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Final Announcement

Meeting Location and Dates

The First Billion Years: Bombardment conference will be held September 30–October 2, 2018 at the High Country Conference Center, 201 W Butler Avenue, Flagstaff, Arizona.

Purpose and Scope

Following planetary accretion there was an extended period of bombardment that may have been punctuated by one or more bursts of collisional activity. The largest impactors produced impact basins hundreds to thousands of kilometers in diameter, completely reshaping the surfaces of the rocky and icy planets. These types of events were not unique to our solar system. Debris disks produced by similar processes have been observed around other stars after they emerged from their natal gas-rich nebulae and up to several hundred million years after they formed.

This topical conference on Bombardment, the third in The First Billion Years series, is designed to investigate the range of collisional events from the late stages of terrestrial planet accretion to the end of the basin-forming epoch on the Moon. Although the Moon may be a central component of the conference due to its exquisitely preserved record, the discussion will necessarily include observations elsewhere, such as on the Hadean Earth, Mercury, Mars, the asteroid belt, outer solar system moons, and planetary systems elsewhere. Because the bombardment may have affected the origin and early evolution of life, discussion will also draw on astrobiological findings.


The topical conference will provide an opportunity to integrate several diverse components of the above topic and, ideally, force reconciliation between scientific fields that may have been exploring these processes independently. The conference will include an assessment of the geologic record of impact cratering throughout the solar system, cosmochemical constraints on any early bombardment, and dynamical models that might explain the flux of debris and potential changes in the flux of debris in our solar system and other planetary systems. Examples of questions to be addressed include:

What are the physical characteristics of the debris disk produced by bombardment a few hundred million years after solar system formation and how do they compare with those debris disks observed with a variety of ground and space facilities?

If impactors are differentiated bodies rather than homogeneous chondritic bodies, how does that affect the crater forming process and distribution of impactor material? Does the outcome differ if the differentiated body is entirely a silicate-metal body (e.g., like Vesta) or a differentiated body involving both icy and silicate-metal materials?

Are collisional velocities uniform throughout the epoch or do the dynamical drivers of the bombardment cause jumps in those velocities? Can any jumps in velocities be verified with observable properties and, if so, what affect does a jump in collisional velocity have on the bombardment of planetary surfaces?

On the Earth, Moon, and Mars, how is the U-Pb system in zircon and other U-Pb phases affected by impact cratering processes on the Earth, Moon, and Mars, and how can that information be used to extract details of the impact flux and its effects on crust from surviving zircon and related phases in the Jack Hills (i.e., on Earth), in impact lithologies (i.e., on the Moon), and among martian meteorites? What complementary insights can be gleaned from other radiometric systems?

On the Moon, how well can we know the flux and sources of early basin-forming impactors? What factors might complicate inferences based on crater size frequency distributions (e.g., velocity, porosity, target temperature)? As crater frequency distributions are the basis for solar system chronologies, what are the implications for the timing of evolutionary events in the rest of solar system?

What is the cadence and magnitude of the bombardment and how does that alter the collisional consequences on a planetary surface, any hydrosphere, and atmosphere? This objective is intentionally designed to provide a transitional baseline for the fourth topical conference in The First Billion Years series, which will feature Habitability:  Where Planets Can Come to Life.

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