Elsevier

Icarus

Volume 281, 1 January 2017, Pages 388-403
Icarus

Radar observations and shape model of asteroid 16 Psyche

https://doi.org/10.1016/j.icarus.2016.08.011Get rights and content

Highlights

  • Radar confirms asteroid 16 Psyche to be the largest metal asteroid in the main belt.

  • A Psyche shape model was generated using radar, adaptive optics, and occultation data.

  • The shape model of Psyche shows evidence of large scale features.

  • Psyche displays significant variations in radar and optical albedo with rotation.

Abstract

Using the S-band radar at Arecibo Observatory, we observed 16 Psyche, the largest M-class asteroid in the main belt. We obtained 18 radar imaging and 6 continuous wave runs in November and December 2015, and combined these with 16 continuous wave runs from 2005 and 6 recent adaptive-optics (AO) images (Drummond et al., 2016) to generate a three-dimensional shape model of Psyche. Our model is consistent with a previously published AO image (Hanus et al., 2013) and three multi-chord occultations. Our shape model has dimensions 279 × 232 × 189 km (± 10%), Deff = 226 ± 23 km, and is 6% larger than, but within the uncertainties of, the most recently published size and shape model generated from the inversion of lightcurves (Hanus et al., 2013). Psyche is roughly ellipsoidal but displays a mass-deficit over a region spanning 90° of longitude. There is also evidence for two ∼50–70 km wide depressions near its south pole. Our size and published masses lead to an overall bulk density estimate of 4500 ± 1400 kgm−3. Psyche's mean radar albedo of 0.37 ± 0.09 is consistent with a near-surface regolith composed largely of iron-nickel and ∼40% porosity. Its radar reflectivity varies by a factor of 1.6 as the asteroid rotates, suggesting global variations in metal abundance or bulk density in the near surface. The variations in radar albedo appear to correlate with large and small-scale shape features. Our size and Psyche's published absolute magnitude lead to an optical albedo of pv = 0.15 ± 0.03, and there is evidence for albedo variegations that correlate with shape features.

Introduction

Asteroid 16 Psyche is the largest Tholen (1984) M-class asteroid and a potential spacecraft target (Elkins-Tanton et al., 2014). The red-slope and generally featureless visible/near-infrared (VISIR) spectra of the M-class were similar to that of meteoritic iron-nickel (Fe-Ni) observed in the laboratory, so the early and canonical interpretation of M-class asteroids is that they are the remnant cores of ancient planetesimals, stripped as a result of cataclysmic collisions (Chapman and Salisbury, 1973, Bell et al., 1989). Additional laboratory work showed that enstatite chondrites are also a possible match (Gaffey, 1976, Gaffey and McCord, 1979) and offers an alternative interpretation which was recently supported by the Rosetta flyby of the M-class asteroid 21 Lutetia (Vernazza et al., 2011).

Because the visible/near-infrared spectral data for the M-class is subtle or ambiguous, radar is a better tool for identifying metallic content in the upper meter or so of the regolith (Cloutis et al., 1990). The first radar observations of Psyche (Ostro et al., 1985) showed it to have a significantly higher radar albedo than other main-belt asteroids, consistent with the metallic-core interpretation. Subsequent radar observations and analysis (Magri et al., 2007a, Shepard, 2008) confirm these findings.

In this paper, we report new radar observations of Psyche using the Arecibo S-band radar (2380 MHz). We use these, previous Arecibo radar observations from 2005 (Shepard et al., 2008), new adaptive-optics (AO) images (Drummond et al., 2016), and constraints from previous lightcurve shape model inversions (Kaasalainen et al., 2002), occultations (Ďurech et al., 2011), and AO images (Hanus et al., 2013) to generate a new shape model for Psyche. In Section 2, we briefly discuss our methods of radar analysis and examine what was previously known of Psyche. In Section 3, we describe the new radar data sets and inversion process. In Section 4, we examine the results, and in Section 5, we discuss future prospects for additional observations.

Section snippets

Radar analysis

In this section we review the conventions and equations that describe the relationship between radar measurements and the physical properties of an asteroid. A more thorough description of these conventions can be found in Ostro et al. (2002).

For our continuous wave radar observations (also known as CW or Doppler-only), each observing cycle or “run” consisted of transmission of a circularly polarized 2380 MHz (12.6 cm) signal for the round-trip light travel time to the target, followed by the

New radar observations

We observed Psyche at Arecibo on eleven days from 28 November 2015 to 9 December 2015. Our primary focus was delay-Doppler imaging and calibrated echo power spectra (Doppler-only) were taken as the opportunity presented. In total, we obtained 18 delay-Doppler images and 6 echo power spectra. Details of the observations are in Tables 1 and 2. Our combined calibrated echo power spectra give a total OC radar cross-section of σOC = 13,700 ± 3500 km2 and a circular polarization ratio of µc 

Analysis

We will reference Psyche's major features with respect to our shape model body-centered longitude (lon) and latitude (lat), where the +a-axis defines 0° longitude, the +b-axis is at 90° longitude, and the +c-axis aligns with the spin axis in the positive (north)-polar direction. Because of our −45° sub-radar aspect, we have good constraints on features in the southern hemisphere, but are blind to features above +45° latitude.

Conclusions and future opportunities

We have convincing evidence that 16 Psyche is the largest metal-rich asteroid in the main-belt. Our shape model is slightly larger but generally consistent with a convex model based primarily on lightcurve inversion (Hanus et al., 2013). Mass estimates and our model suggest that Psyche has a dense interior consistent with Fe-Ni and ∼40% bulk porosity, but we cannot rule out a stony-iron composition and little or no macroporosity. The overall bulk density is similar to the near-surface bulk

Acknowledgments

The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana, and the Universities Space Research Association. The Arecibo Planetary Radar Program is supported by the National Aeronautics and Space Administration under Grant No. NNX12AF24G issued through the Near Earth Object Observations program. We thank the Arecibo operators and staff for their help in

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