Apophis T–9 Years 2020 (LPI Contrib. No. 2242) 2047.pdf

PUTTING NEAR-EARTH ASTEROID (99942) APOPHIS INTO CONTEXT. E. S. Howell1, R. J. Vervack, Jr.2, Y. R. Fernández3, M. L. Hinkle3, C. Magri4, C. Lejoly1, M. C. Nolan1, A. Virkki3, P. A. Taylor5, E. G. Rivera- Valentín5, B. Aponte-Hernández5 ,L. F. Zambrano-Marín3, 1Lunar and Planetary Laboratory, U. Arizona, Tucson AZ ([email protected]), 2JHU/Applied Physics Lab, Laurel, MD, 3U. Central Florida, Orlando, FL, 4U. Maine at Farmington, Farmington, ME, 5Lunar Planetary Institute, Universities Space Research Association, Houston, TX.

Introduction: The asteroid (99942) Apophis has measurements to infer surface roughness. The been extensively studied to determine its properties. polarization ratio at 12.6-cm and 3-cm wavelengths of Using a wide range of instruments and techniques, its Bennu suggested that the surface should be relatively optical, infrared and radar properties have led to smooth and dominated by cm-scale particles [7]. estimates of its size, rotation state, composition and However, Bennu’s surface is in fact dominated by thermal characteristics. Apophis is of taxonomic class meter-scale boulders [8]. The interpretation of radar Sq [1,2], which is one of the most common asteroid scattering for other taxonomic classes may also need to classes seen among the Near-Earth asteroids (NEAs). be reconsidered. The number of NEAs with measured physical Thermal Properties: The infrared flux of Apophis properties is steadily growing, allowing us to re- has been measured and thermal properties reported evaluate the range of properties seen among NEAs, [9,10.11,12]. However, as seen for Bennu and Ryugu, and the relationship of Apophis to other Sq asteroids. the thermal inertia of the larger rocks seems to be quite We will determine how variable the characteristics of low compared to expected values for competent rocks these bodies are, and what we might infer about their [8,13]. One possibility is that these boulders have high parent body or bodies. porosity, and this material does not generally survive Spectral Characteristics: The Bus-DeMeo S- the trip through the Earth’s atmosphere. Thermal complex taxonomic group [3] includes a wide range of inertia derived from remote sensing may not pyroxene and olivine dominated compositions that correspond to dominant particle size in the way we form more of a continuum than clearly separated have been assuming [14]. The roughness values classes. While the closest assumed meteorite analogs derived from thermophysical models suggested cm- of these asteroids may show heterogeneity on scales of scale was the dominant particle size on the surface, cm or mm, the mixing at larger scales (100m-km) which is difficult to reconcile with the surface imaged seems to be more uniform, with rotational spectral on Bennu and Ryugu. Itokawa may be a better analog variation being generally moderate, and collisional for Apophis, but it is likely that Sq NEAs span a wide families being quite spectrally similar [4]. We will range of surface textures as well. The length scales for compare multiple measurements of individual objects thermal roughness, radar roughness and dominant at different aspects to the variability of the Sq class as particle size are complex and overlapping. However, a whole. The relationship of Apophis to others in the comparing derived thermal inertia and roughness for NEA population will help establish our confidence in asteroids within a given size range might still be a inferring its properties. useful way to characterize their structure and derive Radar Properties: The radar properties of NEAs dynamical properties that depend on their thermal have been shown to exhibit some trends with properties. taxonomic class [5]. Apophis has been observed using Putting Apophis into context: We will compare radar and found to have a mean OC radar albedo of NEAs similar to Apophis in composition, taxonomy, 0.25±0.11 and a polarization ratio (SC/OC) of shape, and size in order to better understand how its 0.33±0.11 [6]. (The circular polarization is denoted as properties fit into the broader NEA population. We opposite sense (OC), or same sense (SC) from the will compare the range of spectral composition of a transmitted polarization.) These values are compared single object, with the variance of the Sq class to to other S-type NEAs and found to be consistent in determine the typical compositional variability. The polarization ratio (0.27±0.08), and somewhat higher radar properties of NEAs may also help to predict the than the mean radar albedo for S-type NEAs surface texture, as long as we consider lessons learned (0.15±0.06) [5,6]. Current radar observations cover a from those objects visited by spacecraft. Thermal larger number of objects over a wider size range. In properties are critical for objects like Apophis, where addition, recent spacecraft missions show some the non-gravitational forces are important to predict the surprising surface characteristics that contradict the impact hazard accurately [i.e.15,16]. Considering how standard interpretation of radar polarization Apophis T–9 Years 2020 (LPI Contrib. No. 2242) 2047.pdf

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