Asteroid 16 Psyche: Shape, Features, and Global Map

Asteroid 16 Psyche is the largest Tholen (1984) M-class asteroid and a target of the NASA Discovery mission Psyche (Elkins-Tanton et al. 2017, 2020). Visible and near-infrared spectra (Bell et al. 1989), optical polarimetry (Dollfus et al. 1979), thermal observations (Matter et al. 2013), and radar observations (Ostro et al. 1985; Magri et al. 2007a; Shepard et al. 2008, 2017) all support the hypothesis that its surface to near-surface is dominated by metal (i.e., iron and nickel). However, recent spectral detections of silicates (Hardersen et al. 2005; Ockert-Bell et al. 2008, 2010; Sanchez et al. 2017) and hydrated mineral phases (Takir et al. 2017) have complicated this interpretation and suggest it to be more complex than previously assumed.

Recent studies of Psyche have converged to size estimates (effective diameter) between 220 and 230 km, and to spin poles within a few degrees of (36°, −8°) (ecliptic lon, lat) (Shepard et al. 2017; Drummond et al. 2018; Viikinkoski et al. 2018; Ferrais et al. 2020). Given this size, mass estimates have led to a consensus that Psyche's overall bulk density is between 3400 and 4100 kg m⁻³ (Elkins-Tanton et al. 2020), which places constraints on its internal structure, composition, and possible origin.

Three major hypotheses for the formation and structure of Psyche are currently debated. In simplified terms they are (1) the early interpretation that Psyche is the stripped remnant core of an ancient planetesimal, dominated by an iron composition (e.g., Bell et al. 1989; Asphaug et al. 2006); (2) Psyche is a reaccumulated pile of rock and metal, a type of low-iron pallasite or mesosiderite parent body derived from repeated impacts (Davis et al. 1999; Viikinkoski et al. 2018); and (3) Psyche is either an iron (Abrahams & Nimmo 2019) or a silicate-iron differentiated body (Johnson et al. 2020) with surface eruptions of metallic iron via ferrovolcanism.

The earliest model for Psyche is that it is a remnant metal core left after a large impact stripped the crust and mantle from an early protoplanet (Bell et al. 1989; Davis et al. 1999). However, problems with this interpretation have been difficult to overcome. Davis et al. (1999) suggested that an event of this magnitude was unlikely in the first 500 Ma of solar system formation and would have left dozens of family fragments that should be detectable but have not been found. The presence of silicates, and especially hydrated phases detected on Psyche has also created doubt. Only recently have new hypervelocity impact experiments on iron bodies suggested a way to systematically explain these signatures (Libourel et al. 2019). Perhaps the most problematic datum is Psyche's bulk density. Iron has a grain density of ∼7900 kg m⁻³, yet multiple estimates of Psyche's mass yield an overall bulk density of half that value (Elkins-Tanton et al. 2020). If it is a remnant core, it must also be a rubble pile or extremely porous, and it is not known if this is possible for an object as large as Psyche. Although still possible, it is fair to say that this model for Psyche has fallen out of favor (Elkins-Tanton et al. 2020).

As an alternative, Davis et al. (1999) suggested that while Psyche was shattered by early impacts, it was not completely disrupted. As a result, the surface should preserve a mesosiderite-like mix of metal and silicate material with a still-intact core. The detections of orthopyroxenes and other possible silicates in the spectra of Psyche (Hardersen et al. 2005; Ockert-Bell et al. 2010) are consistent with this. The detection of spectral hydroxyl features (Takir et al. 2017) is inconsistent with this violent early genesis but easily explained by more recent impacts with primitive objects, similar to the scenario envisioned for Vesta (Prettyman et al. 2012; Reddy et al. 2012; Shepard et al. 2015) and later demonstrated experimentally by Libourel et al. (2019).

