Michiya Higa

MEASUREMENTS OF RESTITUTION COEFFICIENTS OF ICE AT LOW TEMPERATURES

Abstract Measurements of the restitution coefficient (e) of a smooth water ice sphere (radius = 1.5 cm) are made in a wide range of impact velocities (1≤υi≤700cms−1) and temperatures (113≤T≤269K). The impact velocity dependence of e is different in the quasi-elastic and inelastic regimes separated by a critical velocity (υc) at which fracture deformation occurs at the impact point of ice samples. In the quasi-elastic regime (υi≤υc), the value of e is almost constant (0.88) and ice samples show no fracture deformations. In the inelastic regime (υi>υc), e decreases with increasing υi and ice samples have fracture patterns. The velocity dependence of e is fitted as e(υ i ) = ( υ i υ c ) − log ( υ i υ c ). vc is shown to increase with decreasing temperature from 25cms−1 (269K) to 180cms−1 (113–215K).

Rapid Growth of Asteroids Owing to Very Sticky Interstellar Organic Grains

We experimentally found interstellar grains covered with organic matter in an asteroid belt, and more importantly, the organic matter played an essential role in the formation of the asteroids. The sticking threshold velocityof 5 m s-1 of the millimeter-sized organic grains was several orders of magnitude higher than those of the coexisting silicate and ice grains. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. In contrast, there was no coagulation of the silicate and ice grains in the terrestrial and Jovian regions, respectively.

Impact cratering of granular mixture targets made of H2O Ice–CO2 Ice–pyrophylite

Abstract Experiments related to impacts onto three-component targets which could simulate cometary nucleus or planetary regolith cemented by ices are presented here. The impact velocities are from 133 to 632 m s −1 . The components are powdered mineral (pyrophylite), H 2 O ice, and CO 2 ice mixed 1:1:0.74 by mass. The porosity of fresh samples is about 0.48. Two types of the samples were studied: nonheated samples and samples heated by thermal radiation. Within the samples a layered structure was formed. The cratering pattern strongly depended on the history of the samples. The craters formed in nonheated targets had regular shapes. The volume was easy to be determined and it was proportional to impact energy E . The crater depth scales as E 0.5 . Impacts on the thermally stratified target led to ejection of a large amount of material from the loose sub-crustal layer. For some particular interval of impact velocity a cratering pattern can demonstrate unusual properties: small hole through the rigid crust and considerable mass transfer (radially, outward of the impact point) within sub-crustal layer.

Measurements of ejection velocities in collisional disruption of ice spheres

Impact experiments are performed on ice spheres to measure the velocity field of ejected ice fragments and the conditions under which the fragments would reaccumulate during accretion in the outer solar system are considered. A single-stage light gas gun set in a cold room at −18°C and an image-converter camera running at 2 × 105-1 × 104 frames per second with a xenon flash lamp are used for observing the collisional phenomena. Spherical projectiles of ice (mp = 1.5 g) collide head-on with spherical targets (Mt = 1.5, 12, 172 g) at 150–690 m s−1. The ejection velocity is observed to vary with the initial position and ranges from 3 to 110 of the impact velocity (Vi). The ejection velocity of fragments at the rear side of the target (Ve) varies with distance from the impact point according to a power law relation, Ve = Va(1D)−n, where Va is the antipodal velocity, l and D are the distance and the target diameter, and n = 1.5–2.0. Va depends on the specific energy (Q) at a constant mass ratio (mpMt = 0.13) and the empirical dependence is written as Va = 0.35 × Q0.52. The ejection velocity of fine fragments formed by the jetting process near the impact point is determined to be 1.7–2.9 times as large as the impact velocity irrespective of the target size and the impact velocity.

Shock pressure attenuation in water ice at a pressure below 1 GPa

Shock pressure attenuation in water ice was studied at an impact pressure below 1 GPa and a temperature of 255 K. The observed shock wave showed a multiple shock wave structure: A precursor wave was followed by a main wave, which had a longer rise time and higher amplitude. The Hugoniot elastic limit (HEL) of water ice was measured to be in the range from 0.1 to 0.3 GPa when associated with precursor waves traveling at 3.86 km/s. The peak amplitude of the main wave Pm was observed to decrease with its propagation x from 3 to 60 mm (from 0.4 to 8 times as large as a projectile radius) in two series of experiments in which initial shock pressures Pi at the impact point were 0.60 and 0.87 GPa. The Pm was described as the power law relation Pm/Pi = (x/2.6 mm)−89. The precursor wave disappears as the Pm attenuated to a pressure <0.1 GPa. The measured wave profiles were used to calculate the loading path of water ice in shock compression between the HEL and 0.6 GPa. The loading path obtained by Lagrangian analysis was closely consistent with previous Hugoniot data regarding water ice.

Direct observations of growing cracks in ice

The crack growth process in the high-velocity impact of water ice was directly observed with an image-converter camera which stored 24 successive images at speeds of 2×105 and 5×105 frames/s. At lower-velocity impacts, the maximum growth velocity of a crack was found to be 1050±130 m/s. At higher-velocity impact experiments, 140–650 m/s, a shear fracture region was observed to grow from the impact point hemispherically with a velocity between 2.0 and 3.5 km/s depending on the impact velocity, which is larger than the shear wave velocity of water ice, 2.05 km/s. Then the fracture velocity gradually decreases, and the growth of the shear fracture region stops when the growth velocity is below 1 km/s, whereas radial cracks continue to grow outside the region. The subsequent radial growth of cracks was similar to that found at lower-velocity impacts.