44th Lunar and Planetary Science Conference (2013) 2835.pdf

INTRODUCING THE EULALIA AND NEW POLANA ASTEROID FAMILIES: RE-ASSESSING THE SOURCE REGIONS OF PRIMITIVE NEAR-EARTH ASTEROIDS. K. J. Walsh1, M. Delbó2, W. F. Bottke1, D. Vokrouhlický3 and D. S. Lauretta4, 1Southwest Research Institute, Boulder, CO, 2Laboratoire Lagrange, UNS- CNRS-Observatoire de la Côte d’Azur, Nice, France, 3Institute of Astronomy, Charles University, V Holěsovičkaćh 2, Prague 8, Czech Republic, 4Lunar & Planetary Laboratory, University of Arizona, Tucson, AZ, USA.

Introduction: The inner main asteroid belt (2.15 AU < times due to the Yarkovsky effect [1]. This effect a < 2.5 AU) is the dominant source region of near- causes a secular drift of the semi-major axis of the or- Earth objects (NEOs). Of those asteroids from this bit due to the emission of the thermal radiation. region that become NEOs, most are delivered via the Whether this effect causes an increase or decrease of

ν6 secular resonance located at ∼2.15 AU. In particular, an asteroid’s semimajor axis depends on its rotation this pathway is the most likely delivery source for state, whereby prograde rotators drift outward and ret- NEOs on low Δ-velocity orbits, such as the proposed rograde rotators drift inward. Thus, radial drift into a space mission targets 1996 RQ36 and 1999 JU3. These resonance, followed by rapid increase in orbital eccen- missions are specifically targeting primitive asteroids - tricity and the eventual interaction with terrestrial those asteroids of C- or B-type taxonomies that are planets, is the typical pathway for Main Belt asteroids thought to be related to Carbonaceous Chondrite mete- to become NEOs [1]. orites. The largest collections of primitive asteroids in the 16 0.3 inner main asteroid belt are found among the Nysa- 14 Polana complex, and the diffuse background of low- 12 albedo bodies. Data from the NASA WISE mission 10 have allowed a closer analysis of all low-albedo, primi- 8 tive asteroids in this region, which is valuable for de- 6 0.2 termining the location and size of asteroid families and inclination (deg) 4 the background population. We have found that the 2 low-albedo component of the Nysa-Polana complex is 0 centered very close to the 3:1 mean motion resonance with Jupiter, with (495) Eulalia as the likely parent. 0.2 The previously assumed parent, (142) Polana, does not 0.1 appear to be a member of the family. Also, the diffuse geometric visible albedo background of primitive bodies appears to be an aster- 0.1 oid family parented by (142) Polana - the “new Po- eccentricity lana” family. It is potentially one of the oldest and most extended primitive asteroid family in this region 0 0 of the asteroid belt. 2.2 2.25 2.3 2.35 2.4 2.45 2.5 Dynamical evolution studies are critical for obtain- semimajor axis (AU) ing the maximum scientific benefit from planned as- Figure 1: The entire inner Main Belt, including all as- teroid sample return missions. Specifically, the teroids that were observed by WISE and whose D and OSIRIS-REx mission will provide the first ground- pV were published in the Preliminary release of Al- truth assessment of the Yarkovsky effect as it relates to bedos and Diameters [2] plotted using computed syn- the chemical nature and dynamical state of an individ- thetic proper elements. The plots show the orbital in- ual asteroid. These studies will provide important input clination and the eccentricity as a function of their to models of the evolution of asteroid families by semimajor axis (AU). The color of each point repre- Yarkovsky drift and delivery to orbital resonances in sents their WISE-determined albedo with values the IMB. shown in the colorbar on the right [2]. The low-albedo component of the Nysa-Polana complex is visible as Inner Main Belt – Primitive Asteroids: The inner the large low-albedo complex at e ∼ 0.15 and i ∼ 3° main asteroid belt (a < 2.5 AU) is bound by the 3:1 extending across almost the entire IMB. mean motion resonance (MMR) with Jupiter (centered at 2.5 AU) and by the ν6 secular resonance at 2.15 AU Within most parts of the Asteroid Belt there are dy- (the ν6 is inclination dependent: see Fig. 1). Asteroids namical families of asteroids - asteroids with closely that are 10-20 km and smaller are likely to experience related orbital elements [3]. Asteroid families are cre- substantial radial drift of their orbits during their life- ated during an asteroid collision, and though the colli- 44th Lunar and Planetary Science Conference (2013) 2835.pdf