Recently, two groups have proposed that Psyche is an object that has experienced a type of iron volcanism, or ferrovolcanism. Abrahams & Nimmo (2019) propose a mechanism in which molten iron erupts onto the brittle surface of a pure iron–nickel remnant as it cools from the outside inward. They predict that iron volcanoes are more likely to be associated with impact craters, or if there is an overlying silicate layer, the iron may be deposited intrusively, as a diapir. Conceivably, these intrusions could later be uncovered by impacts. However, the underlying assumption of this model is that Psyche is largely metallic throughout, possibly covered by a thin layer of silicate impact debris. This seems incompatible with Psyche's overall bulk density.

Johnson et al. (2020) propose that Psyche is a differentiated object with a relatively thin silicate mantle and iron core. As it cooled, volatile-rich molten iron was injected into the overlying mantle and, in favorable circumstances, erupted onto the surface. This model is consistent with Psyche's overall bulk density and the observation of silicates on its surface. They make no specific predictions about the location of eruption centers except that they are more likely where the crust and mantle are thinnest.

In both ferrovolcanic models, the presence of volatiles, e.g., sulfur, is critical for lowering the viscosity of the iron melt and providing the pressure needed to drive the melt onto the surface. In the Johnson et al. model, the thickness of the mantle/crust is also critical; if it is too thick, the melt will never make it to the surface.

In this paper, we refine the size and shape of Psyche using previously published and newly obtained data sets. We examine topographical and optical albedo features discussed in previous work (Shepard et al. 2017; Viikinkoski et al. 2018; Ferrais et al. 2020), produce a global map, and overlay it with radar albedo values, a proxy for metal concentrations in the upper meter of the regolith. We find correlations between centers of high radar and optical albedos and discuss the consequences for these models of Psyche's formation and structure.

In this section, we briefly describe the data sets used in our shape model, our methods, and final model results. Data set details are listed in Tables 1–5. Figures 1–6 illustrate both the image data sets and subsequent model fits to them.

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2.1. Arecibo S-band Delay-Doppler Imaging and Calibrated Echoes

For shape modeling, we use 18 delay-Doppler images acquired by the Arecibo S-band (2380 MHz or 12.6 cm wavelength) radar in 2015 (Shepard et al. 2017). These images have a spatial scale (in range) of 7.5 km/pixel, sample frequency of 50 Hz per pixel (Doppler) and were centered in the southern midlatitudes of Psyche. Details of those observations listed are in Table 1 and illustrated in Figure 1.

Later, we utilize 30 calibrated continuous wave (CW) echoes to examine the radar reflectivity of the upper meter of Psyche's regolith. These include 16 observations in 2005 (Shepard et al. 2008), 5 in 2015 (Shepard et al. 2017), and 9 new observations in 2017. The 2005 and 2015 observations were centered in the southern midlatitudes, and the 2017 observations were the first radar observations to probe the northern midlatitudes. Details of the observations are listed in Table 2, and the newer 2017 observations are illustrated in Figure 2.

