The Composition of Dust in Jupiter-Family Comets as Inferred from Infrared Spectroscopy

We review the composition of Jupiter-family comet dust as inferred from infrared spectroscopy. We find that Jupiter-family comets have 10 micron silicate emission features with fluxes roughly 20-25% over the dust continuum (emission strength 1.20-1.25), similar to the weakest silicate features in Oort Cloud comets. We discuss the grain properties that change the silicate emission feature strength (composition, size, and structure/shape), and emphasize that thermal emission from the comet nucleus can have significant influence on the derived silicate emission strength. Recent evidence suggests that porosity is the dominant parameter, although more observations and models of silicates in Jupiter-family comets are needed to determine if a consistent set of grain parameters can explain their weak silicate emission features. Models of 8 m telescope and Spitzer Space Telescope observations have shown that Jupiter-family comets have crystalline silicates with abundances similar to or less than those found in Oort Cloud comets, although the crystalline silicate mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed with mid-infrared spectroscopy, and we review the evidence for heterogeneous dust properties in the nucleus of comet 9P/Tempel. Models of dust formation, mixing in the solar nebula, and comet formation must be able to explain the observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel, although more work is needed in order to understand to what extent do comets 9P/Tempel and Hale-Bopp represent comets as a whole.Comment: 21 pages, 4 figures, 2 tables. Accepted for publication in Planetary and Space Scienc

Mid-Infrared Spectrophotometric Observations of Fragments B and C of Comet 73P/Schwassmann-Wachmann 3

We present mid-infrared spectra and images from the GEMINI-N (+Michelle) observations of fragments SW3-[B] and SW3-[C] of the ecliptic (Jupiter Family) comet 73P/Schwassmann-Wachmann 3 pre-perihelion. We observed fragment B soon after an outburst event (between 2006 April 16 - 26 UT) and detected crystalline silicates. The mineralogy of both fragments was dominated by amorphous carbon and amorphous pyroxene. The grain size distribution (assuming a Hanner modified power-law) for fragment SW3-[B] has a peak grain radius of a_p ~ 0.5 micron, and for fragment SW3-[C], a_p ~ 0.3 micron; both values larger than the peak grain radius of the size distribution for the dust ejected from ecliptic comet 9P/Tempel 1 during the Deep Impact event (a_p = 0.2 micron. The silicate-to-carbon ratio and the silicate crystalline mass fraction for the submicron to micron-size portion of the grain size distribution on the nucleus of fragment SW3-[B] was 1.341 +0.250 -0.253 and 0.335 +0.089 -0.112, respectively, while on the nucleus of fragment SW3-[C] was 0.671 +0.076 -0.076 and 0.257 +0.039 -0.043, respectively. The similarity in mineralogy and grain properties between the two fragments implies that 73P/Schwassmann-Wachmann 3 is homogeneous in composition. The slight differences in grain size distribution and silicate-to-carbon ratio between the two fragments likely arises because SW3-[B] was actively fragmenting throughout its passage while the activity in SW3-[C] was primarily driven by jets. The lack of diverse mineralogy in the fragments SW3-[B] and SW3-[C] of 73P/Schwassmann-Wachmann 3 along with the relatively larger peak in the coma grain size distribution suggests the parent body of this comet may have formed in a region of the solar nebula with different environmental properties than the natal sites where comet C/1995 O1 (Hale-Bopp) and 9P/Tempel 1 nuclei aggregated.Comment: 31 pages, 5 figure, accepted for publication in A

