Cometary Spectra Research Articles

Optical spectroscopy of comets using Hanle Echelle Spectrograph (HESP)

ABSTRACT Observing the vibrational/rotational lines in a comet’s optical spectrum requires high-resolution spectroscopy, as they are otherwise seen as a blended feature. To achieve this, we have obtained medium and high-resolution (R (λ/Δλ) = 30 000 and 60 000) spectra of several comets, including C/2015 V2 (Johnson), 46P/Wirtanen, 41P/Tuttle–Giacobini–Kresák, and 38P/Stephan–Oterma, using the Hanle Echelle Spectrograph (HESP) mounted on the 2-m Himalayan Chandra Telescope (HCT) in India. The spectra effectively cover the wavelength range 3700–10 000 Å, allowing us to probe the various vibrational bands and band sequences to identify the rotational lines in the cometary molecular emission. We were also able to separate the cometary Oxygen lines from the telluric lines and analyse the green-to-red (G/R) forbidden oxygen [O i] ratios in a few comets. For comets C/2015 V2, 46P, and 41P, the computed G/R ratios, 0.04 ± 0.01, 0.04 ± 0.01, and 0.08 ± 0.02, respectively, point to H2O being a major source of Oxygen emissions. Notably, in the second fibre pointing at a location 1000 km away from the photocentre of comet 46P, the G/R ratio reduced by more than half the value observed in the first fibre, indicating the effects of quenching within the inner coma. We also measured the NH2 ortho-to-para ratio of comet 46P to be about 3.41 ± 0.05 and derived an ammonia ratio of 1.21 ± 0.03 corresponding to a spin temperature of ∼26 K. With these, we present the results of the study of four comets from different cometary reservoirs using medium and high-resolution optical spectroscopy, emphasizing the capabilities of the instrument for future cometary studies.

Dust and Volatiles in the Disintegrating Comet C/2019 Y4 (ATLAS)

C/2019 Y4 (ATLAS) is an Oort cloud comet with an orbital period of ∼5895 yr. Starting in 2020 March, its nucleus underwent disintegration. In order to investigate the gas and dust properties of C/2019 Y4 (ATLAS) during its disintegration, we obtained long-slit spectra at 3600–8700 Å and BVRI multiband images with the Xinglong 2.16 m Telescope in 2020 April. Our observations revealed that C/2019 Y4 (ATLAS) exhibited strong emission bands of CN, C2, C3, and NH2, which are superimposed on a dust-scattering continuum, typical of cometary spectra in the optical. The production rates of CN, C2, and C3 derived using the Haser model and the corresponding C2/CN and C3/CN ratios suggest that C/2019 Y4 (ATLAS) is a “typical” Oort cloud comet under the A’Hearn classification, although it appears less dusty, as revealed by the Af ρ quantities. Its dust-scattering reflectivity is slightly red, with a gradient of ∼5% per 103Å. We model the reflectivity gradient in terms of porous dust and find that the red color is accounted for by porous dust.

Pre-perihelion observations of long-period comet C/2013 X1 (PANSTARRS)

ABSTRACT We present and analyse the results of quasi-simultaneous spectroscopic, photometric, and polarimetric observations of hyperbolic comet C/2013 X1 (PANSTARRS) obtained at the 6-m Big Telescope Alt-azimuth (BTA) telescope Special Astrophysical Observatory (SAO) and 2.6-m Shajn telescope Crimean Astrophysical Observatory (CrAO). A wide fan-shaped structure and a weak tail were detected in the comet. The mean V − R colour of the coma was estimated to be neutral compared to the solar value. The Afρ parameter, a proxy to the dust production in the comet, was about 1672 ± 21 cm in the R band. Emissions of the CN, C2, C3, and NH2 molecules were identified in the cometary spectrum, which covers the wavelength range 3800 – 7100 Å. When the comet was at a distance of 2.66 au from the Sun, the minimum degree of polarization of about −1.5 per cent was detected in the near-nucleus region of the coma, in the range up to about 10 000 km from the optocentre. Further, polarization gradually increased (in absolute value) with distance from the nucleus, reaching −6.5 per cent at about 50 000 km. To reproduce the observed values of linear polarization and the phase-angle dependence of polarization for long-period comets, we used the Sh-matrix method with conjugated Gaussian random particles as light scatters, and the chemical composition of dust particles in the coma of 74 per cent amorphous carbon, 25 per cent of Mg-rich silicates, and 1 per cent of water ice.

