Observing Programmes: Solar System

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Guaranteed Time (Key Programmes)


HSSO: Herschel Solar System Observations

HSSO is using all three Herschel instuments to measure the presence of water and related chemistry in the Solar System with a view to establishing the origin of water and its chemical evolution during the formation of the planets, including how the Earth's oceans were formed.  Observations of comets Mars, Jupiter, Saturn, Uranus and Neptune will help establish the origin and role of water in the planets' atmospheres or on their surfaces.  Herschel will also observe two moons of Saturn, Titan and Enceladus.

Click here to go to the HSSO website.

Lead Scientist: Paul Hartogh (Max Planck Institute for Solar System Research)
UK contact: Bruce Swinyard (Rutherford Appleton Laboratory)

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Guaranteed Time (Round 1)


Herschel in-Situ Asteroid and Comet Observation Programme

In September 2010, 25 years will have passed since the first in-situ encounter of a space probe with a comet. Since this time, NEAR, DS1, Hayabusa, Rosetta, Deep Impact, Stardust-NExT and Dawn satellites have all been launched with the aim of performing flybys, orbiting & sample return. This proposal focuses on using the Herschel Space Observatory to remotely observe a select set of comets and asteroids which are being, have been, or will be observed in-situ by space probes. Observations at a distance and in-situ measurements are considered highly complementary in nature: remote sensing shows the global picture, while in-situ techniques measure physical quantities in a more direct way, but are limited in spatial coverage. The benefit of both combined is that we can compare surface composition, reflectance, albedos & temperatures of in-situ and remote sensing and as a result greatly improve the scientific understanding of the objects in question.

Lead Scientist: Laurence O'Rourke (ESAC)

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A last chance to measure the size of the nucleus of Comet Hale-Bopp directly in the IR

Comet C/1995 O1 (or Hale-Bopp to its friends) was one of the three intrinsically brightest comets of the last 600 years. It is generally agreed to be a giant object with a nucleus of diameter in the range 40-80km compared to the 15x8km of 1P/Halley. It is also one of the most intensively studied objects in history and the only giant comet ever to be studied intensively with modern detectors. In June 2010 Hale-Bopp will passed the orbit of Neptune and now finally appears to be inactive. This allowed us to attack the problem of the one great unknown about the comet: the size of the nucleus. Dozens of size estimates have been published, ranging from under 15km to 250km, but an accurate value for the diameter of the nucleus will allow information on the mass and even the internal structure of the nucleus to be determined. The observations, taken with the PACS instrument, should stand for the next 2500 years!

Lead Scientist: Mark Kidger (ESAC)

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Open Time (Key Programmes)

TNOs are Cool: A survey of the Trans-Neptunian Region

Over 1000 objects have been discovered orbitting beyond Neptune, in a region called the "Kuiper Belt".  These "Trans-Neptunian Objects" were formed from the disk of material out of which the major planets formed.  Observing these objects in our own Solar System gives an insight into what to expect when we observe discs of material around other stars like our Sun.  Their low temperatures, small sizes and large distance makes them difficult to see, and their properties were not well known before Herschel.  By observing 141 different objects, this project will determine the physical characteristics of these objects, such as size, surface type, temperature, and even whether they have their own tiny moons.

Lead Scientist: Thomas Muller (MPE, Garching)
UK contact: Tanya Lim (Rutherford Appleton Laboratory)

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Open Time (Round 1)


HCl, HF and H2O+ in comets : probing solar nebula and coma chemistry

The composition of comets provides a record of the chemistry of the primitive solar nebula, in the region and at the time of their formation. The presence of interstellar-like organics and other exotic gases in cometary nuclei gives the definite impression that comets preserve a record of the interstellar composition characteristic of the presolar cloud, or that cometary and interstellar molecules formed by similar processes. We propose to take benefit of the unique capacities of Herschel in the sub-millimeter domain and the high sensitivity of HIFI to search for cometary molecules of cosmogonic interest that cannot be observed from the ground, namely HCl (both (35)Cl and (37)Cl isotopes) and HF. These species are the main reservoirs of fluorine and chlorine in the ISM, as measured from Herschel, and are locked onto grains in dense molecular clouds. Their survival during the collapse of the presolar cloud is uncertain, but both HCl and HF should have reformed readily from the released atomic Cl and F in the proto-planetary disk. We expect that these molecules condensed onto pre-cometary grains during the cooling phase of the Solar Nebula. In addition, observations of H2O+ are proposed to constrain the H2O chemistry in cometary atmospheres and to measure accurately the frequency of its 111-000 and 202-111 ortho transitions near 1115 and 742 GHz, respectively, for best interpretation of H2O+ interstellar data.

