Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.571s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 571
Other
Aluminum-26, Chondrites, Ordinary, Chondrules, Planetesimal
Scientific paper
A case has already been made favouring chondrites as re-assembled "splash ejecta" following low velocity collisions between molten planetesimals[1]. Here I briefly review this hypothesis, then develop further arguments in its support. The scenario envisaged may be summarized as follows. Planetesimals grew to radii greater than 30 km in less than 1 Ma after the formation of CAIs, and they were heated rapidly by the decay of 26Al. By 2 Ma each planetesimal had a molten interior insulated by a cool, dusty carapace. Low velocity collisions at this stage released enormous, turbulent, expanding clouds of incandescent spray mixed with dust and solid grains from the carapace. The cloud constituted a rather special, transient nebular environment; as it cooled the melt droplets became chondrules. Much of the cloud's contents re-assembled under gravity onto the surface of the hot, residual planetesimal and the accumulated debris became re-heated and metamorphosed. Collisions recurred over the few million years that relative velocities remained low and planetesimals remained molten. Thus, the cumulative debris contained many recycled and broken chondrules. This scenario is apparently reconcilable with chondrule cooling rates, the preservation of clasts of "planetary" rock in chondrites, the retention of volatiles in chondrules, the preservation of solar chemistry and more than a dozen other features. Is it reasonable to claim that 30 km radius bodies existed by 1 Ma, and were substantially molten by 2 Ma? Cameron[2] argued that CAIs were saved from drifting into the sun by their incorporation, soon after formation, into planetesimals whose mass was sufficient to hold them in orbits, decoupled from the drag of nebular gas. Wetherill's models [3] show that many bodies >100 km radius may have formed on a timescale of 10^5 years. In these terms, the proposed 30 km by 1 Ma is quite conservative. Regarding 26Al heating, the remarkably constant initial ratio of 26Al/27Al (5 x 10^-5) in CAIs from different classes of meteorite [4] suggests that 26Al was uniformly distributed in the dust which eventually formed the chondrite parent bodies. This amount of 26Al translates to some 7000 J g^-1. A simple finite element calculation was made to assess the likely thermal evolution of planetesimals of different sizes, starting from 300K at different times. The proposed body of 30 km radius at 1 Ma was found to be a limit for substantial internal melting. Its interior would have remained molten for several million years. Earlier accretion, or larger planetesimals, would have generated even more melt. It seems, therefore, that molten planetesimals were abundant in the early solar system. Moreover, they evidently suffered collision and accretion. If their collision products were not chondrules, then what were they? Two further arguments favouring the proposed scenario concern the age difference of CAIs and chondrules, and the existence of macrochondrules. Cameron's Leonard Award address [2] was stimulated by the inferred time interval of several million years between the formation of CAIs and chondrules. Chondrules were interpreted as dust melted by solar flare activity, the dust having been produced by late collisions between planetesimals. If, as is argued here, the planetesimals were already internally molten, chondrules would have been produced directly, without need to invoke a solar flare heat source. A separate issue is the existence of porphyritic olivine macrochondrules up to 4 cm across [5]. Macrochondrules are not easily reconcilable with chondrule formation by radiative heating in a nebular setting. Such a mechanism predicts an inverse relationship between chondrule diameter and temperature rise, which is not observed. However, in the present scenario macrochondrules are interpreted simply as examples of large blobs of frozen melt. References: [1] Sanders I. S. (1994) Meteoritics, 29, 527. [2] Cameron A. G. W. (1995) Meteoritics, 30, 133-161. [3] Wetherill G. W. (1989) in Asteroids II (R. P. Binzel et al., eds.), 661-680, Univ. of Arizona, Tucson. [4] MacPherson G. J. et al. (1992) Meteoritics, 27, 253-254. [5] Binns R. A. (1967) Mineral. Mag., 37, 319-324.
No associations
LandOfFree
Ordinary Chondrites Viewed as Reassembled 'Splash Ejecta' does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Ordinary Chondrites Viewed as Reassembled 'Splash Ejecta', we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ordinary Chondrites Viewed as Reassembled 'Splash Ejecta' will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830920