An updated fluorescence emission model of CO+ for cometary science

ABSTRACT A new CO$^+$ fluorescence emission model for analysing cometary spectra is presented herein. Accurate line lists are produced using the PGOPHER software for all transitions between the three electronic states (X $^2\Sigma$, A $^2\Pi$, B $^2\Sigma$) with vibrational states up to $v_\textrm {max} = 9, 8, 6$, respectively, and maximum rotational states with rotational quantum numbers $N\le 20$. As a result of improved molecular constants and theoretical transition rates, an expansion of the utilized solar spectrum into the infrared, and the substantial expansion of the included rovibronic states, the model provides an update of the fluorescence efficiencies of the CO$^+$ cation. The dependencies on heliocentric velocity and distance are explicitly included. We report, for the first time, quantification of the fluorescence efficiencies for the ground state rovibrational transitions of CO$^+$ and predict the positions and relative intensities of CO$^+$ lines in windows accessible to both ground- and space-based observatories. The computed fluorescence efficiencies show excellent agreement with UV/optical observations of both C/2016 R2 (PanSTARRS) and 29P/Schwassmann-Wachmann 1. The updated fluorescence efficiencies allow for revised N$_2$/CO abundances for comets 1P/Halley, C/1987 P1 (Bradfield), and C/2016 R2 (PanSTARRS), which can change by up to 30 per cent when accounting for recent improvements to CO$^+$ and N$_2^+$ fluorescence efficiencies. The model code, input files, and fluorescence efficiencies are publicly available and distributed on permanent archives for future uses in cometary analyses.

Optical Imaging Polarimetry of Comet 21P/Giacobini–Zinner during Its 2018 Apparition

We conducted polarimetric observations of comet 21P/Giacobini–Zinner in the optical wavelength region using the polarimetric imager PICO equipped with the 50 cm telescope for Public Outreach at the Mitaka Campus of the National Astronomical Observatory of Japan on UT 2018 September 16, at a solar phase angle of 77° for the comet. We used Gunn’s i′-band filter to cover the wavelength region where sunlight reflected by dust grains dominates cometary spectra. The intensity map of comet 21P/Giacobini–Zinner showed an elongated coma structure along the east–west direction. The linear polarization degree map was almost uniform and showed no evidence of jet or arc structures in the inner coma. The linear polarization degree was 22% in the coma, which is consistent with other optical polarization observations of 21P/Giacobini–Zinner for this and previous apparitions reported in the literature. The measured polarization angles (179°) were nearly perpendicular to the position angle of the scattering plane of 271°. There is no evidence of significant changes in the dust properties (composition, porosity, and size distribution) in the comet from its 1985 to 2018 apparitions. The spectral gradient of the linear polarization degree for the comet was nearly flat in the wavelength range of ∼510–830 nm at a solar phase angle of ∼77° during the 2018 apparition.

