Molecular kinetic simulations are typically used to accurately describe the tenuous regions of the upper atmospheres on planetary bodies. These simulations track the motion of particles representing real atmospheric atoms and/or molecules subject to collisions, the object's gravity, and external influences. Because particles can end up in very large ballistic orbits, upper boundary conditions (UBC) are typically used to limit the domain size thereby reducing the time for the atmosphere to reach steady-state. In the absence of a clear altitude at which all molecules are removed, such as a Hill sphere, an often used condition is to choose an altitude at which collisions become infrequent so that particles on escape trajectories are removed. The remainder are then either specularly reflected back into the simulation domain or their ballistic trajectories are calculated analytically or explicitly tracked so they eventually re-enter the domain. Here we examine the effect of the choice of the UBC on the escape rate and the structure of the atmosphere near the nominal exobase in the convenient and frequently used 1D spherically symmetric approximation. Using Callisto as the example body, we show that the commonly used specular reflection UBC can lead to significant uncertainties when simulating a species with a lifetime comparable to or longer than a dynamical time scale, such as an overestimation of escape rates and an inflated exosphere. Therefore, although specular reflection is convenient, the molecular lifetimes and body's dynamical time scales need to be considered even when implementing the convenient 1D spherically symmetric simulations in order to accurately estimate the escape rate and the density and temperature structure in the transition regime.

分子动力学模拟通常用于准确描述行星体上层大气的脆弱区域。这些模拟跟踪代表真实大气原子和/或分子受到碰撞、物体重力和外部影响的粒子的运动。由于粒子最终会进入非常大的弹道轨道，因此通常使用上边界条件 (UBC) 来限制域大小，从而减少大气达到稳态的时间。在没有清除所有分子的明确高度（例如希尔球）的情况下，经常使用的条件是选择碰撞变得不频繁的高度，以便去除逃逸轨迹上的粒子。其余的要么被镜面反射回模拟域，要么通过分析计算或明确跟踪它们的弹道轨迹，以便它们最终重新进入该域。在这里，我们在方便且常用的一维球对称近似中检查了 UBC 的选择对逃逸率和标称外基地附近大气结构的影响。使用 Callisto 作为示例主体，我们表明，在模拟生命周期与动态时间尺度相当或更长的物种时，常用的镜面反射 UBC 会导致显着的不确定性，例如高估逃逸率和膨胀的外大气层。因此，虽然镜面反射很方便，但分子寿命和体'