Table 2. Psyche Echo Power (CW) Radar Observations

Epoch (UT)	S/N	Lat, Lon (°)	σOC (km2)	μc	Area (km2)	${\widehat{\sigma }}_{\mathrm{OC}}$
2017 Feb 28 04:48	19	+37, 302	20,300	0.18	41,370	0.49
2017 Mar 1 04:44	10	+37, 49	11,400	0.0	41,570	0.27
2017 Mar 2 04:48	13	+37, 144	12,100	0.0	41,190	0.29
2017 Mar 3 05:30	6	+37, 186	9000	0.0	40,040	0.22
2017 Mar 5 04 : 57	14	+36, 75	14,900	0.0	41,910	0.36
2017 Mar 6 04:51	8	+36, 184	10,400	0.2	39,830	0.26
2017 Mar 7 05:09	9	+36, 256	12,000	0.0	42,840	0.28
2017 Mar 8 05:05	9	+36, 6	9600	0.1	39,290	0.29
2017 Mar 11 04:00	11	+35, 41	11,700	0.2	40,690	0.29
2015 Nov 29 05:28	24	−47, 228	13,200	0.09	43,920	0.30
2015 Dec 3 04:42	27	−48, 337	12,800	0.03	44,040	0.29
2015 Dec 4 05:04	12	−48, 41	11,200	0.1	44,230	0.28
2015 Dec 6 04 : 54	22	−48, 262	16,000	0.21	44,960	0.36
2015 Dec 7 04:48	33	−48, 12	18400	0.05	43,710	0.42
2005 Nov 12 05:51	15	−45, 181	12,200	0.18	41,960	0.29
2005 Nov 12 06:05	15	−45, 161	13,200	0.18	42,150	0.31
2005 Nov 13 05:37	17	−45, 302	15,900	0.04	44,140	0.36
2005 Nov 13 05 : 51	16	−45, 282	15,400	0.02	44,420	0.35
2005 Nov 13 06:59	14	−45, 184	10,900	0.00	42,020	0.26
2005 Nov 13 07:14	13	−45, 163	12,200	0.1	42,080	0.29
2005 Nov 14 05:44	22	−45, 32	21,600	0.00	43,330	0.50
2005 Nov 14 05:58	20	−45, 12	21,000	0.18	42,860	0.49
2005 Nov 14 07 : 08	14	−45, 273	16,000	0.16	44,460	0.36
2005 Nov 14 07:22	11	−45, 253	9500	0.4	44,120	0.22
2005 Nov 15 06:50	21	−46, 38	21,300	0.00	43,600	0.49
2005 Nov 15 07:05	24	−46, 18	22,500	0.00	43,020	0.52
2005 Nov 16 05 : 26	17	−46, 259	14,000	0.00	44,360	0.32
2005 Nov 16 05:40	18	−46, 240	14,200	0.03	43,970	0.32
2005 Nov 16 06:46	18	−46, 146	14,400	0.05	42,980	0.34
2005 Nov 16 07:00	20	−46, 126	18,000	0.07	43,750	0.42

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The Earth−Psyche range was 1.73, 1.70, and 2.24 au for the 2005, 2015, and 2017 runs, respectively. Times are UTC midpoints of each run's receive cycle. The signal-to-noise ratio (S/N) is shown for matched filters. Lat/Lon refers to the subradar body-centered latitude and longitude at the time noted (light-time corrected). σ _OC is the OC radar cross section; uncertainties are conservatively estimated to be ±25% based on historic calibration and pointing uncertainties. μ_c is the radar polarization ratio (±0.1 for values with 1 significant figure and ±0.02 for the others). The area is the projected area of the shape model visible at the time of each run (±3%). ${\widehat{\sigma }}_{\mathrm{OC}}$ is the OC radar albedo. Dates/times in bold have high radar albedos. Dates/times in bold italics indicate echoes that appear bifurcated.

2.2. Atacama Large Millimeter Array

2.3. Adaptive Optics Images

2.4. 2019 and 2010 Occultations

2.5. Methods

2.6. Results

Our final model has dimensions (a × b × c) of 278 (−4/+8 km) × 238 (−4/+6 km) × 171 km (−1/+5 km) and an effective spherical diameter of D_eff = 222 −1/+4 km. Our uncertainties reflect our best estimates of the possible range of sizes. They are asymmetric because, while a range of sizes was found to be compatible with the data, our best model fell in the lower half of that range. Our range of uncertainties along the a- and b-axes are based on the highest resolution AO and ALMA images; our uncertainties along the c-axis are tighter because of the exceptional coverage of the 2019 occultation.

Our model is consistent in size and shape with those of Drummond et al. (2018), Viikinkoski et al. (2018), and Ferrais et al. (2020). It is also consistent in size along the major and intermediate axes and in the general shape of the Shepard et al. (2017) radar-derived model, but it is approximately 10% shorter than that model along the c-axis.

Our model's spin axis is (ecliptic) (36°, −8°) ± 2° and is consistent with all recently published models. Our uncertainty is based primarily on the 2019 occultation; even a 1° shift in the pole visibly changed the apparent orientation of our model with respect to the occultation chords.

The properties of the final model are listed in Table 5. Views of the model fit to the radar, ALMA, VLT, Keck, and occultation data are shown in Figures 1 and 3–6. Comparative views along the principal axes for this model are shown along with those of Shepard et al. (2017) and Viikinkoski et al. (2018)/Ferrais et al. (2020) in Figure 7.

