Outer Solar System Debris

• Introduction
  ISM ---> Zodiacal dust

• Kuiper Belt dust
  • Why is it interesting?
  • How much?
  • Production mechanisms
  • Dynamical evolution
  • KB dust model
  • Dust outflow

• Kuiper Belt Objects
  • Dynamical classes
  • Size distribution
  • Outstanding questions

• ISP prospects

ISM ---> Zodiacal dust

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Why is KB dust interesting?

• permeates the [outer] solar system

• source of micrometeoroid impacts [on outer solar system bodies]:
  • affects surfaces of small solid bodies (moons, asteroids)
  • affects giant planet atmospheric chemistry

• radiation [infrared and longer] is annoying for cosmology

• hazard to fast-moving space probes

• source of anomalous cosmic rays

• platform for organic chemistry in space

• source of large particles in the local ISM

• tracer for parent bodies [KBOs], and also for planets [in exo-solar systems]

• closest analog for astronomical debris disks

KB Dust: How much?

• Voyager 1 and 2 (plasma intruments) measured "high concentrations" of micrometer-size particles out to ~50 AU heliocentric distance (Gurnett et al 1997). But the instruments were not calibrated for dust grain detection, thus inconclusive.

• Pioneer 10 (1972--1980) detected dust impacts out to ~18 AU.

• Pioneer 11 (1973--1983) detected dust impacts out to ~13 AU.

Landgraf et al (2002) modelled these data and derived an estimate for the KB dust production rate:

This implies a steady-state dust disk of mass ~ 10^-6M_earth.

KB Dust Production Mechanisms

• Impacts by interstellar dust grains on large KBOs (Yamamoto & Mukai, 1998), produce only small grains ( s < 10 micron), at a rate of
  3 x 10⁵ --- 3 x 10⁷ g/s

• Mutual collisions of KBOs (Stern 1996, Jewitt & Luu 2000) produce dust at a rate
  ~10⁵ --- 3 x 10⁷ g/s for s < 10 micron
  ~10⁶ --- 3 x 10⁸ g/s for s < 3 mm

In either case, dust grains have initial conditions very similar to their parent bodies (relative velocities ~ m/s)

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Dynamical classes

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Size Distribution

• Observed KBOs have radii 10 km < r < 1000 km
  • N(r > 50 km) ~ 5 x 10⁴
  • main belt mass ~0.01M_earth
  • total mass (<50 AU) < 0.03M_earth

• Size-class correlations
  • 'excited' KBOs contain more large objects, fewer small objects compared to the 'classical' KBOs
  • largest CKBO is 1/60th mass of Pluto

• Collisional evolution models indicate that collisions are destructive for objects D < 100-300 km, in the present environment

• The source of short period Jupiter-family comets (JFCs): Uncertain. The scattered disk may be more likely than main belt.

• Accretion models indicate that d > 100 km KBOs must have formed in a dynamically cold environment, ie, (e,i)_initial <~ 0.001
  Some process has disturbed the Kuiper Belt & pumped up KBOs' e's and i's

Outstanding questions

Forces on dust grains:

• solar gravity, F_g ~ m/r²

• solar radiation

  • radiation pressure, F_rad ~ (beta)*F_g
    partially offsets solar gravity ("particle feels a less massive Sun")
    • beta depends upon size and optical properties of the dust grain, and on the stellar SED

    • when a dust grain is released, its orbital elements change relative to that of the parent body
      - small grains -> unbound (beta-meteoroids)
      - large grains -> remain in bound orbits

• Poynting-Robertson light drag

  • circularizes and shrinks the bound orbits (decrease 'e' and 'a')

  • Thus, if there is a steady supply of dust particles at some distant radial location, the particles will drift in and form a dust disk ("debris disk")

• solar wind drag (similar to PR drag)

  

• planetary gravitational perturbations
  • mean motion resonances trap particles in specific radial and azimuthal patterns
  • massive planets can scatter and eject dust particles out of the planetary system, creating dust-free zones

  We can learn about the diversity of planetary systems from studies of debris disk structures!

