9:30 AM - 9:45 AM
[PPS08-03] Cooling rates of chondrules in lightning discharge channels with high solid density
Keywords:chondrules, meteorites, radiative transfer, hydrodynamics
Undifferentiated meteorites, chondrites, preserve the materials in proto-solar nebula thus provide us with the important information on the evolutionary history of the nebula. Especially, chondrites contain abundant chondrules which are mm-sized igneous spherules and occupy 60-80 vol% at most of chondrites. It is a common agreement that precursors of chondrules experienced transient flash heating events to be melted and then solidified. However, we still cannot conclude on the drivers of the heating events.
There are many candidate drivers of chondrule formation and lightning discharge (e.g. Desch & Cuzzi, 2000) is one of them. Lightning is regarded to be a less likely candidate than other heating drivers because previous studies predicted that lightning model leads to too fast cooling rates of chondrules (e.g. Horányi et al., 1995) due to the short length scales of discharge channels and fails to reproduce the chondrule textures (e.g. Desch et al., 2012). This prediction was built on the assumption that discharge channels might be optically thin in protoplanetary disks. However, generation of lightning in protoplanetary disks requires much higher solid density than typical in protoplanetary disks (e.g. Desch & Cuzzi, 2000; Muranushi, 2010; Johansen & Okuzumi, 2018). Some properties of chondrules also suggest their formation in solid-enhanced environments (e.g. Cuzzi & Alexander, 2006; Alexander et al., 2008). Thus, discharge channels can be optically thick, and in this case, lightning model possibly leads to proper cooling rates of chondrules to reproduce their textures.
In this study, we investigated the thermal history of chondrules in an optically thick discharge channel approximated as a column of cylindrical geometry. We calculated expansion of this hot column and cooling rates of chondrules due to radiative transfer. We included evaporation of silicate grains inside the column. Chondrules in this column must interact with surrounding gas and dust by drag force, and as a result, move with them to some degrees. For simplicity, we considered two limiting cases. The first one is the case where drag force is not considered, so chondrules stay at the same positions through the calculations. The second one is the case where chondrules are completely coupled to their surroundings, so they comove with them.
Our main finding is that in either case of the dynamics of chondrules, cooling rates of chondrules are reproduced by lightning model with the typical gas density at a few au in proto-solar nebula, solid density inferred by Alexander et al., 2008, and the typical width of lightning.
There are many candidate drivers of chondrule formation and lightning discharge (e.g. Desch & Cuzzi, 2000) is one of them. Lightning is regarded to be a less likely candidate than other heating drivers because previous studies predicted that lightning model leads to too fast cooling rates of chondrules (e.g. Horányi et al., 1995) due to the short length scales of discharge channels and fails to reproduce the chondrule textures (e.g. Desch et al., 2012). This prediction was built on the assumption that discharge channels might be optically thin in protoplanetary disks. However, generation of lightning in protoplanetary disks requires much higher solid density than typical in protoplanetary disks (e.g. Desch & Cuzzi, 2000; Muranushi, 2010; Johansen & Okuzumi, 2018). Some properties of chondrules also suggest their formation in solid-enhanced environments (e.g. Cuzzi & Alexander, 2006; Alexander et al., 2008). Thus, discharge channels can be optically thick, and in this case, lightning model possibly leads to proper cooling rates of chondrules to reproduce their textures.
In this study, we investigated the thermal history of chondrules in an optically thick discharge channel approximated as a column of cylindrical geometry. We calculated expansion of this hot column and cooling rates of chondrules due to radiative transfer. We included evaporation of silicate grains inside the column. Chondrules in this column must interact with surrounding gas and dust by drag force, and as a result, move with them to some degrees. For simplicity, we considered two limiting cases. The first one is the case where drag force is not considered, so chondrules stay at the same positions through the calculations. The second one is the case where chondrules are completely coupled to their surroundings, so they comove with them.
Our main finding is that in either case of the dynamics of chondrules, cooling rates of chondrules are reproduced by lightning model with the typical gas density at a few au in proto-solar nebula, solid density inferred by Alexander et al., 2008, and the typical width of lightning.