Tuesday, August 4, 2009

A Dangerous Season for Asteroids?

Familiar Tycho, visible to the standard naked eye, is a relative newcomer to the Moon, despite impressive rays that stretch in all directions. From samples, first predicted and then collected by Harrison Schmitt near the landing site of Apollo 17, (2240 kilometers northeast) the Tycho impact event probably occurred 109 million years ago. Its dramatic rays are, therefore, less darkened by the space-weathering forces of optical maturity (OMAT). Literally hundreds of craters just like it are all over the Moon, like most of the ancient features seen in this telescopic view of the southern Highlands, their rays have reddened or worn away, making them at least 900 million years old. When the Tycho-progenitor struck the Moon, dinosaurs roamed Earth. (Eric Soucy, LPOD)

Firefighters sometimes insist the world of humans is divided along an unpredictable line, one that becomes clear only when a home is fully engaged in fire. Some children, they say, upon being awakened by smoke or noise, defy all odds and do all they can to escape, even if they are not ultimately successful. Others simply pull the covers back over their heads and go back to sleep, permanently. These firefighters insist also there is usually no reliable way of predicting ahead of time which child will follow which path, until the danger is undeniable.

As confirmed by direct observations recently sponsored by NASA, Ito and Malhotra (2009) assume a slight majority of impacts on the Moon take place upon it's leading edge, on the hemisphere perpendicular to its direction in orbit. (The same process making meteors more common after midnight on Earth, until the direction of our orbital path around the Sun gradually comes to be more or less directly overhead, at sunrise.)

Dramatic as Tycho was, the crater record seems to indicate such impacts occurred more frequently in the past, and fell off in probability over time as the Solar System stabilized.

Is the present quiet, however, an illusion? The Moon has the answer, though we might not want to think about, or hear, the plain truth of that message.

One of the most compelling reasons to study the Moon closely is it's well-preserved record of the history of the inner and outer Solar System, conveniently located as it is near the Earth, like a rain gauge or temperature probe in our backyard; events recorded there represent conditions in our vicinity.

We have barely scratched the surface.

But what the Moon does have to say is clear enough to read with the naked eye. The history of the Moon, and, therefore, at least the past 4.5 billion years, is the history of Earth, and the testimony is to a history of bombardment.

As such, there is still much disagreement about a few things, or perhaps it would be better to say that through a careful reading of the data we have what appears to show bombardment gradually tapering off in both size and frequency.

It now appears the Moon formed 4,500 million years ago from materials much in common with those found on Earth, ones common to our location and distance from the same Star.

The very first basin-forming impact, some believe, may have been centered only a couple of hundred kilometers north by northwest of the landing site of Apollo 11, and it may have been large enough to create an impact basin larger than 50 percent of the Moon's surface area.

If this Gargantua impact really happened, it's traces can be found in the shape of the Moon itself, higher and more consistent with the materials we think the entire Moon was made of before the massive strike - on the Far Side.

It might also explain the incomplete shape and age of Oceanus Procellarum, for example, but there is no agreement on the date, it may have happened, and it if it happened at all this event was then followed only a hundred million years or so later by a slightly smaller impact on the southern Far Side, forming the largest confirmed basin on the Moon, stretching from the equator on the Far Side all the way south past the South Pole with a massive, mountainous rim that invades five degrees of southern latitude onto the Near Side. This South Pole-Aitken Basin is ringed by the highest landmarks on the Moon, and it also has within it the Moon's lowest points.

Data points to an impact-event around 4 billion years ago.

After this came Nectaris and Imbrium, the latter forming the most clearly discerned "eye" of the snaggle-toothed "Man on the Moon" pattern seen by the naked eye here on Earth., around 3.9 to 3.8 billion years ago.

Data also seems to indicate the period when most of the Moon's known basins formed may have coincided with a, so-called, Grand Bombardment, a period of chaotic struggle where some of the Outer Solar System gas giants, like Uranus and Neptune, and perhaps Saturn, may have swapped positions, causing quite a stir and raining asteroid-sized objects into the Inner Solar System with furious, deadly, mass extinction-causing events.