Impact of Special Collections in JGR Space Physics

Journals occasionally solicit manuscripts for special collections, in which all papers are focused on a particular topic within the journal’s scope. For the Journal of Geophysical Research: Space Physics, there have been 51 special collections from 2005 through 2018, with a total of 1,009 papers out of the 8,881 total papers in the journal over those years (11%). Taken together, the citations to special collection papers, as well as other metrics, are slightly higher than papers not in special collections. Several paper characteristics were examined to assess whether they could explain the higher citation and download values for special collection papers, but they cannot. In addition, indirect methods were conducted for assessing self‐citations as an explanation for the increased citations, but no evidence was found to support this hypothesis. It was found that some paper types, notably Commentaries and Technical Reports, have lower average citations but higher average downloads than Research Articles (the most common type of paper in this journal). This implies that such paper types have a different kind of impact than “regular” science‐result‐focused papers. In addition to having higher average citations and downloads, special collections focus community attention on that particular research topic, providing a deadline for manuscript submissions and a single webpage at which many related papers are listed. It is concluded that special collections are worth the extra community effort in organization, writing, and reviewing these papers.Plain Language SummaryJournals sometimes focus the attention of the research community by having a special collection, even an entire issue, devoted to a single topic. A reasonable question to ask is whether the extra effort of organizing, promoting, and maintaining the special collection is worthwhile. This paper examines paper impact in this journal, the Journal of Geophysical Research Space Physics, separating the special collection papers from those not in special collections. The short answer is, on average, yes, at least based on the metric of citations. Some characteristics of the paper were also assessed, such as the use of a colon in the title, the average author team size, the average number of references in each paper, and the paper type of the articles. None of these factors explains the higher average citations and downloads for papers in special collections. In this analysis, though, it was found that several paper types have lower‐than‐average citations but higher‐than‐average downloads, including Commentaries (personal perspectives articles) and Technical Reports (describing new methods or data sets). This implies that such papers are being read but perhaps not heavily referenced (yet). The overall conclusion is that special collections are worth the extra work.Key PointsJGR Space Physics published 51 special collections from 2006 to 2018, totaling 1,009 papers out of 8,881Average citations and downloads are slightly higher for papers in special collections compared to those not in collectionsPaper attributes thought to influence citations were analyzed, finding no statistically significant effect for special collection papers compared to othersPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153709/1/jgra55389.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153709/2/jgra55389_am.pd

Spitzer Observations of Comet 67P/Churyumov-Gerasimenko at 5.5-4.3 AU From the Sun

We report Spitzer Space Telescope observations of comet 67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet 67P is the primary target of the European Space Agency's Rosetta mission. The Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances similar to our observations. Rotationally resolved observations at 8 and 24 microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and color-temperature of the nucleus are combined with aphelion R-band light curves observed at the Very Large Telescope (VLT) and yield a mean effective radius of 2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The amplitudes of the R-band and mid-infrared light curves agree, which suggests that the variability is dominated by the shape of the nucleus. We also detect the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We find no evidence for recently ejected dust in our images. If the activity of 67P is consistent from orbit to orbit, then we may expect the Rosetta spacecraft will return images of an inactive or weakly active nucleus as it rendezvous with the comet at rh = 4 AU in 2014.Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the Astronomical Journa

Absorption Efficiencies of Forsterite. I: DDA Explorations in Grain Shape and Size

We compute the absorption efficiency (Qabs) of forsterite using the discrete dipole approximation (DDA) in order to identify and describe what characteristics of crystal grain shape and size are important to the shape, peak location, and relative strength of spectral features in the 8-40 {\mu}m wavelength range. Using the DDSCAT code, we compute Qabs for non-spherical polyhedral grain shapes with a_eff = 0.1 {\mu}m. The shape characteristics identified are: 1) elongation/reduction along one of three crystallographic axes; 2) asymmetry, such that all three crystallographic axes are of different lengths; and 3) the presence of crystalline faces that are not parallel to a specific crystallographic axis, e.g., non-rectangular prisms and (di)pyramids. Elongation/reduction dominates the locations and shapes of spectral features near 10, 11, 16, 23.5, 27, and 33.5 {\mu}m, while asymmetry and tips are secondary shape effects. Increasing grain sizes (0.1-1.0 {\mu}m) shifts the 10, 11 {\mu}m features systematically towards longer wavelengths and relative to the 11 {\mu}m feature increases the strengths and slightly broadens the longer wavelength features. Seven spectral shape classes are established for crystallographic a-, b-, and c-axes and include columnar and platelet shapes plus non-elongated or equant grain shapes. The spectral shape classes and the effects of grain size have practical application in identifying or excluding columnar, platelet or equant forsterite grain shapes in astrophysical environs. Identification of the shape characteristics of forsterite from 8-40 {\mu}m spectra provides a potential means to probe the temperatures at which forsterite formed.Comment: 55 pages, 15 figure

