Campo del Cielo Strewn Field: Simulating the Funnel Formation for Vertical & Oblique Impacts
- 1Museum für Naturkunde Berlin, Berlin, Germany (robert.luther@mfn.berlin)
- 2Christian-Albrechts-University, Institute of Geoscience, Kiel, Germany
- 3Planetary Science Institute, Tucson, USA
1. Introduction
The Campo del Cielo strewn field, located in the south of the Chaco province, Argentina, formed about 4000 a ago due to the impact of an octahedrite iron asteroid [1,2,3]. It extends about 14 km and ~3.5 km in impact and lateral direction, respectively. Recent numerical reconstructions of the strewn field yielded impactor parameters of min. 7500 – 8500 tons, 14.5-18.4 km/s entry velocity, and max. 16.5° entry angle [4]. Beside four elliptical impact craters with diameters of 70 - 115 m, penetration funnels with intact meteorites up to 30 tons in weight have been formed during the event [1,2].
In this study, we analyse possible conditions of funnel formation, which may help to constrain the impactor parameters [4].
2. Method
The funnel formation is simulated using the iSALE-2D shock physics code [5], which has been applied to similar problems, previously [6]. Here, we explore the influence of impact velocity and target porosity on the funnel formation processes. Initial conditions are consistent with the atmospheric entry model, i.e. we model a 30 tons fragment at impact velocities between 600 m/s – 1600 m/s. The target is set up with a porosity of 40% to represent the local loess unit at the side of impact. Further, the porosity is varied between 30-50% at a constant impact velocity of 800 m/s.
To test the effect of impact angle on the funnel formation process, we used the 3D version of iSALE to simulate impacts at 600 m/s at varying angles from 15° to 90°.
3. Results
Preliminary analysis of impact experiments [7] have shown that in the Campo del Cielo case (iron projectile and loess as a target) funnel formation and projectile survivability is possible at impact velocities below 1 km/s. Even though the iSALE models confirm this estimate, the 1 km/s is probably the upper limit of impact velocity allowing survivability of high strength iron meteoroids impacting into loess (Figure 1). At this velocity, the fragment is already deformed to a significant extent (Figure 2). A further increase of impact velocity leads to an increase in projectile deformation (pancaking) up to the projectile breakup. At velocities above 1 km/s, funnels are gradually transformed into impact craters due to their increase in diameter. At 1.6 km/s, the cavity widens about 5 times the projectile size and the depth/diameter ratio approaches 1.
Figure 1: Snapshot of the funnel formation after 160 ms into a loess target. On the left is shown the density, on the right is shown the temperature.
Figure 2: Projectile Fate at different impact velocities.
3D models reveal an interesting effect: at shallow angles even a low-velocity projectile is reflected from the surface and an elliptical crater, not a funnel, is formed (Figure 3). An impact angle should be at least 25° to avoid the reflection.
Figure 3: The projectile (dark red) bounces off from the target surface (yellow) for an impact at 15° at 600 m/s. The colour along the floor indicates the density, with 1000 kg/m³ as dark blue to 2000 kg/m³ as red.
4. Conclusion and Discussion
Our findings suggest that the funnels of the Campo del Cielo strewn field are in line with results from recent atmospheric entry simulations [4]. Even though the predicted entry angle of 16.5° is below the threshold for funnel formation (25°), atmospheric deceleration of fragments from cosmic velocities down to 0.6 – 1 km/s steepens their trajectories. Our findings also contradict field estimates of the Campo del Cielo impact scenarios in which impact velocities are in the range of 3-5 km/s at an angle of 9-10° to horizon [1].
Acknowledgements
We gratefully acknowledge the developers of iSALE (www.isale-code.de). The authors acknowledge the funding from the ESA project P3-NEO-VIII and P3-NEO-XXVIII. The authors are grateful to Shawn Wright for the update of Campo del Cielo field observations.
References
[1] Cassidy W. A. and Renard M. L. 1996. M&PS 31:433–448.
[2] Cassidy W. A. et al. 1965. Science 149: 1055–1064.
[3] Liberman R. G. et al. 2002. M&PS 37:295–300.
[4] Schmalen A. et al. 2022. M&PS.
[5] Wünnemann K. et al. 2006. Icarus 180: 514–527.
[6] Luther R. et al. 2017. M&PS 52: 979-999.
[7] Kadono T. 1999. PSS 47: 305–318.
How to cite: Luther, R., Schmalen, A., and Artemieva, N.: Campo del Cielo Strewn Field: Simulating the Funnel Formation for Vertical & Oblique Impacts , Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-636, https://doi.org/10.5194/epsc2022-636, 2022.