Taking "Seven-League Boots" through billions of years of companionship with our Moon, the theory holds that, aside from occasional jostling by passing stars and such, the Solar System gradually settled down to the relative quiet we seem to be experiencing at present. If something was destined to hit something else, it si said, it would most likely have happened by now.

But few, very few, of those who study such things are prepared to make such a bold statement with confidence.

Objects the size of large islands regularly pass by the Earth, often closer than the Moon, and we know the Solar System is both dynamic and stable. It is, therefore, not a matter of whether the bombardment tapered off and the danger is past. It is a simple matter of probability. Sooner or later, Earth will be struck again, just as Jupiter was, as predicted a year earlier in 1994, and was again, this time unpredicted, late last month. Headlines ask "can what happened at Jupiter happen here?" The answer is an emphatic, "of course."

Just knowing where the so-called Near-Earth Objects are is part of the job of understanding the risk. The other part is understanding the probability, and the Moon probably holds that answer; wind, plate tectonics, and especially water have done an excellent job of nearly destroying the same record here on Earth. So, it's especially nice to have the airless Moon so close at hand, probably throughout the past many billions of years.

There is no agreement whether there was any actual increase in bombardment, with the offending objects gradually coming more smaller and less frequent. Fortunately, our understanding of crater-forming events at the Moon has improved vastly in the past 50 years, due in no small part to direct observation from orbit and especially from sampling the surface.

Now comes Takashi Ito and Renu Malhotra, who, like many others, have studied what little we have collected of the Moon's story (which is so much more than we could have guessed at in 1959), and they have come to their own conclusion.

Their conclusion is the "gradually fall-off" in bombardment has not been "symmetric."

A closer look at the fall-off shows, they report, frequent swings, which they now report about in "Asymmetric Impacts of near-Earth asteroids on the Moon." (arXiv:0907.3010)

"Recent lunar crater studies have revealed an asymmetric distribution of rayed craters on the lunar surface," they write as introduction. "The asymmetry is related to the synchronous rotation of the Moon: there is a higher density of rayed craters on the leading hemisphere compared with the trailing hemisphere. Rayed craters represent generally the youngest impacts. The purpose of this paper is to test the hypotheses that (i) the population of Near-Earth asteroids (NEAs) is the source of the impactors that have made the rayed craters, and (ii) that impacts by this projectile population account quantitatively for the observed asymmetry. We carried out numerical simulations of the orbital evolution of a large number of test particles representing NEAs in order to determine directly their impact flux on the Moon."

"The simulations were done in two stages. In the first stage we obtained encounter statistics of NEAs on the Earth's activity sphere. In the second stage we calculated the direct impact flux of the encountering particles on the surface of the Moon; the latter calculations were confined within the activity sphere of the Earth. To represent NEAs' initial conditions, we considered two populations: one is the currently known NEAs, and the other is a synthetic population created by debiasing the orbital distribution of the known NEAs. We find that the near-Earth asteroids do have an asymmetry in their impact flux on the Moon: apex-to-antapex ratio of 1.3-1.4. However, the observed rayed crater distribution's asymmetry is significantly more pronounced: apex-to-antapex ratio of ~1.67."

"Our simulations suggest the existence of an undetected population of slower (low impact velocity) projectiles," the conclude, "such as a population of objects co-orbiting with Earth."

We already know of the existence of a space handful of such co-orbiting objects, (and in remarkably unstable orbits, considering how long Earth, together with our Moon, have been situated where we are).

We are also feverishly looking, high and low, for more, because the implications are staggering for a species with such a short-sighted perspective, based heavily on the time-lines of the lifetimes of both ourselves and even our civilizations (even our species).

All of these are a drop of water, or rather spaces between drops of water in a hurricaine, and those short-lived spaces impress upon us a desired emotional stability, to our cycles of life, though all such stability may, after all, be wholly imaginary.

Read Ito and Malhotra (2009) HERE.

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