Modeling Campo del Cielo strewn field

Description of the CdC

Campo del Cielo (CdC, Figure 1) is a 4000-year-old [1, 2] strewn field in the south of the Chaco province, Argentina, which was caused by an impact of IA iron octahedrite [3]. This strewn field has an extremely elongated pattern extending over an area of ~14 km (downrange) by ~3.5 km (lateral). Other known terrestrial strewn fields are much smaller: the Sikhote-Alin in Siberia is 1.2 km long, the Kaalijarv in Estonia is 1 km, and  the Morasko strewn field in Poland extends over a length of 0.4 km.  Another interesting characteristic of CdC is that a lot of depressions found within the strewn field are not impact craters but penetration funnels [4] in which intact meteorites could be found including 30-ton-weight fragments.

Method

This study presents an attempt to reconstruct the Campo del Cielo impact event, i.e., to estimate the minimal pre-atmospheric mass and velocity of the meteoroid, its fragmentation during the atmospheric entry, and to compare the resulting strewn field with the observed one.

The process of meteoroids entering atmosphere can be described with an ordinary differential equation system, which combines drag, gravity and ablation with some kinematic equations [5]. In addition, most meteoroids fragment along their trajectory due to dynamic (ram) pressure caused by the atmosphere. To get good approximate solutions, with inexpensive computational power, we use semi-analytical approaches, like the Pancake and Separate Fragment Model in combination with statistics, such as the Size-Frequency-Distribution, to describe the fragmentation process.

The impact of dense material onto a porous target results in funnels formation instead of an impact crater [6] if the impact velocity is below a critical value. To simulate funnel formation, we use the iSALE-2D shock physics code [7] with initial conditions consistent with the atmospheric entry model.

 

Atmospheric Model Constraints

• The largest recovered fragments with a mass of ~30 tons each (El Gancedo, 30.8 tons, found in funnel No. 24 and El Chaco, 28.84 tons, found in funnel No. 10) were not fragmented upon landing, but penetrated the surface to form funnels.
• The distance between the largest crater (No. 3) and the most distant small meteorite found in situ (No. 26, 3.09 tons) is ~14 km.
• There are 4 impact craters with a diameter range from 65 to 115 m.

 

Results

Preliminary analysis of impact experiments and numerical models shows that in the CdC case (iron projectile and loess as a target) funnel formation and projectile survivability is possible at impact velocities below 1 km/s. 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. Such low impact velocity requires a specific entry scenario with an extremely shallow entry angle.

Figure 2 shows, that for the vaporization dominated regime (ablation coefficient of 0.01 s²/km²) a fragment with initial mass of 90-100 tons (immediately after fragmentation at an altitude of 20 km) and initial velocity of 16 km/s reaches the surface with velocity below 1 km/s if the entry angle is in the range of 7 - 16° to horizon. Several runs for variated speed of meteoroid at disruption in the range of 12-20 km/s show a possible angle range up to 18.5° and a mass range of 60-150 tons for the largest funnel-forming fragments.

To get a match between calculated and observed expand of the strewn field (~14.03 km), the trajectory angle at the point of disruption should be at least 13° (at a speed of 20 km/s) or up to 15° for lower speed of 12 km/s). 

We also reconstructed the mass and velocity of the four crater-forming fragments using pi-scaling-laws (Figure 3). Their summed mass near the surface is at least 3028 tons (at a speed of 18 km/s) and up to 3692 tons (at a speed of 14 km/s). Its corresponding mass at the disruption point is 5348 and 6779 tons, respectively.

Those results represent just one possible scenario which assumes one fragmentation event from a number of possible scenarios. This scenario suggests that the Campo del Cielo strewn field was formed after an atmospheric entry of a minimal ~9000 tons in mass iron meteoroid (~13 m in diameter) at a shallow entry angle of ~16° and a speed of 14-18 km/s. The meteoroid was severely fragmented during the atmospheric passage: crater-forming fragments (with a final mass of 520 - 1150 tons impacted the surface with a velocity of 4 – 6.5 km/s at an angle of ~14°; funnel-forming fragments (5 – 31 tons, Fig. 4) and small meteorites (3 - 5 tons) landed with velocities < 1 km/s at various angles up to 45° (the smaller the meteorite, the more it is decelerated, and the steeper is the impact angle).

 

 

Discussion: We successfully reproduced the length of the CdC strewn field, but its lateral extension is substantially smaller than the observed one. The main reason of this discrepancy is the uncertainty of the repulsion coefficient in case of catastrophic fragmentation and complex behavior of small fragments. The survivability of large fragments (~30 tons) is indeed possible. However, the dependence of funnel’s length on fragment mass (and hence, its velocity) requires additional runs. Also, post-impact collapse of penetration funnels cannot be resolved without 3D models.

Acknowledgment

The authors acknowledge the funding from the ESA project P3-NEO-VIII. The authors are grateful to Shawn Wright for the update of CdC 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. 37:295–300.

[4] Vesconi M. A. et al. 2011. M&PS 46:935–949.

[5] Artemieva N. A. and Shuvalov V. V. 2001. JGR 106:3297–3310.

[6] Kadono T. 1999. Hypervelocity impact into low density material and cometary outburst. PSS 47: 305–318.

[7] Wünnemann K. et al. 2006. Icarus 180: 514–527.