Editing 16 Psyche (section)

== Characteristics ==
=== Size ===
The first size estimate of Psyche was {{convert|253|km||abbr=}} and came from [[IRAS]] thermal [[infrared]] emission observations.<ref name=JPL/> This is 15% larger than the currently accepted mean value, but was later found to be an accurate estimate for the IRAS viewing aspect because Psyche was viewed pole-on at the time of the measurement.<ref name=Shepard-Richardson-etal-2017/><ref name=Lupishko-1999/>

Psyche has been observed to [[Occultation#Asteroids|occult]] a star on nine occasions.<ref name="Shepard-Richardson-etal-2017" /> Four of these, 2004,<ref name="Shepard-Richardson-etal-2017" /> 2010,<ref name="PSI-asteroid-data-sets" /><ref name="Psyche_occultation_2010" /> 2014,<ref name="PSI-asteroid-data-sets" /> and 2019,<ref name="Psyche_occultation_2019" /> generated [[Chord (astronomy)|multi-chord]] data sets<ref name=PSI-asteroid-data-sets/> and have been used along with adaptive optics imaging and three-dimensional modeling to estimate Psyche's mean diameter, with recent models all converging to an equivalent-volume mean diameter of {{val|222|3}}&nbsp;km.<ref name="Hanuš-Viikinksk-etal-2017"/><ref name=DAMIT-1806-cuni-cz/><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Ferrais-Vernazza-etal-2020" /><ref name="Shepard2021" />

[[File:Psyche asteroid eso.jpg |thumb |left| Multiple views of 16 Psyche imaged by the [[Very Large Telescope]]]]

===Mass and bulk density===
Psyche is massive enough that its gravity observably [[gravitational perturbation|perturbs]] other asteroids' orbits, which can be used to calculate a [[mass]] estimate. Historical values for Psyche's mass have ranged from {{val|1.6|e=19|u=kg}} to {{val|6.7|e=19|u=kg}}.<ref name=Kuzmanoski_Kovacevic_2002/><ref name=Elkins-Tanton_2020/> However, most recent estimates have begun to converge on values near {{val|2.287|0.070|e=19|u=kg}}.<ref name=Elkins-Tanton_2020/><ref name=Siltala_Granvik/> Assuming the mean volume of {{val|5.75|0.19|e=6|u=km3}}, this equates to a bulk density of {{val|3.977|0.253|u=g/cm3}}, which is considerably higher than most [[Small Solar System body|small Solar System bodies]].<ref name=Vernazza2021/><ref name=Elkins-Tanton_2020/><ref name=Siltala_Granvik/>

===Shape and spin pole===
The first published three-dimensional shape model for Psyche was derived from an analysis of numerous 
[[Lightcurve|light curves]].<ref name="Kaasalainen-etal-2002" /> Since then, additional refinements to the shape have been made based on the inversion of lightcurves, adaptive optics observations, radar observations, thermal imaging, and occultations.<ref name="Hanuš-Viikinksk-etal-2017" /><ref name="Shepard-Richardson-etal-2017" /><ref name = Drummond_etal_2018/><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Ferrais-Vernazza-etal-2020" /><ref name="Shepard2021" /><ref name="Vernazza2021" /> The most recent models show that Psyche has a shape consistent with a [[Jacobi ellipsoid]] and dimensions within a few km of 278&nbsp;km x 238&nbsp;km x 171&nbsp;km.<ref name="Viikinksk-Vernza-etal-2018" /><ref name="Vernazza2021" /><ref name="Shepard2021" /><ref name="Ferrais-Vernazza-etal-2020" />

Each shape model provides an estimate of the direction of the north pole (spin axis). All recent models for Psyche suggest it rotates about a pole pointed towards the [[ecliptic coordinate system|ecliptic coordinates]] (long, lat) {{nowrap|''λ'' {{=}} 35°}}, {{nowrap|''β'' {{=}} −8°}},  with a 3° uncertainty.<ref name="Hanuš-Viikinksk-etal-2017"/><ref name="Shepard-Richardson-etal-2017" /><ref name="Drummond_etal_2018" /><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Ferrais-Vernazza-etal-2020" /><ref name="Shepard2021" /> This means that it is essentially tilted toward the ecliptic, with an [[axial tilt]] of 98°.

