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Register a new userLately Exposed Amorphous Water Ice on Comet 49P/Arend-Rigaux
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Comet 49P/ Arend-Rigaux, thought to be a low activity comet since the 1980s was found to be active in its recent apparitions. Recent analysis of the data obtained from Spitzer observation of the comet in 2006 compared with laboratory spectra has revealed amorphous water ice on the surface. In addition, in 2012 a jet was found to appear during its subsequent perihelion passage as witnessed during an observation carried out on 26th March 2012 using the PRL telescope at Mt. Abu. This confirms recent activity of Comet 49P/Arend-Rigaux due to the volatile subsurface materials exposed after several passages close to the Sun. Our result confirms the subsurface ices on cometary nuclei and insists for more observations for a better understanding.
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Comet 49P/Arend-Rigaux is a well known low-activity Jupiter Family comet. Despite the low activity, we have witnessed outgassing activity in 1992, 2004, and 2012. In 2012 a broad tail-like feature (PA$sim270^circ, sim2.3times10^5$ km) and a narrow jet-like feature (PA$sim180^circ, sim9.3times10^4$ km) were seen simultaneously. Using Finson-Probstein (FP) dust dynamical models we determine: grain sizes released in each event; duration of activity; when activity peaked; and velocity of the dust particles, allowing us to make comparisons between the events. We find that the tail feature in 2012 is similar to the tail in 1992 with large grains (40-4000 $mu$m) peaking in activity near perihelion with a long outgassing duration greater than 150 days. The jet feature from 2012, however, is more similar to the 2004 event which we model with small grains (1-8 $mu$m) with a short duration of activity ($sim$1 month). The main difference between these two features is that the 2004 event occurs prior to perihelion, while the 2012 event is post-perihelion. We use the grain sizes from the FP models to constrain ice sublimation models. Between 1985 and 2018 we cover 6 apparitions with 26 nights of our own observations plus data from the literature and the Minor Planet Center, which together, allow us to model the heliocentric light curve. We find that the models are consistent with H$_2$O ice sublimation as the volatile responsible for driving activity over most of the active phases and a combination of H$_2$O and CO$_2$ ices are responsible for driving activity near perihelion. We measure the fractional active area over time for H$_2$O and discover that the activity decreases from an average active area of $sim3%$ to $sim0.2%$. This secular decrease in activity implies that the comet is becoming depleted of volatiles and is in the process of transitioning to a dormant or dead state.
Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comets nucleus. The aim of this work is to search for the presence of H$_2$O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H$_2$O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination conditions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotometrically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 micron absorption band of water ice in the VIRTIS spectral cubes. Out of the 13 selected bright spots, eight of them present positive H$_2$O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H$_2$O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H$_2$O ice and dark terrain, using Hapkes radiative transfer modeling. We also present a detailed analysis of the detected spots.
Water ice-rich patches have been detected on the surface of comet 67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board the Rosetta spacecraft, since the orbital insertion in late August 2014. Among those, three icy patches have been selected, and VIRTIS data are used to analyse their properties and their temporal evolution while the comet was moving towards the Sun. We performed an extensive analysis of the spectral parameters, and we applied the Hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and dark terrains on the three selected water ice rich areas. Study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated to a bimodal distribution of the grains: ~50 {mu}m sized and ~2000 {mu}m sized. In all three cases, after the first detections at about 3.5 AU heliocentric distance, the spatial extension and intensity of the water ice spectral features increased, it reached a maximum after 60-100 days at about 3.0 AU, and was followed by an approximately equally timed decrease and disappearanceat about ~2.2 AU, before perihelion. The behaviour of the analysed patches can be assimilated to a seasonal cycle. In addition we found evidence of short-term variability associated to a diurnal water cycle. The similar lifecycle of the three icy regions indicates that water ice is uniformly distributed in the subsurface layers, and no large water ice reservoirs are present.
Hyperactive comets have high water production rates, with inferred sublimation areas of order the surface area of the nucleus. Comets 46P/Wirtanen and 103P/Hartley 2 are two examples of this cometary class. Based on observations of comet Hartley 2 by the Deep Impact spacecraft, hyperactivity appears to be caused by the ejection of water-ice grains and/or water-ice rich chunks of nucleus into the coma. These materials increase the sublimating surface area, and yield high water production rates. The historic close approach of comet Wirtanen to Earth in 2018 afforded an opportunity to test Hartley 2 style hyperactivity in a second Jupiter-family comet. We present high spatial resolution, near-infrared spectroscopy of the inner coma of Wirtanen. No evidence for the 1.5- or 2.0-$mu$m water-ice absorption bands is found in six 0.8-2.5 $mu$m spectra taken around perihelion and closest approach to Earth. In addition, the strong 3.0-$mu$m water-ice absorption band is absent in a 2.0-5.3 $mu$m spectrum taken near perihelion. Using spectroscopic and sublimation lifetime models we set constraints on the physical properties of the ice grains in the coma, assuming they are responsible for the comets hyperactivity. We rule out pure water-ice grains of any size, given their long lifetime. Instead, the hyperactivity of the nucleus and lack of water-ice absorption features in our spectra can be explained either by icy grains on the order of 1 $mu$m in size with a small amount of low albedo dust (greater than 0.5% by volume), or large chunks containing significant amounts of water ice.
Nereid, Neptunes third largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on spectral characterization of Nereid using its reflectance spectrum from 0.8-2.4 $mu m$, providing the first measurements over the range of 0.8-1.4 $mu m$. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 $mu m$ when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for sub-micron sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of Tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. (2019). Comparison to spectra of large TNOs and satellites of Uranus show that Nereids low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by incomplete understanding of spectral variability among $sim$100km-sized icy bodies.
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