Collisions, Cosmic Radiation and the Colors of the Trojan Asteroids

The Trojan asteroids orbit about the Lagrangian points of Jupiter and the residence times about their present location are very long for most of them. If these bodies originated in the outer Solar System, they should be mainly composed of water ice, but, in contrast with comets, all the volatiles close to the surface would have been lost long ago. Irrespective of the rotation period, and hence the surface temperature and ice sublimation rate, a dust layer exists always on the surface. We show that the timescale for resurfacing the entire surface of the Trojan asteroids is similar to that of the flattening of the red spectrum of the new dust by solar-proton irradiation. This, if the cut-off radius of the size distribution of the impacting objects is between 1mm and 1m and its slope is -3, for the entire size-range. Therefore, the surfaces of most Trojan asteroids should be composed mainly of unirradiated dust.Comment: In press in Icaru

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.Comment: To appear in Space Science Reviews. The final publication is available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-

Activity of comets at large heliocentric distances pre-perihelion

We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous-crystalline ice phase transition (T � 137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive. © 2009 Elsevier Inc

Observational results for eight long-period comets observed far from the Sun

Context. With this work we start a systematic analysis of the distant activity of several long-period comets in order to investigate the evolution of activity throughout the solar system and explore differences between comets that pass their perihelion at far or very close distances from the Sun. Aims. We present observational data for eight long-period comets, observed for the first time beyond r = 5 AU. Three targets have been characterised on their inward orbital branch. The others have passed their perihelion at quite large heliocentric distances (rq from 4.5 to 7.5 AU). Methods. We analyse multicolour broadband images (V,R, and I filters) taken at the Telescopio Nazionale Galileo to characterise the dust coma of the comets and investigate their morphology, photometry, colours, and dust production. Results. The morphological analysis shows many differences among the sample, from the large twisted structure present in the coma of comet C/2005 L3 to the regular coma envelope of C/2010 R1. The colour of the dust coma of all the comets is redder than the Sun. The Afρ value (measured in a reference aperture of radius ρ = 104 km) ranges from 114 ± 2 (C/2005 S4) to 5091 ± 47 (C/2005 L3) cm, depicting a scenario of bodies from moderately to very active. This is confirmed by the first-order quantitative estimate of the dust mass-loss rate for the comets that was obtained from the photometric data: assuming a grain velocity of v = 20 m/s, the dust production rate is comparable with, or even significantly larger than, that measured for many short-period (“old”) comets at much smaller heliocentric distances

Studies of binary layered CH3OH/H2O ices adsorbed on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate binary layered ice systems consisting of methanol deposited on top of various thicknesses of water ice, grown on an underlying graphite surface. RAIRS shows that there is no difference between the RAIR spectra recorded for pure ices and that recorded for the layered ices, suggesting that there is no discernible interaction between the layers during adsorption. However, annealing the ice layers leads to intermixing, as evidenced by a 50 cm-1 downshift of the O-H stretching frequency of the mixed ice, compared to that expected from a simple combination of the pure water and pure methanol O-H stretching bands. TPD shows several new species when compared with TPD spectra for pure ices, confirming that mixing of the ice layers occurs on heating. A TPD peak that can be assigned to the trapping of methanol within the water ice is observed. TPD peaks for the desorption of monolayer and multilayer methanol are also observed

Chemical processing in the coma as the source of cometary HNC

The discovery of hydrogen isocyanide (HNC) in comet Hyakutake with an abundance (relative to hydrogen cyanide, HCN) similar to that seen in dense interstellar clouds raised the possibility that these molecules might be surviving interstellar material1. The preservation of material from the Sun\u27s parent molecular cloud would provide important constraints on the processes that took place in the protostellar nebula. But another possibility is that HNC is produced by photochemical processes in the coma, which means that its abundance could not be used as a direct constraint on conditions in the early Solar System. Here we show that the HNC/HCN ratio determined for comet Hale–Bopp varied with heliocentric distance in a way that matches the predictions of models of gas-phase chemical production of HNC in the coma, but cannot be explained if the HNC molecules were coming from the comet\u27s nucleus. We conclude that HNC forms mainly by chemical reactions in the coma, and that such reactions need to be considered when attempting to deduce the composition of the nucleus from observations of the coma