Lead Scientist: Dominique Bockelee-Morvan

Allocated time: 25.1 hours

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Detecting the Largest Rings in the Solar System--Dust Rings from the Irregular Satellites

We propose to extend observations of Saturn's newly discovered Phoebe ring (Verbiscer et al. 2009) to new wavelengths and greater radial range to better characterize the dust properties and constrain the dynamics and particle size distribution of the ring. We will seek similar rings around both Uranus and Neptune. This work is important to understanding the transfer of material from the irregular satellites to the inner regular satellites, the outermost of which can have their surfaces completely transformed by this process. Furthermore, this dust represents an important analogue to the dust observed in debris disks around other stars. The discovery of the Phoebe ring, as well as work by Turrini et al. (2009), Bottke et al. (2010), and Buratti et al. (1991), among others, suggests that the other giant planets should also possess rings supplied by their irregular satellites. We therefore propose searches for such rings at both Uranus and Neptune. Our proposal is aimed to return important science results for a modest time investment (12.8 hours)

Lead Scientist: Daniel Tamayo

Allocated time: 12.8 hours

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Probing the Enceladus torus with Herschel

Observations of Saturn with HIFI, performed initially in June 2009 and in more details in June 2010 within the framework of the KP-GT ``Water and related chemistry in the Solar System", have revealed unexpected absorptions in the core of several emission lines of water from Saturn's atmosphere (557 GHz, 987 GHz, 1113 GHz and 1670 GHz). These absorptions cannot occur in Saturn itself; rather we show that they are due to absorption from water in the ``Enceladus torus", i.e. a cloud of material originating from Enceladus' active plumes, spreading around Saturn and forming a broad toroidal structure centered around Enceladus' orbit at 4 Saturn radii. Based on a comet-like fluorescent excitation model, our preliminary analysis of these data indicate line-of-sight water column densities of (1-3)x10^13 cm-2 and a radial extent of about 2.5 Saturn radii. This discovery provides an entirely new method to probe physical conditions (density, structure, and composition) in the Enceladus torus. Here we propose a detailed follow-up on these observations. The goals are (i) to monitor the variation of these absorptions with viewing geometry, taking advantage thatthe change of aspect in the Saturn system (with the satellite and ring system becoming progressively more ``open") over the upcoming years (ii) to search for H2O emission directly originating from the torus (iii) to search for several additional compounds such as NH3 (known to be produced by Enceladus' plumes), OH (seen in the UV from HST) and several ionized species (H3O+, H2O+, OH+, expected from torus ionization). The ensemble of data will hopefully provide us with (i) an improved understanding of the excitation conditions in the torus (e.g. on the role of electrons) (ii) an improved understanding of its composition and chemistry (iii) a detailed 3-dimensional view (radial, vertical and longitudinal) of the torus and the ability to directly test physically-based model torus models.

Lead Scientist: Emmanuel Lellouch

Allocated time: 27.2 hours

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Probing the extremes of the outer Solar System: short-term variability of the largest, the densest and the most distant TNOs from PACS photometry

Pluto was believed to be the outermost object in the Solar System until, at the end of the last Millenium, a number of similar objects were found on orbits beyond Neptune. These objects are now being designated as Trans-Neptunian Objects (TNOs) and are believed to be a reminder of a primordial population. The TNO population comprises a wide range of orbital types and spectral features, many of which are of icy nature. Their origin and evolution are still an open question and subject to debate. We propose repeated high-SNR PACS photometry observations of three extreme specimen of the TNO population: (136199) Eris, the largest dwarf planet, (90377) Sedna, the most distant body in the Solar System and (50000) Quaoar, the densest TNO observed so far. We request a total of 51.7 hours of Herschel observing time using different channels, aiming a SNR of 10 or higher. We intend to perform time-series photometry in order to refine physical properties, which have often been determined using coarse thermal models and single-band data in the past, to find hints of thermal IR lightcurves of Eris and Sedna and to make a high SNR observation of the entire lightcurve of Quaoar. Together with available optical lightcurves we can distinguish whether flux variations are due to shape effects or inhomogeneities of the object's surface. We will make use of existing thermal and thermophysical models to refine physical properties, which may be used to recompute density estimations for Eris and Quaoar and to model surface compositions. Herschel is highly suitable for this task, since it offers the highest sensitivity available to date in the far-IR regime, in which TNO emission peaks due their low surface temperatures. Our results will put constraints on some of the important questions concerning the Trans-Neptunian region and will therefore be of vital importance for observers and modelers in Solar System sciences.