{{multiple image
| align = left
| direction = vertical
| total_width = 220
 |image1 = Psyche_ellipsoid_shape.png | caption1 = View of Psyche from its south pole with a best fit ellipsoid overlain. Major deviations from the ellipsoid shape are referred to as Alpha, Bravo, and Charlie.
| image2 = 16PsycheNorthView.gif | caption2 = Animation of a Psyche shape model viewed from 20° north latitude. The red peg is at 0° longitude.
| image3 = 16PsycheSouthView.gif | caption3 = Animation of a Psyche shape model viewed from 20° south latitude. The red peg is at 0° longitude.
}}

===Features===
[[File:Psyche illustration.jpg|thumb|Illustration of Psyche commissioned by NASA]]

Many features have been reported on Psyche. The largest of these are regions of mass-deficits relative to its nominal ellipsoid shape and are reminiscent of the [[Rheasilvia (crater)|Rheasilvia]] basin on [[4 Vesta]].<ref name="Shepard2021" /><ref name="Ferrais-Vernazza-etal-2020" /><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Shepard-Richardson-etal-2017" />

In addition to the large-scale regions of mass-deficit, several apparent craters have been reported. Observers using the [[Very Large Telescope]]'s [[adaptive optics]] [[SPHERE]] imager reported two large craters, on the order of 90&nbsp;km across, which were provisionally named Meroe {{IPAc-en|ˈ|m|ɛr|oʊ|iː}} and Panthia {{IPAc-en|ˈ|p|æ|n|θ|i|ə}},{{citation needed|date=October 2020}}<!-- not clear this is the same name. If the ‘I' is long in Latin, it would be \pan-THY-a\ --> after the twin witches in the Roman novel [[The Golden Ass|''Metamorphoses'' by Apuleius]].<ref name="Zimmerman_mythology"/><ref name=Viikinksk-Vernza-etal-2018/> Observers using the [[Arecibo Telescope|Arecibo Radar Telescope]] reported craters at the south pole (referred to as Delta),<ref name="Shepard-Richardson-etal-2017" /> southern midlatitudes (referred to as Eros),<ref name="Shepard-Richardson-etal-2017" /><ref name="Shepard2021" /> and the north pole (referred to as Foxtrot).<ref name="Shepard2021" /> An analysis of the features present in several independent shape models suggests that craters Panthia and Eros are almost certainly real and Foxtrot is likely to be real. However, there is uncertainty over the existence of Meroe and Delta.<ref name="Shepard2021" />

Early lightcurve studies suggested that Psyche has large variations in its surface brightness.<ref name="Dotto1992"/> These variations became more evident when attempts were made to invert lightcurves to generate shape models.<ref name="Kaasalainen-etal-2002" /><ref name="Shepard-Richardson-etal-2017" /> The most recent shape models based on lightcurve inversions simultaneously solve for surface albedo variations,<ref name="Viikinksk-Vernza-etal-2018" /><ref name="Ferrais-Vernazza-etal-2020" /> and the resulting maps show regions where local albedo differs from the mean albedo by more than 20% in either direction. Notably, the Meroe crater coincides with an area much darker than the mean, and the Panthia crater coincides with an area much brighter than the mean.

Radar observations with the [[Arecibo Telescope|Arecibo Radar Telescope]] revealed that Psyche's "background" [[Albedo#Radar Albedo|radar albedo]] is <math>\hat{\sigma}_{OC}=0.26 </math>, comparable to other [[M-type asteroid]]s like [[21 Lutetia]].<ref name="Shepard2021" /> This value is consistent with a [[Silicate mineral|silicate]] (rocky) regolith enriched in metal phases.<ref name="Shepard2015"/> However, in at least three locations, Psyche's radar albedo is nearly twice this value, suggesting high concentrations of metal phases in these regions.<ref name="Shepard2021" /> One of these locations corresponds with the optically bright Panthia crater, while the other two correspond with regions that have been reported as optically bright.<ref name="Ferrais-Vernazza-etal-2020" /><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Shepard2021" /> This apparent correlation between optical and radar albedos on Psyche has led to the hypothesis that there is a link between the process(es) that create regions of high metal content and brighter terrain.<ref name="Shepard2021" />

