HST UV Observations of Asteroid (16) Psyche

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The Planetary Science Journal, 1:53 (14pp), 2020 December https://doi.org/10.3847/PSJ/abb67e © 2020. The Author(s). Published by the American Astronomical Society.

Tracy M. Becker1 , Nathaniel Cunningham2 , Philippa Molyneux1 , Lorenz Roth3 , Lori M. Feaga4 , Kurt D. Retherford1,5 , Zoe A. Landsman6 , Emma Peavler1,7, Linda T. Elkins-Tanton8 , and Jan-Erik Walhund9 1 Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA; Tracy.Becker@swri.org
2 Nebraska Wesleyan University, Lincoln, NE, USA
3 KTH Royal Institute of Technology, Stockholm, Sweden
4 University of Maryland, College Park, MD, USA
5 University of Texas San Antonio, San Antonio, TX 78249, USA
6 Florida Space Institute, University of Central Florida, Orlando, FL, USA
7 University of California Los Angeles, Los Angeles, CA, USA
8 Arizona State University, Tempe, AZ, USA
9 Swedish Institute of Space Physics, Uppsala, Sweden
Received 2020 April 23; revised 2020 September 8; accepted 2020 September 8; published 2020 October 26

Abstract

The Main Belt Asteroid (16) Psyche is the target object of the NASA Discovery Mission Psyche. We observed the asteroid at ultraviolet (UV) wavelengths (170–310 nm) using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope during two separate observations. We report that the spectrum is very red in the UV, with a blue upturn shortward of ∼200 nm. We find an absorption feature at 250 nm and a weaker absorption feature at 275 nm that may be attributed to a metal-oxide charge transfer band. We find that the red-sloped, relatively featureless spectrum of (16) Psyche is best matched with the reflectance spectrum of pure iron; however, our intimate mixture models show that small grains of iron may dominate the reflectance spectrum even if iron only comprises up to 10% of the material on the surface. We also stress that there is a limited database of reflectances for planetary surface analogs at UV wavelengths for comparison with the spectrum of (16) Psyche. The mid- and far- UV spectra (<240 nm) are markedly different for each of the four asteroids observed at these wavelengths so far, including ones in the same spectral class, indicating that UV observations of asteroids could be used to better understand differences in the composition and processing of the surfaces of these small bodies.

Unified Astronomy Thesaurus concepts: Main belt asteroids (2036); UV astronomy (1736); Hubble Space Telescope (761)

HST UV Observations of Asteroid (16) Psyche

1. Introduction

The asteroid (16) Psyche (hereafter, Psyche) is the target of the NASA Discovery Mission Psyche, expected to launch in 2022 (Elkins-Tanton et al. 2017). Psyche is the largest of the asteroids designated as an M-type in the Tholen taxonomic classification (Tholen 1984), which are defined by their featureless, red-sloped spectra in the visible and near-infrared (near-IR) wavelengths. The relatively higher radar albedo measurements of many M-types suggest a composition dominated by iron–nickel (Ostro et al. 1985; Shepard et al. 2008, 2010), leading to the hypothesis that these asteroids are the remnant metallic cores of larger, differentiated protoplanets, exposed after a series of hit-and-run collisions stripped the bodies of their mantles (Chapman & Salisbury 1973; Bell et al. 1989); Asphaug et al. 2006; Asphaug 2010; Sarid et al. 2015). Psyche is the archetype of this class of asteroids.

The shape, effective diameter (226 ± 23 km), and radar albedo (0.37 ± 0.09) of Psyche were derived from observations taken by the Arecibo Observatory (Shepard et al. 2017). These measure- ments, combined with estimates of its mass ((2.72±0.75)× 1019 kg) from observations of gravitational perturbations on other asteroids(Viateau2000;Kuzmanoski&Kovacĕvić2002;Carry 2012), indicate that its bulk density is ∼3990±260kg/m−3, consistent with an Fe–Ni composition with 40% macroporosity or a stony iron with almost no macroporosity (Britt & Consolmagno 2003; Viikinkoski et al. 2018). However, initial estimates of an elevated thermal inertia that would be indicative of high metal content (Matter et al. 2013) may be contradicted by a more moderate thermal inertia derived using data from the Spitzer Space Telescope (Landsman et al. 2017).