Lead Scientist: Esa Vilenius

Allocated time: 29.2 hours

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Properties of Jupiter Trojans from far-IR spectroscopy and photometry

The two swarms of Jovian Trojan asteroids, which librate around Jupiter's Lagrangian points L4 and L5, occupy an intriguing place between the predominantly rocky inner Solar System and its icy outer regions. Their dynamical and physical properties provide crucial constraints on our understanding of the origin and early evolution of the Solar System. When Spitzer enabled the first spectroscopic observations of these objects at mid-IR wavelengths, the spectral signature of fine-grained silicates was found on a number of bright Trojans (Emery et al., 2006; Mueller et al., 2010). This provided a surprising piece of observational evidence between the hypothesized genetic connection between Trojans and cometary nuclei. Thanks to the spectroscopic capabilities of PACS, a completely unexplored wavelength range is now opening up that contains diagnostic features of elusive materials such as crystalline water, hydrous silicates, or organic irradiation residues. We here propose PACS spectroscopy observations of 4 Jupiter Trojans, the respectively two brightest objects in the L4 and L5 swarms: Hektor, Agamemnon, Patroclus, and Aneas. We also propose a small amount of time for PACS photometric observations in support of spectroscopy. Our sample is sufficiently large to provide our results credibility, but small enough to keep our observing time request modest. Our observations will provide new constraints on the amount of water ice on the surface (if any) and will shed light on the hypothesized previous cometary activity of these objects.

Lead Scientist: Esa Vilenius

Allocated time: 44.1 hours

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An Irregular Dust Cloud around Uranus

The Solar System's irregular satellites have been grinding themselves to oblivion since their capture by the Giant planets billions of years ago. We have developed an evolutionary model of this collisional evolution that matches the known irregular populations and predicts that the cloud of small icy fragments created could be detectable. These circumplanetary clouds may also be detectable around extrasolar planets. We propose to use the SPIRE instrument on Herschel for 16.5 hours to observe a ~1 degree square region around Uranus in search of this cloud of icy particles. Because we expect the surface brightness to be fainter than the Zodiacal, galactic, and cosmic background this observation requires subtraction of two images taken one year apart. The first image is centered on Uranus, but a year later the planet has moved by 4 degrees and the same background is imaged the second time. Discovery of this cloud will be concrete evidence that the irregulars are collisionally evolved and allow a much better estimate of the size distribution between micron size grains and the largest irregulars. Dust cloud structure and asymmetry will provide information on grain properties and their fate, many of which are thought to coat the surfaces of outer regular satellites. The knowledge gained of the irregular satellite populations will allow more informed models of extrasolar circumplanetary swarms and pave the way for their discovery. It would be particularly apt that dust around Uranus is discovered by Herschel, as Uranus itself was discovered by William Herschel in 1781.

Lead Scientist: Grant Kennedy

Allocated time: 16.5 hours

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Variability in Ice Giant Stratospheres: Implications for Radiative, Chemical and Dynamical Processes

We will assess the rotational variability in the stratospheres of the ice giants, Uranus and Neptune, to understand the dynamical and chemical variability of the atmospheric structure of both planets as a function of longitude. This effort follows up observations by the Spitzer IRS that shows consistent evidence for rotational variability of stratospheric hydrocarbons in Uranus and intermediate-term variability in Neptune's emissions, neither of whose origins are not well understood. Herschel provides an opportunity to follow up these observations with its unparalleled sensitivity. Over the 17-hour periods characterizing the equatorial rotation periods of both planets a series of eight PACS dedicated line scans will be made of strategic lines of HD, methane and water vapor. An efficient scheme takes advantage of the simultaneous availability of Uranus and Neptune in Herschel's visibility window. These will assess the variability of hydrocarbons vs temperatures in both atmospheres to an unprecedented accuracy. The results will be analyzed by a team consisting of many members of the Key Project on ``Water and Related Chemistry in the Solar System'' who will apply their expertise with the data and its analysis, as well as researchers who discovered the Spitzer variability and ground-based inhomogeneity. The data will be examined in the context of models by team members who are experts in radiative transfer, photochemistry, and dynamical modeling of circulation and zonal thermal wave structure. By refining quantitative models for interactions between radiative, dynamical and chemical processes in these two cold but radiatively and dynamically diverse planets, a baseline will be created that will be useful in the interpretation of variability in the spectra of giant exoplanets. This work will also be programmatically useful in the evaluation of the variability of radiation from Uranus and Neptune, both which are key members of the Herschel flux calibration system.