{{multiple image
| align = right
| direction = vertical
| total_width = 220
| image1 = Psyche features Shepard2021.jpg | caption1 = Shape model of asteroid 16 Psyche, with some of the observed surface features indicated.
| image2 = Psyche albedo Shepard2021.jpg |thumb | caption2 = Optical albedo map of asteroid 16 Psyche with superimposed radar albedo measurements (circles).
}}

=== Composition ===
The bulk density of Psyche ({{val|3.9|0.3|u=g/cm3}}) places constraints on its overall composition. The iron-nickel found in most [[iron meteorite]]s has a bulk density of {{val|7.9|u=g/cm3}}. If Psyche were the remnant [[Planetary core|core]] of an early [[planetesimal]], it would have to have an overall porosity of 50%. Given Psyche's size, this is considered highly improbable.<ref name="Elkins-Tanton_2020" /> However, there are other metal-rich [[meteorite]] types that have been suggested as Psyche analogs, each of which have bulk densities that are similar to Psyche's, including [[enstatite chondrite]]s, [[bencubbinite]]s, and [[mesosiderite]]s.<ref name="Gaffey_asteroidsII" /><ref name="Viikinksk-Vernza-etal-2018" /><ref name="Elkins-Tanton_2020" />

Several observers have reported the presence of [[silicate mineral]]s on the surface of Psyche.<ref name="Hardersen-Gaffey-Abell-2005" /><ref name="Ockert-Bell"/><ref name="Sanchez"/> Spectra taken in October&nbsp;2016 at the [[NASA Infrared Telescope Facility]] at the [[Mauna Kea Observatories]] showed evidence (~3&nbsp;μm absorption feature) of [[Hydroxy group|hydroxyl]] ions on the asteroid that may suggest the presence of hydrated silicates.<ref name=Takir-Reddy-etal-2016/> Since Psyche is thought to have formed under dry conditions without the presence of water, the hydroxyl may have reached Psyche via past impacts from smaller [[carbonaceous asteroid]]s.<ref name=Takir-Reddy-etal-2016/><ref name="Shepard2015" />

Psyche's [[Albedo#Radar Albedo|radar albedo]] varies considerably over the surface, ranging from 0.22 to 0.52,<ref name="Shepard2021" /> values that are two to four times as high as most main-belt asteroids.<ref name="Magri2007"/> Models of radar reflection equate this range of values with regolith bulk densities of 2.6 to 4.7 {{val|u=g/cm3}}.<ref name="Shepard2015" /> This range is consistent with most of the metal-rich [[Meteorite classification|meteorite]] classes noted above and the spectroscopic detection of silicate minerals. It is inconsistent with a pure [[Iron meteorite|iron-nickel]] regolith unless it is highly porous.

[[File:16 Psyche orbit.png|thumb|The orbit of Psyche, between Mars and Jupiter, with eccentricity of 0.140: much higher than most planets, but typical for an asteroid.]]

===Origin===
Several possible origins have been proposed for Psyche. The earliest of these was that Psyche is an exposed [[Planetary core|metallic core]] resulting from a collision that stripped away the crust and [[Mantle (geology)|mantle]] of an originally larger [[Planetary differentiation|differentiated]] parent body some 500&nbsp;kilometers in diameter.<ref name="Bell_AsteroidsII" /> Other versions of this include the idea that it was not the result of a single large collision but multiple (more than three) relatively slow sideswipe collisions with bodies of comparable or larger size.<ref name=Asphaug-Reufer-2014/> However, this idea has fallen out of recent favor as mass and density estimates are inconsistent with a remnant core.<ref name="Elkins-Tanton_2020" />

A second hypothesis is that Psyche was disrupted and gravitationally re-accreted into a mix of metal and silicate.<ref name=Davis-Farinella-Francesco-1999/> In this case, it may be a candidate for the parent body of the [[mesosiderite]]s, a class of [[stony–iron meteorite]]s.<ref name=Davis-Farinella-Francesco-1999/>

A third hypothesis is that Psyche may be a differentiated object, like [[Ceres (dwarf planet)|1 Ceres]] and [[4 Vesta]], but has experienced a type of iron volcanism, also known as ferrovolcanism, while still cooling.<ref name="Johnson_ferrovolcanism"/> If true, this model predicts that metal would be highly enriched only in those regions containing (relic) volcanic centers. This view has been bolstered by recent radar observations.<ref name=Shepard2021/>