sional remnants’ orbital elements remain close in or- bital element space, they do spread out in semi-major axis over time due to the thermal forces of the Yark- ovsky effect. Depending on the families’ location and age, the smallest bodies may have time to drift into major reso- nances (such as the Jupiter 3:1 MMR and the ν6) insti- gating rapid and dramatic orbital change. This process can lead to the correlated production of NEOs, such that certain families can dominate the production of certain types of NEOs for long periods of time. Low- albedo asteroid families have been claimed to be the primary source of some primitive and low-albedo NEOs [4,5,6]. However, it has recently been noted that the background, or diffuse grouping of asteroids that seemingly have no family correlation, is more efficient than the familes in delivering low-albedo asteroids to near- Earth space via the ν6 resonance [5,6]. However, the origin of the low-albedo background remain a mys- tery.

Figure 2: The absolute magnitude, H, for all WISE- The Eulalia and new Polana families: In order to observed asteroids with albedo pV below 0.1, eccentric- study a major source region for primitive NEOs, we ities between 0.1 and 0.2 and inclinations below 10°, used measurements from the NASA WISE mission to plotted as a function of their semi-major axes (AU). focus on only low-albedo inner main belt asteroids. The bottom panel is the same as the top, except lines Then a subset of asteroids centered on the Nysa-Polana are drawn to outline the Yarkovsky “V-shape” in the complex was extracted with orbital eccentricities be- data. ° tween 0.1 and 0.2 and inclinations below 10 . Figure 2 shows that the distribution of the absolute magnitude H Of particular note with these two primitive families as function of semimajor axis has the typical “V” we are interested in their delivery of asteroids via the shape of asteroid families [1]. The V-shape is due to Yarkovsky-effect to the ν6 resonance due to its effi- the size dependance of the Yarkovsky-effect for the ciency at delivering NEOs. The new Polana family, family members, where smaller asteroids drift faster, being older, is delivering asteroids up to H∼16, while thus have drifted further from the center of the family. the Eulalia family is only now contributing bodies of Two V-shapes are visible clearly indicating the pres- H 17.5. Both have also contributed a large fraction of ence of two familes of asteroids of low albedo in the ∼ their total families into the 3:1 MMR. region. The first family is that consisting of the low-albedo component of the Nysa-Polana complex (bounded by References: red lines in Fig. 2). This subset of that large complex [1] Bottke, W. F., Jr., Vokrouhlicky ́, D., Rubincam, D. appears to be a single family with a center very near P., and Nesvorny ́, D. (2006). AREPS, 34, 157. [2] Ma- the 3:1 MMR with Jupiter. Analysis of the possible siero, J. R., et al. 2011. ApJ, 741 68. [3] Cellino, A. et parent asteroids finds that the previously considered al. (2001) Icarus, 152, 225. [4] Campins, H. et al. parent, (142) Polana, is not a likely family member. (2010). ApJ, 721, L53. [5] Gayon-Markt, J. et al., Instead asteroid (495) Eulalia is the likely parent. Its (2012). MNRAS, 424, 508-518. [6] Campins, H. et proximity to the 3:1 MMR is responsible for substan- al.(2012). LPI Contributions 1667 6452. tial orbital chaos, which is why it has not previously been linked to this family. The size of the observable Acknowledgements: KJW thanks NLSI for funding, family suggests that it was formed 900-1500 Myr due and the Gaia-GREAT program of the European Sci- to the breakup of a 100–160 km diameter parent body. ence Foundation. MD thanks the Centre National This family is older and therefore more diffuse. The d’Etudes Spatiales (CNES) for financial support. DSL second family appears to be parented by (142) Polana. and WFB thank the NASA New Frontiers Program for Similar analysis yields an estimate age for this family supporting this research (Contract NNM10AA11C). of older than 2000 Myr, though there is no reliable DV was supported by the Grant Agency of the Czech estimate for the size of the original parent body from Republic through grant 205/08/0064. the current analysis.