44 research outputs found
Water Ice and Dust in the Innermost Coma of Comet 103P/Hartley 2
On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI)
successfully encountered comet 103P/Hartley 2, when it was at a heliocentric
distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were
acquired in the 1.05-4.83 micron wavelength range using the HRI-IR
spectrometer. We present spectral maps of the inner ~10 kilometers of the coma
collected 7 minutes and 23 minutes after closest approach. The extracted
reflectance spectra include well-defined absorption bands near 1.5, 2.0, and
3.0 micron consistent in position, bandwidth, and shape with the presence of
water ice grains. Using Hapke's radiative transfer model, we characterize the
type of mixing (areal vs. intimate), relative abundance, grain size, and
spatial distribution of water ice and refractories. Our modeling suggests that
the dust, which dominates the innermost coma of Hartley 2 and is at a
temperature of 300K, is thermally and physically decoupled from the
fine-grained water ice particles, which are on the order of 1 micron in size.
The strong correlation between the water ice, dust, and CO2 spatial
distribution supports the concept that CO2 gas drags the water ice and dust
grains from the nucleus. Once in the coma, the water ice begins subliming while
the dust is in a constant outflow. The derived water ice scale-length is
compatible with the lifetimes expected for 1-micron pure water ice grains at 1
AU, if velocities are near 0.5 m/s. Such velocities, about three order of
magnitudes lower than the expansion velocities expected for isolated 1-micron
water ice particles [Hanner, 1981; Whipple, 1951], suggest that the observed
water ice grains are likely aggregates.Comment: 51 pages, 12 figures, accepted for publication in Icaru
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Uncorrelated Volatile Behavior during the 2011 Apparition of Comet C/2009 P1 Garradd
The High Resolution Instrument Infrared Spectrometer (HRI-IR) on board the Deep Impact Flyby spacecraft detected H2O, CO2, and CO in the coma of the dynamically young Oort Cloud comet C/2009 P1 (Garradd) post-perihelion at a heliocentric distance of 2 AU. Production rates were derived for the parent volatiles, Q_(H2O) = 4.6 ± 0.8 × 10^(28), Q_(CO2) = 3.9 ± 0.7 × 10^(27), and Q_(CO) = 2.9 ± 0.8 × 10^(28) molecules s^(–1), and are consistent with the trends seen by other observers and within the error bars of measurements acquired during a similar time period. When compiled with other observations of Garradd's dominant volatiles, unexpected behavior was seen in the release of CO. Garradd's H_2O outgassing, increasing and peaking pre-perihelion and then steadily decreasing, is more typical than that of CO, which monotonically increased throughout the entire apparition. Due to the temporal asymmetry in volatile release, Garradd exhibited the highest CO to H_2O abundance ratio ever observed for any comet inside the water snow line at ~60% during the HRI-IR observations. Also, the HRI-IR made the only direct measurement of CO_2, giving a typical cometary abundance ratio of CO_2 to H_2O of 8% but, with only one measurement, no sense of how it varied with orbital position
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru