Two Distinct Populations of Dark Comets Delineated by Orbits and Sizes

Small bodies are capable of delivering essential prerequisites for the development of life, such as volatiles and organics, to the terrestrial planets. For example, empirical evidence suggests that water was delivered to the Earth by hydrated planetesimals from distant regions of the Solar System (1). Recently, several morphologically inactive near-Earth objects (NEOs) were reported to experience significant nongravitational accelerations inconsistent with radiationbased effects, and possibly explained by volatile-driven outgassing (2, 3). However, these “dark comets” display no evidence of comae in archival images, which are the defining feature of cometary activity. Here we report detections of nongravitational accelerations on seven additional objects previously classified as inactive (doubling the population) that could also be explainable by asymmetric mass loss. A detailed search of archival survey and targeted data rendered no detection of dust activity in any of these objects in individual or stacked images. We calculate dust production limits of ∼ 10, 0.1, and 0.1 kg s−1 for 1998 FR11, 2001 ME1, and 2003 RM with these data, indicating little or no dust surrounding the objects during the observations. This set of dark comets reveals the delineation between two distinct populations: larger, “outer” dark comets on eccentric orbits that are end members of a continuum in activity level of comets, and smaller, “inner” dark comets on near-circular orbits that could signify a new population. These objects may trace various stages in the life cycle of a previously undetected, but potentially numerous, volatile-rich population that may have provided essential material to the Earth.

小型天体可为类地行星输送生命演化所需的关键前提物质，例如挥发物（volatiles）与有机物。例如，已有实证证据表明，地球的水来自太阳系远端含水星子（planetesimals）的输送（文献1）。近期有研究报道，数个形态无活动的近地天体（near-Earth objects, NEOs）出现了显著的非引力加速度（nongravitational accelerations），该现象与辐射相关效应不符，或可通过挥发物驱动的排气过程予以解释（文献2、3）。但这类暗彗星（dark comets）在存档图像中并未呈现彗发（comae）——彗发正是彗星活动的标志性特征。本文报道了另外7个此前被归类为无活动天体的非引力加速度探测结果，该样本使已知暗彗星种群规模翻倍，此类加速度同样可通过不对称质量损失予以解释。我们对存档巡天数据与靶向观测数据开展了详尽检索，在单幅图像与叠加图像中均未在这批天体中发现尘埃活动的迹象。基于这批数据，我们测算出1998 FR11、2001 ME1与2003 RM的尘埃产生率上限分别约为10、0.1与0.1 kg·s⁻¹，表明观测期间这些天体周围几乎不存在或完全不存在尘埃。这批暗彗星揭示了两类截然不同的种群之间的分界：一类是轨道偏心率较高的大型“外区暗彗星”，属于彗星活动强度连续谱的末端成员；另一类是轨道接近圆形的小型“内区暗彗星”，或代表一类全新的天体种群。这类天体或可反映一类此前未被发现但数量可能众多的富挥发物种群的演化周期各个阶段，而该种群曾为地球输送过关键物质。