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Table 5. Psyche Model Characteristics

Parameter	Value
Maximum dimensions (km)	278 × 238 × 171
Uncertainties (km)	−4/+8, −4/+6, −1/+5
Deff (km)	222 −1/+5 km
DEEVE (km)	274 × 234 × 171
Surface area (km2)	1.62 ± 0.05 × 105
Volume (km3)	5.75 ± 0.19 × 106
Mean visual albedo, pv	0.16 ± 0.01
Sidereal rotation period (hr)	4.195 948 ± 0.000001
Ecliptic pole (λ,β)	(36°, −8°) ± 2°
Equatorial pole (α,δ)	(363, +61) ± 2°
W0 (TDB)	2704

Note. Based on a final model of 1652 vertices and 3300 facets. Uncertainties indicate our best estimates of the possible sizes and are asymmetric for reasons described in the text. D_eff is the diameter of a sphere with the same volume as the model. DEEVE is the dynamically equivalent equal-volume ellipsoid, the ellipsoid with the same volume and moments of inertia as the model. Mean visual albedo assumes an absolute magnitude H = 5.90 (JPL Small Body Database). Pole coordinates are given in ecliptic and converted to the equatorial system to comply with the recommended IAU format for spin characteristics. W₀ is the location of the prime meridian (major axis) at J2000 Barycentric Dynamical Time (TDB). The Psyche shape file (.obj format) is included in the supplemental materials.

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Using a mass estimate for Psyche of 22.87 ± 0.70 × 10¹⁸ kg (Baer & Chesley 2017; Elkins-Tanton et al. 2020), we calculate a bulk density of 4000 kg m⁻³. This is consistent with previously published estimates (Elkins-Tanton et al. 2020). The most recent mass estimate is 22.21 ± 0.78 × 10¹⁸ kg (Siltala & Granvik 2021), which is within the uncertainties of the Baer and Chesley (2017) value. We therefore adopt a bulk density of 4000 ± 200 kg m⁻³.

Data for the Viikinkoski/Ferrais et al. models can be found at https://astro.troja.mff.cuni.cz/projects/damit/asteroid_models/view/1806 (Durech et al. 2010) and https://observations.lam.fr/astero/16Psyche/3DModel/.

Here we examine the largest topographical features evident in our model and those reported by others and evaluate whether they are likely to be real or possible artifacts of data processing and inversion. While there are many subtle depressions evident in our model, it is plausible that some are real and equally plausible that some are noise artifacts. For this reason, we will ignore them unless they are pertinent to the discussion. Where relevant, we will also note optical albedo features reported by others.

To better organize our description of individual features, we adopt names from the International Civil Aviation Organization (ICAO) phonetic alphabet commonly used by radio operators. ⁹ Details of each feature are listed in Table 6. In Figures 1 and 3–6, topographical features are indicated and identified by their first letter.

We reference Psyche's major features with respect to our shape model body-centered longitude and latitude, where the +a-axis (major) defines 0° longitude, the +b-axis (intermediate) is at 90° longitude, and the +c-axis (minor) aligns with the spin axis in the positive (north) polar direction.

Ferrais et al. (2020) (their Figure A.6.) noted three equatorial regions of missing mass relative to an ellipsoidal reference figure and refer to these "depressions" as A, B, and C. We follow suit but refer to them as Alpha, Bravo, and Charlie, respectively. They are visible in our model and are illustrated in Figure 8. Alpha falls around longitude 270° and is the only one that appears to be a depression while the others are large, flat areas. Bravo is the missing mass region first identified by Shepard et al. (2017) and falls between longitudes 340°–50°. Charlie falls between longitudes 90° and 150°.

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Although it is common to treat the shape of asteroids as variations on ellipsoids, our model of Psyche is also well described (along the major and intermediate axes) by a rounded rectangular shape with only one corner deviating significantly from this figure (Figure 8). The side lengths differ by only ∼10%. From this perspective, Bravo and Charlie disappear, and Alpha remains and becomes considerably wider. We will continue to refer to the smaller region as Alpha and the extended area as West Alpha.

