Physical characteristics of very small meteoroids

Abstract

We have optically recorded faint meteors using a large aperture LLLTV (low light level television) system based on second generation image intensifiers. These data consist of 42 two-station meteors of which 13 were captured during an observing campaign near London, Ontario (Canada) in May 2004, and 29 during a campaign near Kiruna (Sweden) in October 2007. Among 13 meteors recorded in the London campaign, where the baseline between the two sites was 5 km, only four meteors satisfied our requirement for complete lightcurves and deceleration profiles by starting in the field of view, of at least one station and also ending in the field of view of at least one station. From the second set of 29 meteors captured in Sweden, with a baseline of 117.7 km, only two satisfied these criteria. The cameras used in both campaigns had fields of view of 6 degrees, which with an assumed range of 100 km, gives a scale of 13 m/pixel at ∼60 interlaced fields per second. This resolution allows precise measurement of the deceleration of very faint meteors. The limiting magnitude for meteors on these systems is near V = +8, while +11th magnitude stars are visible in the individual fields. The meteors detected in these two campaigns have peak brightnesses between absolute magnitude +6.2 and +7.4. Their photometric masses range from 4.2 mg to 0.35 mg. An ablation model was applied to fit each complete two-station event using the high-precision metric and photometric data as a constraint, in an attempt to compute bulk meteoroid densities. Interestingly, a large proportion of our faint meteor events were found to ablate at low altitudes, a result partly of our observing biases. The orbits of these events are consistent with either asteroidal or a Jupiter-family comet origin. The meteoroids' physical properties, as determined through model fits, suggest high densities, which favors an asteroidal interpretation. The high percentage of apparently dense asteroidal meteoroids at these small sizes may call into question earlier findings that only ∼1% of meteoroids at these masses are asteroidal in origin. Our results are similar to others that find ∼15% of faint TV meteors had spectra consistent with pure iron meteoroids. We find that many of these apparently asteroidal objects also undergo extensive fragmentation, which may reflect melting and spraying of droplets rather than mechanical fragmentation. Some individual cases in our small dataset will be highlighted where high bulk densities (approaching that of iron) are required to adequately match the end height, peak brightness, and observed deceleration. We speculate that these meteoroids may represent metallic condensates from impact-processed asteroid regolith.

Bulk density of small meteoroids

“…However, it is difficult to determine meteoroid density from meteor observations, because the meteoroid does not in general ablate uniformly, and does not remain as a single body during its entire flight in the Earth's atmosphere. It experiences fragmentation, which, along with its unknown shape, structure, and chemical composition, complicates any effort of ablation modeling (Kikwaya et al 2009). …”

“…Campbell- Brown & Koschny (2004) presented an ablation model which follows these steps: (a) the surface temperature of the meteoroid reaches a certain critical point; (b) the binding matrix of the meteroid disrupts (the penetration of the heat through the meteoroid is governed by thermal conductivity); (c) grains are released (fragmentation). Kikwaya et al (2009) applied this model of ablation to six meteors with masses between 10 −6 and 10 −8 kg recorded with narrow-field intensified cameras. They evaluated the bulk density by simultaneously fitting the lightcurve and the deceleration measurements of each of them.…”

“…He found values ranging from 400 kg m −3 to 7800 kg m −3 . Bellot Rubio et al (2002) pointed out two questionable assumptions (see Kikwaya et al 2009, for details on the discussion) which led Babadzhanov to estimate these high densities, the most important being the fact that Babadzhanov's (2002) method was only based on fitting the lightcurve, neglecting the meteoroid dynamics. Working on the same data, Bellot Rubio et al (2002), using the single body theory which neglects fragmentation entirely, found bulk densities ranging from 400 kg m −3 to 4800 kg m −3 .…”

“…Working on the same data, Bellot Rubio et al (2002), using the single body theory which neglects fragmentation entirely, found bulk densities ranging from 400 kg m −3 to 4800 kg m −3 . The complete model of meteoroid ablation must take into account fragmentation and dynamical properties of the meteoroids simultaneously (Borovicka 2005), as described and applied by Borovicka et al (2007), and Kikwaya et al (2009).…”

“…In this work, following Kikwaya et al (2009), we use the dustball theory as modelled numerically by Campbell- Brown & Koshny (2004), and we apply it to faint (magnitude +2.5 to +6.0) meteors. We will explore the entire solution space of each of 8 free model parameters, and we will produce for each meteor typically hundreds of thousands of solutions.…”