I’ve selected this weeks article because it reaches an interesting conclusion through some fine logical reasoning. The authors describe the detection of two ancient dust showers (periods of relatively large dust fluxes into the atmosphere) and then convincingly argue that the more recent of the two must have come from an family of objects known as Veritas in the Main Asteroid Belt. To reach their conclusion, the authors use geologic coring and isotope analysis, astronomical observations, and computer simulations. Their conclusions provide yet more reason for us to be aware of the solar system in which we live; though our planet and its atmosphere protect us from much, we are very definitely not isolated from what goes on around us.
Citation (online at CiteULike.org):
Farley, K.A., Vokrouhlicky, D., Bottke, W.F., and Nesvorny, D. (2006). A Late Miocene Dust Shower from the Break-Up of an Asteroid in the Main Belt. Nature 439(7074), pp. 295-297.
Synopsis:
3He is not a common element on the Earth’s surface, so the presence of high levels of 3He in layers of rock is indicative of extraterrestrial inputs. Over the last 70 million years, there were two such events where the levels of 3He increased markedly over the background, as shown in the figure on the left. The data for this graph was collected from a variety of sources including two new rock samples from the Indian (site 757) and Atlantic (site 926) oceans. The authors remark that the Late Eocene peak at about 35 MYa (million years ago) coincides with two large impact craters: the Popigai and the Cheapeake Bay. Thus the increase in 3He in the rock core they analyzed can be explained by asteroid impacts.
The 3He spike in the Late Miocene Epoch approximately 8.2 MYa cannot be explained in the same fashion, because there are no known impact craters near this time. However, in the asteroid belt there is a cloud of debris and smaller asteroids known as the Veritas family that is thought to have been produced by a collision at approximately that time. The collision would have produced a distribution of fragments, many of them smaller than 1,000 micrometers. Those smaller than 1,000 micrometers would gradually spiral inward toward the sun over a period of 100,000 years due to the solar wind and radiation drag (known as Yarkovsky drag).
Some of these tiny interplanetary dust particles (IDPs) would then rain on the Earth. Those smaller than about 10 micrometers would survive atmospheric entry without heating excessively and losing any 3He that may have stuck to it while being bombarded by the solar wind. Farley and others assert that their models show that continued collisions between grains 1-5 mm in diameter would replenish the dust shower and produce a distribution of dust like that shown in the graph to the right. The relatively smooth line is their model prediction of increases in IDP flux that has been overlain on the detailed measurements of 3He. The two different graphs correspond to the two different samples reported by the authors.
The line between the letters “I” and “K” in the figure above corresponds to an abrupt transition between two forms of clay sediments (Illinite and Kaolinite) that indicates a sudden change in climate regime. The authors note that the coincidental match between near-peak IDP fluxes and a global cooling event is very suggestive. However, they caution against attributing the cooling to the dust influx because the quantities of dust should not have been sufficient to cause any noticeable impact on Earth’s climate.
Context:
This article presents a really neat piece of work. The logical chain of argument goes as follows: 1) 3He spikes observed in the rock, 2) no known impact crater to cause this, 3) an asteroid breakup occurred around the same time, 4) solar-wind and radiation-induced drag caused some of the smaller particles to rain in on the Earth as IDPs, 5) based on a model of continued IDP production, the increase and gradual decrease in 3He is matched very well by the expected evolution of IDP influx.
This type of chain of reasoning is how some of the very best science is done, especially geology. The clues to geological mysteries are often either destroyed or so tenuous that such mysteries can go unsolved for decades before someone has a breakthrough in reasoning. For instance, it took a Physicist, Luis Alvarez, in tandem with his geologist son, Walter Alvarez, to realize that the presence of Iridium in sediments 65 million years old meant a large impact. Why? Because at the time, the Chixculub crater had not been discovered, Mother Nature buries her secrets deep.
General Explanations:
The Veritas Family
The Veritas Family is a group of main-belt asteroid fragments located at about 3.17 AU (1 AU is the Earth-Sun distance) that continues to produce approximately 10% of the inner-solar system dust. This dust is created when fragments of the parent body of Veritas collide and release a cloud of smaller chunks. By accurately measuring the orbits of some of the larger pieces of Veritas and then following those orbits backward in time, the approximate date of the collision that produced it is 8.5 Mya. When that collision occurred, millions of chunks of varying sizes were released in all directions. Those larger than about 1 mm (1000 micrometers) continue to orbit the sun. Farley and others reasoned that only the smaller IDPs from the Veritas ever reached Earth because pieces not affected by drag forces would have required enormous ejection velocities to reach the Earth (the asteroid belt orbits significantly more slowly than does the Earth). There are orbital resonances (location at which the orbital velocity, given by Kepler’s Third Law, of an object falls in some integer ratio with a much larger body, i.e. 11:2, or 5:3. So, 11 asteroid fragment orbits for every 2 Jupiter orbit, and so on.) with Jupiter that could have, over millions of years, sent larger chunks towards us. But, these resonances are known to be poor-producers of Earth-bound asteroids, so it is highly likely that only IDPs from the Veritas family ever reached the Earth.
