16 Psyche

Mass, size and shape

Psyche is massive enough that its gravitational perturbations on other asteroids can be observed, which enables a mass measurement. As of 2017 the best mass estimate is 7000272000000000000♠2.72±0.75×10¹⁹ kg.^[2][8]

Lightcurve Inversion model DAMIT 1806 and occultation chords from two observations of 2010 and 2014.

The first size estimate of Psyche came from IRAS thermal infrared emission observations. They showed that it had a diameter of about 253 km,^[1] although it was likely an overestimate as Psyche was viewed pole-on at that time.^[2]

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Light curve analysis indicates Psyche appears somewhat irregular in shape. There is a pronounced mass deficit near the equator at about 90° longitude comparable to Rheasilvia basin on Vesta. There are also two additional smaller (50–70 km in diameter) crater-like depressions near the south pole. that Psyche's north pole points towards either ecliptic coordinates (β, λ) = (−7°, 34°) with a 10° uncertainty.^[10] This gives an axial tilt of 95°.^[2]

Observations of two multi-chord stellar occultations of 2010 and 2014^[11] allow the matching of light curve inversions model DAMIT 1806 that give an equivalent Volume mean diameter of 216+/-12 km, and an equivalent Surface mean diameter of 227 +/- 13 km.

The density of Psyche that follows from size and mass estimates mentioned above – 7003450000000000000♠4500±1400 kg/m³ – is consistent of that of metallic object with a porosity of about 40%.^[12] This is higher than most previous estimates.^[13][2]

Composition and origin

Radar observations indicate that Psyche has a fairly pure iron–nickel composition,^[14][15] consistent with it having one of the highest radar albedos in the asteroid belt (6999370000000000000♠0.37±0.09).^[2] Psyche seems to have a surface that is 90% metallic (iron),^[16] with small amounts – 7000600000000000000♠6±1% – of orthopyroxene.^[17][18]

The NASA Infrared Telescope Facility at the Mauna Kea Observatories released evidence (~3 μm absorption feature) of the presence of water or hydroxyl ions on the asteroid in October 2016. The water is believed to have reached Psyche via past impacts from smaller asteroids containing volatile substances such as carbon, hydrogen and water.^{[19][20][21][22]}

Based on its composition Psyche appears to be an exposed metallic core or a fragment of a metallic core^[17] from a larger differentiated parent body some 500 kilometers in diameter. If Psyche is indeed one, there could be other asteroids on similar orbits. However, Psyche is not part of any identified asteroid family.^[23] One hypothesis is that the collision that formed Psyche occurred very early in the Solar System's history, and all the other remnants have since been ground into fragments by subsequent collisions or had their orbits perturbed beyond recognition. However, this scenario is considered to have a probability of just 1%. An alternative is that Psyche was broken by impacts, but not catastrophically torn apart. In this case, it may be a candidate for the parent body of the mesosiderites, a class of stony–iron meteorites.^[23]

Another possibility is that Psyche may be an endmember of diverse relic bodies left by the inner planet formation. The asteroid's mantle may have been stripped away not by a single collision but by multiple (>3) relatively slow hit-and-run collisions with bodies of comparable or larger size. What is left is a metallic core covered by thin layer of silicates, which reveal itself spectrally. In such a case Psyche would be analogous to Mercury but much less massive.^[24]