Physical Characterization of Near -Earth Asteroid 52768 1998 OR2 Evidence of Shock Darkening Impact Melt Characterization of NEA 52768 1998 OR2

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Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock

Darkening/Impact Melt

Characterization of NEA (52768) 1998 OR2

Adam Battle

, Vishnu Reddy1, Juan A. Sanchez

, Benjamin Sharkey1, Neil Pearson2, Bryn

Bowen1

Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Boulevard, Tucson, AZ 85721-0092, USA

Planetary Science Institute, 1700 East Fort Lowell Road, Tucson, AZ 85719, USA

Abstract:

We conducted photometric and spectroscopic characterization of near-Earth asteroid (52768) 1998

OR2 during a close approach to the Earth in April of 2020. Our photometric measurements confirm

the rotation period of the asteroid to be 4.126 ± 0.179 hours, consistent with the previously

published value of 4.112 ± 0.001 hours. By combining our visible spectroscopic measurements

(0.45–0.93 µm) with archival MITHNEOS near infrared spectra (0.78–2.49 µm), we classify the

asteroid as an Xn-type in the Bus-DeMeo taxonomy. The combined spectrum shows two weak

absorption bands: Band I at 0.926 ± 0.003 µm and Band II at 2.07 ± 0.02 µm with band depths of

4.5 ± 0.15% and 4.0 ± 0.21%, respectively. The band area ratio is 1.13 ± 0.05. These spectral band

parameters plot at the tip of the S(IV) region of the Gaffey S-asteroid subtypes plot suggesting an

affinity to ordinary chondrite meteorites. We calculated the chemistry of the olivine and pyroxene

using the Band I center to be 20.1 ± 2.3 mol% fayalite and 18.2 ± 1.5 mol% ferrosilite, consistent

with H chondrites. Principal component analysis of 1998 OR2’s combined visible-NIR spectrum

fall on the C/X-complex side of the α-line, near the end of the shock darkening trend, consistent

with its weak absorption bands (band depth < 5%). We use an aerial mixing model with lab

measurements of the shock darkened H5 chondrite, Chergach, to constrain the amount of shock

darkened material on the asteroid’s surface at ~63% dark lithology and ~37% light lithology.

1. INTRODUCTION

S-complex asteroids dominate the km-sized near-Earth asteroid (NEA) population and make up

the majority of NEAs at all sizes (DeMeo et al. 2009; Binzel et al. 2019; Ieva et al. 2020). Several

studies have linked these asteroids to ordinary chondrite meteorites (e.g., Gaffey et al. 1993;

Vernazza et al. 2008; Binzel et al. 2009, 2019; DeMeo et al. 2009; Nakamura et al. 2011; Thomas

et al. 2011). Spectral band parameters measured from visible and near infrared (VNIR; 0.45 – 2.5

m) spectra of S-type asteroids are diagnostic of the surface mineralogy of these asteroids (Reddy

et al. 2015). This allows more in-depth and quantitative comparisons of asteroid surfaces and their

proposed meteorite analogs. However, non-compositional effects including grain size, phase

angle, and alteration mechanisms such as space weathering and impact processes, can affect our

ability to make these asteroid-meteorite linkages.

Lunar-style space weathering is an alteration of the surface of airless bodies that occurs due to

long periods of time exposed to the space environment (Gaffey 2010). Nanophase iron particles

created from micrometeorite impacts, solar wind implantation, and other space environment

factors are the main source of the effects of lunar-style space weathering as described in Pieters &

Noble (2016) and references within. These effects have been studied for over 50 years, since before

the Apollo Moon landing (Zeller & Ronca 1967), but shock darkening and impact melt alteration

processes have not been studied as long, with one of the first references being Britt & Pieters

(1989). Whereas studies of space weathering persisted throughout the development of asteroid

science (Hapke 2001; Brunetto et al. 2006; Hiroi et al. 2006; Gaffey 2010; Noguchi et al. 2011),

studies of shock darkening and impact melt only picked up again after the 2013 Chelyabinsk event

(Kohout et al. 2014, 2020a, 2020b; Reddy et al. 2014).

