Effects of Friction and Plastic Deformation in Shock‐Comminuted Damaged Rocks on Impact Heating

Abstract

Hypervelocity impacts cause significant heating of planetary bodies. Such events are recorded by a reset of 40Ar‐36Ar ages and/or impact melts. Here we investigate the influence of friction and plastic deformation in shock‐generated comminuted rocks on the degree of impact heating using the iSALE shock‐physics code. We demonstrate that conversion from kinetic to internal energy in the targets with strength occurs during pressure release, and additional heating becomes significant for low‐velocity impacts (<10 km s−1). This additional heat reduces the impact‐velocity thresholds required to heat the targets with the 0.1 projectile mass to temperatures for the onset of Ar loss and melting from 8 and 10 km s−1, respectively, for strengthless rocks to 2 and 6 km s−1 for typical rocks. Our results suggest that the impact conditions required to produce the unique features caused by impact heating span a much wider range than previously thought.

Implantation of Martian Materials in the Inner Solar System by a Mega Impact on Mars

“…The impact velocity of the debris to pre-existing asteroids in the Hungarian region and in the main belt is expected to be larger than 5 km s −1 (Figure 4), while the nominal collision velocity between asteroids is ∼5 km s −1 (Bottke et al 1994). Such a high energetic impact can be recorded as a reset of 40 Ar-39 Ar age and/or impact melts (Kurosawa & Genda 2018), which is also expected in the case of the Moon-forming giant impact (Bottke et al 2015). Since the timing of a mega impact on Mars is likely different from that of the Moon-forming giant impact (∼100 Myr after CAI condensation, Touboul et al 2007), one of the multiple signatures for 40 Ar resetting age in chondrites (Bottke et al 2015) would be attributed to a mega impact on Mars.…”

Slow Impacts on Strong Targets Bring on the Heat

“…At a strain ε of order 1, the heat per unit mass added by deformation is thus the lesser of εμP/ρ and εY m /ρ, where ρ is density. As shown by Kurosawa and Genda (2017), this can easily translate to temperature increases of 1,000 K in impacts as slow as 6 km/sec. We note, however, that while Kurosawa and Genda (2017) emphasized the role of the friction coefficient in their text, stresses in their computation for large friction coefficients were in fact limited by the maximum yield stress of about 1.7 GPa (see their Figure S1).…”

“…In their recent paper, Kurosawa and Genda () bring to light a source of heat in impact‐deformed rocks that has, astonishingly, escaped explicit attention for decades. Indeed, both of the present authors' (who refereed the paper) initial reaction was approximately “how could we have been so blind?” Not only is this oversight surprising, this heat source is quite important for impacts at speeds less than about 15 km/sec, and it should not be overlooked in any future modeling of impact craters.…”

“…Kurosawa and Genda () are not the first to note the role of strength in enhancing heating of the target. They noted that Quintana et al () also saw an increase in heating in their numerical simulations, although they did not trace this heating back to material strength.…”

“…In their recent paper, Kurosawa and Genda (2017) bring to light a source of heat in impact-deformed rocks that has, astonishingly, escaped explicit attention for decades. Indeed, both of the present authors' (who refereed the paper) initial reaction was approximately "how could we have been so blind?"…”