Motivation

1
MICROMETEORITE ANNEALING OF OUTER PLANET ICY
SATELLITE SURFACES
S. B. Porter1, S. J. Desch1, J. C. Cook2, 1School
of Earth and Space Exploration, Arizona State
University (simon.porter_at_asu.edu) 2Southwest
Research Institute
Motivation

• The surfaces of the satellites of Saturn,
  Uranus, and Neptune are mostly H2O ice, which
  appears to be crystalline, based on the 1.65
  micron absorption feature in telescopic spectra
  (Bauer et al. 2002, Brown et al. 1998, Grundy et
  al. 2006).
• But galactic cosmic rays (GCRs) and solar
  ultraviolet (UV) should break down the ices
  crystalline structure into an amorphous state
  over very short timescales (1.5 Myr at 40 AU
  Cook et al. 2007)
• Some process, therefore, must be actively
  crystallizing the ice within the last few Myr.
• Cryovolcanism may emplace crystalline ice, as
  suggested for Quaoar (Jewitt Luu 2004) and
  Charon (Cook et al. 2007) but many icy
  satellites with crystalline ice are not obviously
  cryovolcanic.
• The ice may instead be annealed by localized
  heating by micrometeorite impacts.

Methodology
Figure 1 Ice annealing time as a function of
distance from planet

• Pioneer 10 data shows a near-constant dust flux
  between 5 and 18 AU, implying that the Kuiper
  Belt is the chief source of interplanetary dust
  in the outer solar system (Hume 1980). The dust
  flux around Saturn, Uranus, and Neptune should
  thus be same as for the inner Kuiper Belt, though
  at higher orbital velocities.
• Previous work (Cook et al. 2007) showed that the
  slower impact velocities of the Pluto system
  resulted in micrometeorite annealing being
  significantly less effective than solar UV and
  GCR amorphization.
• Giant planet satellites, however, are exposed to
  dust that is both higher in heliocentric velocity
  and gravitationally focused by the planet. The
  increases in the dust particle density and
  velocity are (Krivov et al. 2003, Thiessenhusen
  et al. 2002, Colombo et al. 1966)
• ,
• Where Mp and Rp are the planets mass and radius,
  a is distance from the planet, and the unfocused
  dust velocity is assumed to be (e2 i2)1/2 Vp,
  where e i 0.3 and Vp is the orbital velocity
  of the planet.
• We then estimated the instantaneous kinetic
  energy flux onto the satellite surfaces by
  assuming the micrometeorite impacts to be
  isotropic on the scale of the satellite
• where vsat is the orbital velocity of the
  satellite around its host planet. For each
  satellite, this kinetic energy flux is integrated
  over a complete orbit, accounting for orbital
  eccentricity.
• Cook et al. (2007) calculated how heat deposited
  by a micrometeorite impact would thermally
  diffuse through ice. One third of the kinetic
  energy of impact is assumed to go into mechanical
  work, the remainder being deposited as heat.
  Initially, a small volume is heated to gt200 K,
  leading to rapid annealing within it as the heat
  spreads, larger volumes are heated for longer
  times, but to lower temperatures. Eventually the
  heat is too diffuse to anneal the ice on the
  relevant timescales.
• For a reasonable range of ice thermal
  conductivities, the mass of ice that is annealed
  scales as a constant times the kinetic energy of
  the impact (about 10 times the mass of the
  impactor for an impact speed of 1.8 km/s). We
  then calculate the timescale for annealing as
• This is to be compared to the timescale for
  amorphization. For GCRs, we assume a constant
  tamor 1.5 Myr, appropriate for 40 AU (Cook et
  al. 2007). Amorphization rates due to solar UV
  irradiation are less certain. The timescale at
  40 AU was calculated by Cook et al. (2007) to be
  tamor 50 kyr. Timescales at the other planets
  are shortened because of the higher UV flux, but
  also lengthened due to the higher temperatures,
  in a complicated way. We adopt values from Cook
  (2007).
• The fraction of ice that is crystalline was then
  calculated as tamor/(tamor tanneal).