Lead Scientist: Glenn Orton

Allocated time: 17.1 hours

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Nitrogen, Phosphorus and Sulphur Chemistry in Saturn's Atmosphere: Internal and External Origins for HCN, HCP and CS

Our understanding of some of the fundamental physiochemical processes at work within Saturn’s gaseous atmosphere is presently limited by the difficulties associated with detection of a number of atmospheric species. Based on our new understanding of Saturn’s bulk composition and chemistry from the Cassini mission, Herschel/HIFI offers an unprecedented opportunity to detect these species for the first time, and to place constraints on their origins. Radiative transfer calculations have been used in tandem with chemical modelling to select optimal transitions of HCN, HCP and CS for study by HIFI. These species have never been detected before, but are expected to be important secondary repositories for nitrogen, phosphorus and sulphur in Saturn’s atmosphere. Furthermore, the superb spectral resolution of heterodyne spectroscopy is ideal for distinguishing between broad tropospheric absorptions and narrow stratospheric emissions, allowing us to distinguish between internal and external origins for each species. Tropospheric abundances will be compared to expectations from state of the art thermochemical and photochemical models, in addition to predictions of lightning-induced shock chemistry. Stratospheric abundances will be interpreted in terms of external supply of N, P and S-bearing materials, either from large asteroidal/cometary impacts (where shock chemistry in impact plumes is also important) or influx of material from Enceladus, the rings or interplanetary dust particles. As a result, the HIFI search for the first signatures of Saturn’s HCN, HCP and CS abundances will serve as vital constraints on internal chemistry and the coupling between Saturn’s cold atmosphere and external environment, revealing the fundamental processes at work in the cold outer reaches of our Solar System.

Lead Scientist: Leigh N. Fletcher

Allocated time: 8 hours

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Analysis of OPRs and D/Hs of hydrogen sulfide in comets: Understanding their natal origin and constraining their place of formation in the protoplanetary disk.

Measuring the relative amounts of individual species present in the nucleus of a comet can provide information on their formation mechanism, and thus on cometary origins. We propose to investigate ortho-to-para ratios (OPRs) of hydrogen sulfide and further survey its isotopologues (e.g. HDS) to evaluate deuterium-to-hydrogen ratios (D/Hs) of all suitable comets that become available over the performance period of this OT1. We seek to obtain a quantitative analysis of production rates of sulfur species, isotopic fractionation in H2S (HDS/H2S), and the ratio of nuclear spin species for H2S. This can provide a measure of the temperature at which the nuclear spins were last set prior to being incorporated into the nucleus. Comparison of these measurements in comets with those found in interstellar cloud cores, aided with predictions of nebular chemistry, will test the presence of legacy ice from the natal cloud core and the degree of processing experienced by pre-cometary ices.

Lead Scientist: Lucas Paganini

Allocated time: 20 hours

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The chlorine cycle on Jupiter and Saturn

Hydrogen halides provide key insights into giant planet atmospheres but their detection has so far remained elusive. Herschel/HIFI's low noise and high spectral resolution provides a unique opportunity to detect these species for the first time - with an estimated sensitivity at the sub part per trillion level - an improvement of over three orders of magnitude on the best measurements currently available. Observing hydrogen halide species provides an exciting new avenue for studying chemical and dynamical processes at work on the giant planets. This proposal will focus on HCl - the halide with the highest predicted abundance and detectablity. HCl could have an internal or external origin - although models predict that an external origin from influx of extraplanetary material is the most likely. The observed HCl abundances will be used to: (1) Determine the magnitude of external Cl sources and compare exogenic flux environments and physiochemical processes between Jupiter and Saturn. (2) For Jupiter, by comparing the Cl flux to the O flux (from H2O), we can determine the excess Cl flux and measure the proportion of Io's plasma torus that enters the top of the jovian atmosphere. This unique measurement would have implications for the whole Jupiter system. (3) Use the gradient of the vertical profile to determine the efficiency of HCl scavenging by NH3 in the stratosphere and constrain the stratospheric chlorine cycle. (4) Provide the most stringent test of interior thermochemical models to date, by determining limits on the abundance of HCl in the troposphere. This will help constrain the speed at which internal material is dredged up and the efficiency of HCl depletion by formation of ammonia salts. Comparing HCl abundances between Jupiter and Saturn will allow us to probe these processes under different internal and external environments, providing further insight.

