Revisiting the search for the parent body of the Tagish Lake meteorite -Case of a T/D asteroid 308 Polyxo-

The Tagish Lake meteorite which fell early in 2000 have turned out to be the most primitive meteorite in our collections. T. Hiroi et al. (Science, 293, 2234, 2001) identified the D asteroids, especially those in the main belt, as the most probable candidate for the parent body of the meteorite. After examining more details of reflectance spectra of dark asteroids and Tagish Lake meteorite samples, we have found that the search for the parent body should be expanded beyond just the type D of the Tholen\u27s classification used by Hiroi et al. (ibid, 2001) to include the T type. Among the D and T asteroids we examined for their possibility to be the parent body of the Tagish Lake meteorite, a T/D asteroid 308 Polyxo has turned out to be one of the best candidates because of its similarity to the Tagish Lake sample we use in reflectance spectral shape, albedo, and the presence of 3-μm water band

A Comparative Study of Infrared Asteroid Surveys: IRAS, AKARI, and WISE

We present a comparative study of three infrared asteroid surveys based on the size and albedo data from the Infrared Astronomical Satellite (IRAS), the Japanese infrared satellite AKARI, and the Wide-field Infrared Survey Explorer (WISE). Our study showed that: (i) the total number of asteroids detected with diameter and albedo information with these three surveyors is 138,285, which is largely contributed by WISE; (ii) the diameters and albedos measured by the three surveyors for 1,993 commonly detected asteroids are in good agreement, and within +/-10% in diameter and +/-22% in albedo at 1sigma deviation level. It is true that WISE offers size and albedo of a large fraction (>20%) of known asteroids down to a few km bodies, but we would suggest that the IRAS and AKARI catalogs compensate for larger asteroids up to several hundred km, especially in the main belt region. We discuss the complementarity of these three catalogs in order to facilitate the use of these data sets for characterizing the physical properties of minor planets.Comment: 21 pages, 19 figures, and 2 tables, accepted for publication in PAS

Oblique impact cratering experiments in brittle targets: Implications for elliptical craters on the Moon

Most impact craters observed on planetary bodies are the results of oblique impacts of meteoroids. To date, however, there have only been very few laboratory oblique impact experiments for analogue targets relevant to the surfaces of extraterrestrial bodies. In particular, there is a lack of laboratory oblique impact experiments into brittle targets with a material strength on the order of 1 MPa, with the exception of ice. A strength on the order of 1 MPa is considered to be the corresponding material strength for the formation of craters in the 100 m size range on the Moon. Impact craters are elliptical if the meteoroid's trajectory is below a certain threshold angle of incidence, and it is known that the threshold angle depends largely on the material strength. Therefore, we examined the threshold angle required to produce elliptical craters in laboratory impact experiments into brittle targets. This work aims to constrain current interpretations of lunar elliptical craters and pit craters with sizes below a hundred meters. We produced mortar targets with compressive strength of 3.2 MPa. A spherical nylon projectile (diameter 7.14 mm) was shot into the target surface at a nominal velocity of 2.3 km/s, with an impact angle of 5°‐90° from horizontal. The threshold angle of this experiment ranges from 15° to 20°. We confirmed that our experimental data agree with previous empirical equations in terms of the cratering efficiency and the threshold impact angle. In addition, in order to simulate the relatively large lunar pit craters related to underground cavities, we conducted a second series of experiments under similar impact conditions using targets with an underground rectangular cavity. Size and outline of craters that created a hole are similar to those of craters without a hole. Moreover, when observed from an oblique angle, a crater with a hole has a topography that resembles the lunar pit craters. The relation between the impact velocity of meteoroids on the Moon and the probability of elliptical crater formation was investigated based on our experimental results and an existing empirical equation. The results suggest a distinct possibility that most craters in the 100 m size range on the Moon, given their elliptical shape, originated as secondary craters. © 2016 The Author