An Archaean heavy bombardment from a destabilized extension of the asteroid belt

The barrage of comets and asteroids that produced many young lunar basins (craters over 300 kilometres in diameter) has frequently been called the Late Heavy Bombardment (LHB). Many assume the LHB ended about 3.7 to 3.8 billion years (Gyr) ago with the formation of Orientale basin. Evidence for LHB-sized blasts on Earth, however, extend into the Archaean and early Proterozoic eons, in the form of impact spherule beds: globally distributed ejecta layers created by Chicxulub-sized or larger cratering events4. At least seven spherule beds have been found that formed between 3.23 and 3.47?Gyr ago, four between 2.49 and 2.63?Gyr ago, and one between 1.7 and 2.1?Gyr ago. Here we report that the LHB lasted much longer than previously thought, with most late impactors coming from the E belt, an extended and now largely extinct portion of the asteroid belt between 1.7 and 2.1 astronomical units from Earth. This region was destabilized by late giant planet migration. E-belt survivors now make up the high-inclination Hungaria asteroids. Scaling from the observed Hungaria asteroids, we find that E-belt projectiles made about ten lunar basins between 3.7 and 4.1?Gyr ago. They also produced about 15 terrestrial basins between 2.5 and 3.7?Gyr ago, as well as around 70 and four Chicxulub-sized or larger craters on the Earth and Moon, respectively, between 1.7 and 3.7?Gyr ago. These rates reproduce impact spherule bed and lunar crater constraints.     

A late Miocene dust shower from the break-up of an asteroid in the main belt

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter approximately 1-1,000 microm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales. Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of approximately 4 times pre-event levels, and dissipated over approximately 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past approximately 10(8) yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower or asteroid collision remains uncertain.     

统计学在天文学中的挑战

数子巡天、来自轨道望远镜的数据及计算的发展，使近几年天文数据数量和质量大大增加，这就要求更敏捷的统计和分析研究技能。幸运的是近几年统计学方法论也有了长足进展，更加促进了天文学家和统计学家对天文统计学问题及其途径的兴趣，使他们形成了强有力的联合。     

《崇祯历书》中的开普勒物理天文学思想

开普勒的新天文学是西方天文学从传统的数学天文学转变为物理天文学的标志。明末崇祯改历过程中介绍了许多西方天文学的知识,其中也包含了开普勒的物理天文学思想。《崇祯历书》中介绍的开普勒的物理天文学思想包括了开普勒的物理机械论,即通过一种普遍的自然力来解释以太阳为中心的整个行星体系的运动,太阳是宇宙的几何中心、运动中心和动力中心。本文通过分析开普勒的物理天文学思想在西方的接受情况、传教士向中国传播的情况以及中国改历的需要,说明开普勒的物理天文学对崇祯历书的影响,以及没有引进开普勒行星运动三定律的原因。