Using pit crater chains to probe regolith depth and development on asteroid 433 Eros

NASA Discovery Data Analysis Program

Start Date: 06/01/2021
Project #: 1809
End Date: 05/31/2025
Award #: 80NSSC21K1014

PSI Personnel

Project Description

Scientific Objectives

Pit craters are a ubiquitous surface feature on many asteroids visited by spacecraft. Pit craters present as linear chains of similarly- sized, often overlapping craters, and are frequently associated with visible ridges or grooves. They also lack the raised rims and ejecta blankets indicative of impact craters. Instead, pit craters are believed to form via the drainage of loose regolith into an underlying extensional fault, producing a chain of pits whose diameters and spacing are proportional to the depth of the overlying regolith layer. Pit crater chains thus offer a unique means for estimating an asteroid’s regolith depth at a specific location, especially when the chain consists of several pits and sampling statistics can be utilized. Additionally, when multiple pit crater chains have been identified on an asteroid, regional variations in regolith depth can be explored.

On fractured monolith asteroids (asteroids that have not been previously disrupted and gravitationally reaggregated to form a rubble pile), impact cratering is the primary means by which regolith is both produced (via ejecta deposits and crater collapse) and lost (via ejecta launch to space). Initially, regolith production dominates loss, such that regolith depth will gradually increase until an equilibrium state is reached, wherein losses to space from small impacts are offset by regolith production from occasional large impacts. As such, regolith depth can be used as an indicator of exposure age to an impactor flux. Similar to crater count methods, which can only be used to date a surface up to the point that crater density equilibrium is reached, this regolith depth method can only be used to date a surface up to the point that regolith depth equilibrium is reached. However, regolith depth equilibrium generally occurs long after crater density equilibrium, thus extending the measurable age. Additionally, regional variations in regolith depth are indicative of either global regolith migration trends or an irregular ejecta deposition pattern, primarily due to specific, large impacts. Asteroid regolith depth estimates thus offer an extremely useful indicator of a particular body’s surface history.

Methodology

The first aim of this investigation is to conduct a detailed morphometric study of the dozens of previously identified (but not measured) pit crater chains on asteroids 433 Eros, visited by the NEAR Shoemaker spacecraft. These measurements will be used to produce a regional-scale regolith depth map across all parts of the surface for which sufficient observations of pit crater chains are available.

In the second part of this investigation, these regolith depth estimates, along with previously conducted crater counts, will be used
as constraints in two types of numerical asteroid surface models: the first used to explore the impact ejecta landing locations and thicknesses associated with specific, individual impacts; and the second used to explore the gradual development of a global regolith layer due to exposure to a given impactor flux. The overall goal of this work will be to obtain a regional-scale understanding of the asteroid’s impact cratering surface properties and history.

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