[PPS08-P05] Oblique impact experiments of ice spheres simulating icy planetesimals
Keywords:icy planetesimals, collisional disruption, oblique impact, impact strength
We carried out impact experiments by using two-stage H2 gas gun at Kobe University. We prepared spherical targets with the diameter of 60 to 80 mm by pouring a tap water into the mold and freezing them in the refrigerator at the temperature of -25 oC. The porosity was ~0%. We used two types of projectiles, a polycarbonate spherical projectile with the diameter of 4.7 mm and a glass spherical projectile with the diameter of 2 mm (for only a head-on impact), and they were accelerated at 0.8 to 4 km/s. The impact angle, θ, was changed from 15o to 90o (a head-on impact). The ice target was set in a vacuum chamber installed in a cold room at the temperature of -15 oC. The vacuum chamber was evacuated below 200 Pa. The collisional phenomena were observed by a high-speed digital video camera at the frame rate of 105 fps and the shutter speed of 380 ns.
First, we measured the mass of all recovered fragments to examine the effect of impact angle on the size frequency distribution of impact fragments and the impact strength. The cumulative number distributions of all recovered fragments also matched very well at the θ larger than 30o, while the number of small fragments decreased as the θ became smaller. Next, we examined the relationship between the largest fragment mass normalized by the original target mass, ml/Mt, and the impact angle, θ. As a result, the ml/Mt was almost consistent with each other and it was about 0.05 at the θ from 30o to 90º. In the case of θ smaller than 30o, the ml/Mt increased abruptly as the θ decreased: the ml/Mt was about 0.25 at θ=20o and it was about 0.8 at θ=15o.
The impact strength, Q*, is defined as an energy density, Q, when the largest fragment mass is half of the original target mass. The energy density, Q, is defined as Q=mpVi2/2Mt, where mp is the projectile mass and Vi is the impact velocity, that is, the kinetic energy of projectile per unit mass of original target. At a head-on impact, the impact strength for ice sphere was 17.0 J/kg, much smaller than those of porous gypsum sphere and glass sphere obtained by Yasui et al. , about 1000 J/kg. As the impact angle decreased, the impact strength became larger.
Next, we measured the fragment velocity ejected from the antipodal point of impact point, Va, as a representative of the ejection velocity of impact fragments (We call this velocity as an antipodal velocity). As a result, the Va was proportional to the impact angle. At θ=20o, the Va was almost consistent, about 1 m/s, irrespective of energy density, and at θ=50o, the Va for 280 J/kg was about 6 m/s, twice as large as that for 135 J/kg.
Finally, we introduced the effective energy density, Qeff, to represent the relationship between the impact strength, Q*, or the antipodal velocity, Va, and the impact angle, θ, quantitatively. The effective energy density, Qeff, is defined as Qeff=Qsin2θ. As a result, we obtained two empirical equations about the normalized largest fragment mass and the antipodal velocity, ml/Mt=7.54Qeff-1.06 and Va=0.05Qeff0.94, and furthermore, the effective impact strength, Qeff*, which is defined as Q* when the largest fragment mass is half of the original target mass, was obtained to be 12.8 J/kg.
[References] Fujiwara and Tsukamoto , Icarus 44, 142-153; Yasui et al. , DPS meeting abstract #48, id. 318.12.