Gerard Beaudin

Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s–1 in early June 2014 to 1.2 kg s–1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K–1 m–2 s–0.5), consistent with a thermally insulating powdered surface, is inferred.

Spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko observed from Rosetta/MIRO in August 2014

Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the (H2O)-O-16 rotational transition line at 556.936 GHz observed from Rosetta/MIRO in August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the (H2O)-O-16 outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7-9, 2014 and August 18-19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 x 1025 molecules s(-1) sr l to 3.0 x 10(25) molecules s(-1) sr, the terminal gas expansion velocity varies by 0.17 km s(-1) from 0.61 km s(-1) to 0.78 km s(-1), and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.

MIRO observations of subsurface temperatures of the nucleus of 67P/Churyumov-Gerasimenko

Observations of the nucleus of 67P/Churyumov-Gerasimenko in the millimeter-wave continuum have been obtained by the Microwave Instrument for the Rosetta Orbiter (MIRO). We present data obtained at wavelengths of 0.5 mm and 1.6 mm during September 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27 AU. The data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters, including thermal inertia and absorption properties at the MIRO wavelengths. MIRO brightness temperatures on the irregular surface of 67P are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet’s rotation axis with respect to the Sun. The MIRO emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. The data are quantitatively consistent with very low thermal inertia values, between 10–30 J K -1 m -2 s -1/2 , with the 0.5 mm emission arising from 1 cm beneath the surface and the 1.6 mm emission from a depth of 4 cm. Although the data are generally consistent with simple, homogeneous models, it is difficult to match all of its features, suggesting that there may be some vertical structure within the upper few centimeters of the surface. The MIRO brightness temperatures at high northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, based on observations of gas and dust production, ice is known to be sublimating.

Dark side of comet 67P/Churyumov-Gerasimenko in Aug.-Oct. 2014. MIRO/Rosetta continuum observations of polar night in the southern regions

The high obliquity (similar to 50 degrees) of comet 67P/Churyumov-Gerasimenko (67P) is responsible for a long-lasting winter polar night in the southern regions of the nucleus. We report observations made with the submillimeter and millimeter continuum channels of the Microwave Instrument onboard the Rosetta Orbiter (MIRO) of the thermal emission from these regions during the period August-October 2014. Before these observations, the southern polar regions had been in darkness for approximately five years. Subsurface temperatures in the range 25 50 K are measured. Thermal model calculations of the nucleus near-surface temperatures carried out over the orbit of 67P, coupled with radiative transfer calculations of the MIRO channels brightness temperatures, suggest that these regions have a thermal inertia within the range 10-60 Jm(-2) K-1 s(-0.5). Such low thermal inertia values are consistent with a highly porous, loose, regolith-like surface. These values are similar to those derived elsewhere on the nucleus. A large difference in the brightness temperatures measured by the two MIRO continuum channels is tentatively attributed to dielectric properties that differ significantly from the sunlit side, within the first few tens of centimeters. This is suggestive of the presence of ice(s) within the MIRO depths of investigation in the southern polar regions. These regions started to receive sunlight in May of 2015, and refinements of the shape model in these regions, as well as continuing MIRO observations of 67P, will allow refining these results and reveal the thermal properties and potential ice content of the southern regions in more detail.

Heterodyne instrument for FIRST (HIFI): preliminary design

We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 103 up to 107 (300 - 0.03 km/s) and deep line observations.

European Minor Constituent Radiometer: A New Millimeter Wave Receiver for Atmospheric Research

EMCOR is a heterodyne receiver for the frequency range of 201 to 210 GHz. It has been designed for ground-based measurements of various minor constituents of the stratosphere involved in ozone chemistry. Since the aim was the detection of faint spectral lines, a superconducting tunnel junction has been chosen as mixer element and special care has been taken in developing the calibration unit of the system. The front-end is completed by a quasi-optical system, a solid state local oscillator with electronic tuning and a HEMT pre-amplifier. In the back-end an acousto-optical spectrometer is employed to analyse the signal. A PC controls the whole system. The instrument has been installed at a high mountain site in the Swiss Alps.

The Mars flyby of ROSETTA: an opportunity for infrared and microwave high-resolution sounding

Abstract The Mars flyby of ROSETTA will provide a valuable opportunity for sounding, at high spatial resolution, the Mars atmosphere and surface in the infrared and microwave range. The VIRTIS infrared imaging spectrometer should be able to determine the surface mineralogy and temperature of the observed areas, the abundances of minor constituents (H2O, CO), and possibly to study the atmospheric thermal profile. VIRTIS will complement the OMEGA and PFS infrared spectrometers on-board the Mars Express mission, expected to operate in Mars orbit at the time of the ROSETTA flyby. The MIRO microwave spectrometer is expected to provide information on the thermal profile, the H2O vertical distribution, the temperature of the subsurface, and possibly the atmospheric winds. In addition, the Mars flyby of ROSETTA will provide the first opportunity for testing the VIRTIS and MIRO instruments, in particular for wavelength/frequency and photometry/radiometry calibration.

A low-noise cryogenically-cooled 8–12 GHz HEMT Amplifier for future space applications

A two-stage 8–12 GHz (X-band) cryogenically-cooled Low-Noise Amplifier (LNA) has been developed with a commercial pseudomorphic HEMT on AsGa substrate. In a first step, different commercial transistors have been fully characterized from 300 K to 20 K using a new method to measure the four noise parameters. Preliminary results have allowed the selection of the best device. This enabled the design of the two-stage LNA with the help of a microwave CAD software.In a second step, the LNA has been characterized at 300 K, 30 K and 4 K. As the physical temperature decreased from 300 K to 30 K, the LNA exhibited an average gain increase of 2 dB and as much as a fourfold reduction of noise temperature. A noise figure of 22.5 K and a gain of 23 dB have been achieved at 30 K around 10 GHz. The noise temperature has been furthermore reduced to 20 K by cooling the amplifier at the liquid helium temperature (4.2 K). Different methods to measure the noise characteristics of the amplifier are widely developed in this paper.

Models of superconducting microstrip and coplanar elements for submillimeter applications

Some models which predict the microwave properties of superconducting microstrip lines, radial stubs, coplanar lines and coplanar waveguides are presented. These models take into account the dispersion which occurs in superconducting lines when the frequency of operation becomes close to the gap frequency of the superconductros (700 GHz for niobium). The effects of fringing fields are also taken into consideration. These models are also valid for high-temperature superconductors. They have been developed in the specific case of elements for which the characteristic dimension (the dielectric thickness for microstrip lines, the spacing between conductors for coplanar elements) is of the same order of magnitude as the typical length of penetration of the magnetic field in the metal. The length of penetration is the skin depth for normal metals and the London penetration depth for superconductors. The consequence of this particular topology is that the propagation of signals in such a transmission line is slow-wave and its microwave properties are modified. These models are also valid for classical line configurations (dielectric thickness much higher than penetration length).

A 230GHz Low Noise Cooled Receiver for Radioastronomy Applications

A new radioastronomical cooled receiver is presented here. It works in the range 210-240 GHz with a DSB system noise of 560 < presently and a 600 MHz instantaneous bandwidth. It contains a local oscillator (klystron + varactor tripler), a Schottky diode mixer with a cooled FET amplifier and uses quasi-optical techniques for signal injection. A microprocessor drives the whole system and is linked to a computer. The microprocessor allows self-calibration and frequency tuning. Details are given on the quasi-optical techniques, the frequency tripler, the mixer and the microprocessor.