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

球粒陨石是已知的构成类地行星的最原始的材料，因为它们的元素丰度与我们在太阳中发现的一样。其他陨石群，如无球粒陨石和铁陨石，分别表现出不同程度的分化，类似于地球的玄武岩地壳和铁芯。传统上，这两类陨石被解释为分别来自更原始的（球粒陨石）和更分化的（无球粒陨石和铁）母体。然而，另一种解释认为，即使已分化的母体也能保留未分化的球粒质外壳。这一理论与观测到的小行星21 Lutetia一致，该小行星表面有球粒陨石组成，但似乎是内部分化的[3]。这种情况可能出现在经历短暂放射性核素衰变的内部加热的小型星子中。部分熔融和内部岩浆活动将促进内部的分化，而外壳保持完整和未加工。然而，这种机制可能会受到母体的一定大小和形成年龄的限制。在早期或较大的星子中，更强烈的加热和内部动力学可能导致球粒陨石盖的构造破坏和/或火山重新表面。岩浆通过变形的韧性-脆性岩石运输的新一代多相反应-运输模型[4-6]将使我们能够在无球粒质母体上测试球粒质地壳的假设稳定性！

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!