ATMOSPHERIC EFFECTS ON CRATERING EFFICIENCY

Laboratory experiments permit quantifying the effects of an atmosphere on cratering efficiency by hypervelocity impacts. Three separable processes have been identified: ambient atmospheric pressure, aerodynamic drag, and projectile-atmosphere interactions. The effect of ambient atmosphere can be described by a dimensionless pressure ratio, P/deltanu2 for a pressure P, target density delta, and impact velocity. The use of a helium atmosphere minimized potential effects of aerodynamic drag (low density) and projectile-atmosphere interactions (low mach numbers) and revealed a power-law exponent of -0.23 for the dimensionless pressure term for impacts into pumice, Similar exponents but different degrees of reduced cratering efficiency are observed for targets composed of loose fine-grained sand, low-density microspheres, and mixtures of coarse sand with small amounts (<10%) of fine-grained particulates. Consequently, cratering efficiency in an atmosphere does not appear to depend on material properties of the target (internal angle of friction) but on atmospheric pressure and constituent grain size. Correcting the data for atmospheric pressure allowed testing for the possible role of aerodynamic drag through the use of higher-density atmospheres (same pressure) and various targets with contrasting ejecta sizes but the same bulk density. The results reveal that the effect of aerodynamic drag deceleration d can be viewed as a correction to gravity g in the dimensionless gravity-scaling pi2 parameter. The derived power law exponent for drag-controlled crater growth in an atmosphere closely matched the exponent for gravity-controlled crater growth in a vacuum. If it is further assumed that a combination of pressure and drag jointly limit crater growth, then the empirical power law exponent reasonably matches values derived from dimensional analysis and a coupling parameter theory. After correcting for pressure and drag, systematic offsets of the data remained and appeared to depend on Mach number. The possible role of the supersonic wake trailing the projectile was tested by performing projectile-less impacts where only the wake was allowed to interact with the target. The colliding projectile wake was found to affect crater scaling in two ways. At low mach numbers, the trailing wake gases augmented cratering efficiency by creating backpressure in the transient cavity; that is, the effect of the projectile wake was decoupled from the impact and could be viewed as a negative ambient pressure. At high mach numbers, the disturbed wake gas became coupled with the impactor, thereby changing the effective size of the energy source. Both effects could be documented in the data and in observed phenomena during impact. These results not only reconcile previous studies where atmospheric pressure effects were found to be minimal but could have potential implications for interpreting the cratering record on planets with atmospheres.