Asteroids 2867 Steins (Jorda et al. 2012), 101955 Bennu (Barnouin et al. 2019), and 162173 Ryugu (Watanabe et al. 2019) all display shapes with a significant polygonal character, and for the latter two, this is thought to be a consequence of their rubble-pile nature (Michel et al. 2020). It is unclear whether that is possible for an asteroid the size of Psyche, some two orders of magnitude larger than Bennu or Ryugu. The surprising fit to a rounded rectangle may be a coincidence, or it may reveal something about Psyche's internal structure.

Figure 9 shows screenshots from an animation (MP4) to aid in visualization. The figure on the left shows the model at a subobserver latitude and longitude of +20°, 290° and the one on the right at −20°, 290°. The red peg indicates the +a-axis (lon 0°). Several major features of interest are labeled on the screenshots but not in the animation.

The presence of Alpha is driven primarily by the deconvolved VLT images of the north polar region (Figure 4) and is less obvious in the raw data. This flattened-to-concave area should be visible in the ALMA data but is not (Figure 3). Instead, the ALMA data suggest there is additional topography at this longitude (270°) that is not readily evident in the VLT or Keck data. The Arecibo observations were not favorably aligned to support either possibility. The ALMA and AO data are complementary in that regions that are dark in visible and near-infrared (AO) will be bright in the thermal IR (ALMA), and vice versa, so perhaps this region is optically dark. We will refer to this possible feature as X-ray. Its presence is consistent with the shape outlined by the 2010 occultation (Figure 6), and the Viikinkoski et al. (2018) albedo map does show a dark region at this longitude. We conclude that the depression Alpha is likely a real feature, though not a certainty, and may include unmodeled topography.

The features referred to as Bravo and Charlie are observed in the radar data, the AO observations from Keck and VLT, and the shape outlined by the 2010 occultation. The evidence for these regions is convincing.

Shepard et al. (2017) noted two dynamical depressions in Psyche's southern hemisphere, which they labeled D1 and D2. These were regions where the topography, rapid rotation, and estimated gravity of that model conspired to create regions of higher gravitational force or dynamical depressions. These are locations where fines might preferentially pond, and they appeared to correspond with purely topographical depressions. In our model, we find hints but no clear evidence for a topographical depression that coincides with the dynamical depression shown as D1 in that paper (here referred to as Delta) (Figure 9). As a result, we conclude that a significant topographical depression at the south pole is possible but indeterminate.

The dynamical depression referred to as D2 in the Shepard et al. (2017) model is evident in the topography and is likely to be a large impact crater. It is consistently visible in the delay-Doppler images (Figure 1) as a pocket of low-S/N pixels, typically one to three standard deviations lower than the surroundings. It is also suggested (a side view) in some of the ALMA images (Figure 3). We conclude that the evidence for it is convincing and will refer to this depression as Eros. ¹⁰ In our model, it is centered at longitude ∼290°, latitude ∼−65°, and appears to be between 50 and 75 km wide and ∼4 km deep (Figure 9). From some viewing aspects, there are indications that it might be two smaller overlapping depressions.

Our model shows a significant topographical depression at the north pole of Psyche, and we refer to it as Foxtrot (Figure 9). It was not noted by Viikinkoski et al. (2018) or Ferrais et al. (2020), but signs of it are found in the deviation from the ellipsoid figure shown in Ferrais et al. (their Figure A.7). This region was not visible in the data sets used in the Shepard et al. (2017) model. Evidence of its presence can be seen in the midline depression seen in the ALMA images (Figure 3), a few of the 2019 VLT images (Figure 4), and the 2019 occultation (Figure 6). We conclude that the existence of Foxtrot is likely, though not certain. If confirmed, measurements on our model suggest it to be ∼50 km wide and a few kilometers deep.

Viikinkoski et al. (2018) described two regions that they referred to as Meroe and Panthia. Meroe is located around longitude 90° just north of the equator and was noted to be significantly darker (optically) than its surroundings. Their model indicated it to be a crater some 80–100 km in size. Our model shows no significant depression at this location and we conclude that a crater here is possible but indeterminate.