Lead Scientist: Nicholas Teanby

Allocated time: 6.4 hours

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Confirmation of the first detection of HNC on Titan

Observations of Titan were performed on June 14, 2010 with Herschel/HIFI, as part of the Herschel guaranteed time key programme "Water and related chemistry in the Solar System" (PI: P. Hartogh). These measurements, targetted to the H2O 556.935 GHz line, have shown in addition an unanticipated line at 543.897 GHz. We attribute this emission to HNC(6-5), which would represent the first detection of HNC in Titan's atmosphere. Preliminary interpretation of the data suggests that HNC is confined to the upper atmosphere (above at least 300 km, and may be even higher). HNC is a plausible species in Titan's atmosphere, expected to be produced by dissociative recombination reaction of the ionospheric ion HCNH+ at altitudes above 1000 km. The loss process considered is HNC protonation by reaction with H-bearing ions and H atoms, yielding HCN. An accurate knowledge of the vertical distribution of HNC and HCN at altitudes above 800 km would provide a major constraint for the photochemical formation scheme of HNC. The goal of this proposal are (i) to spectroscopically confirm the presence of HNC in the upper atmosphere of Titan by observing another transition at 906 GHz (ii) to measure the narrow component of HCN at 532 GHz, in order to retrieve its abundance abundance profile over 400-800 km. The so-constrained HNC/HCN ratio in the upper atmosphere will permit us to discriminate between the different possible formation/loss schemes of HNC.

Lead Scientist: Raphael Moreno

Allocated time: 12.4 hours

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Probing the atmospheres of Uranus, Neptune and Titan with CH4 lines

Methane is a key species in the Outer Planets. It is the third most abundant molecule in all four Giant Planets, with an abundance of about 2 % in Uranus and Neptune, and reaching 5% (of N2) at the surface of Titan. Because of its large abundance, methane plays a dominant role in governing the atmospheric chemistry of all these planets. Indeed, the photolysis of methane by solar photons initiates a complex chemistry, giving rise to a wealth of hydrocarbons. In the case of Titan, the photochemistry of methane is even more complex, because of the coupled CH4-N2 chemistry taking place in Titan's upper atmosphere. Initial observations of Neptune and Titan, performed in the framework of the GT-KP ``Water and related chemistry in the Solar System'' (PI: P. Hartogh) have allowed the detection of CH4 emission at 119.6 micron and in several other lines, but with a low spectral resolution. These measurements have constrained the stratospheric abundance of CH4. In the case of Uranus, the PACS measurements of the CH4 line at 159 microns shows only tropospheric absorption, and with a low signal-to-noise ratio. The goal of this proposal is to use the high spectral resolution of HIFI in order to resolve the 1882 GHz methane lines on Neptune and Titan. These optically thick lines will allow to constrain the vertical temperature profiles in their stratospheres, and in Neptune's case, the vertical distribution of methane. For Uranus, we propose to observe again the methane line at 159 micron with PACS, but with a gain of a factor 3 in sensitivity, in order to confirm the detection and better constrain its abundance.

Lead Scientist: Raphael Moreno

Allocated time: 26.3 hours

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Herschel portrait of Pluto: measuring the thermal lighcurve and the emissivity of volatile ices

Beyond the orbit of Neptune there exists a population of remnant bodies from the formation of the Solar System; i.e. the Transneptunian objects (TNOs) in the Kuiper belt. Scientific interest in these bodies arises because they are considered to retain the most pristine and least altered material of the Solar System. Improving the knowledge of these distant bodies thus extends the understanding of the origin and evolution of the Solar System. The Pluto/Charon system plays a key role in the study of the Transneptunian region. Much of what we understand of the physical constitution, composition and evolution of the objects in the Kuiper Belt is put into context by studies of Pluto. Herschel is the only facility which gives the opportunity to draw a portrait of the Pluto/Charon system using the thermal wavelength region. In particular we propose to measure Pluto/Charon thermal lightcurve and perform spectroscopic measurements of the system in the range (50-220) micron. These combined measurements will constrain thermal properties (thermal inertia) and emissivities (spectral and bolometric) of the different terrains on Pluto. The observations will provide constraints on the composition, the physical character (grain sizes, mixing characteristics, texture) and the temperature profile within the near-surface layers of Pluto. We intend also to search for signatures of yet unknown surface ices. Furthermore these observations give us the possibility to answer some open questions like: (i) is Pluto's surface changing? (ii) are nitriles and carbon dioxide ices present on the surface of Pluto as predicted? Answering these questions will provide a benchmark for understanding Pluto and all the large TNOs in the Kuiper Belt. Herschel gives the unique opportunity to complement NASA's New Horizons mission to Pluto, expected to arrive in 2015.

