INVESTIGATING A POSSIBLE LUNAR COLD SPOT FORMATION MECHANISM: MODELING GRANULAR WAVES IN SURFACE REGOLITH USING SOFT SPHERE DISCRETE ELEMENT METHOD

Abstract

Lunar Cold Spots (LCS) are recently classified ephemeral features appearing as halos around fresh impact craters on the Moon. LCS can only be observed by examining nighttime surface regolith temperatures; there is no other indicator of their presence. They appear cold as compared to the background, a suspected consequence of reduced thermal inertia regolith (to about 40 cm depth) in the halo. The reduction in thermal inertia implies that there is some event associated with the impact that dilates (‘fluffs up’) surface material over a large radial extent. The LCS halo extends a much greater distance than could be explained by ejecta alone (the average extent is 100 crater radii), with the halo being occasionally punctured by low thermal inertia rays that extend even further. Cold Spots are ubiquitous, occurring across all terrains of the Moon and representing an approximated 1% of total surface area. However, the LCS formation mechanism is unknown. In this work, we use the Soft Sphere Discrete Element Method (SSDEM) to model granular wave propagation in conditions analogous to the lunar surface and near-surface. Our investigated hypothesis is that LCS form as the result of a granular wave propagating radially outward from an impact site.

First, we characterize the behavior of near surface grains experiencing a laterally propagating granular wavefront when they are exposed to vacuum and low-gravity. We simulate piston impacts into long channels filled randomly with Hertzian particles and observe the emergence of a solitary wave (SW, packetized energy propagation) which lofts particles in its wake. We term this mechanism SW Induced Dilation (SID) and see that the amount of fluffing that occurs is enhanced with larger piston impact speeds and as the initial compaction (packing fraction) of the bed increases. We initially observed this effect in an idealized scenario and so we undertake a scaling analysis to predict how SID would manifest in lunar conditions. We show that particle forces experienced in a 3D granular wavefront follow the same scaling relationships as in a 1D chain. We then balance 3D wavefront forces with gravitational overburden to determine an equation that predicts lofting depth (depth to which bulk dilation could be expected to occur) as a function of material properties and the magnitude of particle-particle collisional velocities along the channel floor. Our loft depth equation agrees well with simulated results and predicts a depth within the same order of magnitude as the 40 cm LCS halo depth. However, the existence of a surface SW (and thus, dilation) requires external energy input to be driven along the floor of the channel.Second, we consider a possible driver for the surface dilating wave by characterizing the decay of an initial pulse through chains of buried boulders. On the Moon, the surface regolith (a uniform layer of fine, <100 micron sized particles) covers an underlying layer of larger scale ejecta (boulders). In granular assemblies, the decay of a wave is typically governed by a fixed number of particles since energy is lost through collisions. We suspect that meter scale boulders may be able to support energy propagation over the km scale distances of the LCS halo. Through measuring wave decay in scaled up versions of the channels from our initial work, we see that the decay rate in 3D systems is governed by the same concepts as in a 1D chain: particle number governs decay distance, power law decay in time. We use the lofting equation to estimate the requisite collisional velocity (4 m/s) of particles within the buried layer’s wavefront needed to initiate SID in the surface layer. We see this velocity threshold exceed over hundreds of meters, within an order of magnitude of LCS halo scale distances.

The results of this thesis point to the following formation mechanism: cold spot regolith is fluffed up by a surface solitary wave driven from below by the decay of impact energy throughboulders at the regolith-megaregolith boundary. However, this hypothesis requires experimental validation. Understanding cold spot formation has implications for both science and engineering efforts on the Moon. Cold spot regolith fluffing represents yet another surface modification processes altering the already complex history of the Moon (and solar system) entrained in the regolith. If our proposed mechanism is proven to be true, cold spots can also reveal details about local subsurface topography such as the size distribution of buried boulders. The location of buried features may be an important consideration for future large scale construction efforts. Finally, our proposed mechanism suggests that high speed impacts on any low-gravity, low-pressure planetary surface covered with regolith and possessing an underlying layer of larger boulders would display cold spots around young craters.