The region referred to as Panthia is in the northern midlatitudes from longitudes ∼280°–320° and was found to contain areas much brighter (optically) than the surroundings (Viikinkoski et al. 2018). Our model also shows this region to contain a relatively wide (∼80–90 km) and shallow depression consistent with the Viikinkoski et al. description (Figures 4 and 9). Based on its appearance in both models, we conclude that the evidence for Panthia is convincing.

One of the primary reasons for visiting Psyche is to investigate an object of a type not yet seen—an object that could be the remnant metallic core of an ancient planetesimal. Fortunately, we have 30 calibrated radar echoes that can provide some insight into the potential concentration of metals in the near-surface (Table 2).

For calibrated radar echoes, we transmit a circularly polarized continuous wave (CW) signal to the asteroid and measure the echo power in the same (SC) and opposite (OC) senses of polarization. The OC echo is dominated by first surface reflections and is typically the stronger. Reported measurements include the OC radar cross section, σ_oc, an estimate of the cross-sectional area of a metallic sphere that would produce the same echo power, and the more intuitive OC radar albedo, ${\widehat{\sigma }}_{\mathrm{OC}}$, the ratio of the power received from the target relative to that which would be measured from a metallic sphere of the same cross-sectional area at the same distance. To calculate this, we divide the OC radar cross section by the projected area of the asteroid at the time of its observation.

The ratio of the SC to OC echo is referred to as the circular polarization ratio, or μ_c. It is generally interpreted to be an indicator of near-surface roughness. A low μ_c (0.0 to ∼0.3) is often interpreted to indicate surfaces relatively smooth at the wavelength scale and dominated by first surface reflections. Higher values are associated with surfaces that are thought to be rough at the wavelength scale, display significant volume or multiple scattering, or some combination of these.

In general, radar albedo is thought to be correlated with the near-surface (upper ∼meter) bulk density of an object (e.g., Ostro et al. 1985; Shepard et al. 2015). Given the assumption that larger main-belt asteroids are generally covered by regolith, higher bulk densities imply a greater concentration of iron–nickel in the regolith. Published radar albedos for asteroids range from lows of ${\widehat{\sigma }}_{\mathrm{OC}}$ ∼0.04 for primitive type (e.g., P) asteroids to highs of ∼0.5 for several of the M-class asteroids (Magri et al. 2007a; Shepard et al. 2015). The relatively common S- and C-class main-belt asteroids have mean radar albedos of ${\widehat{\sigma }}_{\mathrm{OC}}=0.14\pm 0.04$ and 0.13 ± 0.05, respectively (Magri et al. 2007a).

Using the average of calibrated echo power spectra from 2005, 2015, and 2017, and revised areal cross-sections of Psyche from this shape model, we estimate Psyche's mean OC radar albedo to be ${\widehat{\sigma }}_{\mathrm{OC}}=0.34\pm 0.08$ although there is considerable variation over the surface. This value is slightly lower than previously reported, ${\widehat{\sigma }}_{\mathrm{OC}}=0.37\pm 0.09$ (Shepard et al. 2017), because most of the new observations in the northern hemisphere have ${\widehat{\sigma }}_{\mathrm{OC}}\lt 0.30$. Nevertheless, this value is still more than twice as high as the mean MBA S- and C- class radar albedos. The most reasonable interpretation of this observation is that the near-surface (upper ∼meter) of Psyche has a bulk density significantly higher than the typical S- or C-class asteroid, probably caused by an enrichment of iron and nickel (Shepard et al. 2010).

We estimate the mean circular polarization ratio for the 2017 observations to be μ_c = 0.1 ± 0.1. This value is consistent with a relatively smooth surface at the wavelength scale and echoes dominated by first surface reflections. These measurements are consistent with those measured in 2005 (0.06) and 2015 (0.11) (Shepard et al. 2008, 2017).