Space weathering affects reflectance spectra of S-type asteroids primarily by lowering the visual

albedo, suppressing absorption bands, and reddening their VNIR spectrum (Hiroi et al. 2006;

Gaffey 2010; Noguchi et al. 2011; Pieters & Noble 2016). Shock darkening also reduces the albedo

and suppresses mineralogical band features, but typically does not redden the spectrum (Kohout

et al. 2014). Shock darkening also changes physical aspects of meteorites and asteroid regolith,

such as porosity and magnetic susceptibility, which cannot be detected through current remote

sensing techniques (Kohout et al. 2014, 2020b, 2020a; Reddy et al. 2014).

Shock darkening observed in the meteorite collection is the result of partial or complete troilite or

FeNi metal melting and mobilization due to impact-related heating. Two stages of shock darkening

are observed at different pressure ranges. The first stage with lower amounts of darkening occurs

at ~40-60 GPa with injection of melts into silicate grains. The second stage occurs at pressures

between 90-150 GPa where bulk melting takes place and troilite and metal are finely dispersed

within the silicate material. The latter is sometimes referred to as impact melt to differentiate the

two phases of shock darkening (Kohout et al. 2020a, 2020b). The two phases cannot effectively

be distinguished with remote sensing and we primarily use the term shock darkening to refer to

both types of impact heating-related darkening of an asteroid surface.

Analysis of shock darkening in meteorites has been used to derive trends in principal component

space that can help distinguish between space weathering and shock darkening (e.g., Binzel et al.

2019). Where space weathering is believed to alter asteroid spectra from Q-type to S-type, shock

darkening and impact melt trends observed in meteorite spectra would result in asteroid spectra

moving from S- or Q-type to the C/X-complex in principal component space (DeMeo et al. 2009;

Kohout et al. 2014; Reddy et al. 2014; Binzel et al. 2019).

Here we show spectroscopic measurements of asteroid (52768) 1998 OR2 as evidence of possible

shock darkened material on the surface of an asteroid in the NEA population. This implies that

some S-complex NEAs may be altered by shock processes into resembling spectral properties of

lower-albedo NEAs.

1.1 Background on 1998 OR2

1998 OR2 is an Apollo class asteroid classified as a potentially hazardous asteroid [JPL Small

Body Database]. The most recent results from the NEOWISE survey of 1998 OR2 use an absolute

magnitude, H = 16.0, and estimate a diameter of 2.51 ± 0.81 km and a visible geometric albedo of



 (Masiero et al. 2021). Arecibo radar measurements give a diameter of approximately

2.16 km (Devogèle et al. 2020). The radar estimated diameter can be used in the diameter-albedo

relationship from Pravec & Harris 2007 (see their equation 2) to confirm the albedo.

  

 (1)

Using the Arecibo estimated 2.16 km diameter, the derived visible geometric albedo is 0.15 which

is consistent with the NEOWISE values. These albedo values are lower than the average for S-

types in the NEA population, 

, and are closer to the low/moderate-albedo C-type average

albedo of 

 (Thomas et al. 2011). The albedo of 1998 OR2 is, however, consistent with

the average main belt S-type albedo of 0.174 ± 0.039 found by Ryan & Woodward (2010).

The earliest rotational period estimate for 1998 OR2 was 3.198 ± 0.006 h from Betzler & Novaes

(2009). Koehn et al. (2014) provided a new period estimate of 4.112 ± 0.002 h which has been

confirmed with optical observations by Warner & Stephens (2020) [4.1114 ± 0.0002 h] and Franco

et al. (2020) [4.111 ± 0.001 h] as well as sequences of delay-doppler images from Arecibo

Observatory

[reported as 4.1 h] during the asteroid’s close approach in 2020. Variations in

lightcurve amplitudes are seen on different nights of 1998 OR2’s close approach in Warner &

Stephens (2020), likely due to rapidly changing viewing conditions of the topography on the

asteroid. Most recently, Colazo et al. (2021) claims a 4.01 ± 0.02-h rotation period which does not

agree with most previously published values.

