Poster presentations and abstracts

Introduction: On January 24^th, 2016, 15:27 UTC, a bright daytime fireball was observed in Florida and registered by weather radar and a dash-board camera. Eight fusion-crusted stones with a total mass of 1.1 kg were found [1]. Osceola is now the 33^rd meteorite (and the 9^th L chondrite) with a published orbit [2].

Methods. Orbit. The atmospheric trajectory was reconstructed based on a dash-cam video recording with account for atmospheric conditions, using the methods described in [3,4]. The orbit (a=1.486 au; e=0.3406, i=13.20°, W=303.9°, w=169.0) was calculated based on this trajectory with the open source software Meteor Toolkit [5]. Noble gases. Four chips (total mass = 118.2 mg) were analyzed at ETH Zurich after problems were encountered during the analysis of the first two chips. Pending detailed analysis, we only report here the results for the second two chips (Os-3 and Os-4). Extraction was done by total fusion in a single temperature step (Table 1), and each measurement was bracketed by blanks. We measured ^3,4He, ^20,21,22Ne, ^36,38,40Ar, ⁸⁴Kr, ^129,132Xe and some potentially interfering (isobaric) species [6]. We calculate meteoroid radius, shielding depth and cosmic-ray exposure (CRE) ages using [7,8] and the bulk chemistry for L chondrites given therein. Radiogenic gas retention (RGR) ages are based on typical abundances of 0.013, 0.043, 825 ppm adopted for U, Th, K, respectively [9]. Cosmogenic radionuclides. Results are forthcoming and will be presented and discussed at the conference.

Results & Discussion. Orbit. The aphelion of Osceola’s barely Earth-crossing orbit is located just inside the inner edge of the asteroid belt, at ca. 2 au, i.e. the orbit is dynamically evolved (Figure 1). In that respect, it is comparable, among L chondrites, only to Creston [10]. Noble gas inventory. Unsurprising for an equilibrated L chondrite, the noble gases in Osceola are cosmogenic and radiogenic for He and Ne, with only minor trapped Ar, and Q-like ⁸⁴Kr/³⁶Ar, ¹³²Xe/³⁶Ar. Meteoroid size and cosmic history. The cosmogenic ²²Ne/²¹Ne ratio of ~1.07 in both chips suggests irradiation in a meteoroid with a radius >50 cm, at a depth of >40 cm [7], consistent with the size estimate based on the fireball deceleration using the mass calculation method detailed in [11]. Under these shielding conditions, we expect a cosmogenic ³He/²¹Ne ratio of ~4.5, which fits well with the values measured in Os-3 and -4 (4.60 and 4.40), suggesting no significant loss of He relative to Ne during cosmic-ray exposure. Using ²²Ne/²¹Ne-based production rates from [8], ³He and ²¹Ne in both chips give a consistent CRE age o 18±2 Ma, while ³⁸Ar yields ~22 Ma. The former is our preferred age, since ³⁸Ar can be more affected by sample inhomogeneities (i.e., distribution of Ca, which is the primary target for production of ³⁸Ar) than ³He and ²¹Ne. This CRE age does not fall on any prominent peak in the CRE-age-histogram of the L chondrites (e.g., [12]). The CRE age is compatible with the expected collisional lifetime of a R >50 cm meteoroid in the asteroid belt (>14 Ma [13]). The U,Th-He retention age (⁴He corrected for the contribution of cosmogenic ⁴He = ~6 × ³He) of the two chips is 460 and 420 Ma, respectively, close to the age of the L chondrite parent body disruption event ca. 470 Ma ago [14]. Given the uncertainties inherent in these ages, it seems at least plausible that Osceola, like a large fraction of the L chondrites falling today, was affected by this shock event. The K-Ar retention ages for the two chips is are 1.5 and 1.1 Ga. The higher age from the K-Ar system might either be due to a contribution of atmospheric ⁴⁰Ar (given that the measured ⁴⁰Ar/³⁶Ar ratios of the two chips are lower than the atmospheric value), or incomplete degassing of radiogenic ⁴⁰Ar during the shock event 470 Ma ago. Dissimilar shock-degassing patterns for the U,Th-He and K-Ar systems are frequently observed for meteorites with shock stages S3 and S4 [15], consistent with the shock classification of Osceola of S4 [1].

