1,983 research outputs found
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
Survey of cometary CO2, CO, and particulate emissions using the Spitzer Space Telescope: Smog check for comets
We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space
Telescope in wide filters centered at 3.6 and 4.5 microns. Emission in the 3.6
micron filter arises from sunlight scattered by dust grains; these images
generally have a coma near the nucleus and a tail in the antisolar direction
due to dust grains swept back by solar radiation pressure. The 4.5 micron
filter contains the same dust grains, as well as strong emission lines from CO2
and CO gas; these show distinct morphologies, in which cases we infer they are
dominated by gas. Based on the ratio of 4.5 to 3.6 micron brightness, we
classify the survey comets as CO2+CO "rich" and "poor." This classification is
correlated with previous classifications by A'Hearn based on carbon-chain
molecule abundance, in the sense that comets classified as "depleted" in
carbon-chain molecules are also "poor" in CO2+CO. The gas emission in the IRAC
4.5 micron images is characterized by a smooth morphology, typically a fan in
the sunward hemisphere with a radial profile that varies approximately as the
inverse of projected distance from the nucleus, as would apply for constant
production and free expansion. There are very significant radial and azimuthal
enhancements in many of the comets, and these are often distinct between the
gas and dust, indicating that ejection of solid material may be driven either
by H2O or CO2. Notable features in the images include the following. There is a
prominent loop of gas emission from 103P/Hartley 2, possible due to an outburst
of CO2 before the Spitzer image. Prominent, double jets are present in the
image of 88P/Howell. A prominent single jet is evident for 3 comets. Spirals
are apparent in 29P and C/2006 W3; we measure a rotation rate of 21 hr for the
latter comet. Arcs (possibly parts of a spiral) are apparent in the images of
10P/Tempel 2, and 2P/Encke.Comment: accepted for publication in Icaru
A survey of debris trails from short-period comets
We observed 34 comets using the 24 micron camera on the Spitzer Space
Telescope. Each image contains the nucleus and covers at least 10^6 km of each
comet's orbit. Debris trails due to mm-sized or larger particles were found
along the orbits of 27 comets; 4 comets had small-particle dust tails and a
viewing geometry that made debris trails impossible to distinguish; and only 3
had no debris trail despite favorable observing conditions. There are now 30
Jupiter-family comets with known debris trails, of which 22 are reported in
this paper for the first time. The detection rate is >80%, indicating that
debris trails are a generic feature of short-period comets. By comparison to
orbital calculations for particles of a range of sizes ejected over 2 yr prior
to observation, we find that particles comprising 4 debris trails are typically
mm-sized while the remainder of the debris trails require particles larger than
this. The lower-limit masses of the debris trails are typically 10^11 g, and
the median mass loss rate is 2 kg/s. The mass-loss rate in trail particles is
comparable to that inferred from OH production rates and larger than that
inferred from visible-light scattering in comae.Comment: accepted by Icarus; figures compressed for astro-p
1I/2017 U1 (`Oumuamua) is Hot: Imaging, Spectroscopy and Search of Meteor Activity
1I/2017 U1 (`Oumuamua), a recently discovered asteroid in a hyperbolic orbit,
is likely the first macroscopic object of extrasolar origin identified in the
solar system. Here, we present imaging and spectroscopic observations of
\textquoteleft Oumuamua using the Palomar Hale Telescope as well as a search of
meteor activity potentially linked to this object using the Canadian Meteor
Orbit Radar. We find that \textquoteleft Oumuamua exhibits a moderate spectral
gradient of , a value significantly lower
than that of outer solar system bodies, indicative of a formation and/or
previous residence in a warmer environment. Imaging observation and spectral
line analysis show no evidence that \textquoteleft Oumuamua is presently
active. Negative meteor observation is as expected, since ejection driven by
sublimation of commonly-known cometary species such as CO requires an extreme
ejection speed of m s at au in order to reach the
Earth. No obvious candidate stars are proposed as the point of origin for
\textquoteleft Oumuamua. Given a mean free path of ly in the solar
neighborhood, \textquoteleft Oumuamua has likely spent a very long time in the
interstellar space before encountering the solar system.Comment: ApJL in pres
Near-UV OH Prompt Emission in the Innermost Coma of 103P/Hartley 2
The Deep Impact spacecraft fly-by of comet 103P/Hartley 2 occurred on 2010
November 4, one week after perihelion with a closest approach (CA) distance of
about 700 km. We used narrowband images obtained by the Medium Resolution
Imager (MRI) onboard the spacecraft to study the gas and dust in the innermost
coma. We derived an overall dust reddening of 15\%/100 nm between 345 and 749
nm and identified a blue enhancement in the dust coma in the sunward direction
within 5 km from the nucleus, which we interpret as a localized enrichment in
water ice. OH column density maps show an anti-sunward enhancement throughout
the encounter except for the highest resolution images, acquired at CA, where a
radial jet becomes visible in the innermost coma, extending up to 12 km from
the nucleus. The OH distribution in the inner coma is very different from that
expected for a fragment species. Instead, it correlates well with the water
vapor map derived by the HRI-IR instrument onboard Deep Impact
\citep{AHearn2011}. Radial profiles of the OH column density and derived water
production rates show an excess of OH emission during CA that cannot be
explained with pure fluorescence. We attribute this excess to a prompt emission
process where photodissociation of HO directly produces excited
OH*() radicals. Our observations provide the first direct
imaging of Near-UV prompt emission of OH. We therefore suggest the use of a
dedicated filter centered at 318.8 nm to directly trace the water in the coma
of comets.Comment: 21 page
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