Research Article

The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt

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Science  13 Feb 2020:
eaay6620
DOI: 10.1126/science.aay6620
  • Fig. 1 Dynamic geophysical environment at the surface of Arrokoth is determined by its gravity and rotation.

    (A) Effective surface gravity in Arrokoth’s rotating frame, overlain on the shape model (8). (B) Gravitational slope, i.e., the difference between the local effective gravity vector and the surface normal to the global shape model. Arrows indicate the tilt of the local gravitational slope; the steepest slopes occur in or near the neck region [figure S1 in (8)]. In both (A) and (B) a uniform density of 500 kg m−3 is assumed for both lobes; red dot indicates center of mass/rotation axis. The background grid is in 1 km intervals.

  • Fig. 2 The compressive or tensile stress supported at Arrokoth’s neck depends on the body's density.

    (A) The solid blue line separates the unconfined compression and tension regimes. For bulk densities ρ ≳ 250 kg m−3, the neck is in compression. For a nominal cometary cohesion of 1 kPa (dashed black line) and internal friction angle of 30° (33), the upper limit density of Arrokoth is ~500 kg m−3; otherwise the neck region would collapse. Greater strengths are compatible with greater bulk densities. For ρ ≲ 250 kg m−3, the neck is in tension (shown on an expanded scale in B). For a nominal cometary tensile strength of 100 Pa (33) (dashed black line), the lower limit density of Arrokoth remains close to 250 kg m−3. Much lower densities (≲50 kg m−3), for which the forces between the lobes vanish, are not considered physical. Strength estimates scale inversely with the assumed contact area (we adopted 23 km2 [8]).

  • Fig. 3 The mean gravitational slope of Arrokoth as a function of assumed bulk density.

    The minimum mean slope occurs for a bulk density of ~240 kg m−3 (cf. Fig. 1B, which assumes ρ = 500 kg m−3). If Arrokoth's topography behaves similarly to that of asteroids and cometary nuclei (39), this may be the approximate density of Arrokoth. The minimum is quite broad, however, which is consistent with a range of densities considered appropriate to cometary nuclei (29).

  • Fig. 4

    Numerical N-body calculations of collisions between spherical bodies of the scale and approximate mass ratio of the LL and SL lobes of Arrokoth. The larger lobe (LL) is represented by green and the smaller lobe (SL) by blue particles, respectively. A bulk density of 500 kg m−3 is assumed for both bodies. (A) At a collision speed of 10 m s−1 and a moderately oblique angle, the impact severely disrupts both bodies, leaving a long bridge of material stretched between them. As the simulation progresses, this connection breaks as SL moves farther from LL and ultimately escapes. Movie 1 shows an animated version of this panel. (B) At 5 m s−1 and for the same impact angle of 45°, the impact creates a contact binary, but with an asymmetric, thick neck and a lopsided SL. Movie 2 shows an animated version. (C) At 2.9 m s−1 and an oblique impact angle of 80°, both lobes remain intact, and the contact area between them forms a well-defined, narrow neck. Movie 3 shows an animated version. Interparticle friction between the particles is assumed in all cases; in A and B the interparticle cohesion is 1 kPa (a value thought typical for comet-like bodies; see text) and zero cohesion is assumed in C. No initial spin is assumed in A and B, whereas the lobes in C are set to rotate synchronously before collision.

  • Fig. 5 Possible initial stages in the formation of a contact binary in the Kuiper Belt, illustrated by numerical models.

    (A) Overdense particle concentrations in the protosolar nebula self-amplify by the streaming instability, which then leads to gravitational instability and collapse to finer scale knots. Snapshot from a numerical simulation in (60) illustrates vertically integrated particle density, Σpar, viewed perpendicular to the nebular midplane, relative to the initially uniform surface density, 〈Σpar〉; lighter colors mean greater particle density, H is nebular scale height, and 0.02H is the initial particle scale height [adapted from (60); ©AAS, reproduced with permission]. (B) Outcomes of an example collapsing, gravitationally unstable particle cloud, from N-body simulations [reproduced from (58) with permission]. Arrokoth may have formed as a binary planetesimal in such a collapsing particle cloud, either as a contact, or more likely, a co-orbiting binary.

  • Fig. 6 The inertial axes of Arrokoth and its two lobes are aligned.

    (A) Viewed down the spin axis, arrows indicate the maximum (c, or red), intermediate (b, or green), and minimum (a, or blue) principal axes of inertia for each lobe (thin vectors), and the body as a whole (thick vectors). Vectors originate from the center of mass of each component. Background grid is in 1-km intervals. (B) Oblique view of the same, matching the geometry of the CA06 image (8). Alignment of the maximum principal axes of inertia, and of SL’s minimum principal axis of inertia with that of Arrokoth as a whole, is unlikely to be due to chance alone (see text).

  • Fig. 7 Illustration of the protosolar nebula headwind interacting with a co-orbiting equal mass binary.

    The averaged torque is proportional to the product of the lobe orbital velocity and the differential velocity between the nebular gas and the binary’s center-of-mass about the Sun.

  • Movie 1. Animated version of Fig. 4A.

    Merger of spherical components at 5 m s−1 and impact angle 45°, using a rubble-pile model with 198,010 particles total. Material parameters correspond to rough surfaces with a friction angle of ~40° and a cohesion of 1 kPa. For this model there was no initial component rotation. Particle color indicates body of origin – green particles are from the large lobe (representing LL), while blue particles are from the small lobe (representing SL).

  • Movie 2. Animated version of Fig. 4B.

    Merger of spherical components at 10 m s−1 and impact angle 45° degrees using a rubble-pile model. Particle size and density and material parameters are identical to the simulations in Movie 1. For this model there was no initial component rotation. Particle color indicates body of origin.

  • Movie 3. Animated version of Fig. 4C.

    Merger of spherical components at 2.9 m s−1 and impact angle 80° using a rubble-pile model. Particle size and density and material parameters are identical to the simulations in Movies 1 and 2. For this model, each component has an initial spin period of 9.2 hours in the same sense as the orbit, in order to produce synchronous rotation. Particle color indicates body of origin.

  • Movie 4. Peak accelerations during the merger of spherical components at 2.9 m s−1 and an impact angle of 80°.

    This is the same simulation as in Movie 3, but now particle colors correspond to the maximum acceleration experienced by each particle up to the time shown. Darkest blues correspond to 3.5 × 10−4 m s−2 and darkest reds to 8.8 × 10−1 m s−2 on a linear scale. Although some disturbance is experienced by loose surface particles globally (each sphere is settled individually before they experience each other’s gravity), the maximum disturbance during the simulation is concentrated in the narrow contact area, or “neck,” between the two bodies.

Supplementary Materials

  • The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt

    W. B. McKinnon, D. C. Richardson, J. C. Marohnic, J. T. Keane, W. M. Grundy, D. P. Hamilton, D. Nesvorný, O. M. Umurhan, T. R. Lauer, K. N. Singer, S. A. Stern, H. A. Weaver, J. R. Spencer, M. W. Buie, J. M. Moore, J. J. Kavelaars, C. M. Lisse, X. Mao, A. H. Parker, S. B. Porter, M. R. Showalter, C. B. Olkin, D. P. Cruikshank,
    H. A. Elliott, G. R. Gladstone, J. Wm. Parker, A. J. Verbiscer, L. A. Young, the New Horizons Science Team

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Team Members and Affiliations
    • Materials and Methods
    • Supplementary Text
    • Table S1
    • References

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