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[PPS07-02] Effect of impact angle on post shock heat due to impact cratering on comet nucleus
Keywords:Impact crater, Post shock heat, Comet, Oblique Impact
Introduction: Comet nuclei may have highly porous structure containing organic substance, anhydrous minerals and icy materials. As revealed by recent spacecraft missions, aqueous altered materials were also detected [e.g.1]. Detection of these aqueous altered minerals suggests that comet nuclei once had liquid water. Post shock heat induced by impact may be one of the heat sources to induce liquid water because impact heating is highly effective on porous material [2,3]. Laboratory cratering experiments to measure the post shock temperature on porous H2O ice have been conducted for the study of comet nuclei [3]. However, the laboratory experiments for oblique impacts on ice is quite limited and those for oblique impacts on cohesive porous target suggest that the crater formation efficiency and its oblateness are affected by impact angle [4]. Here we present the results on crater size and post shock heat on porous ice and discuss the effects of impact angle.
Method: Porous ice targets were prepared in a cold room ( -15°C), controlling their porosity 40% using sieved grain crushed from ice cube. Post shock heat was measured using K-type thermocouples carefully placed in certain depths. Al projectile (Φ=2 mm) is propelled by two-stage light gas gun facility at ISAS. Impact experiments were conducted at impact velocities of v=3.0, 4.2, 6.0 km/s, and at impact angles of θ= 15°, 30°, 45°. The target was placed in a vacuum chamber (~ 200 Pa).
Result: Oblique impact crater becomes more elliptical at lower velocities and at lower angles. The crater size was studied by using the conventional crater scaling law to clarify the effects of oblique impacts on porous water ice (Fig.1). The π scaling law for crater size in the strength regime was used in this study and it is written as follows [5],
πR = HπY-μ/2π4(1-3ν)/3,
where πR, πY, and π4 are nondimensional parameters described as follows,
πR =R(ρ/m)1/3, πY =Y/ρv2 and π4 =ρ/δ,
where R is a crater radius, ρ is a target density, m is a projectile mass, and δ is a projectile density. The target compressive strength Y for our targets was assumed to be 628 KPa. The μ, ν and H are constants depending on the material physical properties: ν= 0.4 [5]. A bold line in Fig.1 is a fitted line of the data obtained for vertical impacts, θ=90°. To consider the effects of oblique impacts, we used minor axis of elliptical crater, Rminor for R and v = v sin θ for πY. This improved πY’ was applied to the relationship at all impact angles as shown in Fig.1, and succeeded to scale the impact angle.
The temperature change was measured by a thermocouple at different distances, L, from the crater center. The maximum value of temperature change measured by a thermocouple is ΔTmax, and the relationship between ΔTmax and the normalized distance L /LR (where LR is the thermal diffusion distance) is shown in Fig.2. The equivalent radius is used for LR in this study. The results for θ=45° are almost the same as our previous results shown as the bold line for the normal impact, θ=90°. On the other hand, the data of θ=30° scatters around this line. The data of θ=15° is smaller than the resolution (0.1K) of our data logger and indicates that the temperature rise is smaller than the fitted line for θ=90°. The reason why the temperature rises at θ=30° and 15° are lower than that at θ= 90° is speculated to be as follows. The crater depth was shallower at lower impact angles, so the measurement points of the temperature were farther from the crater wall. The reason why a part of the temperature at θ= 30° was higher than that at θ= 90° is considered as follows: There were local melt pockets under the crater wall where the temperature was locally high.
[1] Altwegg et al. (2016), Sci. Adv. 2, e1600285.
[2] Yasui et al. (2021) Communications Earth & Environment, 2, 95.
[3] Sasai et al. (2021) LPS LIV, Abstract #1974.
[4] Suzuki et al. (2021) Planetary & Space. Sci., 195, 105141.
[5] Housen & Holsapple et al. (2011) Icarus, 211, 856-875.
Method: Porous ice targets were prepared in a cold room ( -15°C), controlling their porosity 40% using sieved grain crushed from ice cube. Post shock heat was measured using K-type thermocouples carefully placed in certain depths. Al projectile (Φ=2 mm) is propelled by two-stage light gas gun facility at ISAS. Impact experiments were conducted at impact velocities of v=3.0, 4.2, 6.0 km/s, and at impact angles of θ= 15°, 30°, 45°. The target was placed in a vacuum chamber (~ 200 Pa).
Result: Oblique impact crater becomes more elliptical at lower velocities and at lower angles. The crater size was studied by using the conventional crater scaling law to clarify the effects of oblique impacts on porous water ice (Fig.1). The π scaling law for crater size in the strength regime was used in this study and it is written as follows [5],
πR = HπY-μ/2π4(1-3ν)/3,
where πR, πY, and π4 are nondimensional parameters described as follows,
πR =R(ρ/m)1/3, πY =Y/ρv2 and π4 =ρ/δ,
where R is a crater radius, ρ is a target density, m is a projectile mass, and δ is a projectile density. The target compressive strength Y for our targets was assumed to be 628 KPa. The μ, ν and H are constants depending on the material physical properties: ν= 0.4 [5]. A bold line in Fig.1 is a fitted line of the data obtained for vertical impacts, θ=90°. To consider the effects of oblique impacts, we used minor axis of elliptical crater, Rminor for R and v = v sin θ for πY. This improved πY’ was applied to the relationship at all impact angles as shown in Fig.1, and succeeded to scale the impact angle.
The temperature change was measured by a thermocouple at different distances, L, from the crater center. The maximum value of temperature change measured by a thermocouple is ΔTmax, and the relationship between ΔTmax and the normalized distance L /LR (where LR is the thermal diffusion distance) is shown in Fig.2. The equivalent radius is used for LR in this study. The results for θ=45° are almost the same as our previous results shown as the bold line for the normal impact, θ=90°. On the other hand, the data of θ=30° scatters around this line. The data of θ=15° is smaller than the resolution (0.1K) of our data logger and indicates that the temperature rise is smaller than the fitted line for θ=90°. The reason why the temperature rises at θ=30° and 15° are lower than that at θ= 90° is speculated to be as follows. The crater depth was shallower at lower impact angles, so the measurement points of the temperature were farther from the crater wall. The reason why a part of the temperature at θ= 30° was higher than that at θ= 90° is considered as follows: There were local melt pockets under the crater wall where the temperature was locally high.
[1] Altwegg et al. (2016), Sci. Adv. 2, e1600285.
[2] Yasui et al. (2021) Communications Earth & Environment, 2, 95.
[3] Sasai et al. (2021) LPS LIV, Abstract #1974.
[4] Suzuki et al. (2021) Planetary & Space. Sci., 195, 105141.
[5] Housen & Holsapple et al. (2011) Icarus, 211, 856-875.