CO2 as the main parent source of atomic oxygen in comet C/2017 K2 (Pan-STARRS)

C/2017 K2 (Pan-STARRS) is an Oort cloud comet discovered in May 2017. Ground observations have revealed that this long-period comet was active at heliocentric distance of 35 au. At such a distance, activity cannot be driven by the sublimation or the exothermic crystallization of water ice. We assume that the activity of comet C/2017 K2 may be driven by the sublimation of super-volatile ices such as CO and CO2. The nature of parent molecules driving the unprecedented activity of comet C/2017 K2 can be investigated by analyzing its spectrum. In particular, the analysis of atomic oxygen emission lines and the calculation of the green (5577.339 Å) to red (6300.304 and 6363.776 Å) line intensity ratio (hereafter G/R) can reveal the nature of molecules driving cometary activity. We report on the detection of atomic oxygen lines in the spectra of comet C/2017 K2 (Pan-STARRS) from high-resolution spectra obtained at the Telescopio Nazionale Galileo (TNG) on 24 June and 2 July 2022 using the High Accuracy Radial velocity Planet Searcher North (HARPS-N) echelle spectrograph. We found G/R ratios equal to 0.29±0.02 (24 July 2022) and 0.27±0.01 (2 July 2022), which are consistent with a cometary activity driven by CO2, even if located at 2.8 au from the Sun. To better define the nature of the driving molecule, we measured the width of the three oxygen lines and found that the green line in the spectrum of 24 July 2022 is wider than either of the two red lines. This allows us to argue that CO2 is dissociated by highly energetic solar photons that produce O(1S) with a large excess velocity.

The N2 production rate in comet C/2016 R2 (PanSTARRS)

ABSTRACT Observations of comet C/2016 R2 (PanSTARRS) have revealed exceptionally bright emission bands of N$_2^+$, the strongest ever observed in a comet spectrum. Alternatively, it appears to be poor in CN compared to other comets, and remarkably depleted in H2O. Here, we quantify the N2 production rate from N$_2^+$ emission lines using the Haser model. We derived effective parent and daughter scale lengths for N2 producing N$_2^+$. This is the first direct measurement of such parameters. Using a revised fluorescence efficiency for N$_2^+$, the resulting production rate of molecular nitrogen is inferred to be Q(N2) ∼ 1 × 1028 molecules s−1 on average for 2018 February 11, 12, and 13, the highest for any known comet. Based on a CO production rate of Q(CO) ∼ 1.1 × 1029 molecules s−1, we find Q(N2)/Q(CO) ∼ 0.09, which is consistent with the N$_2^+$/CO+ ratio derived from the observed intensities of N$_2^+$ and CO+ emission lines. We also measure significant variations in this production rate between our three observing nights, with Q(N2) varying by plus or minus 20 per cent according to the average value.

T-matrix and Hapke Modeling of the Thermal Infrared Spectra of Trojan Asteroids and (944) Hidalgo: Implications for Their Regolith Particle Size and Porosity

Trojan asteroids (911) Agamemnon, (1172) Aneas, and (624) Hektor and primitive asteroid (944) Hidalgo share a common thermal infrared spectral feature: a prominent 10 μm plateau that is also present in cometary comae spectra. To fit these asteroid spectra, we modeled individual minerals using the light-scattering multiple sphere T-matrix (MSTM) and Hapke reflectance models. Modeled mineral spectra were then combined using a weighted least-squares (WLS) model that included a spectral library of varied particle sizes and porosities. We later refined our method by using the mineral abundances, particle sizes, and porosities computed by WLS as an input to rerun the MSTM and Hapke models. We were able to model the asteroid spectral features using a mixture of olivine components, fine particles, and lunar-like porosities. The Trojan asteroids and (944) Hidalgo are comparable in mineral composition and particle size to spectrally similar bodies such as comet Hale-Bopp and CO3 and CY chondrite meteorites. While the required porosities for modeling are like those present on the lunar surface, they are lower than those in the meteorites and higher than those in comets.