Lead Scientist: Silvia Protopapa

Allocated time: 31.3 hours

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The latitudinal and seasonal variations of D/H and O2 on Mars

We propose to use HIFI to determine latitudinal and seasonal variations of HDO, H2-18O, O2 and CO on Mars. The water data will be used to retrieve the D/H ratio and study its variations with latitude and season. The martian D/H ratio, presently poorly known, is a an important parameter as its excess on Mars (about 5 times the terrestrial value) is interpreted as the signature of an early outgassing of the martian atmosphere. In addition, models predict a possible variation of D/H with latitude and season, as an effect of condensation processes and surface/atmosphere interactions. The O2 and CO data will be used to study the cycle of these two non-condensible species as a function of the solar longitude. Results will be compared with climate models which predict significant variations of all these species. Our data will provide important constraints to photochemical models. H2O is known to be maximum at high northern latitudes during northern summer, while CO and O2 are expected to be maximum at high southern latitudes at the same time. Some CO variations have been observed in the infrared but not in the millimeter/submillimeter range so far. No information is presently known about the possibile variations of O2. Herschel is unique in its capability to observe O2 and H2O. We propose to observe Mars in three positions (North, Center and South) at two different seasons, with Ls close to 50 deg. and 120 deg. respectively. We will use AOT II-2 mode (raster scan with DBS) limited to 3 points. We have chosen high-frequency transitions to get the maximum spatial resolution. We propose to observe 3 settings: (1) H218O and HDO around 1630 GHz, (2) O2 and HDO around 1815 GHz and (3) 13CO and C18O around 1867 GHz. The total observing time is 26.3 hours.

Lead Scientist: Therese Encrenaz

Allocated time: 26.3 hours

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Open Time (Round 2)


The water D/H ratio in a TOO long-period comet

Having retained and preserved pristine material from the Solar Nebula at the moment of their accretion, comets contain unique clues to the history and evolution of the Solar System. Important diagnostics of how and where cometary materials formed are contained in isotopic ratios, since isotopic fractionation is very sensitive to chemical and physical conditions. Following the discovery of an Ocean-like D/H ratio in the water of the Jupiter-family comet 103P/Hartley 2 using HIFI (Hartogh et al. 2011, Nature, in press), we propose to obtain a high-accuracy D/H measurement in a long-period comet from the Oort cloud. Since measurements in Jupiter-family comets have only been acquired with Herschel, it is important to get a set of measurements for Oort cloud comets with the same instrumentation. Confirming the observed dichotomy in D/H ratio for long-period versus Jupiter-family comet would be the first clear compositional difference between these two dynamical classes of comets. The values and statistics of distribution of the D/H ratio in the two classes will improve our understanding of their origin in the Solar System, with implications for the dynamical evolution of the early Solar System and for the delivery of water and other volatiles to the Earth.

Lead Scientist: Dominique Bockelee-Morvan

Allocated time: 7.7 hours

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Galactic Background Calibrations for OT1_ddan01_1

This is a follow-up proposal to our ongoing OT1 proposal "Detecting the Largest Rings in the Solar System--Dust Rings from the Irregular Satellites" (OT1_ddan01_1). We seek to re-image each field visited by our OT1 observations once the planet and its rings have moved out of the scan. This will allow us to accurately subtract the galactic background that currently obscures our data. The cirrus emission represents the last barrier to accurate photometry at uninvestigated wavelengths of the newly discovered Phoebe ring at Saturn(Verbiscer et al. 2009), as well as the possible discovery of analogous rings around Uranus and Neptune. The total time required (including overheads) is 11.8 hours as estimated by HSpot.

