Radar observations and shape model of asteroid 16 Psyche
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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