FALLING SKY: THE HADEAN AND ARCHEAN EARTH ASTEROID IMPACT FLUX (Invited Presentation)

Advances have recently been made in understanding the nature and rate of collisions between the early Earth and planetesimals leftover of planetary accretion (e.g., Marchi et al 2014; Morbidelli et al 2018). Of particular interest is the collisional history during the Hadean and Archean (~4.0-2.5 Ga) eons, which were characterized by drastic environmental changes culminating in the rise of atmospheric oxygen at ~2.4 Ga (e.g., Lyons et al 2014).

A combination of geophysical observations, geochemical data, and dynamical models have been used to constrain the oldest (>3.8-3.9 Ga) Earth-Moon collisional history (e.g., Marchi et al 2012, 2014; Morbidelli et al 2012). These models primarily rely on observational constraints such as lunar cratering rates inferred from crater counts and sample radiometric ages, as well as concentrations of highly siderophile elements in lunar and terrestrial rocks. Although significant uncertainties remain (e.g., existence of a late heavy bombardment), available collisional models tend to converge at ~3.8-3.9 Ga thanks to absolute calibration from Apollo/Luna samples (e.g., Morbidelli et al 2018).

The subsequent ~3.5-2.5 Ga impact flux can be constrained thanks to preserved records of terrestrial collisions, such as impact spherule layers (Bottke et al 2012). The spherule layers likely represent an incomplete record of impacts during this time interval (e.g., Glass & Simonson 2013); however, they do provide a useful lower limit for the bombardment flux. There are currently 16 known impact spherule layers identified in outcrops in the 3.5-2.5 Ga time range, some of which have only very recently been discovered. Additional spherule layers have been found in drill cores from the Barberton Belt, South Africa; however, it remains unclear whether they represent new, independent impact events or are duplicate of known spherule layers (Schulz et al 2017).

In this talk I will review pros and cons of current Hadean and Archean asteroid bombardment models.

Bottke W.F., et al. Nature 485, 78, 2012; Glass B. & Simonson B. 10.1007/978-3-540-88262-6, 2013; Lyons T. W., et al. Nature 506, 307, 2014; Marchi S., et al. Nature 511, 578, 2014; Marchi S., et al. EPSL 325, 27, 2012; Morbidelli A., et al. EPSL 355, 144, 2012; Morbidelli A., et al. Icarus 305, 262, 2018; Schulz T., et al. GCA 211, 322, 2017.