M. R. Leese

In situ measurements of the physical characteristics of Titan's environment

On the basis of previous ground-based and fly-by information, we knew that Titans atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity (‘lightning’) was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 ± 0.25 K, and the pressure was 1,467 ± 1 hPa.

Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10−10 to 10−7 kilograms, and 48 grains of mass 10−5 to 10−2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturns largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titans atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

DENSITY AND CHARGE of PRISTINE FLUFFY PARTICLES FROM COMET 67P/CHURYUMOV-GERASIMENKO

The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA’s Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov–Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to <1 kg m-3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds <1 m s-1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.

Evolution of the Dust Size Distribution of Comet 67P/Churyumov–Gerasimenko from 2.2 au to Perihelion

The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67Ps trail. At sizes <1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

During the period between 15 September 2014 and 4 February 2015, the Rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67P/Churyumov-Gerasimenko (67P). The Grain Impact Analyzer and Dust Accumulator (GIADA) onboard Rosetta moni- tored the 67P coma dust environment for the entire period. Aims. We aim to describe the dust spatial distribution in the coma of comet 67P by means of in situ measurements. We determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification. Methods. We analyzed GIADA data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribu- tion of 67P, to monitor its activity, and to retrieve information on active areas present on its nucleus. We combined GIADA detection information with calibration activity to distinguish different types of particles that populate the coma of 67P: compact particles and fluffy porous aggregates. By means of particle dynamical parameters measured by GIADA, we studied the dust acceleration region. Results. GIADA was able to distinguish different types of particles populating the coma of 67P: compact particles and fluffy porous aggregates. Most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet’s neck region of the nucleus, the so-called Hapi region. This resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 AU heliocentric distance. The speed of these particles, having masses from 10−10 to 10−7 kg, ranged from 0.3 to 12.2 m s−1 . The variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. The influence of solar radiation pressure on micron and submicron particles was studied. The integrated dust mass flux collected from the Sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly from the comet nucleus. The awakening 67P comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008.

Huygens' Surface Science Package

The design and performance of the Surface Science Package (SSP) on the Huygens probe are discussed. This instrument consists of nine separate sensors that are designed to measure a wide range of physical properties of Titans lower atmosphere, surface, and sub-surface. By measuring a number of physical properties of the surface it is expected that the SSP will be able to constrain the inferred composition and structure of the moons near-surface environment. Although the SSP is primarily designed to sense properties of the surface, some of its sensors will also make measurements of the atmosphere along the probes entry path and will complement the data gathered by other experiments on the Huygens probe.

Laboratory calibration of the Cassini Cosmic Dust Analyser (CDA) using new, low density projectiles

The Cassini Cosmic Dust Analyser (CDA), developed from the Galileo and Ulysses dust instruments with the addition of a Chemical Analyser, is currently travelling outward from the Earth (collecting data from March 1999 onward) to the Saturnian system (arrival 2004) via Jupiter. The Chemical Analyser will provide information on the elemental composition of impacting micrometeoroids through impact ionisation, time-of-flight mass spectrometry. A rigorous calibration programme primarily focussed upon the Chemical Analyser is in progress at the University of Kent at Canterbury. A 2-MV Van de Graaff electrostatic accelerator and CDA laboratory model are used to simulate impacts. Acceleration of revolutionary low density, polymer dust particles has enabled an insight into the response of CDA to molecularly bonded material with increasing event velocity. These conducting polymer coated polystyrene latex particles represent significantly better analogues for carbonaceous cosmic grains than more traditionally accelerated projectiles (e.g. iron) and have enabled complex organic spectra to be produced in the laboratory. The current status of an ongoing programme is reported. Three samples are presented, two polypyrrole coated latexes of differing size and one PEDOT-coated latex sample.

Optical and radiometric models of the NOMAD instrument Part I: The UVIS channel

The NOMAD instrument has been designed to best fulfil the science objectives of the ExoMars Trace Gas Orbiter mission that will be launched in 2016. The instrument is a combination of three channels that cover the UV, visible and IR spectral ranges and can perform solar occultation, nadir and limb observations. In this series of two papers, we present the optical models representing the three channels of the instrument and use them to determine signal to noise levels for different observation modes and Martian conditions. In this first part, we focus on the UVIS channel, which will sound the Martian atmosphere using nadir and solar occultation viewing modes, covering the 200-650nm spectral range. High SNR levels (>1000) can easily be reached for wavelengths higher than 300nm both in solar occultation and nadir modes when considering binning. Below 300nm SNR are lower primarily because of the lower signal and the impact of atmospheric absorption.

Design and characterisation of the new CIS115 sensor for JANUS, the high resolution camera on JUICE

JUICE, the Jupiter Icy Moon Explorer, is a European Space Agency L-class mission destined for the Jovian system. Due for launch in 2022, it will begin a science phase after its transit to Jupiter that will include detailed investigations of three of the Galilean moons: Ganymede, Callisto and Europa. JUICE will carry payloads to characterise the Jovian environments, divided into in situ, geophysical and remote sensing packages. A key instrument in the remote sensing package is JANUS, an optical camera operating over a wavelength range of 350 nm to 1064 nm. JANUS will be used to study the external layers of Jupiter’s atmosphere, the ring system and the planetary bodies. To achieve the science goals, resolutions of better than 5 m per pixel are required for the highest resolution observations during the 200 km altitude orbit of Ganymede, whilst the system is operated with a signal to noise ratio of better than 100. Jupiter’s magnetic field is a dominant object in the solar system, trapping electrons and other charged particles leading to the radiation environment around Jupiter being very hostile, especially in the regions closest to Jupiter in the Ganymede orbit. The radiation tolerance of the focal plane detector in JANUS is therefore a major concern and radiation testing is vital to confirm its expected performance after irradiation will meet requirements set by the science goals. JANUS will be using a detector from e2v technologies plc, the CMOS Imaging Sensor 115 (CIS115), which is a device manufactured using 0.18 μm Imaging CMOS Process with a 2000 by 1504 pixel array each 7 μm square. The pixels have a 4T pinned photodiode pixel architecture, and the array is read out through four differential analogue outputs. This paper describes the preliminary characterisation of the CIS115, and results obtained with the CIS107 precursor sensor.