Guy J. Consolmagno

Composition and evolution of the eucrite parent body: evidence from rare earth elements

Abstract Eucrites are extraterrestrial plagioclase-pigeonite basalts. Experimental studies suggest that they were produced by partial melting of an olivine (Fo 65 )-pigeonite (Wo 5 En 65 )-plagioclase (An 94 )-spinel-metal source region. Quantitative modeling of the evolution of REE abundances in the eucrites indicates that the main group of eucrites (e.g. Juvinas) may be produced by approximately 10% equilibrium partial melting of a source region with initial REE abundances which were chondritic relative and absolute. Other eucrites appear to represent greater (e.g. Sioux County—15%) or smaller (e.g. Stannern—4%) degrees of melting. Moore County and Serra de Mage appear to be cumulates of pyroxene and plagioclase produced by fractional crystallization of a Juvinas-like melt. Nuevo Laredo may represent a residual liquid after such fractional crystallization. Our calculations are consistent with the conclusion that the eucrites were derived from a single type of source region. The close correspondence of the age of the eucrites (⋍ 4.6 AE) to the age of the solar system appears to preclude the possibility of extensive chemical differentiation of the eucrite parent body prior to the event which produced the eucritic melts. Thus our calculations have yielded not only the mode of the source region but, assuming homogeneous accretion, the mode and hence the bulk composition of the eucrite parent body as well. We are unable to estimate quantitatively the ratio of metal to olivine in the parent body. If no metal is present, the bulk composition (in oxide wt%) is Na 2 O—0.04, MgO—29.7, Al 2 O 3 —1.8, SiO 2 —39.0, CaO—1.2, FeO—28.3. If, in contrast, the parent body contained 30% metal, the bulk composition of the silicate portion of the eucrite parent body is Na 2 O—0.06, MgO—28.0, Al 2 O 3 —2.6, SiO 2 —41.3, CaO—1.9, FeO—26.3. Relative abundances of the meteorites suggest that the eucrite parent body is still intact. The solar system object most closely resembling the eucrites is asteroid 4 Vesta. Because Vesta is unique among the asteroids, we have license to conclude that it is the source of the eucrites and its bulk composition is close to the analyses given above.

The evolution of icy satellite interiors and surfaces

Abstract The satellites of the outer solar system planets are thought to be mixtures of ices and rocky material, in which decay of radioactive nuclides can lead to internal melting and solid-state convection. Time-dependent models indicate that melting will reach its maximum extent approximately 2.0 GYr after formation; bodies of radius 4 ices on any Saturnian satellite except Titan; photolysis explains the absence of NH 3 ices on these bodies, and possibly the absence of water ice on the surface of Callisto. The photolysis rate of CH 4 also implies a crustal reservoir of CH 4 on Titan.

Optical Spectroscopy of the Large Kuiper Belt Objects 136472 (2005 FY9) and 136108 (2003 EL61)

We present high signal precision optical reflectance spectra of the large Kuiper Belt objects 2005 FY9 and 2003 EL61. The spectrum of 2005 FY9 exhibits strong CH4 ice bands. A comparison between the spectrum and a Hapke model indicates that the CH4 bands are shifted 3.25 ± 2.25 A relative to pure CH4 ice, suggesting the presence of another ice component on the surface of 2005 FY9, possibly N2 ice, CO ice, or Ar. The spectrum of 2003 EL61 is remarkably featureless. There is a hint of an O2 ice band at 5773 A; however, this feature needs to be confirmed by future spectroscopic observations of 2003 EL61 with a higher continuum signal precision sufficient to detect a second, weaker O2 ice band at 6275 A.

