| There is abundant inferential evidence for massive collisions in the early solar system [1]:
Mercury’s high density; Venus’ retrograde spin; Earth’s Moon; Mars’ North/South
hemispherical cratering anisotropy; Vesta’s igneous origin [2]; brecciation in meteorites [3];
and Uranus’ spin axis located near the plane of the ecliptic. Recent work [4] analyzing
Spitzer mid-IR spectra has demonstrated the presence of large amounts of amorphous
silica and SiO gas produced by a recent (within 103 – 104 yrs) large (MExcess >
MPluto) hypervelocity impact collision around the young (~12 Myr old) nearby star
HD172555, at the right age to form rocky planets. Many questions still remain
concerning the location, lifetime, and source of the detected silica/SiO gas, which should
not be stable in orbit at the estimated 5.8 AU from the HD172555 A5V primary
for more than a few decades, yet it is also highly unlikely that we are fortuitously
observing these systems immediately after silica formation A tabulation of the amount
counts in the fine silica dust is decidedly Fe and Mg-atom poor compared to solar
[4].
Three possible origins for the observed silica/SiO gas seem currently plausible
:
(1) A single hyperevelocity impact (>10km/s in order to produce silica and vaporize SiO
at impact) creating an optically thick circumplanetary debris ring which is overflowing or
releasing silica-rich material from its Hill sphere. Like terrestrial tektites, the Fe/Mg poor
amorphous silica rubble is formed from quick-quenched molten/vaporized rock created
during the impact. The amount of dust detected in the HD172555 system is easily enough to
fill and overflow the Hill sphere radius of 0.03 AU for a Pluto-sized body at 5.8 AU from an
A5 star, unless it is optically thick (> 1 cm in physical depth). Such a disk would
provide a substantial fraction of the observed IR flux, and will be dense enough
to self-shield its SiO gas, greatly extending its photolytic lifetime. The lifetime
for such a system versus re-condensation into a solid body like the Moon is short,
though, ~ 103 to 104 yrs [5]. Credence is lent to this scenario by observations of the
Jovian impact in July 2009 [6], where absorption features due to silica have been
found superimposed on those of hot ammonia at the > 60 km/s impact site (Fig.
1).
(2) Ongoing multiple small hypervelocity impacts continuously grinding down a
distribution of large circumstellar particles above the blowout size limit (the “rubble”
identified in [4]) and releasing silica rich material and SiO gas. This model would require
a massive (>1 MMoon) belt of 10 μm – 1 cm particles with inclinations spread
out over at least ±45o [4] or dust on highly eccentric orbits [7]. The amount of
material implied by the relative amplitude of the rubble spectral feature is consistent
with the amount needed to collisionally produce the fine silica dust [4, 8]. A body
rapidly re-accreting in a debris ring after collisional disruption (like the Moon) would
have similar behavior (lots of impacts for some time, producing gas and little melt
droplets).
(3) A single impact onto a silica-rich object with already highly differentiated
surface layers. For a very young system at 10 - 20 Myr when we expect planets
to be rapidly accreting, a Mercury or larger-sized rocky body covered in an SiO
rich magma ocean is very likely by the Jeans energy criterion [9], even without
considering additional heating input by 26Al and other radioactives. For the lowest
expected impact velocities,v MercuryEscape = 4 km/s, a pre-existing magma ocean
in equilibrium with a surrounding SiO atmosphere would be required; at higher
velocities the impacting body could be the formative mechanism for the magma ocean
[10].
Further evidence for excess circumstellar emission due to silica dust have now been
found. The youngest of these, HD154263, at ~20 Myr age shows evidence for SiO gas and
amorphous + crystalline silica. The 2 older systems, HD23514 at ~100 Myr age, and
HD15407 at ~2 Gyr, conspicuously do not show any evidence for SiO gas while exhibiting
strong features mainly due to crystalline silica. HD23514 also shows evidence for large
amounts of amorphous carbon, PAHs, and nanodiamonds, due to a strongly enhanced C-atom
abundance in impactor or impactee. HD15407, the oldest system, also does not
show any conclusive evidence for the presence of large dark particles (“rubble”). |