Catastrophic disruptions as the origin of bilobate comets

Several comets observed at close range have bilobate shapes, including comet 67P/Churyumov–Gerasimenko (67P/C–G), which was imaged by the European Space Agency’s Rosetta mission. Bilobate comets are thought to be primordial because they are rich in supervolatiles (for example, N₂ and CO) and have a low bulk density, which implies that their formation requires a very low-speed accretion of two bodies. However, slow accretion does not only occur during the primordial phase of the Solar System; it can also occur at later epochs as part of the reaccumulation process resulting from the collisional disruption of a larger body, so this cannot directly constrain the age of bilobate comets. Here, we show by numerical simulation that 67P/C–G and other elongated or bilobate comets can be formed in the wake of catastrophic collisional disruptions of larger bodies while maintaining their volatiles and low density throughout the process. Since this process can occur at any epoch of our Solar System’s history, from early on through to the present day, there is no need for these objects to be formed primordially. These findings indicate that observed prominent geological features, such as pits and stratified surface layers, may not be primordial

Inherited risk factors for venous thromboembolism

Venous thromboembolism (VTE) has important heritable components. In the past 20 years, knowledge in this field has greatly increased with the identification of a number of gene variants causing hypercoagulability. The two main mechanisms are loss-of-function of anticoagulant proteins and gain-of-function of procoagulants, the latter owing to increased synthesis or impaired downregulation of a normal protein or, more rarely, to synthesis of a functionally hyperactive molecule. Diagnosis of thrombophilia is useful to determine the causes of VTE, recognizing that this multifactorial disease can also be influenced by various acquired factors including cancer, surgery, trauma, prolonged immobilization, or reproduction-associated risk factors. Diagnosis of inherited thrombophilia rarely affects the acute or long-term management of VTE. However, the risk of recurrent VTE is increased in anticoagulant-deficient patients and in homozygotes for gain-of-function mutations. Screening for inherited thrombophilia in thrombosis-free individuals is indicated only for relatives of a proband who is anticoagulant-deficient or has a family history of VTE. In families with thrombophilia and VTE, primary antithrombotic prophylaxis during risk situations lowers the rate of incident VTE. In this Review, we discuss the main causes of inherited thrombophilia, the associated clinical manifestations, and the implications for antithrombotic prophylaxis in the affected individuals