2. PHOTOMETRIC STUDY

2.1. Observations and Data Reduction

Photometric observations of 1998 OR2 were obtained at LEO Observatory (MPC Code V17) on

2020 April 16 and 17 UTC. These observations were made prior to the asteroid’s close approach

to Earth on 2020 April 29 UTC at a nominal distance of 0.042 AU or approximately 16 lunar

distances [JPL Small Body Database]. The 0.52-m, f/2.9 telescope is equipped with a 4k x 4k, 9

http://www.naic.edu/~pradar/press/1998OR2.php

µm-pixel detector and a filter wheel with Sloan g’, r’, i’, and z’ filters resulting in a field of view

of 1.4 x 1.4 degrees with a 1.2” px-1 plate scale. Images were taken with 30 s exposure times in

each of the four Sloan filters. The data were calibrated using standard CCD reduction techniques

of dark- and bias-subtraction with flat-field division (Howell 2006). The number of images in each

filter and other observing conditions are shown in Table 1.

Table 1. Observational circumstances for photometric observations

Telescope

Date

UTC

Start

UTC

End

UTC

Airmass

Mag

(deg)

Filters

No. of

Img.

(g’, r’,

i’, z’)

LEO

2020

April 16

02:44

07:03

1.01 – 2.06

12.98

105.6

SDSS g’,

r’, i’, z’

78, 77,

77, 77

LEO

2020

April 17

02:50

06:17

1.02 – 1.62

12.85

105.8

SDSS g’,

r’, i’, z’

74, 73,

74, 64

Photometric extraction was performed with a custom in-frame photometry pipeline written in

Python which utilizes astropy, photutil, TKinter, and glob in addition to standard Anaconda

distribution packages (Robitaille et al. 2013; Price-Whelan et al. 2018; Bradley et al. 2019;

Anaconda Python Distribution 2021). Photutils version 0.7.2 was used for source extraction,

including source deblending, aperture photometry, and error propagation from the background

estimation process via the image segmentation package.

Astroquery was used to interface with astrometry.net (Lang et al. 2010) to plate solve each frame

from the telescope so that astronomical coordinates would be available in the file. After source

detection in pixel and sky plane coordinates, stellar sources were matched with the SDSS12 catalog

(Alam et al. 2015). Astroquery was also used to query the asteroid ephemeris from the Minor

Planet Center to confirm the target was properly identified in each frame. Traditional in-frame

aperture photometry was performed for each frame in each filter, as outlined below. For a more

in-depth review of in-frame photometry, differential photometry, and aperture photometry

methods, please refer to Howell (2006).

A local background map was computed using a two-dimensional Photutils filter after masking

point sources in the image. Aperture sizes for each image were selected using point spread function

information measured by Photutils. The number of counts within each aperture was summed and

the estimated local background subtracted. The resulting net number of counts was divided by the

exposure time to provide a flux from each star and the target asteroid. Instrumental magnitude was

then calculated for each catalog star and the median difference between the instrumental magnitude

and catalog magnitude for all in-frame stars provides the zero point magnitude. Each frame

included at least 50 stars to make this estimation more robust. The zero point magnitude for each

frame was added to the instrumental magnitude estimated for the target asteroid in order to

calculate the calibrated SDSS magnitude of the target used for constructing the lightcurves. Using

this method of in-frame photometry makes the brightness measurements more robust to

atmospheric variation throughout the night and allows night-to-night comparisons.

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PhysicalCharacterizationofNear-EarthAsteroid(52768)1998OR2:EvidenceofShockDarkening/ImpactMeltCharacterizationofNEA(52768)1998OR2AdamBattle1,VishnuReddy1,JuanA.Sanchez2,BenjaminSharkey1,NeilPearson2,BrynBowen11LunarandPlanetaryLaboratory,UniversityofArizona,1629E.UniversityBoulevard,Tucson,AZ85721-0...

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Physical Characterization of Near -Earth Asteroid 52768 1998 OR2 Evidence of Shock Darkening Impact Melt Characterization of NEA 52768 1998 OR2

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