R. Moreno

Ocean-like water in the Jupiter-family comet 103P/Hartley 2

For decades, the source of Earths volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10−4, has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites suggests a dry proto-Earth with subsequent delivery of volatiles by local accretion or impacts of asteroids or comets. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10−4. The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10−4, together with dynamical simulations, led to models in which asteroids were the main source of Earths water, with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10−4. This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System.

Localized sources of water vapour on the dwarf planet (1) Ceres

The ‘snowline’ conventionally divides Solar System objects into dry bodies, ranging out to the main asteroid belt, and icy bodies beyond the belt. Models suggest that some of the icy bodies may have migrated into the asteroid belt. Recent observations indicate the presence of water ice on the surface of some asteroids, with sublimation a potential reason for the dust activity observed on others. Hydrated minerals have been found on the surface of the largest object in the asteroid belt, the dwarf planet (1) Ceres, which is thought to be differentiated into a silicate core with an icy mantle. The presence of water vapour around Ceres was suggested by a marginal detection of the photodissociation product of water, hydroxyl (ref. 12), but could not be confirmed by later, more sensitive observations. Here we report the detection of water vapour around Ceres, with at least 1026 molecules being produced per second, originating from localized sources that seem to be linked to mid-latitude regions on the surface. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks.

Herschel measurements of the D/H and 16 O/ 18 O ratios in water in the Oort-cloud comet C/2009 P1 (Garradd) ⋆

The D/H ratio in cometary water is believed to be an important indicator of the conditions under which icy planetesimals formed and can provide clues to the contribution of comets to the delivery of water and other volatiles to Earth. Available measurements suggest that there is isotopic diversity in the comet population. The Herschel Space Observatory revealed an ocean-like ratio in the Jupiter-family comet 103P/Hartley 2, whereas most values measured in Oort-cloud comets are twice as high as the ocean D/H ratio. We present here a new measurement of the D/H ratio in the water of an Oort-cloud comet. HDO, H_2O, and H_2^(18) lines were observed with high signal-to-noise ratio in comet C/2009 P1 (Garradd) using the Herschel HIFI instrument. Spectral maps of two water lines were obtained to constrain the water excitation. The D/H ratio derived from the measured H_2^(16)O and HDO production rates is (2.06 ± 0.22) × 10^(-4). This result shows that the D/H in the water of Oort-cloud comets is not as high as previously thought, at least for a fraction of the population, hence the paradigm of a single, archetypal D/H ratio for all Oort-cloud comets is no longer tenable. Nevertheless, the value measured in C/2009 P1 (Garradd) is significantly higher than the Earth’s ocean value of 1.558 × 10^(-4). The measured ^(16)O/^(18)O ratio of 523 ± 32 is, however, consistent with the terrestrial value.

Chemical Composition Diversity Among 24 Comets Observed at Radio Wavelengths

We present a comparative study on molecular abundances in comets based on millimetre/submillimetre observations made with the IRAM 30-m, JCMT, CSO and SEST telescopes. This study concerns a sample of 24 comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observed from 1986 to 2001 and 8 molecular species (HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, H2S). HCN was detected in all comets, while at least 2 molecules were detected in 19 comets.

The 1995-2002 Long-Term Monitoring of Comet C/1995 O1 (HALE-BOPP) at Radio Wavelength

The bright comet Hale-Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nancay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to r h = 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r h = 14 AU.

TNOs are Cool: A survey of the trans-Neptunian region III. Thermophysical properties of 90482 Orcus and 136472 Makemake