Viikinkoski et al. (2018) generated their shape model from AO observations and lightcurves by simultaneously optimizing for both shape and albedo variegations. They allowed albedo variations of ∼20% higher or lower than the mean (p_v = 0.16) and produced a global albedo map. Figure 10 shows a refinement of that map in Ferrais et al. (2020; see Figure A.4 in both references). Our shape model corresponds closely to theirs (poles are within 2°, major axes within 3°, see Figure 7), so it is a fair representation of where brighter and darker regions can be found on our shape model. Ferrais et al. reported observing three additional bright features in the 2019 AO images that were not indicated on the map. The sizes of those features must have been significant but were not reported, so we have placed stylized spot figures in the locations they report.

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Figure 10 shows Psyche to be optically dichotomous, a feature noted in early lightcurve studies (e.g., Dotto et al. 1992 and references therein). It is brighter in a band from the equator to the southern midlatitudes over roughly 180° of longitude ranging from Bravo to West Alpha. A wide belt of bright material between longitudes 300° and 330° also extends upward into the northern mid-latitude region referred to as Panthia. It is brighter than average in the northern midlatitudes around longitude 120° and the high northern latitudes. The rest of the asteroid is ∼average-to-dark except for the two bright spots located just south of the equator at longitudes of ∼120° and ∼160°.

We have overlain the Figure 10 map with the locations of the subradar centers of each calibrated radar CW observation and assigned symbol sizes based on radar albedos. We divided our radar observations into three categories: radar albedos ${\widehat{\sigma }}_{\mathrm{OC}}\lt 0.3$ are a typical background value (although roughly twice as high as a typical S-class asteroid), radar albedos of ${\widehat{\sigma }}_{\mathrm{OC}}0.3\mbox{--}0.4$ are elevated with respect to the background, and radar albedos of ${\widehat{\sigma }}_{\mathrm{OC}}\gt 0.4$ are significantly higher than background.

Although the map is coarse, it does reveal at least three areas with significantly higher radar albedos of ${\widehat{\sigma }}_{\mathrm{OC}}\gt 0.4$ (Table 2, bolded entries) and at least one region where the radar echoes are consistently elevated (${\widehat{\sigma }}_{\mathrm{OC}}$ of 0.3–0.4) and bifurcated (Table 2, bold/italic entries). As a caveat, we note that radar albedo is a whole-disk average; however, the echo will usually be most heavily weighted by the radar properties of features nearest (within ∼30°–40°) the subradar point.

The first region of high radar albedo is centered on latitude +37° and longitude 303° and corresponds to the topographical depression and optically bright region of Panthia (Table 6, Figures 9, 10). The regolith may be enriched in metal, or there may be a radar-focusing effect because of the depression, or both.

The second region of high radar albedo is centered on latitudes of −45° between longitudes 12° and 50°. Multiple observations on separate dates show it to be a reproducible feature. It falls in an area south of Bravo, and we refer to it as Golf. There is some evidence for a broad and shallow depression near this region (centered at longitude ∼50° south of the equator, Figure 8) but it also could be an artifact of our fit. Golf is just south of a region much brighter (optically) than the mean (Table 6, Figure 10).

The third region of high radar albedo is centered at (−46°, 128°) and is referred to as Hotel (Table 6, Figure 10). Again, multiple observations on separate dates show it to be a reproducible feature. There is no obvious topographical structure here, and the region appears generally flat. Overall, this region is optically darker than the mean except for the nearby bright spots noted by Ferrais et al. (2020).

The fourth area of interest is in the southern midlatitudes between longitudes of 230° and 300°, and we refer to it as India (Table 6, Figure 10). It is notable for two reasons: almost all of the echoes in this region have elevated radar albedos and four of them, between longitudes ∼260° and ∼280°, are bifurcated (Table 2, entries with asterisks).

Bifurcated echoes are often associated with a contact binary structure (e.g., Kleopatra; Ostro et al. 2000, Shepard et al. 2018), but that is not the case here. Instead, there are likely two or more radar-bright regions separated by a region that is less reflective. As with Golf and Hotel, all the echoes were seen on different dates but fall in the same geographic region, i.e., the behavior is reproducible and is evidence that there is something unusual here.

