[论文解读] Physical characterisation of near-Earth asteroid (1620) Geographos. Reconciling radar and thermal-infrared observations

B. Rozitis, Simon Green|Open Research Online (The Open University)|Jul 8, 2014

Astro and Planetary Science参考文献 51被引用 6

一句话总结

本研究结合光变曲线、雷达和热红外数据，对近地小行星(1620) Geographos进行了统一的热物理建模，以调和雷达与热红外观测之间的不一致。研究发现其热惯量为340⁺¹⁴⁰₋₁₀₀ J m⁻² K⁻¹ s⁻¹/²，整体密度为2100⁺⁵⁵⁰₋₄₅₀ kg m⁻³，并确认其为碎石堆结构，雷达数据因近赤道观测几何条件而高估了z轴尺寸。

ABSTRACT

The Yarkovsky (orbital drift) and YORP (spin state change) effects play important roles in the dynamical and physical evolution of asteroids. Thermophysical modelling of these observed effects, and of thermal-infrared observations, allows a detailed physical characterisation of an individual asteroid to be performed. We perform a detailed physical characterisation of near-Earth asteroid (1620) Geographos, a potential meteor stream source and former spacecraft target, using the same techniques as previously used in Rozitis et al. (2013) for (1862) Apollo. We use the advanced thermophysical model (ATPM) on published light-curve, radar, and thermal-infrared observations to constrain the thermophysical properties of Geographos. The derived properties are used to make detailed predictions of the Yarkovsky orbital drift and YORP rotational acceleration, which are then compared against published measurements to determine Geographos's bulk density. We find that Geographos has a thermal inertia of 340 +140/-100 J m-2 K-1 s-1/2, a roughness fraction of >50%, and a bulk density of 2100 +550/-450 kg m-3 when using the light-curve-derived shape model with the radar-derived maximum equatorial diameter of 5.04 +/- 0.07 km. It is also found that the radar observations had overestimated the z-axis in Geographos's shape model because of their near-equatorial view. This results in a poor fit to the thermal-infrared observations if its effective diameter is kept fixed in the model fitting. The thermal inertia derived for Geographos is slightly higher than the typical values for a near-Earth asteroid of its size, and its derived bulk density suggests a rubble-pile interior structure. Large uncertainties in shape model z-axes are likely to explain why radar and thermal-infrared observations sometimes give inconsistent diameter determinations for other asteroids.

研究动机与目标

• 解决近地小行星(1620) Geographos的雷达反演与热红外观测之间的不一致问题。
• 通过统一模型确定Geographos的热物理特性，包括热惯量、粗糙度和整体密度。
• 通过将推导出的整体密度与已知陨石密度进行比较，评估其内部结构。
• 评估形状模型z轴不确定性（尤其是）对热红外与雷达数据拟合的影响。
• 利用推导出的物理参数预测Yarkovsky与YORP效应，并与观测测量结果进行比较。

提出的方法

• 应用高级热物理模型（ATPM）同时拟合光变曲线、雷达和热红外观测数据。
• 使用光变曲线反演的形状模型，并结合雷达反演的最大赤道直径5.04 ± 0.07 km。
• 在模型拟合过程中，将热惯量和表面粗糙度作为自由参数。
• 将模型预测的Yarkovsky漂移率与YORP旋转加速度与已发表的测量值进行比较，以约束整体密度。
• 对形状模型z轴不确定性的敏感性分析，识别出因近赤道观测几何条件导致雷达高估z轴。
• 采用统计拟合方法，推导热惯量、粗糙度和整体密度的置信区间。

实验结果

研究问题

• RQ1 为何雷达与热红外观测对(1620) Geographos的直径估计存在不一致？
• RQ2 Geographos的真实热惯量与表面粗糙度是多少？它们如何影响热辐射建模？
• RQ3 Geographos的整体密度是多少？这对其内部结构有何含义？
• RQ4 形状模型z轴的不确定性在多大程度上影响其对热红外数据的拟合？
• RQ5 Yarkovsky与YORP效应在多大程度上主导Geographos的长期动力学演化？

主要发现

• 小行星(1620) Geographos的热惯量为340⁺¹⁴⁰₋₁₀₀ J m⁻² K⁻¹ s⁻¹/²，略高于典型千米级近地小行星的值。
• 整体密度被约束在2100⁺⁵⁵⁰₋₄₅₀ kg m⁻³，表明其具有碎石堆结构，宏观孔隙率约为37%。
• 雷达观测因近赤道观测几何条件而高估了形状模型的z轴尺寸，导致在固定直径条件下与热红外数据拟合效果差。
• YORP效应预计将在Geographos的长期演化中占主导地位，约700万年内使其自转周期减半，并使其黄赤交角趋于180°。
• Yarkovsky漂移速率为27.4 ± 5.7 m yr⁻¹，与推导出的整体密度一致，支持模型的内部一致性。
• 推导出的物理参数表明，由于整体密度较低且热惯量较高，Geographos表面松散物质在地球飞掠期间比以往认为的更容易被抛射。