The European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a relatively small and lightweight millimeter-submillimeter spectrometer ...instrument, the first of its kind launched into deep space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively, the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids.The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species – CO, CH3OH, NH3 and three, oxygen-related isotopologues of water, H216O, H217O and H218O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system.
•Five years worth of 2.2-cm wavelength maps of Saturn are presented.•Maps represent the spatial distribution of ammonia vapor in Saturn’s atmosphere.•The average ammonia relative humidity of Saturn ...is found to be 70±15%.•Spatial pattern of ammonia indicates a Hadley-like circulation near the equator.•The 2010–2011 northern storm and SH storms have very low ammonia RH.
This work focuses on determining the latitudinal structure of ammonia vapor in Saturn’s cloud layer near 1.5 bars using the brightness temperature maps derived from the Cassini RADAR (Elachi et al. 2004, Space Sci. Rev. 115, 71–110) instrument, which works in a passive mode to measure thermal emission from Saturn at 2.2-cm wavelength. We perform an analysis of five brightness temperature maps that span epochs from 2005 to 2011, which are presented in a companion paper by Janssen et al. (Janssen, M.A., Ingersoll, A.P., Allison, M.D., Gulkis, S., Laraia, A.L., Baines, K., Edgington, S., Anderson, Y., Kelleher, K., Oyafuso, F. 2013. Icarus, this issue). The brightness temperature maps are representative of the spatial distribution of ammonia vapor, since ammonia gas is the only effective opacity source in Saturn’s atmosphere at 2.2-cm wavelength. Relatively high brightness temperatures indicate relatively low ammonia relative humidity (RH), and vice versa. We compare the observed brightness temperatures to brightness temperatures computed using the Juno atmospheric microwave radiative transfer (JAMRT) program which includes both the means to calculate a tropospheric atmosphere model for Saturn and the means to carry out radiative transfer calculations at microwave frequencies. The reference atmosphere to which we compare has a 3× solar deep mixing ratio of ammonia (we use 1.352×10−4 for the solar mixing ratio of ammonia vapor relative to H2; see Atreya 2010. In: Galileo’s Medicean Moons – Their Impact on 400 years of Discovery. Cambridge University Press, pp. 130–140 (Chapter 16)) and is fully saturated above its cloud base. The maps are comprised of residual brightness temperatures—observed brightness temperature minus the model brightness temperature of the saturated atmosphere.
The most prominent feature throughout all five maps is the high brightness temperature of Saturn’s subtropical latitudes near ±9° (planetographic). These latitudes bracket the equator, which has some of the lowest brightness temperatures observed on the planet. The observed high brightness temperatures indicate that the atmosphere is sub-saturated, locally, with respect to fully saturated ammonia in the cloud region. Saturn’s northern hemisphere storm was also captured in the March 20, 2011 map, and is very bright, reaching brightness temperatures of 166K compared to 148K for the saturated atmosphere model. We find that both the subtropical bands and the 2010–2011 northern storm require very low ammonia RH below the ammonia cloud layer, which is located near 1.5 bars in the reference atmosphere, in order to achieve the high brightness temperatures observed. The disturbances in the southern hemisphere between −42° and −47° also require very low ammonia RH at levels below the ammonia cloud base. Aside from these local and regional anomalies, we find that Saturn’s atmosphere has on average 70±15% ammonia relative humidity in the cloud region. We present three options to explain the high 2.2-cm brightness temperatures. One is that the dryness, i.e., the low RH, is due to higher than average atmospheric temperatures with constant ammonia mixing ratios. The second is that the bright subtropical bands represent dry zones created by a meridionally overturning circulation, much like the Hadley circulation on Earth. The last is that the drying in both the southern hemisphere storms and 2010–2011 northern storm is an intrinsic property of convection in giant planet atmospheres. Some combination of the latter two options is argued as the likely explanation.
•Five years worth of high-resolution maps of Saturn at 2.2-cm λ are presented.•They show variations in both cloud and sub-cloud ammonia concentration.•They reveal features never before seen, or ...viewed in a way never before possible.•Comparison indicates a remarkable degree of stability in most regions.•Future multi-wavelength comparisons can address circulations in a new way.
