Campo del Cielo Strewn Field: Simulating the Funnel Formation for Vertical & Oblique Impacts&#160;
Authors: Luther Robert , Schmalen Andrea , Artemieva Natasha 
Published: September 2022

&lt;p&gt;&lt;strong&gt;1. Introduction&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;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 &amp;#8211; 8500 tons, 14.5-18.4 km/s entry velocity, and max. 16.5&amp;#176; entry angle [4]. Beside four elliptical impact craters with diameters of 70&amp;#160; - 115 m, penetration funnels with intact meteorites up to 30 tons in weight have been formed during the event [1,2].&lt;/p&gt;
&lt;p&gt;In this study, we analyse possible conditions of funnel formation, which may help to constrain the impactor parameters [4].&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;2. Method&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;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 &amp;#8211; 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.&lt;/p&gt;
&lt;p&gt;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&amp;#176; to 90&amp;#176;. &amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;3. Results&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;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.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Figure 1:&lt;/strong&gt; 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.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Figure 2:&lt;/strong&gt; Projectile Fate at different impact velocities.&lt;/p&gt;
&lt;p&gt;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&amp;#176; to avoid the reflection.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Figure 3:&lt;/strong&gt; The projectile (dark red) bounces off from the target surface (yellow) for an impact at 15&amp;#176; at 600 m/s. The colour along the floor indicates the density, with 1000 kg/m&amp;#179; as dark blue to 2000 kg/m&amp;#179; as red.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;4. Conclusion and Discussion&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;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&amp;#176; is below the threshold for funnel formation (25&amp;#176;), atmospheric deceleration of fragments from cosmic velocities down to 0.6 &amp;#8211; 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&amp;#176; to horizon [1].&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;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.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
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