N2+fluorescence spectrum of comet C/2016 R2 (PanSTARRS)

Context.N2is rarely found in comets, or when it is present, it is found only in small quantities despite its abundance on the surfaces of different outer Solar System objects. A few comets presented N2+emission lines in their optical spectra. One of them, C/2016 R2, showed an unusually high abundance of this species in 2018, with dozens of bright N2+emission lines. A robust model of the N2+fluorescence in comets would permit us to perform a detailed quantitative analysis of this species and enlarge our comprehension of the surprisingly wide range of N2abundances in comets.Aims.The goal of this work is to provide the necessary tools to interpret N2+spectra. Computing production rates for a cometary species necessitates a good knowledge of the number of molecules located along the line of sight of the spectrometer. This in turn requires a good modeling of the emission spectrum with detailed fluorescence efficiencies for the different bands.Methods.We developed a model based on available laboratory data and new theoretical results relative to the N2+to compute the emission spectrum of this species observed in 8.2m Very Large Telescope high-resolution spectra of comet C/2016 R2. Because of some significant differences between spectra obtained on the nucleus and at a cometocentric distance of about 6000 km, it became apparent that a classic fluorescence equilibrium spectrum could not be used. A synthetic spectrum based on a Monte Carlo method and producing spectra at different times from an initial relative population was developed and compared to our observational data.Results.Our modeling of the cometary N2+emission spectrum satisfactorily fits our observed spectra of comet C/2016 R2, leading to the first modeling at high resolution. Different fluorescence efficiency factors are computed.

Accurate Einstein coefficients for electric dipole transitions in the first negative band of N2+

Context. The N2+ fluorescence spectrum of comet C/2016 R2 is modelled in a companion paper. That work relies on accurate Einstein coefficients for electric dipole transitions between the B2∑u+ and X2∑g+ electronic states of N2+. Aims. These coefficients are provided in the present paper. Methods. The potential energy curves and transition dipole moments were computed at a high level of ab initio theory and include relativistic corrections. Rovibrational wavefunctions were then obtained without assuming separability of vibrational and rotational motions. Results. Vibrationally and rotationally resolved Einstein coefficients are presented in a convenient three-parameter functional form for three isotopologues. A possible explanation is given for the large variation in the experimental radiative lifetimes.

Photodissociation Rate, Excess Energy, and Kinetic Total Energy Release for the Photolysis of H2O Producing O(1S) by Solar UV Radiation Field

Forbidden atomic oxygen lines in emission are ubiquitous for cometary spectra in the visible region, and the oxygen atoms in metastable states causing the forbidden emission lines are considered as a proxy of H2O in coma. However, the photodissociation rate and related quantities for the dissociation reaction producing O(1S) from H2O have never been estimated based on experimental studies. Based on the recent laboratory study of the photodissociation reaction of H2O producing O(1S) by Chang et al., we derived the photodissociation rates of the reactions for both the O(1S) and O(1D) channels, consistent with the green-to-red line ratios observed in comets so far. Furthermore, the total kinetic energies released for the photodissociation products are also consistent with the intrinsic line widths of forbidden atomic oxygen emission lines observed in comets. The photodissociation rates of H2O leading to O(1S) and O(1D) calculated here do not significantly change the previous estimates of CO2/H2O in comets based on the green-to-red line ratios of the comets if we use the photodissociation rates of CO2 (calculated elsewhere) with a correction for the difference of solar UV spectra used for calculating photodissociation rates of H2O and CO2.

Ultraviolet Observations of Comet 96/P Machholz at Perihelion

Abstract Ultraviolet spectra of Comet 96/P Machholz were obtained during its 2002 perihelion with the UltraViolet Coronagraph Spectrometer instrument on board the SOHO satellite. Emission from H i, C ii, C iii, and O i is detected near the nucleus. The outgassing rate is in line with the value extrapolated from rates at larger distances from the Sun, and abundances of C and O are estimated. Reconstructed images show a nearly spherical cloud of H i Lyα emission and an ion tail seen in C iii. Radiation pressure on the hydrogen atoms produces a modest distortion of the shape of the Lyα cloud as seen from SOHO and Doppler shifts up to 30 km s−1 in the outer parts of the cloud. We estimate a ratio of C to H2O similar to what is observed in other comets, so low carbon abundance does not account for the anomalously low C2 and C3 ratios to NH2 observed at optical wavelengths.