Lead Scientist: Daniel Tamayo

Allocated time: 4.3 hours

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Pluto's seasonal evolution and surface thermal properties

In 2015, Pluto and its system of satellites will be explored by the New Horizons space mission. Pluto is the most prominent representative of large Kuiper-belt objects with volatile ices (N2, CH4, CO) on their surfaces. Due to its elliptic orbit and strong polar inclination, the large seasonal and spatial variability of the insolation is expected to drive global scale transport of the volatile ices, whose thermal balance determines the atmospheric state. Recent observations of Pluto of various kinds have revealed that it is undergoing seasonal evolution, with changes in its atmosphere, surface spectrum, and thermal emission. In particular, Spitzer observations over 2004-2007 indicated a surprising dimming of the dwarf planet at thermal wavelengths, most probably associated with extension of the coldest, N2-ice dominated, regions. We propose to measure the thermal lightcurve of the Pluto/Charon system (i.e. the rotational variation of its thermal emission) with PACS and SPIRE in order to (i) assess the changes in the ice distribution (ii) determine the emissivities (spectral and bolometric) of Pluto's different terrains (iii) determine their thermal inertia on seasonal timescales. These parameters are needed to constrain volatile transport and general circulation models, and to put the upcoming New Horizons measurements in a broader, time-evolving, perspective.

Lead Scientist: Emmanuel Lellouch

Allocated time: 10.2 hours

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Probing the Enceladus torus with Herschel

Observations of Saturn with HIFI, performed in 2009 and 2010 have revealed absorptions in the core of several emission lines of water from Saturn's atmosphere. They are due to absorption from water in the “Enceladus water torus”, a cloud of material originating from Enceladus' active plumes and spreading around Saturn to form a broad toroidal structure centered at Enceladus' orbit. These data permit to determine water column densities in the torus equatorial plane and allow a first estimate of the torus vertical extent and dynamical structure. Comparison with physical models indicates an Enceladus source rate of ~10^28 water molecules s^-1, and suggests that Enceladus' activity is the ultimate source of water in Saturn's atmosphere (Hartogh et al. 2011). These H2O observations thus provide an entirely new method to probe physical conditions (density, structure, kinematics) in the Enceladus torus and to monitor Enceladus’ venting activity. As such, they are highly complementary to the Cassini measurements, which characterize the source region but cannot sample water away from Enceladus and have led to contradictory results as to the stability/variability of plume activity. Additional observations of these H2O lines have been acquired in December 2010 and July 2011. We propose to (i) continue to monitor the variation of these absorptions with viewing geometry and time, taking advantage of the change of aspect in the Saturn system over the upcoming years (ii) search for H2O emission directly originating from the torus (iii) monitor the water emission from Saturn, which from the July 2011 observations was shown to be enhanced in relation by to the current major storm developing in Saturn's Northern hemisphere. The ensemble of data, along with torus models developed by our team, will provide us with an improved understanding of the torus three-dimensional structure, the excitation conditions for water in the torus, and the variation timescales of Enceladus’ cryo-volcanic activity.

Lead Scientist: Emmanuel Lellouch

Allocated time: 11.7 hours

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Herschel Space Observatory observations of outburst and split comets

Our primary goal is to increase our knowledge about the outburst and split comets using the both the Herschel Space Observatory and ground-based telescopes in order to combine the thermal infrared and visible domain data. Our comet targets are selected from different orbit classes as like Ecliptic Comets (ECs), Nearly-isotropic Comets (NICs), and Main Belt Comets (MBCs). These are 17/PHolmes (EC), the most puzzling comet underwent already two superoutbursts, 133P/Elst-Pizarro (MBC) which shows recurrent cometary activity, 213P/Van Ness (EC) is a split comet with two components, and C/2010 X1 (Elenin) is a split and outburst comet. Comparison of the thermal infrared and visible range observations allows to investigate the shape and possible surface inhomogenities using infrared flux curves and light curves in the visible. We will use the unique capability of the PACS instrument of the Herschel Space Observatory. The measurements for all targets are performed in scan map mode, observations are done priimarilz in the PACS blue (70 micron) band. By means of this observational technique we will reach 1/1.5 mJz in the 70 micron band. The proposal will cover total observing time schedule of 13.9 h.

Lead Scientist: Imre Toth

Allocated time: 13.9 hours

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A Search for Gaseous H2O in Asteroids (24) Themis & (65) Cybele

The detection of water ice on Asteroids (24) Themis and (65) Cybele and comet-like activity on some asteroids (so-called main belt comets) have recently provided evidence for water ice in the outer asteroid belt. This supports the suggestion that water in the Earths oceans may have been delivered from the outer asteroid belt.

In 2010, for both (24) Themis and (65) Cybele, results were published from rotationally resolved near-IR spectra that indicated the presence of widespread ice on their surfaces, a detection which served to imply a wider prevalence of water among minor Solar System bodies. However, this detection has since been challenged in 2011 with an alternative interpretation of the absorption at 3.1 microns being suggested.