In the Bus-DeMeo asteroid taxonomy (DeMeo et al. 2009), Psyche is classified as Xk due to the presence of an absorption feature at 0.95μm (Hardersen et al. 2005) attributed to orthopyroxenes on the surface. IR studies using the NASA Infrared Telescope Facility (IRTF) measured a 3 μm absorption feature, which was attributed to water or OH (hydroxyl) on the surface (Takir et al. 2017), though measurements from the AKARI satellite did not detect this feature (Usui et al. 2019). An absorption feature has also been observed at 0.43 μm on Psyche, which may be associated with chlorites and magne- sium-rich serpentines or pyroxenes (Fornasier et al. 2010). The presence of these materials suggests a possible alternate formation scenario for Psyche or has implications for exogenic material emplaced on the asteroid’s surface. Despite the noted absorption features, the current best meteorite analog for Psyche based on visible and IR observations is the relatively featureless iron meteorite MET101A (Fornasier et al. 2010).

To better assess the composition of Psyche, we consider the ultraviolet (UV) spectrum of the asteroid. Laboratory studies by Cloutis et al. (2008) show that the UV spectral region can be more sensitive to some mineral properties than the longer wavelengths and can therefore be extremely useful in the compositional analysis of planetary surfaces. Although it is a

Figure 1. Orientation of Psyche at the time of the two HST observations assuming the shapes and pole orientations determined by Hanuš et al. (2017; top row) and Kaasalainen et al. (2002; bottom row). The data were taken 12.499 rotations apart and observe different longitudes on the surface, though the relative orientation of the asteroid was such that much of the northern hemisphere was observed during both data acquisitions. The angular size of Psyche was ∼0 13 for both observations, well within the 0 2 width of the slit. Along the slit, Psyche occupies ∼5 pixels, using the 0 025 plate scale for STIS. Figures were generated using the Interactive Service for Asteroid Models found athttp://isam.astro.amu.edu.pl/ (Marciniak et al. 2012).

target of significant interest, Psyche had not been observed in the UV since the asteroid observation campaign conducted by the International Ultraviolet Explorer (IUE) in the 1980s (Butterworth & Meadows 1985).

Here we report the analysis of high-resolution UV observa- tions of Psyche taken in 2017 April by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) that extends the spectral coverage of Psyche down to ∼170 nm. In Sections 2 and 3 we discuss the observations and data reduction, with a discussion of our analysis of the data in Section 4. We present potential spectral analogs to Psyche in Section 5. In Section 6 we consider implications for the surface composition of Psyche and the need for more UV observations of asteroids. We summarize our findings in Section 7.

HST UV Observations of Asteroid (16) Psyche

2. Observations

We observed Psyche on 2017 April 4 and 6 using the HST STIS first-order NUV Multi-Anode Microchannel Array (MAMA) G230L grating mode with the 52×0 2 slit. The bandpass for this grating is 158–310 nm, though the signal-to- noise ratio (S/N) shortward of 170 nm is very low for these observations. The observations were taken such that Psyche, which has a rotation period of ∼4.2 hr, had rotated 12.499 times between the HST visits. This was planned so that

comparisons could be made between hemispheres, though the orientation of the pole angle relative to the Earth meant that much of the northern hemisphere was observed in both observations, as shown in Figure 1. Each hemisphere was observed for the duration of one HST orbit after acquisition, resulting in exposure times of 2288 s or approximately 54° of the asteroid’s rotation. The HST slit was oriented 58° and 67° east of north on the first and second observation, respectively. Details of the observations are included in Table 1.

Psyche was at a distance of 2.40 and 2.42 au from the Earth during the first and second HST visits, resulting in an angular diameter of 0 130 and 0 129, respectively. The 0 025 plate scale of the STIS NUV mode meant that we collected signals across the asteroid’s diameter over ∼5 pixels, enabling a simple analysis of potential spatial variability.

3. Data Reduction

The raw 2D spectra were reduced to calibrated, background- subtracted 1D flux spectra (“x1d” files) by the HST software pipeline, using CALSTIS software version 3.4.1. This proces- sing is designed for sources that are pointlike at the STIS resolution. Though Psyche is not a point source, its 0 130 diameter, covering 5 spatial pixels on the STIS MAMA detector, is well contained within the 0 2 wide slit and 11 pixel

high spectral extraction region. Aligning the spectral features (of solar origin) in common between the spectra from the two visits required shifting the wavelength scale of the first visit by −0.12 nm; wavelength scales of both spectra were further shifted by −0.07 nm to best match the solar spectra described below, in order to minimize artifacts when dividing. These two shifts cannot be accounted for by expected Doppler shifts, as the pipeline already corrects for Doppler shifts due to HST’s orbital motion and the motion of Earth around the Sun; and the difference in radial velocity of Psyche with respect to the Sun between these two visits is too small to account for such a shift. For present purposes, with fluxes subsequently binned in 1 nm or wider bins, such shifts are negligible. We binned the 1D flux spectra to improve S/N into 1 and 3nm bins (from the pipeline’s original 0.16 nm bins for this observing mode). Flux uncertainties of 1σ including statistical and instrumental contributions were also produced by the pipeline and propagated in the binning process.

