The Conductive Cooling of Planetesimals With Temperature-Dependent Properties

Modeling the planetary heat transport of small bodies in the early Solar System allows us to understand the geological context of meteorite samples. Conductive cooling in planetesimals is controlled by thermal conductivity, heat capacity, and density, which are functions of temperature (T). We investigate if the incorporation of the T-dependence of thermal properties and the introduction of a nonlinear term to the heat equation could result in different interpretations of the origin of different classes of meteorites. We have developed a finite difference code to perform numerical models of a conductively cooling planetesimal with T-dependent properties and find that including T-dependence produces considerable differences in thermal history, and in turn the estimated timing and depth of meteorite genesis. We interrogate the effects of varying the input parameters to this model and explore the nonlinear T-dependence of conductivity with simple linear functions. Then we apply non-monotonic functions for conductivity, heat capacity, and density fitted to published experimental data. For a representative calculation of a 250 km radius pallasite parent body, T-dependent properties delay the onset of core crystallization and dynamo activity by similar to 40 Myr, approximately equivalent to increasing the planetary radius by 10%, and extend core crystallization by similar to 3 Myr. This affects the range of planetesimal radii and core sizes for the pallasite parent body that are compatible with paleomagnetic evidence. This approach can also be used to model the T-evolution of other differentiated minor planets and primitive meteorite parent bodies and constrain the formation of associated meteorite samples. Plain Language Summary Meteorites are fragments of the earliest planetary bodies in our Solar System. Meteorite samples record snapshots of the temperatures and cooling rates experienced inside these small rocky bodies before they were broken apart in collisions. By taking the cooling rate recorded in a meteorite and comparing it to the cooling rates expected at different depths inside parent bodies (based on computational modeling), we can estimate what size the parent body might have been and how deep inside it the meteorite formed. Properties like thermal conductivity control how the body cools: these properties are temperature dependent, so their value changes as the body cools down. We find that including this temperature-dependence is important when modeling meteorite parent bodies, and that assuming these properties are constant can result in different interpretations of meteorite samples. As an example, we model the parent body of stony-iron meteorites called pallasites. We find that if you include temperature-dependent properties, the iron core freezes 40 million years later than if you use constant thermal properties, which in turn affects in which meteorite samples you expect to find paleomagnetism records of core dynamo activity. This result has implications for the thermal evolution of other meteorite parent bodies and other minor planets in the Solar System, and the code developed can be adapted to investigate these other small bodies.

Automated summary

Modeling the planetary heat transport of small bodies in the early Solar System is key to understand the geological context of meteorite samples. Including temperature-dependence in the models is crucial for accurate interpretations of the origin of different classes of meteorites. The thermal properties of parent bodies play a significant role in the timing and depth of meteorite genesis.