This proposal requests 2 hours of the Herschel Space Observatory’s HIFI instrument scheduling time, 1 hour observation per asteroid, to provide conclusive evidence of the presence of water ice on these asteroids through the detection of outgassing via sublimation of this water; a result which has profound implications on the understanding of the origin of water on our planet.

Lead Scientist: Laurence O'Rourke

Allocated time: 2 hours

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Titan's Ammonia Cycle

Titan's atmosphere has one of the most complex photochemical cycles in the solar system and results in a vast array of organic compounds containing H, C, N, and O. Understanding Titan's atmosphere has been a major scientific goal since the Voyager era and was one of the main drivers for the Cassini mission. When Cassini first flew through Titan's upper-most atmosphere its mass spectrometer discovered unprecedented levels of ammonia (NH3), which were orders of magnitude greater than those predicted by the most up-to-date photochemical models. This discovery suggests serious deficiencies in our present understanding Titan's photochemical cycle.

Based on the Cassini upper atmosphere measurements, it is now suspected that NH3 is present throughout Titan's bulk atmosphere in much greater quantities than previously expected. This has recently lead to an additional NH3 production pathway being hypothesised - which we aim to test in this proposal. By observing NH3 spectral emission lines around 1764 GHz with Herschel's HIFI instrument it will be possible to determine the bulk atmosphere NH3 abundance for the first time. This will allow us to probe missing links in Titan's ammonia and nitrogen cycles and dramatically increase our understanding of Titan's atmosphere and photochemical processes in general.

Lead Scientist: Nicholas Teanby

Allocated time: 8.1 hours

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The origin of CO in Uranus

One of the Infrared Space Observatory's most important results is the detection of H2O and CO2 in the giant planet stratospheres. The presence of a condensible species (H2O) above the tropospheric cold trap implies an external origin of H2O. This oxygen supply, which manifests itself also through the presence of CO2 and CO in these atmospheres, may have several sources: a permanent flux from interplanetary dust particles, local sources (rings, satellites), or large comet impacts. However, observing CO in the stratospheres of the giant planets does not automatically imply only an external origin of this species because there is no condensation sink at the tropopause for CO. Thus, it can be transported from the deep hot interior of the planet to the stratosphere. CO can have either an internal origin, an external origin or a combination of both. Spectrally-resolved, high signal-to-noise ratio and spatially-resolved (when possible) observations are needed to disentangle the various sources. While Jupiter, Saturn and Neptune have all an external source of CO, probably of cometary origin, and an internal source (not yet demonstrated but likely in Saturn), the situation remains unclear at Uranus despite the detection of CO from its 5 micron fluorescence. So far, we have not detected CO from the ground in the (sub)millimeter range but we obtained a tentative detection of the CO(8-7) line with HIFI in July 2011 in the framework of the HssO Key Programme. Because external sources of CO seem to be active for all the other giant planets, we propose to observe Uranus at the frequency of the CO(8-7) line with HIFI in order to confirm our tentative detection in the submillimeter range. We will obtain a spectrally-resolved and high signal-to-noise ratio observation of the lineshape that will enable us to retrieve information on its vertical distribution. Thus, we will be able to discuss the origin of CO in Uranus from this observation.

Lead Scientist: Thibault Cavalie

Allocated time: 8 hours

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Probing the temperature and chemistry of Saturn's storm with Herschel

Saturn's usually slowly evolutive seasonal cycle has been disrupted in December 2010 between 20°N and 50°N by the outbreak of an unexpected huge storm system. First Cassini/CIRS and ground-based observations have shown that temperatures, winds and chemistry have been rapidly affected by the storm in the stratosphere. For instance, a temperature increase of 50K over 60° in longitude has been measured by Cassini/CIRS in May 2011. We propose to take advantage of this rare opportunity to use Herschel’s mapping capability with HIFI and PACS to probe the vertical structure of this unique storm and derive constraints on its formation processes. We will map the emission of H2O at 66 and 67 microns and CH4 at 120 microns and 1882 GHz to measure the temperature between 0.1 and 100 mbar in the stratosphere and to check for any disturbance in the H2O vertical profile. Such disturbance would be due to the injection of massive amounts of tropospheric H2O into the stratosphere by the storm. Because this storm seem to undergo a slow evolution, we propose to monitor its temporal evolution by observing the proposed set of maps once in each of the three remaining observability windows of Saturn.

Lead Scientist: Thibault Cavalie

Allocated time: 11.7 hours

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