Physical and dynamical characterization of hyperbolic comet C/2017 U7 (PANSTARRS)

We present here a dynamical and observational study of the comet C/2017 U7 (PANSTARRS). This comet was discovered in 2017 and found to have a hyperbolic orbit. Our dynamical analysis shows that the object has probably originated in the Oort cloud, however an interstellar origin cannot be discarded. The observations were obtained in 2018 and 2019 using the Goodman High Throughput Spectrograph (GHTS) at the SOAR telescope. We obtained visible spectra covering the wavelength range of 0.5−0.9μm and also images in the SDSS filters system. Both the low-resolution reflectance spectrum and the reflectance spectra derived from the SDSS filters show an atypical band at ∼0.595μm. We conducted a comparative study of the colors and reflectance spectra of different small body populations (e.g., comets, Centaurs, and trans-Neptunian objects or TNOs) from the literature and concluded that the spectra and the colors of this comet are atypical, showing only some overlap with those of some known members of the TNOs and Centaurs, within the large uncertainties of the measurements of those populations. It is found that the feature and overall spectral shape can be reproduced by laboratory spectra of kerite, a template for aliphatic-rich hydrocarbons that has been previously identified in NIR cometary spectra absorptions. It is tentatively proposed that the unusual spectral shape is the result of a particle size distribution of dust grains in the coma or on the surface that has arisen due to a low grain ejection velocity from the surface and large nucleus size.

Volatile Abundances, Extended Coma Sources, and Nucleus Ice Associations in Comet C/2014 Q2 (Lovejoy)

Abstract High-resolution infrared spectra of comet C/2014 Q2 Lovejoy were acquired with NIRSPEC at the W. M. Keck Observatory on two post-perihelion dates (UT 2015 February 2 and 3). H2O was measured simultaneously with CO, CH3OH, H2CO, CH4, C2H6, C2H4, C2H2, HCN, and NH3 on both dates, and rotational temperatures, production rates, relative abundances, H2O ortho-to-para ratios, and spatial distributions in the coma were determined. The first detection of C2H4 in a comet from ground-based observations is reported. Abundances relative to H2O for all species were found to be in the typical range compared with values for other comets in the overall population to date. There is evidence of variability in rotational temperatures and production rates on timescales that are small compared with the rotational period of the comet. Spatial distributions of volatiles in the coma suggest complex outgassing behavior. CH3OH, HCN, C2H6, and CH4 spatial distributions in the coma are consistent with direct release from associated ices in the nucleus and are peaked in a more sunward direction compared with co-measured dust. H2O spatial profiles are clearly distinct from these other four species, likely due to a sizable coma contribution from icy grain sublimation. Spatial distributions for C2H2, H2CO, and NH3 suggest substantial contributions from extended coma sources, providing further evidence for distinct origins and associations for these species in comets. CO shows a different spatial distribution compared with other volatiles, consistent with jet activity from discrete nucleus ice sources.

A high-spectral-resolution catalog of emission lines in the visible spectrum of comet C/2020 F3 (NEOWISE)

Aims. Comet C/2020 F3 (NEOWISE) is considered to be the brightest comet observed in the northern hemisphere since the passage of comet C/1995 O1 (Hale-Bopp) in 1997. Since the study of comets offers a unique opportunity to investigate the early stages of the formation and evolution of our Sun and the Solar System, we obtained high-resolution optical spectra (R = λ/Δλ = 11 5000) of comet NEOWISE. The unique passage and its brightness yielded spectra with a large number of emission lines, providing information on the coma composition and the physical and chemical processes occurring in the nucleus. The spectra have been used to generate a catalog of emission lines to be used for future studies of comets since there are no catalogs in the literature with such a high spectral resolution. Methods. Two high-resolution spectra of comet NEOWISE were obtained, on 26 July 2020 (geocentric distance of 0.7 AU) and 5 August 2020 (geocentric distance of 0.89 AU), with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) echelle spectrograph installed on the 360 cm Telescopio Nazionale Galileo. The spectra cover the range between 383 and 693 nm, and have been extracted using the HARPS-N Data Reduction Pipeline. To analyze the spectra and compile the high-resolution catalog, we collected several laboratory molecular line lists that cover the same wavelength range as that of our spectra. To validate the final identification, we compared our catalog with other atlases that resulted from the spectral analysis of other comets. Results. We generate a high-spectral-resolution catalog of emission lines observed in comet NEOWISE, providing the identification for 4488 lines. We found cometary lines due to CN, CH, C2, C3, and NH2 and atomic lines due to NaI and [OI].