• Lorentz forces from heliospheric magnetic fields

• physical destruction or comminution
  

  - collisions, sublimation, sputtering

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Solar System Kuiper Belt Dust Disk Model

Release dust grains (massless test particles) from planetesimals (ICs) and follow their evolution (in a numerical model) until the particles disappear due to sublimation, ejection or accretion onto the planets.

• Numerical model accounts for:
  • solar gravity and solar radiation pressure
  • PR and solar wind drag
  • planetary perturbations (including mutual perturbations)

• Model does NOT account for:
  • gas drag
  • grain-grain collisions
  • grain erosion (sputtering)
  • Lorentz forces from heliospheric B fields

• Dust ring with gap near Neptune (due to trapping in MMRs)
• Depleted zone interior to r ~ 10 AU (due to scattering by Jupiter and Saturn)
• Density contrasts more prominent for larger particle sizes (more efficient resonance trapping)

• Bright ring at 10-15 AU
• Steep increase of brightness in inner 5 AU (combination of decreasing particle density and increasing temperature)

In the absence of planetary perturbation, there would be a nearly uniform surface density of dust particles.

More Results (Moro-Martin & Malhotra, 2002,2003)

• Particle size distribution - radial dependence

  Assume initial power law: n(s)ds = n₀s^-qds (q=3.5 for classical collisional cascades)

  • Radiation forces: power law index decreased from q to ~q-1 (at distances smaller than the aphelion of the parent bodies). No large particles are to be found at larger distances, consequently the size distribution is very steep at the larger distances.
  • Planetary perturbations: at smaller distances, the power law index increases (q=2.5 -> 2.9), but planetary scattering affects severely the size distribution at large distances.
  • Future: investigate the potential of this effect to detect and characterize planets in exo-solar debris disks.

• Velocity field of KB dust grains in the inner and outer solar system: useful for planning dust detectors on future spacecraft missions (New Horizons, InterStellar Probe)

• Heliospheric B field effects: cause a random walk in the semimajor axis of charged dust grains. For particles larger than a few microns, this effect is negligible compared to the systematic PR drift in semimajor axis.

  

• KB as a source of IDPs collected at Earth?

  Due to the dynamically "hot" state of the present Kuiper Belt, KB dust grains arrive at 1 AU with high eccentricities and inclinations, similar to cometary rather than asteroidal dust. Thus, their encounter velocities and capture rates are similar to cometary dust - CONTRARY to previous results).

  No more than ~25% of IDPs captured by Earth have cometary or KB origin.

  Only ~1-2% of the collected IDPs have a KB origin.

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• Dust outflows from planetary systems (Moro-Martin & Malhotra, 2004)

  

  Massive planets scatter and eject dust grains out of the planetary system

  • Small dust grains (beta>0.5) are ejected due to radiation pressure (beta-meteoroids)

  • Large dust grains (beta<0.5) remain initially on bound orbits, but in the Solar system 80-90% are eventually ejected by gravitational scattering from Jupiter and Saturn

    Dynamical models of other planetary systems yield 50-99% ejection rate.

  • May contribute significantly, or even dominate, the clearing of circumstellar debris in planetary systems

  • Links the interplanetary environment to the galactic environment of a star

    Planetary systems are prime sites for large particle formation; as such, they can contaminate the immediate vicinity of star-forming regions via these dust outflows, affecting the particle size distribution and gas-to-dust abundance of their local ISM

  • Important for the interpretation of in-situ dust detections by space probes in the outer Solar system
    • Ulysses and Galileo made the surprising discovery of large interstellar (IS) particles (implying higher dust/gas ratio in local ISM exceeding cosmic abundances?!)
    • How are the ISPs identified? By their hyperbolic velocities.
    • But large interplanetary grains can also be found on hyperbolic orbits due to gravitational ejection!

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ISP prospects

• KBO Imaging System
  • survey for small KBOs
  • physical studies of large KBO surfaces

• Dust detector
  • spatial and size distribution of dust
  • chemical composition