The bifurcated echoes are centered at the longitude of Alpha, although it is north of our subradar points. One echo lobe (on the negative Doppler frequency side) includes reflections from Eros and the bright area and bright albedo spot north of India. The area corresponding to the other lobe (positive Doppler) aligns with the optically bright region just south of West Alpha. There is also some evidence for a broad depression in our model here, just south of the equator and centered at longitude 240°.

There is a possible bifurcated echo in the 2017 observations (northern hemisphere). This echo has an elevated radar albedo like those observed in the south and does fall in a region of modestly higher optical albedo, but there are no obvious structures present.

Based on this admittedly small sample, there appears to be a positive correlation between radar and optical albedos, i.e., the optically brighter regions are more radar reflective and darker regions less so. If correct, the most likely reason is that the regolith in the brighter regions has a higher bulk density, and by inference, a higher concentration of metal.

Here we discuss our two main conclusions and briefly discuss their ramifications for the current models of the formation and evolution of Psyche.

5.1. Psyche is Not Uniformly Radar Bright

5.2. Radar Albedo is Correlated with Optical Albedo

5.3. Consequences for Psyche Model Interpretations

The earliest interpretation of M-class asteroids like Psyche is that they are the remnant metal cores of ancient protoplanets (Bell et al. 1989). The consensus after many years of data gathering is that this is unlikely (Elkins-Tanton et al. 2020). Nevertheless, given the recent experiments of high-velocity impacts on metal substrates (Libourel et al. 2019), we find that our results cannot rule it out.

If Psyche is a mixed silicate-metal world, our results are best explained by a differentiated object with a mixed silicate and metal regolith (e.g., enstatite or CH/CB chondritic analogs), peppered with local regions of high metal content from ferrovolcanic activity as proposed by Johnson et al. (2020). This model provides both a mechanism for concentrating metal in local regions and several possible mechanisms for increasing optical albedos in the vicinity.

Unfortunately, radar observations of Psyche at Arecibo are no longer possible. Goldstone radar can achieve S/Ns of ∼15/day in 2025, but these are too low for delay-Doppler imaging of Psyche. Prior to the arrival of the Psyche mission in early 2026, Psyche does have favorable oppositions in 2022 March (2.23 au, similar to the 2017 encounter), 2023 May (2.24 au), 2024 August (1.70 au), and 2025 December (1.69 au, similar to the 2015 encounter) that may provide opportunities for additional AO, ALMA, and other spectral and thermal observations.

We are saddened by the recent loss of the unique Arecibo radar observatory and would like to thank the Arecibo operators and staff for their years of service to the entire astronomical community. Like many others, we advocate for its replacement as an indispensable tool for both characterizing objects within the solar system and for planetary defense against near-Earth objects.

At the time of the 2015 and 2017 radar observations, Arecibo Observatory was operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968) in alliance with Ana G. Méndez-Universidad Metropolitana and Universities Space Research Association. The Arecibo planetary radar system was supported by the National Aeronautics and Space Administration under grant No. NNX12AF24G issued through the Near-Earth Object Observations program. Additional support for radar data analysis and publication is provided by NASA grant No. 80NSSC19K0523.

This work made use of the JPL Horizons ephemeris service and NASA's Astrophysics Data System.

This paper makes use of the following ALMA data: ADS/JAO.ALMA\#2018.1.01271.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

We thank M. Viikinkoski for providing the optical albedo data used in Figure 10. We also gratefully acknowledge the use of observations made at the ESO Telescopes at the La Silla Paranal Observatory (VLT) under program 199.C-0074 (PI Vernazza). These data are available at http://observations.lam.fr/astero/.

We thank A. Conrad, J. Drummond, and W. Merline and gratefully acknowledge the use of observations from the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

We thank E. Cloutis for conversations regarding the spectral nature of metallic minerals.

We also are grateful to the following for observing the 2019 October 24 stellar occultation by Psyche: D. Palmer, P. Maley, D. Dunham, J. Dunham, T. George, S. Herchak, R. Jones, R. Reaves, D. Stanbridge, W. Thomas, T. Blank, P. Yeargain, and R. Wasson.