Atomic Iron and Nickel in the Coma of C/1996 B2 (Hyakutake): Production Rates, Emission Mechanisms, and Possible Parents

Abstract Two papers recently reported the detection of gaseous nickel and iron in the comae of over 20 comets from observations collected over two decades, including interstellar comet 2I/Borisov. To evaluate the state of the laboratory data in support of these identifications, we reanalyzed archived spectra of comet C/1996 B2 (Hyakutake), one of the nearest and brightest comets of the past century, using a combined experimental and computational approach. We developed a new, many-level fluorescence model that indicates that the fluorescence emissions of Fe I and Ni I vary greatly with heliocentric velocity. Combining this model with laboratory spectra of an Fe-Ni plasma, we identified 22 lines of Fe I and 14 lines of Ni I in the spectrum of Hyakutake. Using Haser models, we estimate the nickel and iron production rates as Q Ni = (2.6–4.1) × 1022 s−1 and Q Fe = (0.4–2.8) × 1023 s−1. From derived column densities, the Ni/Fe abundance ratio log10[Ni/Fe] = −0.15 ± 0.07 deviates significantly from solar abundance ratios, and it is consistent with the ratios observed in solar system comets. Possible production and emission mechanisms are analyzed in the context of existing laboratory measurements. Based on the observed spatial distributions, excellent fluorescence model agreement, and Ni/Fe ratio, our findings support an origin consisting of a short-lived unknown parent followed by fluorescence emission. Our models suggest that the strong heliocentric velocity dependence of the fluorescence efficiencies can provide a meaningful test of the physical process responsible for the Fe I and Ni I emission.

Knowledge Gaps in the Cometary Spectra of Oxygen-bearing Molecular Cations

Abstract Molecular cations are present in various astronomical environments, most notably in cometary atmospheres and tails where sunlight produces exceptionally bright near-UV to visible transitions. Such cations typically have longer-wavelength and brighter electronic emission than their corresponding neutrals. A robust understanding of their near-UV to visible properties would allow these cations to be used as tools for probing the local plasma environments or as tracers of neutral gas in cometary environments. However, full spectral models are not possible for characterization of small, oxygen-containing molecular cations given the body of molecular data currently available. The five simplest such species (H2O+, CO2 +, CO+, OH+, and O 2 + ) are well characterized in some spectral regions but are lacking robust reference data in others. Such knowledge gaps hinder fully quantitative models of cometary spectra, specifically hindering accurate estimates of physical-chemical processes originating with the most common molecules in comets. Herein, the existing spectral data are collected for these molecules, and the places where future work is needed are highlighted, specifically where the lack of such data would greatly enhance the understanding of cometary evolution.

FUV Observations of the Inner Coma of 46P/Wirtanen

Abstract Far-ultraviolet observations of comets yield information about the energetic processes that dissociate the sublimated gases from their primitive surfaces. Understanding which emission processes are dominant, their effects on the observed cometary spectrum, and how to properly invert the spectrum back to the composition of the presumably pristine surface ices of a comet nuclei are all critical components for proper interpretation and analysis of comets. The close approach of comet 46P/Wirtanen in 2018–2019 provided a unique opportunity to study the innermost parts of a cometary coma with the Hubble Space Telescope Cosmic Origins Spectrograph, rarely accessible with remote observations, at length scales (hundreds of kilometers) and wavelengths (900–1430 Å) previously probed only by the European Space Agency’s Rosetta spacecraft. Our observations show a complex picture for the inner coma; atomic production rates for H and O that show water is the dominant source of both, an abundance of atomic sulfur that is difficult to explain with the lifetimes of common sulfur parent molecules, and a density distribution that is poorly fit with both Haser and vectorial models.

