Presentation Information
[PPS04-P08]Impact experiments on wet gypsum targets simulating water-bearing planetesimal: Measurements of impact-induced residual temperature
*Seitaro Otsubo1, Minami Yasui1, Masahiko Arakawa1, Hatsune Okawa1, Haruna Toyoshima1, Reia Kakinoki1, Ryosuke Kikukawa1, Ohashi Takuma1, AYAKA OHNISHI1 (1.Kobe University)
Keywords:
Impact heating,Hydrated porous materials,Aqueous alteration,Crater formation
Particles returned from C-type asteroids and comets contain hydrated minerals and organic materials (Sandford et al., 2006), indicating aqueous alteration within their parent bodies. Mineralogical analyses of Ryugu samples suggest alteration temperatures of ~40 ºC (Yokoyama et al., 2022).
The heat source responsible for this alteration remains debated. Although decay heat from the short-lived radionuclide 26Al has been considered the primary candidate, it may not have provided sufficient heating depending on the formation timing and size of the parent body. Impacts between small bodies were ubiquitous in solar system history and may have acted as localized heat sources. Previous experiments using dry porous gypsum targets characterized the spatial distribution of impact-generated residual heat (Yasui et al., 2021). However, actual parent bodies likely contained internal water, and the characteristics of impact-generated residual heat under water-bearing conditions remain poorly constrained.
This study evaluates whether impact-generated residual heat in water-bearing porous bodies can serve as a heat source for aqueous alteration. Impact experiments using hydrated gypsum targets were conducted to directly measure post-impact temperature evolution and determine its spatial and temporal distributions, and to compare the results with dry targets.
Porous gypsum targets were prepared in dry and hydrated conditions. Hydrated targets were produced by saturating dried targets with water to fill pore spaces between gypsum grains. Impacts were performed using a horizontal two-stage light-gas gun at Kobe University. 4.7-mm polycarbonate projectiles were launched at normal incidence at 2–3 km/s under reduced pressure below 3000 Pa. Internal temperatures were measured with embedded K-type thermocouples (five per target, 11–33 mm from the impact point), and temperature histories were recorded using a data logger. Surface temperature distributions were measured using an infrared camera. Crater morphology and internal structures were examined by X-ray CT before and after impact.
Craters were strength-dominated, consisting of a central bowl-shaped transient crater and a surrounding spall region. Hydrated targets exhibited a distinct delamination-like structure beneath the crater floor. Crater diameters in hydrated targets were 1.5–2 times larger than in dry targets under identical impact velocities, suggesting that shock attenuation was suppressed by water filling the pore spaces, allowing more efficient energy transmission.
In hydrated targets, internal temperatures increased rapidly immediately after impact, but the elevated temperature duration was limited to ~1.3 s, shorter than in dry targets, which showed gradual heating followed by slower cooling. This behaviour differs from dry targets, where temperature increases primarily due to thermal conduction from residual heat along the crater walls, and instead suggests heating associated with advection of hot materials during crater growth. Infrared measurements showed that dry targets reached surface temperatures exceeding 100 ºC immediately after impact, whereas hydrated targets reached only ~40 ºC, with a temperature increase of 28 ºC, significantly lower than in dry targets. Furthermore, high-temperature regions formed during impact were rapidly removed from the surface by ejecta excavation and emplacement processes, resulting in little residual heating on the target surface.
The heat source responsible for this alteration remains debated. Although decay heat from the short-lived radionuclide 26Al has been considered the primary candidate, it may not have provided sufficient heating depending on the formation timing and size of the parent body. Impacts between small bodies were ubiquitous in solar system history and may have acted as localized heat sources. Previous experiments using dry porous gypsum targets characterized the spatial distribution of impact-generated residual heat (Yasui et al., 2021). However, actual parent bodies likely contained internal water, and the characteristics of impact-generated residual heat under water-bearing conditions remain poorly constrained.
This study evaluates whether impact-generated residual heat in water-bearing porous bodies can serve as a heat source for aqueous alteration. Impact experiments using hydrated gypsum targets were conducted to directly measure post-impact temperature evolution and determine its spatial and temporal distributions, and to compare the results with dry targets.
Porous gypsum targets were prepared in dry and hydrated conditions. Hydrated targets were produced by saturating dried targets with water to fill pore spaces between gypsum grains. Impacts were performed using a horizontal two-stage light-gas gun at Kobe University. 4.7-mm polycarbonate projectiles were launched at normal incidence at 2–3 km/s under reduced pressure below 3000 Pa. Internal temperatures were measured with embedded K-type thermocouples (five per target, 11–33 mm from the impact point), and temperature histories were recorded using a data logger. Surface temperature distributions were measured using an infrared camera. Crater morphology and internal structures were examined by X-ray CT before and after impact.
Craters were strength-dominated, consisting of a central bowl-shaped transient crater and a surrounding spall region. Hydrated targets exhibited a distinct delamination-like structure beneath the crater floor. Crater diameters in hydrated targets were 1.5–2 times larger than in dry targets under identical impact velocities, suggesting that shock attenuation was suppressed by water filling the pore spaces, allowing more efficient energy transmission.
In hydrated targets, internal temperatures increased rapidly immediately after impact, but the elevated temperature duration was limited to ~1.3 s, shorter than in dry targets, which showed gradual heating followed by slower cooling. This behaviour differs from dry targets, where temperature increases primarily due to thermal conduction from residual heat along the crater walls, and instead suggests heating associated with advection of hot materials during crater growth. Infrared measurements showed that dry targets reached surface temperatures exceeding 100 ºC immediately after impact, whereas hydrated targets reached only ~40 ºC, with a temperature increase of 28 ºC, significantly lower than in dry targets. Furthermore, high-temperature regions formed during impact were rapidly removed from the surface by ejecta excavation and emplacement processes, resulting in little residual heating on the target surface.