Results

• The degrees of crystallinity for selected
  satellites are presented in the table below.
• As can be seen in the plot above, the key
  geometric factor in determining the gravitational
  focusing of dust is the ratio of the satellites
  semimajor axis to the planets radius (Krivov et
  al. 2003, Colombo et al. 1966). Annealing rates
  are much higher for satellites orbiting close to
  their host planets, and tend to be somewhat
  higher at Saturn than at Neptune.
• An eccentric orbit can also enhance the kinetic
  energy flux onto a satellite because although the
  satellite spends a small fraction of its time
  near the planet at high velocity, the flux scales
  as velocity cubed. Nereids eccentricity of e
  0.75 enhances the kinetic energy flux impacting
  it by a factor of 2 (over the flux if Nereid had
  a circular orbit).
• The range of orbital parameters leads to a range
  of annealing times. Phoebes slow orbit and weak
  focusing lead to an annealing timescale 0.6Myr,
  whereas the ice on Mimas can be annealed by
  micrometeorite impacts on timescales as short as
  500 years. In all cases, annealing is competitive
  with the amorphization due to GCRs but is not,
  generally, competitive with the maximum possible
  amorphization by solar UV.
• Annealing by micrometeorites may therefore
  explain why the water ice on satellites is
  generally crystalline.

Discussion

• Amorphization by solar UV potentially may be
  effective on 104 year timescales (Cook et al.
  2007) if it is then it will dominate over
  micrometeorite annealing. But further experiments
  on the amorphization of ice by UV irradiation
  similar to those of Kouchi Kuroda (1990), Leto
  Baratta (2003),and Leto et al (2005) are needed
  to assess the ability of UV photons to amorphize
  ice.
• On Kuiper Belt Object surfaces, like Charon, ice
  should be amorphous unless UV irradiation is
  ineffective and/or dust fluxes are about an order
  of magnitude higher than those inferred from
  Pioneer 10 measurements out to 18 AU. To the
  extent that this is true, the presence of
  crystalline water ice on KBO surfaces strongly
  implies the existence of cryovolcanism (Cook et
  al. 2007).
• Other factors may come into play in planetary
  environments. Energetic particles trapped in
  planetary magnetospheres may amorphize ice more
  quickly than we have calculated. On the other
  hand, our analysis has assumed impacts by
  interplanetary dust particles only, but the
  production of Saturns E-Ring by Enceladus (Spahn
  et al. 2006) creates a dust cloud through which
  Dione, Tethys, and Mimas must orbit (Kurth et al.
  2006). Particles may also be ejected from the
  other planets satellites and considerably
  enhance the micrometeorite flux onto their icy
  satellites, more rapidly annealing their ice.
  Finally, GCRs will be stopped by solar wind and
  planetary magnetospheres, and should take gt 1.5
  Myr to amorphize ice on satellites. Further study
  is needed to assess the net effect of these many
  factors.
• A test of micrometeorite annealing should be
  possible by obtaining an improved NIR spectrum of
  the satellite of the KBO 2003 EL61. The KBO
  itself has a crystalline water ice surface
  (Barkume et al. 2006), and while its satellite is
  known to be predominantly water ice 11, its
  crystallinity is unknown. If this satellite is
  found to have amorphous water ice, this would
  argue strongly against the idea that the surface
  of 2003 EL61 is crystalline because of annealing
  by micrometeorites. The presence of crystalline
  water ice on this satellite, however, would
  strongly suggest annealing by micrometeorites,
  since it is too small (200 km) to support
  cryovolcanism. Spectra of other KBOs could be
  similarly analyzed to test the micrometeorite
  annealing hypothesis.

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Table 1 Fraction of surface ice annealed to
crystalline