Optical observations of near isotropic comet C/2006 OF2 (Broughton) at two different heliocentric distances

Spectra and images of comet C/2006 OF2 (Broughton) were obtained with the 4.2-m William Herschel Telescope (La Palma, Spain) and with the 6-m Big Telescope Alt-azimuth (SAO RAN, Russia) at two observing epochs on October 15, 2007 and December 05, 2008.The comet was at heliocentric distances of 4.24 au and 2.59 au before and after the perihelion, respectively. The emissions CN 0–0, 0–1 (B2Σ+ − X2Σ+), C3 (A1Πu − X1Σg+) bands and C2 0–0 (A3Πg – X3Πu) of the Swan band system were identified in the post-perihelion cometary spectrum. Using the simple Haser model, the gas production rates of CN, C2 and C3 molecules were estimated at level of 1.35 × 1024 mol/s, 0.45 × 1024 mol/s, and 0.12 × 1024 mol/s, respectively. The gas production rate ratios, log(C2/CN) = −0.48 and log(C3/CN) = −1.05, point out that comet C/2006 OF2 probably belongs to the group of “carbon-chain depleted” comets. The ratios between C2, C3 and CN abundances of comet C/2006 OF2 are similar to those recently measured for interstellar comet 2I/Borisov at comparable heliocentric distances. The normalized reflectivity gradients of 12.7 ± 0.2%/1000 Å and − 2.3 ± 0.2%/1000 Å were calculated from the post perihelion spectrum in the intervals 4450 Å – 5500 Å and 5500–6935 Å, respectively, using the spectral regions free from comet emissions. The continuum slope, measured in the interval 4200 Å – 4800 Å, is even steeper, reaching 32.7 ± 0.2%/1000 Å. The normalized reflectivity gradient measured from a pair of the pre-perihelion V and R-images varies between 8%/1000 Å and 11%/1000 Å in the inner part of the coma, which exceeds the value obtained from the similar spectral interval of the post-perihelion spectrum, indicating that the coma became bluer after the perihelion passage. The morphology of the pre-perihelion images of the comet is governed by faint morphological features hidden in the bright coma. The color of the jets isolated from the pre-perihelion images is redder than the color of the ambient coma. The pre- and post-perihelion V-band Afρ parameters were found to be 1990 ± 90 cm and 2500 ± 100 cm for heliocentric distances of 4.24 au and 2.59 au, respectively.

Lyα Observations of Comet C/2013 A1 (Siding Spring) Using MAVEN IUVS Echelle

Abstract The close approach of comet C/2013 A1 (Siding Spring) to Mars in 2014 October provided a unique opportunity to observe a dynamically new Oort cloud comet with potential for interaction with a planet’s atmosphere. The water-originating hydrogen coma of the comet extended to over 20 million km from the nucleus. Determining the properties of this coma contributes to characterizing the comet’s water content and production rate. The present study analyzes a unique data set of high spectral resolution UV observations of comet C/2013 A1 Siding Spring measured by the Mars Atmosphere and Volatile Evolution spacecraft. The Siding Spring observations capture Lyα emissions from the Martian corona, the interplanetary medium, as well as the cometary H and D reservoirs. The isolated cometary spectra are analyzed to reveal a velocity distribution of H atoms that are consistent with model estimates of H2O photodissociated H emissions and of OH photodissociated H emissions, Doppler shifted from the main comet H emission line center by 18 km s−1 and 8 km s−1, respectively. The variations in comet H brightness with distance from the nucleus are used to constrain cometary water production to a rate of 0.5 × 1028 molecules s−1 at a time when Siding Spring was at 1.5 au, pre-perihelion.