Provenance of north Gondwana Cambrian-Ordovician sandstone: U-Pb SHRIMP dating of detrital zircons from Israel and Jordan

A vast sequence of quartz-rich sandstone was deposited over North Africa and Arabia during Early Palaeozoic times, in the aftermath of Neoproterozoic Pan-African orogeny and the amalgamation of Gondwana. This rock sequence forms a relatively thin sheet (1–3 km thick) that was transported over a very gentle slope and deposited over a huge area. The sense of transport indicates unroofing of Gondwana terranes but the exact provenance of the siliciclastic deposit remains unclear. Detrital zircons from Cambrian arkoses that immediately overlie the Neoproterozoic Arabian–Nubian Shield in Israel and Jordan yielded Neoproterozoic U–Pb ages (900–530 Ma), suggesting derivation from a proximal source such as the Arabian–Nubian Shield. A minor fraction of earliest Neoproterozoic and older age zircons was also detected. Upward in the section, the proportion of old zircons increases and reaches a maximum (40%) in the Ordovician strata of Jordan. The major earliest Neoproterozoic and older age groups detected are 0.95–1.1, 1.8–1.9 and 2.65–2.7 Ga, among which the 0.95–1.1 Ga group is ubiquitous and makes up as much as 27% in the Ordovician of Jordan, indicating it is a prominent component of the detrital zircon age spectra of northeast Gondwana. The pattern of zircon ages obtained in the present work reflects progressive blanketing of the northern Arabian–Nubian Shield by Cambrian–Ordovician sediments and an increasing contribution from a more distal source, possibly south of the Arabian–Nubian Shield. The significant changes in the zircon age signal reflect many hundreds of kilometres of southward migration of the provenance

Comet Dust: The Diversity of "Primitive" Particles and Implications

Comet dust is primitive and shows significant diversity. Our knowledge of the properties of primitive particles has expanded significantly through microscale investigations of cosmic dust samples ( IDP's(Interplanetary Dust Particles) and AMM's (Antarctic Micrometeorites)) and of comet dust samples (Stardust and Rosetta's COSIMA), as well as through remote sensing (spectroscopy and imaging) via Spitzer and via spacecraft encounters with 103P/Hartley 2 and 67P/Churyumov-Gerasimenko. Microscale investigations show that comet dust and cosmic dust are particles of unequilibrated materials, including aggregates of materials unequilibrated at submicron scales. We call unequilibrated materials "primitive" and we deduce they were incorporated into ice-rich (H2O-, CO2-, and CO-ice) parent bodies that remained cold, i.e., into comets, because of the lack of aqueous or thermal alteration since particle aggregation; yet some Stardust olivines suggest mild thermal metamorphism. Primitive particles exhibit a diverse range of: structure and typology; size and size distribution of constituents; concentration and form of carbonaceous and organic matter; D-, N-, and O- isotopic enhancements over solar; Mg-, Fe-contents of the silicate minerals; the compositions and concentrations of sulfides, and of less abundant mineral species such as chondrules, CAIs and carbonates. The uniformity within a group of samples points to: aerodynamic sorting of particles and/or particle constituents; the inclusion of a limited range of oxygen fugacities; the inclusion or exclusion of chondrules; a selection of organics. The properties of primitive particles imply there were disk processes that resulted in different comets having particular selections of primitive materials. The diversity of primitive particles has implications for the diversity of materials in the protoplanetary disk present at the time and in the region where the comets formed

Dust in Comet C/2007 N3 (Lulin)

We report optical imaging, optical and near-infrared polarimetry, and Spitzer mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations were obtained in R (0.676 micron) at phase angles from 0.44 degrees to 21 degrees with simultaneous observations in H (1.65 micron) at 4.0 degrees, exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows typical negative polarization in the optical as well as a similar negative branch near-infrared wavelengths. The 10 micron silicate feature is only weakly in emission and according to our thermal models, is consistent with emission from a mixture of silicate and carbon material. We argue that large, low-porosity (akin to Ballistic Particle Cluster Aggregates) rather absorbing aggregate dust particles best explain both the polarimetric and the mid-infrared spectral energy distribution.Comment: 18 pages, 9 figures, 3 table