Table 1: He, Ne, Ar in Osceola

References. [1] Bouvier et al., 2017, M&PS 52:2411. [2] www.meteoriteorbits.info, acc. 7.3.20. [3] Lyytinen & Gritsevich, 2016, Proc. Int. Meteor Conf., 159-163. [4] Lyytinen & Gritsevich, 2016, Planet. Space Sci. 120:35. [5] Dmitriev et al., 2015, Planet. Space Sci. 117:223. [6] Meier et al., 2017, M&PS 52:1561. [7] Leya & Masarik, 2009, M&PS 44:1061. [8] Dalcher et al., 2013, M&PS 48:1841. [9] Wasson & Kallemeyn, 1988, Phil. Trans. Roy. Soc. 325:535. [10] Jenniskens et al., 2019 M&PS 54:699. [11] Gritsevich, 2009, Adv. Space Res. 44:323. [12] Marti & Graf, 1992, Ann. Rev. Earth Planet. Sci. 20:221. [13] Farinella et al., 1998, Icarus 132:378. [14] Korochantseva et al., 2007 M&PS 42:113. [15] Stöffler et al., 1991 Geochim. Cosmochim. Acta 55:3845.

How to cite: Meier, M. M. M., Gritsevich, M., Welten, K. C., Lyytinen, E., Plant, A. A., Maden, C., and Busemann, H.: Orbit, Meteoroid Size and Cosmic History of the Osceola (L6) Meteorite, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-730, https://doi.org/10.5194/epsc2020-730, 2020.

Apollo-type asteroids Bennu and Ryugu are currently targets of sample-return missions. The goal of OSIRIS-REx mission (NASA) is to explore asteroid Bennu and Ryugu is being probed by JAXA’s Hayabusa2 mission. Observations of Bennu in January 2019 revealed ejecting material in the close proximity of the asteroid. Here we peresent our results of studying orbital evolution of potential meteoroid streams along the orbits of Bennu and Ryugu by integrating over 5000 test particles each for 1000 yr. We searched for their approaches to the Earth and we were also interested in evolution of their Earth MOIDs in order to estimate possible activity of potential meteor showers. Our results indicate possible observability from the Earth approximately for next 400 - 500 yr in both cases. Theoretical radiants for both asteroids and their potential meteor showers were also calculated.

How to cite: Kováčová, M., Nagy, R., Kornoš, L., and Tóth, J.: Bennu and Ryugu: Dynamical modelling of ejected particles to the Earth, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-659, https://doi.org/10.5194/epsc2020-659, 2020.

The InSight lander (Banerdt et al. 2020) on Mars is equipped with two cameras capable of sky imaging both of which have been used opportunistically to search for meteors. The rate of occurrence of Martian meteors has not been directly measured and initial reports of an imaged meteor by Selsis et al. 2005 were likely incorrect (Domokos et al. 2007). The meteor search is part of the investigation into the flux of impactors at Mars (Daubar et al. 2018).

The InSight cameras have been described by Maki et al. (2018). The Instrument Deployment Camera (IDC) can be aimed by a robotic arm and has a 45-degree square field of view (FOV). However, this camera was typically unavailable, and has only been used twice. The Instrument Context Camera (ICC) has a 120-degree fisheye FOV. It is aimed downward, but sees a broad section of the southern sky to around 20-degrees elevation angle.

IDC images are shown in Fig. 1. They were aimed to the southwest at an elevation of about 35 degrees, and on sols 126 and 176 (5 April and 27 May 2019), four 5-minute exposures were acquired. Stars are visible in the images, and will be used to determine the sensitivity. Many cosmic rays were seen (e.g., Fig. 2)—long ones can be mistaken for meteors, but they have a distinctive morphology with a narrow end and a diffuse end due to the charge diffusion process after the charged particles pass through the detector (Fisher-Levine and Nomerotski, 2015). Despite the Sun being 60 degrees down, diffuse sky brightness was visible (Banfield et al. 2020). No meteors were detected.

ICC images are shown in Fig. 3—about 75% of the images do not include sky. On 25 sols from 254 to 432 (15 August 2019 to 6 February 2020), the ICC acquired four, 5-minute exposures. No meteors were seen.

We will present an analysis of the results and their implication for the meteor rate at Mars. While no meteors were seen, the upper limit is likely to be constraining. Total exposure time is 540 minutes, using cameras more sensitive than the limiting exposures of Domokos et al. (2007) and with wider FOVs. However, the complex geometry and the time variable atmospheric dust extinction will be considered.