effects. The weather in North America certainly was odd. > ... probably much smaller than > the ruckus raised by (for example) the massive erruptions that occurred > in what is now Yellowstone... Actually, massive volcanic activity is one of the "dark horse" theories of massive extinctions. There is one province of India that mostly sits on a layer of lava *hundreds of meters* thick; that must have been one #@$%&@ of an eruption, and it has been cited as a possible cause for the Cretaceous- Tertiary event that wiped out the dinosaurs. > I suspect that the impact of a 5-mile-diameter comet would have been at > least as impressive... especially if it happened to strike in an area > with a substantial body of magma at a shallow depth (on a mid-ocean > ridge, for example). There was a suggestion a while ago that Iceland is the remains of the impact that caused the Cretaceous-Tertiary extinction, in fact. > I saw a mention somewhere within the past couple of years that > there is historical evidence that people actually saw the impact > of a good-sized object on Luna... I believe the event you're thinking of is the observation by English monks in the 1100s (?) of a lunar impact that may have been the formation of the crater Giordano Bruno. Recently there has been a report that Bruno, the Tunguska event early in this century, and an impact storm observed by the Apollo seismometers early in the 70s, are all consistent with a single swarm of debris in a specific orbit, presumably the remains of an extinct comet. It's been tentatively dubbed the Canterbury Swarm, after the monks' observation. There is some chance of observing the Swarm at close range; it will make a near-miss on us within the next few years. (A specific date was given, but this is all from memory and I don't remember it.) -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
pamp@bcsaic.UUCP (pam pincha) (01/15/86)
In article <6282@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: > >> ... probably much smaller than >> the ruckus raised by (for example) the massive erruptions that occurred >> in what is now Yellowstone... > >Actually, massive volcanic activity is one of the "dark horse" theories of >massive extinctions. There is one province of India that mostly sits on >a layer of lava *hundreds of meters* thick; that must have been one #@$%&@ >of an eruption, and it has been cited as a possible cause for the Cretaceous- >Tertiary event that wiped out the dinosaurs. > This Indian example would not be a very good example either, since it is a classic example of a series of flood basalts -- which are very mild in nature, as opposed to andesitic lavas found at Yellowstone, St.Helens, Tambor, Karakatoa, El Chichon, Mt. Mazama(now known as Crater Lake),etc.etc.etc. Basalts are what make up the Hawaiin Islands. They are characterized as very fluid, high magnesium/iron lavas that rarely have significant ash falls associated with them. They tend to form long lava flows eminating from fissures,cracks, and holes that sometimes form impresive fire fountains. Very little atmospheric influence. The deposits in India (and the Columbia Plateau of eastern Washington state that seems to be comprised of flood basalt deposits (basalt lava flood of great quantites from long fissures int the ground) are around 10,000 feet thick) would have little atmospheric components despite their size. Now there are some very massive ash eruption deposits of the much more explosive andesite lavas, that would be prime candidates for mass extinction episodes. The Mt. Mazama explosion put out a massive amount that spread ash all over the US and beyond. Next to its remains (Crater Lake) there are fields of pumice that are 200-500 feet thick. (And as these sort of eruptions go, this was only a moderate sized one.) P.M.Pincha-Wagener volcanic type m
space@ucbvax.UUCP (01/24/86)
That was an interesting article. A piece of friendly advice, though. Most
people refer to our large orbiting satellite as "The Moon". We perceive the
name "Luna" to be the jargon of science fiction. It is, at the least,
distracting having to "read around" your use of the name Luna in what I assume
was intended to be a serious article. I talked to two people who simply hit
their 'j' key and junked it. Assuming it was the ravings of a science fiction
nut. Remember, when talking to a general audience, use the words they are
accustomed to hearing. You may not like it, but that is what communication
is all about.
--Brian
PS - It really was interesting. I don't know a whole lot about the existing
mass extinction theories, but maybe I will have to learn about them.