Evidence for cometary bombardment episodes

Evidence for cometary bombardment episodes

Evidence is found that large terrestrial impacts tend to cluster in discrete episodes, with characteristic separations 25–30 Myr and durations of about 1–2 Myr. The largest impactors are strongly concentrated within such events, and the Cretaceous–Tertiary extinctions occurred within one of them. The evidence also indicates the presence of a weak periodicity, which might be ∼24, ∼35 or ∼42 Myr ...

An Inquiry about Evidence for the Late Heavy Bombardment

Introduction: Tera et al. [1] first proposed a "terminal cataclysm" or Late Heavy Bombardment (LHB) based on an apparent spike in lunar rock resetting ages; the LHB has more recently been advocated on the basis of a spike in ages of lunar impact melts, or at least an absence of secure impact melt ages prior to 4 Ga [2-5]. Dates for lunar impact basins [6] from Nectaris (3.90-3.92 Ga) to Orienta...

Dating Howardite Melt Clasts: Evidence for an Extended Vestan Bombardment?

Cometary Diversity and Cometary Families 1.1 Introduction

Comets are classified from their orbital characteristics into two separate classes: nearly-isotropic, mainly long-period comets and ecliptic, short-period comets. Members from the former class are coming from the Oort cloud. Those of the latter class were first believed to have migrated from the Kuiper belt where they could have been accreted in situ, but recent orbital evolution simulations sh...

Cometary Rotation: an Overview

I discuss our current understanding of the spin states of cometary nuclei. Cometary spin influences the temporal and spatial patterns of outgassing from the nucleus (through diurnal and seasonal effects) and is in turn influenced by outgassing-driven torques. The current challenge to cometary astronomers is to quantify the interactions between the spin, the outgassing, and the resultant torques...