Keith Grogan

The size-frequency distribution of the zodiacal cloud: evidence from the solar system dust bands

Recent observations of the size–frequency distribution of zodiacal cloud particles obtained from the cratering record on the LDEF satellite are the latest evidence for a significant large particle population (100-μm diameter or greater) near 1 AU. Our previous modeling of the Solar System dust bands, features of the zodiacal cloud associated with the comminution of Hirayama family asteroids, has been limited by the fact that only small particles (25-μm diameter or smaller) have been considered. This was due to the prohibitively large amount of computing power required to numerically analyze the dynamics of larger particles. The recent availability of inexpensive, fast processors has finally made this work possible. Models of the dust bands are created, built from individual dust particle orbits, taking into account a size–frequency distribution of the material and the dynamical history of the constituent particles. These models are able to match both the shapes and amplitudes of the dust band structures observed by IRAS in multiple wavebands. The size–frequency index, q, that best matches the observations is approximately 1.4, a distribution in which the surface area (and hence the infrared emission) is dominated by large particles. However, in order to successfully model the “ten degree” band, which is usually associated with collisional activity within the Eos family, we find that the mean proper inclination of the dust particle orbits has to be approximately 9.35°, significantly different from the mean proper inclination of the Eos family (10.08°).

SPITZER MID-IR SPECTRA OF DUST DEBRIS AROUND A AND LATE B TYPE STARS: ASTEROID BELT ANALOGS AND POWER-LAW DUST DISTRIBUTIONS

Using the Spitzer/Infrared Spectrograph (IRS) low-resolution modules covering wavelengths from 5 to 35 μm, we observed 52 main-sequence A and late B type stars previously seen using Spitzer/Multiband Imaging Photometer (MIPS) to have excess infrared emission at 24 μm above that expected from the stellar photosphere. The mid-IR excess is confirmed in all cases but two. While prominent spectral features are not evident in any of the spectra, we observed a striking diversity in the overall shape of the spectral energy distributions. Most of the IRS excess spectra are consistent with single-temperature blackbody emission, suggestive of dust located at a single orbital radius—a narrow ring. Assuming the excess emission originates from a population of large blackbody grains, dust temperatures range from 70 to 324 K, with a median of 190 K corresponding to a distance of 10 AU. Thirteen stars however, have dust emission that follows a power-law distribution, F_ν = F 0λ^α, with exponent α ranging from 1.0 to 2.9. The warm dust in these systems must span a greater range of orbital locations—an extended disk. All of the stars have also been observed with Spitzer/MIPS at 70 μm, with 27 of the 50 excess sources detected (signal-to-noise ratio > 3). Most 70 μm fluxes are suggestive of a cooler, Kuiper Belt-like component that may be completely independent of the asteroid belt-like warm emission detected at the IRS wavelengths. Fourteen of 37 sources with blackbody-like fits are detected at 70 μm. The 13 objects with IRS excess emission fit by a power-law disk model, however, are all detected at 70 μm (four above, three on, and six below the extrapolated power law), suggesting that the mid-IR IRS emission and far-IR 70 μm emission may be related for these sources. Overall, the observed blackbody and power-law thermal profiles reveal debris distributed in a wide variety of radial structures that do not appear to be correlated with spectral type or stellar age. An additional 43 fainter A and late B type stars without 70 μm photometry were also observed with Spitzer/IRS; results are summarized in Appendix B.

Orbital Evolution of Interplanetary Dust

The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical features include the offset of th e center of symmetry of th e cloud from the Sun, the radial gradient of the ecliptic polar brightness at th e Earth, and th e warp of th e cloud. The dust bands provide th e st rongest evidence th at a substantial and possibly dominant fraction of the cloud originate s from aster oids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of th ese large particles towards th e inner Solar System due to Poynting Robert son light drag and their slow passage through secular resonances at the inner edge of the asteroid belt result s in large increases in th eir eccent ricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and comet ary type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between ast eroidal and cometary particles on dynamical grounds alone.

RESONANT STRUCTURE IN THE KUIPER DISK: AN ASYMMETRIC PLUTINO DISK

In order to develop a dynamical model of the Kuiper disk, we run numerical integrations of particles originating from source bodies trapped in the 3 : 2 external mean motion resonance with Neptune to determine what percentage of particles remain in the resonance for a variety of particle and source body sizes. The dynamical evolution of the particles is followed from source to sink with Poynting-Robertson light drag, solar wind drag, radiation pressure, the Lorentz force, neutral interstellar gas drag, and the effects of planetary gravitational perturbations included. We find that the number of particles in the 3 : 2 resonance increases with decreasing � (i.e., increasing particle size) for the cases in which the initial source bodies are small (� 10 km in diameter) and that the percentage of particles in resonance is not significantly changed by either the addition of the Lorentz force, as long as the potential of the particles is small (� 5 V), or the effect of neutral interstellar gas drag. The brightness of the entire Kuiper disk is calculated using a model composed of 500 lm diameter particles and fits well with upper limits to the Kuiper disk brightness and previous estimates. A disk with a size-frequency distribution weighted toward large particles, which are more likely to remain in resonance, may have a stronger, more easily identifiable resonant signature than a disk composed of small particles.

Sources and Orbital Evolution of Interplanetary Dust Accreted by Earth

We review observational and theoretical constraints on the relative contributions of asteroids and comets to interplanetary dust particles (IDPs) in the zodiacal cloud. The estimated contributions span a broad range but the most abundant unambiguous sources are asteroid families, the progenitors of the observed zodiacal dust bands. Other features of the zodiacal cloud indicate additional contributions from non-family asteroids and short-period comets. Numerical modeling of the orbital evolution of IDPs from all these sources reveals natural mechanisms which bias the terrestrial dust accretion rate heavily in favor of asteroidal IDPs, in particular, those originating in the Eos, Themis, and Koronis asteroid families. Over an extended time scale the accretion rate of IDPs from all asteroidal sources should vary by a factor of two to three and display a 100 kyr periodicity that is anti-correlated with Earth’s orbital eccentricity. Extraterrestrial 3He concentrations in deep-sea sediments have a similar periodicity but are 50 kyr out of phase with the predicted variations. Possible expla- nations of this 180° phase lag are discussed.

Scientist-engineering interactions across the project lifecycle

The roles and responsibilities of a Principal Investigator (PI) and/or Project Scientist (PS) and the Project Systems Engineer (PSE) on a NASA mission change across the project lifecycle, as do the nature and focus of their interactions. There are also interactions between the Investigation Scientist (IS) and the Payload System Engineer (PLSE) and Flight System Systems Engineer (FSSE). In order to achieve the best results for the mission, the interactions between the science community and the engineering community need to be rich, focused and frequent. In this paper, the author describes the roles and responsibilities of the PI, PS, IS, PSE, PLSE and FSSE, and the nature of the interactions between them in each phase of the lifecycle. She also describes the primary focus of each role in each phase, and gives specific examples and anecdotes of how solid interactions addressed key issues in a timely, productive manner. The paper concludes with some insights and recommendations for scientist-engineering interactions.