Masters of disguise: can achondrite parent bodies hide beneath a chondritic cover?

陨石球粒陨石是已知的最原始的类地行星的组成部分，因为它们显示出太阳中仍然存在的元素丰度[1]。其他类型的陨石包括无球粒陨石和铁陨石，它们显示出不同程度的分异，分别类似于地球的玄武岩地壳和铁核。传统上，这些类别的陨石被解释为分别来自更原始的(球粒陨石)和更具差异性的(非球粒陨石和铁)母体。然而，另一种解释认为，即使分化的母体也可以保留未分化的软骨结壳[2]。这一理论与观测到的小行星21 Lutetia是一致的，该小行星表面有球粒状成分，但似乎是内部分化的[3]。这种情况可能发生在经历了短暂放射性核素衰变的内部加热的小行星上。部分熔融和内部岩浆作用将促进内部的分化，而外层地壳保持完好和未处理。然而，这一制度可能会被限制在一定的母体大小和形成年龄。在较早或较大的行星体中，更强烈的加热和内部动力可能导致构造破裂和/或火山重新浮出球粒体盖。新一代用于岩浆通过变形的韧性-脆性岩石的多相反应-运输模型[4-6]将使我们能够检验非球粒陨石母体上的球粒陨石结壳的假设稳定性！

Chondrite meteorites are the most primitive known building blocks of terrestrial planetsas they show elemental abundances as still found in our Sun [1]. Other groups of meteorites knownas achondrites and iron meteorites show various degrees of differentiation resembling the Earth'sbasaltic crust and iron core, respectively. Traditionally, these categories of meteorites have beeninterpreted to derive from more primitive (chondrites) and more differentiated (achondrites, and irons)parent bodies, respectively. However, an alternative interpretation has it that even differentiatedparent bodies can retain an undifferentiated chondritic crust [2]. This theory is consistent withobservation of asteroid 21 Lutetia that has a chondritic surface composition but appears to beinternally differentiated [3]. That scenario could arise in small planetesimals that experience internalheating from short-lived radionuclide decay. Partial melting and internal magmatism would facilitatedifferentiation of the interior while an outer crust remains intact and unprocessed. However, thatregime will likely be restricted to a certain size and formation age of parent bodies. In earlier or largerplanetesimals, more intense heating and internal dynamics might lead to tectonic disruption and/orvolcanic resurfacing of the chondritic lid. A new generation of multi-phase reaction-transport modelsfor magma transport through deforming ductile-brittle rock [4-6] will allow us to put the hypotheticalstability of a chondritic crust on an achondrite parent body to the test!