Depth of Cracking beneath Impact Craters: New Constraint for Impact Velocity

Abstract

Abstract. Both small-scale impact craters in the laboratory and less than 5 km in diameter bowlshaped craters on the Earth are strength (of rock) controlled. In the strength regime, crater volumes are nearly proportional to impactor kinetic energy. The depth of the cracked rock zone beneath such craters depends on both impactor energy and velocity. Thus determination of the maximum zone of cracking constrains impact velocity. We show this dependency for small-scale laboratory craters where the cracked zone is delineated via ultrasonic methods. The 1 km-deep cracked zone beneath Meteor Crater is found to be consistent with the crater scaling of Schmidt (1) and previous shock attenuation calculations. INTRODUCTIONImpact-induced fractures in rock have long been recognized beneath terrestrial impact craters. Simmons et al. (2) pointed out that the multiply impacted near-surface rocks on the Moon were fractured to depths of tens of kilometers and the fracture density controls the near-surf ace seismic velocity structure. Dvorak and Phillips (3) discovered cracks beneath young fresh craters that were unfilled on the Moon, or filled with air or water on the Earth gave rise to significant (negative) gravity anomalies. Ahrens and Rubin (4) demonstrated that the seismic velocity of rocks beneath small laboratory, strength-limited, craters increased with depth. This results from the near-surface zone of cracking beneath craters.The intensity of cracking decreases with increasing depth reflecting the decreasing dynamic tensile stresses with depth in the rock. The volume of strengthcontrolled impact craters has been demonstrated to be nearly proportional to impactor energy for soft and hard rocks (5).

Could Pantheon Fossae be the result of the Apollodorus crater-forming impact within the Caloris basin, Mercury?

“…Senft and Stewart (2007) used a shock physics code to reproduce final crater shapes and damage zones from laboratory to planetary scales. They found that the depth to the base of the damage zone is~1/4 the final crater diameter, a result in qualitative agreement with data from terrestrial craters (Ahrens et al, 2001). For the 40-km-diameter Apollodorus crater, that scaling would imply a damage zone of about 10 km in vertical thickness.…”

The theoretical plausibility of central pit crater formation via melt drainage

“…Impact induced fractures and voids have been inferred beneath many terrestrial impact craters based on gravity observations (Innes, 1961;Pilkington and Grieve, 1992;Henkel et al, 2010) and seismic studies (Ahrens et al, 2002;Koeberl et al, 2007). Geologic mapping studies of eroded craters where the central uplift is exposed show that the intensity of faulting increases towards the center of the crater (e.g.…”

“…Based on a logarithmic fit to data in Table 1 of Ahrens et al (2002) with an R 2 value of 0.94, the depth of the fractured region in kilometers, d frac , can be estimated by:…”

Dynamic tensile strength of terrestrial rocks and application to impact cratering

“…According to the criteria of Holsapple (1993), Meteor Crater is within the gravity regime. However, according to the definition of Ahrens et al (2002), Meteor Crater, with a diameter of about 1 km, is within the strength regime in the damage depth versus crater diameter space. We use both equations to calculate the volume of Meteor Crater.…”