Context. The goal of the Herschel open time programme “TNOs are Cooll” is to derive the physical and thermal properties for a large sample of Centaurs, and trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects. Aims. Based on observations of two targets we tried (i) to optimise the SPIRE observing technique for faint (close to the background confusion noise), slowly moving targets; (ii) to test different thermal model techniques; (iii) to determine radiometric diameter and albedo values; (iv) to compare with Spitzer results whenever possible. Methods. We obtained SPIRE photometry on two targets and PACS photometry on one of the targets. Results. We present results for the two targets, (90482) Orcus and (136472) Makemake, observed with SPIRE and for one of those targets, Makemake, observed with PACS. We adopt pv = 0.27 and D = 850 km as our best estimate of the albedo and diameter of Orcus using single terrain models. With two-terrain models for Makemake, the bright terrain is fitted by, 0.78 \textless pv \textless 0.90, and the dark terrain 0.02 \textless pv \textless 0.12, giving 1360 \textless D \textless 1480 km. Conclusions. A single terrain model was derived for Orcus through the SPIRE photometry combined with MIPS data. The Makemake data from MIPS, PACS and SPIRE combined are not compatible with a single terrain model, but can be modelled with a two-terrain fit. These science demonstration observations have shown that the scanning technique, which allows us to judge the influence of background structures, has proved to be a good basis for this key programme.

A HERSCHEL STUDY OF D/H IN WATER IN THE JUPITER-FAMILY COMET 45P/HONDA-MRKOS-PAJDUŠÁKOVÁ AND PROSPECTS FOR D/H MEASUREMENTS WITH CCAT

We present Herschel observations of water isotopologues in the atmosphere of the Jupiter-family comet 45P/Honda–Mrkos–Pajdusakova. No HDO emission is detected, with a 3σ upper limit of 2.0 × 10^(−4) for the D/H ratio. This value is consistent with the earlier Herschel measurement in the Jupiter-family comet 103P/Hartley 2. The canonical value of 3 × 10^(−4) measured pre-Herschel in a sample of Oort-cloud comets can be excluded at a 4.5σ level. The observations presented here further confirm that a diversity of D/H ratios exists in the comet population and emphasize the need for additional measurements with future ground-based facilities, such as CCAT, in the post-Herschel era.

Complex organic molecules in comets C/2012 F6 (Lemmon) and C/2013 R1 (Lovejoy): detection of ethylene glycol and formamide ⋆

A spectral survey in the 1 mm wavelength range was undertaken in the long-period comets C/2012 F6 (Lemmon) and C/2013 R1 (Lovejoy) using the 30 m telescope of the Institut de radioastronomie millimetrique (IRAM) in April and November−December 2013. We report the detection of ethylene glycol (CH_2OH)_2 (aGg’ conformer) and formamide (NH_2CHO) in the two comets. The abundances relative to water of ethylene glycol and formamide are 0.2–0.3% and 0.02% in the two comets, similar to the values measured in comet C/1995 O1 (Hale-Bopp). We also report the detection of HCOOH and CH_3CHO in comet C/2013 R1 (Lovejoy), and a search for other complex species (methyl formate, glycolaldehyde).

Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy).

First detections of ethyl alcohol and glycolaldehyde in a comet provide new constraints on solar system chemical complexity. The presence of numerous complex organic molecules (COMs; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. These COMs may be part of the material from which planetesimals and, ultimately, planets formed. Comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. Molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few COMs, such as ethylene glycol and glycine. We report the detection of 21 molecules in comet C/2014 Q2 (Lovejoy), including the first identification of ethyl alcohol (ethanol, C2H5OH) and the simplest monosaccharide sugar glycolaldehyde (CH2OHCHO) in a comet. The abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar-type protostars. Overall, the high abundance of COMs in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk.

First detection of hydrogen isocyanide (HNC) in Titan’s atmosphere

We report on the first identification of hydrogen isocyanide (HNC) in Titans atmosphere, from observations using the HIFI instrument on the HerschelSpace Observatory. An emission line from the HNC J = 6 → 5 rotational transition at 543.897 GHz was measured in Titan on June 14 and December 31, 2010. Radiative transfer modeling indicates that the bulk of HNC is located above 400 km, with a column density in the range (0.6−1.5) × 10 13 cm −2 , but the observations cannot establish its vertical profile. In particular HNC could be restricted to the upper thermosphere (∼1000 km), in which case its local abundance relative to HCN could be as high as ∼0.3. HNC is probably formed mostly at ionospheric levels (950-1150 km) from dissociative recombination of HCNH + and possibly other heavier nitrile ions. Ionospheric loss of HNC occurs by protonation with XH + ions. Additional formation (e.g. from N( 4 S) + 3 CH2) and loss routes (e.g. from isomerization to HCN) in the neutral atmosphere remain to be investigated.