We used solar flux spectra from the Solar Radiation and Climate Experiment (SORCE)/Solar–Stellar Irradiance Compar- ison Experiment (SOLSTICE) instrument (McClintock et al. 2005) acquired on dates matching those of our STIS observations but was shifted by one day to correct for solar rotation. The solar spectra were binned in the same fashion as the STIS spectra and were used to calculate geometric albedo p in each wavelength bin according to Equation (1):

where D is the Earth–Psyche distance in astronomical units, Δ is Sun–Psyche distance in km, RP=113 km is the effective radius of Psyche, f is a unitless phase correction factor, F(λ)P is the flux of Psyche from STIS at each wavelength, and F(λ)Sun is the SOLSTICE solar flux observed at 1 au at each wavelength, where the flux is given in erg cm-1 s-1 Å-1 . We assume a phase curve similar to that measured at visible wavelengths, which would indicate a phase correction factor of 1.8 for these observations (Lupishko et al. 1980). This is also consistent with the rough UV phase curve for Psyche presented by Roettger & Buratti (1994) with corrections made using IUE spectra of Psyche by Butterworth & Meadows (1985), though the wavelength-dependent phase curve could be different at shorter wavelengths. The 3% uncertainty in the SOLSTICE- measured solar flux (Sparn et al. 2005) has not been included in these albedo calculations.

HST UV Observations of Asteroid (16) Psyche

We present the derived geometric UV albedo of Psyche in Figure 2, binned to spectral resolutions of 1 nm and 3 nm. Here we average the spectra from both HST STIS data sets. The error bars represent the 1σ propagated uncertainties from the averaged spectrum of Psyche. We overplot an inverted solar spectrum on the 1 nm resolution plot for reference. The apparent outlier data points near 280 and 285 may be explained by the inadequate removal of the Mg II doublet near 280 nm and the Mg I line at 285.2nm in the solar spectrum. The outliers at longer wavelengths may be due to the series of sharp solar features longward of 290 nm.

Psyche’s UV spectrum is red overall with an albedo minimum near 200 nm and a blue slope shortward of ∼200 nm. Inspection of Figure 2(b) reveals possible absorption features in the spectrum near 250 and 275 nm. There is also a potential feature centered near 220 nm; however, its proximity to the FUV upturn at 200 nm makes it hard to distinguish from the broader spectral shape and thus we do not attempt to measure it in this work. Our spectral analysis of Psyche relies on the broader features and the slope of the spectrum, so we will use the 3nm spectral binning for reference hereafter, unless otherwise explicitly stated. Similarly, our analysis will use the geometric albedo derived from the combination of the data, as shown in Figure 2(b), from the two HST visits, except when explicitly stated.

4. Data Analysis

For our analysis of Psyche, we assess potential spatial heterogeneity of the asteroid’s surface, identify and measure spectral features, and compare the UV spectrum with spectral mixing models in order to better understand the surface composition of the asteroid. We note that observations at mid- UV wavelengths (∼200–300 nm) have a sensing depth of tens to hundreds of nanometers and so our study is of the uppermost surface layer.

4.1. Surface Variability

As discussed in Section 2, we designed the two HST observations so that we could observe nearly opposite hemi- spheres of Psyche in order to look for variations in brightness or spectral features. In Figure 3 we show the spectra from each visit, binned to 3 nm spectral resolution, as well as the ratio between the data sets. Longward of 200 nm, the slopes and locations of spectral features (Section 4.2) are very similar. This is consistent with the lack of statistically significant rotational variation in the metal abundance in the visible—near-IR study of Psyche completed by Sanchez et al. (2017), though that study observed the opposite hemisphere than these observations. We note, however, that Takir et al. (2017) did find rotational variability in the 3 μm feature. Both UV spectra appear to display an upturn into the FUV, where the reflectance increases with decreasing wavelengths. However, for Visit 2, the upturn occurs near 200 nm, while for Visit 1 the upturn occurs closer to 180 nm, where the data is noisier. The position of this upturn may be related to space weathering or differences in the composition or roughness of the surface (see Section 4.4).

We note that our observations do not cover the southern hemisphere of Psyche, where a mass-deficit region was detected by radar observations (Shepard et al. 2017) and where rotational variations in the pyroxene chemistry were observed by Sanchez et al. (2017). Further, these variations were detected using subtle differences in the band depth and location

HST UV Observations of Asteroid (16) Psyche

Figure 2. UV spectrum of Psyche binned to 1 nm (left) and 3 nm (right) spectral resolution, with 1σ uncertainties. We overplot the inverted solar spectrum in orange to show where solar features may not have been adequately removed during the data reduction, resulting in several outlier points at wavelengths longer than 270 nm.