We present well-calibrated, high-resolution maps of Saturn’s thermal emission at 2.2-cm wavelength obtained by the Cassini RADAR radiometer through the Prime and Equinox Cassini missions, a period covering approximately 6years. The absolute brightness temperature calibration of 2% achieved is more than twice better than for all previous microwave observations reported for Saturn, and the spatial resolution and sensitivity achieved each represent nearly an order of magnitude improvement. The brightness temperature of Saturn in the microwave region depends on the distribution of ammonia, which our radiative transfer modeling shows is the only significant source of absorption in Saturn’s atmosphere at 2.2-cm wavelength. At this wavelength the thermal emission comes from just below and within the ammonia cloud-forming region, and yields information about atmospheric circulations and ammonia cloud-forming processes. The maps are presented as residuals compared to a fully saturated model atmosphere in hydrostatic equilibrium. Bright regions in these maps are readily interpreted as due to depletion of ammonia vapor in, and, for very bright regions, below the ammonia saturation region. Features seen include the following: a narrow equatorial band near full saturation surrounded by bands out to about 10° planetographic latitude that demonstrate highly variable ammonia depletion in longitude; narrow bands of depletion at −35° latitude; occasional large oval features with depleted ammonia around −45° latitude; and the 2010–2011 storm, with extensive saturated and depleted areas as it stretched halfway around the planet in the northern hemisphere. Comparison of the maps over time indicates a high degree of stability outside a few latitudes that contain active regions.
We report measurements in laboratory conditions of the relative complex permittivity (hereafter permittivity) of porous material on a large range of frequencies from 50MHz to 190GHz. Such ...measurements, developed in preparation of the Rosetta mission to comet 67P/Churyumov–Gerasimenko, specifically for the MIRO radiometric experiment, were obtained with different instrumentations in three frequency bands: 50–500MHz, 2.45–12GHz and 190GHz (center-band frequency of the millimeter receiver of MIRO, specially developed for our purpose). Considering the expected properties of cometary nuclei, they were carried out with porous granular materials of volcanic origin, with various sizes ranging from a few to 500μm, i.e. Etna׳s ashes and NASA JSC Mars-1 martian soil simulant. The samples were split into several sub-samples with different size ranges and bulk densities. The real part and the imaginary part of the permittivity remain respectively in the 2.1–4.0 range and in the 0.05–0.31 range. Volume scattering becomes significant for the measurements at 190GHz when the mean grain size of sub-samples is greater than about 200μm and implies an increase of the real part and the imaginary part of the permittivity. Without this effect, for any sub-sample, the results are consistent over the frequency range. From 50MHz to 190GHz, evidence is provided for a slight decrease of the real part of the permittivity. Bulk densities of the sub-samples, being in the 800–1300kgm−3 range, were determined during the measurements at 190GHz. Taking into account the expected bulk density of the nucleus (100–370kgm−3), as well as temperature for the surface and subsurface (in the 30–300K range) and its composition (consisting both of silica-rich dust and ices, mostly of water), these first series of results allow an estimate of the real part and the imaginary part of the permittivity of the near-surface of the cometary nucleus: the real part is likely to be lower than 1.6 for non-icy regions and lower than 1.4 for icy regions; the imaginary part is likely to be below 0.09. These estimates represent upper limits relevant for the interpretation of the future data of MIRO.
•Permittivity of granular matter has been measured in support of the Rosetta mission.•Three instrumentations were used to cover frequencies from 50MHz to 190GHz.•The real part of the permittivity remains in the 2.1–4.0 range.•The imaginary part of the permittivity remains in the 0.05–0.31 range.•Results at 190GHz provide upper limits, relevant for the MIRO data on Rosetta.
