Presentation Information
[PPS04-P28]Thermal radiation during the initial expansion stage of vapor plumes after mid-scale impacts on the Earth
*Mao Arakawa1, Kosuke Kurosawa1,2 (1.Department of Environment and Sustainability, Faculty of Global Human Sciences, Kobe University, 2.Planetary Exploration Research Center, Chiba Institute of Technology)
Keywords:
Hypervelocity impacts,Thermal radiation,Ultraviolet,Hydrocode
Cometary and/or asteroidal impacts have produced a wide range of environmental consequences throughout Earth's history. In this study, we focus on mid-scale impact events by asteroids with diameters of 0.1-3 km. Such impactors are large enough to avoid disruption during atmospheric traverse and strike the Earth's surface at hypervelocity (e.g., 1). Peak pressures beneath the impact point exceeds 100 GPa, resulting in a vaporization of both the projectile and target materials (e.g., 2). Then, the high-pressure (P) and -temperature (T) plume adiabatically expands into a surrounding atmosphere (e.g., 2).
During the impact, the atmosphere is also heated, leading to a radiatively complex environment. As the plume expands into the atmosphere, it halts at a characteristic radius (3). At this time, the plume remains hotter than the atmosphere and continues thermal radiation over an extended period.
Thermal radiation from such events has been evaluated by assuming that the radiation during the adiabatic expansion can be neglected (3). Although the plume temperature initially exceeds ~10000 K, the surrounding partially ionized atmosphere is optically thick, inhibiting the radiative escape of thermal energy. As the shocked atmosphere becomes optically thin (transparent temperature ~3000 K), thermal radiation from the plume can reach the environment. (3) modeled this late-stage radiation. For a 2-km-diameter impactor, the late-stage radiative phase lasts several hundred seconds, which is much longer than the duration of the adiabatic expansion (~10 s).
This framework, however, neglects spectral information. Even during the early expansion phase (EPP), the optically-thick atmospheric layer, which is concentrated around the plume-atmosphere boundary, would emit thermal energy. Such emission may include UV radiation, with potential effects on biological systems.
In this study, we investigated thermal radiation emitted during EPP. To examine the interaction between the plume and the atmosphere, we performed iSALE simulations (4-6).
We found that the shocked atmospheric layer initially reaches temperatures of >10000 K. Moreover, the high-T atmospheric layer that is ahead of the plume. Cooling of this high-T layer to the transparent temperature takes ~10 s.
Although a full treatment of radiative transfer within the high-T layer is required for accurate evaluation, we adopt a simplified approach in which thermal radiation is assumed to be emitted as blackbody radiation from the location of the maximum T. The total radiative energy during EPP can reach up to ~1018 J. This corresponds to ~0.5% of the initial internal energy and up to 50% of the thermal radiation energy estimated by (3).
Radiation in wavelength bands that are relatively weakly absorbed by the atmosphere (170-215 nm and 295-400 nm) may reach energies of up to ~1017 J. These wavelength ranges span the ranges from UV-A to UV-C, which are of particular biological significance (7). Our results suggest that the role of early-stage thermal radiation should be re-examined to assess the thermal and radiative effects after hypervelocity impacts.
Acknowledgements: We would like to thank G. Collins, K. Wunnemann, B. Ivanov, H. J. Melosh, D. Elbeshausen and T. Davison, the developers of iSALE, for their contributions. The numerical simulations were performed using the computational and analysis servers of the CfCA, NAOJ.
References: [1] Melosh 1989, Impact cratering: A geologic process, Oxford University Press. [2] Ahrens & O'Keefe 1972, The Moon, 4, 214. [3] Collins et al. 2005, MaPS, 40, 817. [4] Amsden et al. 1980, LANL Report, LA-8095:101p. [5] Ivanov et al. 1997, Int. J. of Impact Eng., 20, 411. [6] Wunnemann et al. 2006, Icarus, 180, 514. [7] Sasaki 2006. National Institute for Environmental Studies, Report M018.
During the impact, the atmosphere is also heated, leading to a radiatively complex environment. As the plume expands into the atmosphere, it halts at a characteristic radius (3). At this time, the plume remains hotter than the atmosphere and continues thermal radiation over an extended period.
Thermal radiation from such events has been evaluated by assuming that the radiation during the adiabatic expansion can be neglected (3). Although the plume temperature initially exceeds ~10000 K, the surrounding partially ionized atmosphere is optically thick, inhibiting the radiative escape of thermal energy. As the shocked atmosphere becomes optically thin (transparent temperature ~3000 K), thermal radiation from the plume can reach the environment. (3) modeled this late-stage radiation. For a 2-km-diameter impactor, the late-stage radiative phase lasts several hundred seconds, which is much longer than the duration of the adiabatic expansion (~10 s).
This framework, however, neglects spectral information. Even during the early expansion phase (EPP), the optically-thick atmospheric layer, which is concentrated around the plume-atmosphere boundary, would emit thermal energy. Such emission may include UV radiation, with potential effects on biological systems.
In this study, we investigated thermal radiation emitted during EPP. To examine the interaction between the plume and the atmosphere, we performed iSALE simulations (4-6).
We found that the shocked atmospheric layer initially reaches temperatures of >10000 K. Moreover, the high-T atmospheric layer that is ahead of the plume. Cooling of this high-T layer to the transparent temperature takes ~10 s.
Although a full treatment of radiative transfer within the high-T layer is required for accurate evaluation, we adopt a simplified approach in which thermal radiation is assumed to be emitted as blackbody radiation from the location of the maximum T. The total radiative energy during EPP can reach up to ~1018 J. This corresponds to ~0.5% of the initial internal energy and up to 50% of the thermal radiation energy estimated by (3).
Radiation in wavelength bands that are relatively weakly absorbed by the atmosphere (170-215 nm and 295-400 nm) may reach energies of up to ~1017 J. These wavelength ranges span the ranges from UV-A to UV-C, which are of particular biological significance (7). Our results suggest that the role of early-stage thermal radiation should be re-examined to assess the thermal and radiative effects after hypervelocity impacts.
Acknowledgements: We would like to thank G. Collins, K. Wunnemann, B. Ivanov, H. J. Melosh, D. Elbeshausen and T. Davison, the developers of iSALE, for their contributions. The numerical simulations were performed using the computational and analysis servers of the CfCA, NAOJ.
References: [1] Melosh 1989, Impact cratering: A geologic process, Oxford University Press. [2] Ahrens & O'Keefe 1972, The Moon, 4, 214. [3] Collins et al. 2005, MaPS, 40, 817. [4] Amsden et al. 1980, LANL Report, LA-8095:101p. [5] Ivanov et al. 1997, Int. J. of Impact Eng., 20, 411. [6] Wunnemann et al. 2006, Icarus, 180, 514. [7] Sasaki 2006. National Institute for Environmental Studies, Report M018.