Radar constraints on asteroid regolith properties using 433 Eros as ground truth

Abstract

Abstract-Radar data enable us to estimate an asteroid's near-surface bulk density, thus providing a joint constraint on near-surface porosity and solid density. We investigate two different approaches to simplifying this joint constraint: estimating solid densities by assuming uniform porosities for all asteroids; and estimating porosities by assuming uniform mineralogy within each taxonomic class. Methods used to estimate asteroids' near-surface solid densities from radar data have not previously been calibrated via independent estimates. Recent spacecraft results on the chondritic nature of 433 Eros now permit such a check, and also support porosity estimation for S-class objects.We use radar albedos and polarization ratios estimated for 36 main-belt asteroids and nine nearEarth asteroids to estimate near-surface solid densities using two methods, one of which is similar to the uncalibrated algorithms used in previous studies, the other of which treats Eros as a calibrator. We also derive porosities for the same sample by assigning solid densities for each taxonomic class in advance. Density-estimation results obtained for Eros itself are consistent with the uncalibrated method being valid in the mean; those derived for the full sample imply that uncalibrated solid densities are, at most, a few tens of percent too large on average. However, some derived densities are extremely low, whereas most porosity estimates are physically plausible. We discuss the relative merits of these two approaches.

Shape model and surface properties of the OSIRIS-REx target Asteroid (101955) Bennu from radar and lightcurve observations

“…The distribution of energy in the two orthogonal polarizations gives an estimate of the surface roughness, or the amount of light multiply scattered compared to that returned by specular reflectance. Other compositional factors may also influence the polarization ratio (Benner et al 2008;Magri et al, 2001). Ephemeris improvements are routinely made for all asteroids observed with radar.…”

“…All other values were measured by the authors using standard techniques from archival data. The Uncalibrated and Eros-Calibrated near-surface bulk densities are computed from the 12.6-cm OC albedo and polarization ratio using the method of Magri et al (2001), equation 11, except that the porosity factor 1/(1-p) has been excluded; thus these are bulk densities rather than solid densities. Similarly, the "Eros-calibrated" near-surface bulk density has been calculated using the assumption that Eros has a near-surface bulk density of 3.75/2 = 1.875, i.e., half that of L-chondrite meteorites, and that near-surface bulk density scales with OC radar albedo, as in Magri et al, but with no attempt to adjust for porosity.…”

“…Similarly, the "Eros-calibrated" near-surface bulk density has been calculated using the assumption that Eros has a near-surface bulk density of 3.75/2 = 1.875, i.e., half that of L-chondrite meteorites, and that near-surface bulk density scales with OC radar albedo, as in Magri et al, but with no attempt to adjust for porosity. a Ostro et al, 2004. b Magri et al, 2001. c Value is standard deviation of 9 days of observation, not uncertainty. …”

“…The details are complicated, but the most useful properties are that rough surfaces tend to depolarize an incident polarized ray due to multiple and irregular scattering, and that specular reflections, which preserve polarization, occur from changes in the index of refraction at a surface boundary, which is typically related to material density. Magri et al (2001) used these properties to attempt to determine the bulk and grain densities of surface material of near-Earth asteroids using 433 Eros and L-chondrite meteorites as "ground truth". Magri et al derive two relationships for density.…”

Asteroid 1566 Icarus’s Size, Shape, Orbit, and Yarkovsky Drift from Radar Observations

“…∼1.44 km and a measured radar cross-section of 0.03 km 2 is less than 2% (see Table 3), which we believe to be the lowest radar albedo ever measured (e.g., Magri et al 1999;Magri et al 2001Magri et al , 2007a.…”