Evidence of N2-ice on the surface of the icy dwarf Planet 136472 (2005 FY9)

Abstract We present high signal precision optical reflectance spectra of 2005 FY9 taken with the Red Channel Spectrograph and the 6.5-m MMT telescope on 2006 March 4 UT (5000–9500 A; 6.33 Au2009pixel −1 ) and 2007 February 12 UT (6600–8500 A; 1.93 Au2009pixel −1 ). From cross-correlation experiments between the 2006 March 4 spectrum and a pure CH 4 -ice Hapke model, we find the CH 4 -ice bands in the MMT spectrum are blueshifted by 3 ± 4 A relative to bands in the pure CH 4 -ice Hapke spectrum. The higher resolution MMT spectrum of 2007 February 12 UT enabled us to measure shifts of individual CH 4 -ice bands. We find the 7296, 7862, and 7993 A CH 4 -ice bands are blueshifted by 4 ± 2, 4 ± 4, and 6 ± 5 A. From four measurements we report here and one of our previously published measurements, we find the CH 4 -ice bands are shifted by 4 ± 1 A. This small shift is important because it suggest the presence of another ice component on the surface of 2005 FY9. Laboratory experiments show that CH 4 -ice bands in spectra of CH 4 mixed with other ices are blueshifted relative to bands in spectra of pure CH 4 -ice. A likely candidate for the other component is N 2 -ice because its weak 2.15 μm band and blueshifted CH 4 bands are seen in spectra of Triton and Pluto. Assuming the shift is due to the presence of N 2 , spectra taken on two consecutive nights show no difference in CH 4 /N 2 . In addition, we find no measurable difference in CH 4 /N 2 at different depths into the surface of 2005 FY9.

Equivalence of equations describing trace element distribution during equilibrium partial melting

Abstract The mathematical equivalence of several equations used to calculate the evolution of trace element abundances during equilibrium partial melting is demonstrated.

Lorentz scattering of interplanetary dust

Abstract Charged dust grains in a turbulent magnetic field will see a Lorentz force due to the convection of the solar magnetic field past them at the solar wind velocity. Since the sign of this magnetic field is randomly varying, the direction of the force will be random, and the net effect will be to randomly scatter the orbital elements of these particles. The square roots of the mean square change in semimajor axis, inclination, and eccentricity are determined as a function of the particles original orbital elements. Particles 3 μm in radius and smaller will have their motions strongly perturbed or dominated by Lorentz scattering. This scattering will have an effect comparable to, or greater than, the Poynting-Robertson effect on these particles for time scales comparable to their Poynting-Robertson lifetimes.

26Al and the partial ionization of the solar nebula

Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of26Al will result in strong coupling of the gas to magnetic fields. In the absence of26Al,40K will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.

1998 SM165: A large Kuiper belt object with an irregular shape

The recent discovery of an ancient reservoir of icy bodies at and beyond the orbit of Neptune—the Kuiper belt—has opened a new frontier in astronomy. Measurements of the physical and chemical nature of Kuiper belt objects (KBOs) can constrain our ideas of the processes of planet formation and evolution. Our 1.8-m Vatican Advanced Technology Telescope and charge-coupled device camera observations of the KBO 1998 SM165 indicate its brightness periodically varies by 0.56 magnitudes over a 4-h interval. If we assume a uniform albedo of 0.04, which is typical of values found in the literature for a handful of KBOs, and an “equator-on” aspect, we find 1998 SM165 has axes of length 600 × 360 km. If our assumptions are correct, such dimensions put 1998 SM165 among the largest elongated objects known in our solar system. Perhaps long ago, two nearly spherical KBOs of comparable size coalesced to form a compound object, or perhaps 1998 SM165 is the residual core of a catastrophic fragmentation of a larger precursor.

Apollo Samples and the Geochemical Determination of Basaltic Achondrite Parent Bodies

Our understanding of the origin and evolution of basaltic meteorites has been strongly linked to how we understand the origin and evolution of Moon rocks. At one point, basaltic meteorites were thought to have come from the Moon; when Apollo samples were found to be chemically distinct from the HED and SNC meteorite families, new ideas for origins were demanded. Yet the very techniques that were developed to understand the evolution of lunar basalts have been found most useful when applied to extra-lunar basalts; likewise, perhaps a deeper understanding of basaltic asteroids such as 4 Vesta will eventually provide insights into lunar evolution.