From Nanotubes to Debris Disks

Micrometeoric ferrihydrite was certainly much more reactive than the single crystals of the synthetic variety used in the simulation experiments (cf. Sect. 7.2). One cause of this much enhanced chemical reactivity would be related to the filamentary texture of micrometeoritic ferrihydrite probably made of an entanglement of "nanotubes," which fill up the interstices and voids between the grains (Fig. 3.9). They were probably implicated in a still-unexplored

Fig. 3.9. Transmission electron microscope micrograph of an ultra-microtomed section of a Cap-Prudhomme Antarctic micrometeorite. The fibrous material filling the voids between crystals is very likely made of "nanotubes" of ferrihydrite. The associated electron diffraction pattern clearly shows that they are made of ferrihydrite (Courtesy G. Matrajt).

Fig. 3.9. Transmission electron microscope micrograph of an ultra-microtomed section of a Cap-Prudhomme Antarctic micrometeorite. The fibrous material filling the voids between crystals is very likely made of "nanotubes" of ferrihydrite. The associated electron diffraction pattern clearly shows that they are made of ferrihydrite (Courtesy G. Matrajt).

area of prebiotic chemistry involving interactions of organics with the astonishing world of "nanomaterials." In particular, the huge specific area of the nanotubes should have at least drastically amplified their characteristic of adsorbent of organics and salts.

The conjuncture of Hartmann, which predicts a huge amplification (x106) of the impactor flux at the time of formation of the Moon, looks validated within a factor 2-3. Indeed, it rightly predicts the total amounts of volatiles in the atmosphere, and the right content of iridium and sulfur in the upper Earth's mantle. This is quite astonishing because Hartmann constantly warns that lunar cratering rates beyond 3.9 Gyr ago are known only within a factor of 10, and Tolstikhin and Marty (1998) quote even a factor 100! By assuming that micrometeorites are delivered by periodic comets from the Edgeworth-Kuiper belt Morbidelli concluded that "most models seem to produce a maximum flux of comets, which is smaller by several orders of magnitude to the value conjectured with the Hartman's curve during the first 100 Myr that followed the formation of the Moon. Is it possible that this apparent contradiction suggests that the configuration of the young solar system was very different of both the present day configuration and from all we can conceive"? In brief, we still do not understand the huge amplification of the impactors flux in earlier times (cf. Maurette and Morbidelli, 2001).

During its evolution to the main sequence along a T-Tauri phase, a young Sun-like star is surrounded by a disk-shaped parent nebula that lasts for about 1-20 Myr. Since 2001, astronomers investigating the thermal emission of dust around stars with IR telescopes got progressively convinced that a high proportion of stars (about 45%) are "debris disk" stars (DD stars), where dust would have been continuously regenerated by the collisions of planetesimals after the clearing of the early stellar nebula (Greases, 2005). It was first thought that such disks could hardly survive beyond a few 100 Myr. But it has been recently found that they can persist for much of the lifetime of the stars. Therefore, a Sun like star with a short-lived DD might represent a minor fraction of the stars (—15%).

Greaves (2005) even suggested that the debris phase might be analogous to Earth's early period with a high-impact rate called the heavy bombardment. Moreover, after the formation of the Moon, it is likely that the rocky plan-etesimals formed in the inner solar system were essentially locked in planetary embryos, which merged as to fabricate the terrestrial planets and the possibly rocky cores of the giant planets, i.e., a total mass of material of about 50 Earth mass. Therefore, the residual planetesimals still present during the post-lunar period in the large volume of the solar DD (such disks have radius ranging from about 50 to 500 AU) were essentially comets that accreted in the outer solar system, and that we still observe today.

In this case, the basic assumption of EMMA (i.e., the flux of micrometeorite scales as the declining flux of lunar impactors) just becomes an ordinary ineluctable outcome of the early Sun being a DD-star with a relatively normal lifetime of a few 100 Myr, with regard to the wide range of values observed today, i.e., 10-20 Myr for the so-called transition objects to 100-10,000 Myr for main sequence stars (see Greaves, her Table 1). Indeed, if one increases the number of comets, one also increases the fraction of both small comets with perihelion smaller than 2.5 AU that release interplanetary dust during the sublimation of their dirty ices, and giant comets that fragment into multiple bodies. This concept of debris disk might help tackling the stumbling problem of the high lunar cratering rates conjectured by Hartman (1999).

A promising approach related to this concept has just been proposed by Gomes et al. (2005) - see also Levinson et al. (2001). The key process is a powerful mean motion resonance (resonance crossing) between the orbits of Neptune and Jupiter, which did destabilize in particular the EdK-belt about 3.8 Gyr ago, thus firing a huge spike of comets to the inner solar system. It thus ejected 97% of the comets from the belt. A back of the envelope estimate suggests that if an additional early spike was occurring before this late spike, it could have both delivered the right mass of cometary micrometeorites to the Earth and formed the lunar megaregolith, which extends up to depths of -20 km.

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