Figure 3. Comparison between observed hemispheres of Psyche. At longer wavelengths, the hemispheres appear to have very little heterogeneity. Differences are more apparent at wavelengths <200 nm, where the FUV upturn begins, which is potentially indicative of composition, space weathering, or a combination of the two; however, this region is also much noisier than in the NUV.

of the 0.9 micron feature at eight different rotation phases. With only two rotation phases, we do not detect significant changes but also do not rule out the possibility of small longitudinal variations.

We were able to spatially resolve Psyche across ∼5 spectral pixels. To further assess spatial variability of the asteroid, we use the calibrated, background-subtracted 2D spectral images produced by calstis (“x2d” files). We assume the brightest 4 pixels contain the majority of the signal from Psyche and compare the two sides of the asteroid captured within the field of view by summing the top 2 and bottom 2 pixels along the slit containing that signal. The line spread function for the STIS MAMA grating at a wavelength of 240 nm for both an extended source and a point source is ∼2 pixels; thereby we encompass the entire line spread function in the summed signal. In Figure 4, we show the ratio of the two sides during each visit. Here we focus on wavelengths longer than 200 nm because of the lower S/N in the FUV. Visit 2 may have a somewhat sharp difference in spectra near 250 nm, which may be attributed to the strength of the Fe–O charge transfer band (see Section 4.2). Both visits show some small spectral difference between the sides near 300 nm, perhaps suggesting a feature in the northern hemisphere, since that side of the asteroid was captured during both visits due to the asteroid orientation (Figure 1). There is a difference, however, in the position of the FUV upturn along the slit during Visit 1 (see Section 4.4). These differences may suggest variability of the composition of the uppermost surface layer of Psyche.

4.2. Absorption Features

As discussed in Section 3, we identified two clear spectral features centered near 250 and 275nm. To measure the strength of these features, we determine the background continuum slope by fitting a line to the spectrum on either side of the feature and then dividing by that baseline to remove the local continuum. For the 250nm feature, we use the average albedo between 233.5 and 239.5nm and between 257.5 and 263.5 nm to establish the baseline (Figure 5(a)). For

Figure 4. Ratio of the upper two and lower two STIS pixels containing data of Psyche for each visit, smoothed using a boxcar average. For Visits 1 and 2, the FUV upturn appears to differ along the slit. For Visit 2, there may also be a difference in the strength of a feature at 250 nm. Both visits show a possible weak feature near 300 nm in only one half of the slit. In both visits the bottom of the slit, which mostly captures the northern hemisphere of Psyche, has a lower reflectance than the more equatorial region that is rotating through the top of the slit by approximately 10% and 20% for Visit 1 and 2, respectively.

the 270 nm feature, we find the average albedo between 263.5 and 269.6 nm and between 284.5 and 290.5 nm to find the baseline (Figure 5(c)).

After dividing by the established baseline for the local spectral slope, we then fit a parabola to the data points that fall below the average continuum (Figures 5(b) and (d)). Using the minima of those parabolas, we determine estimates for the band depth and position of the feature.

We find that the first absorption feature is centered at 249.8 nm with a band depth of ∼5.6% and an approximate bandwidth of 18nm. We measure a sharper, shallower second absorption feature near 275 nm. Assuming the fitted parabola minimum as the band center and depth, we find that it is centered at 275.6 nm with a band depth of 2.5%. However, a parabola does not produce a good fit to this feature, which presents itself more like the check mark-shaped absorption features sometimes detected at 3 μm on asteroids (Landsman et al. 2015). If we instead assume the lowest data point represents the band center and depth, the absorption feature is centered at 272.5 nm with a band depth of 3.3% and a bandwidth of approximately 13 nm.

Laboratory observations of a variety of minerals show a number of spectral features in the UV (e.g., Wagner et al. 1987; Cloutis et al. 2008). While many of the materials measured by Cloutis et al. (2008) displayed bands at 250 nm or 275 nm, no single material they investigated displayed bands at both of those wavelengths. An absorption feature near 250nm is consistent with Fe2+ –O or Ti4+ –O charge transfer bands (Cloutis et al. 2008). Most of the materials (some pyroxenes, olivines, and plagioclase feldspars) with a 275 nm minimum in the Cloutis et al. (2008) study had another reflectance minima between 220 and 225nm. It is possible that a very weak 220 nm absorption feature can be seen in the Psyche spectrum, but it is not distinguishable from the slope of the spectrum…”

[Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.]

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