The European Space Agency (ESA) Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko (C-G), carries a complementary set of instruments on both the orbiter and lander ...(Philae) portions of the spacecraft, to measure the composition of the Comet C-G. The primary composition measuring instruments on the Orbiter are Alice, COSIMA, ICA, MIRO, OSIRIS, ROSINA and VIRTIS. These instruments collectively are capable of providing compositional information, including temporal and spatial distributions of important atomic, molecular, and ionic species, minerals, and ices in the coma and nucleus. The instruments utilize a variety of techniques and wavelength ranges to accomplish their objectives. This paper provides an overview of composition measurements that will be possible using the suite of orbiter composition measuring instruments. A table is provided that lists important species detectable (depending on abundances) with each instrument.
The European Space Agency's Rosetta spacecraft made a close flyby of asteroid (21) Lutetia on July 10, 2010. The spacecraft carries a dual-band radiometer/spectrometer instrument, named MIRO, which ...operates at 190GHz (1.6mm) and 560GHz (0.5mm). During the flyby, the MIRO instrument measured the temperature of Lutetia in both the northern and southern hemispheres. At the time of the flyby, the northern hemisphere was seasonally sun-lit and warmer than the southern hemisphere. Subsurface (depths from ∼2mm to ∼2cm) temperatures ranged from ∼200K on the northern hemisphere to ∼60K on the southern hemisphere. A lunar-like regolith – very low thermal inertia<20J/(Km2s0.5) in the upper 1–3cm overlaying a layer of rapidly increasing density and thermal conductivity – is required to explain the observations. A spectroscopic search was made for H2O, CO, CH3OH, and NH3 in Lutetia's exosphere but none of the molecules were detected. An upper limit to the water column density was estimated to be <5×1011molecules/cm2 at the time of the flyby.
► Lutetia's regolith has a very low thermal inertia, similar to the lunar regolith. ► Microgardening of Lutetia's surface is proposed. ► Upper limit on water vapor in Lutetia's exosphere is reported. ► Near surface temperatures of ∼60K measured at Lutetia's winter pole.
Results of the first year of data from the differential microwave radiometers on the Cosmic Background Explorer are presented. Statistically significant structure that is well described as ...scale-invariant fluctuations with a Gaussian distribution is shown. The rms sky variation, smoothed to a total 10-deg FWHM Gaussian, is 30 +/-5 micro-K for Galactic latitude greater than 20-deg data with the dipole anisotropy removed. The rms cosmic quadrupole amplitude is 13 +/-4 micro-K. The angular autocorrelation of the signal in each radiometer channel and cross-correlation between channels are consistent and give a primordial fluctuation power-law spectrum with index of 1.1 +/-0.5, and an rms-quadrupole-normalized amplitude of 16 +/-4 micro-K. These features are in accord with the Harrison-Zel'dovich spectrum predicted by models of inflationary cosmology.
An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and ...including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500–900
km region. The 300–360, 540–660 and 1080–1280
GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200
km using rotational lines of CH
4, CO, and HCN. Winds can be measured over the 200–1200
km altitude range with an accuracy of 3–5
m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H
2O, NH
3, CH
3C
2H, CH
2NH, and several nitriles (HC
3N, HC
5N, CH
3CN, and C
2H
3CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.
The European Space Agency Rosetta Spacecraft passed within 803
km of the main belt asteroid (2867) Steins on 5 September 2008. The Rosetta Spacecraft carries a number of scientific instruments ...including a millimeter and submillimeter radiometer and spectrometer. The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190
GHz (1.6
mm) and 562
GHz (0.53
mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients. A 4096 channel CTS (chirp transform spectrometer) having 180
MHz total bandwidth and ∼44
kHz resolution is also connected to the submillimeter receiver. We present the continuum observations of asteroid (2867) Steins obtained during the fly-by with the MIRO instrument. Spectroscopic data were also collected during the fly-by using the MIRO spectrometer fixed-tuned to rotational lines of several molecules. Results of the spectroscopic investigation will be the topic of a separate publication.
Comparative thermal models and radiative transfer calculations for Steins are presented. Emissivities of Steins were determined to be 0.6–0.7 and 0.85–0.9 at wavelengths of 0.53 and 1.6
mm, respectively. The thermal inertia of Steins was estimated to be in the range 450–850
J/(m
2
s
0.5
K). Assuming that the emissivity of Steins is determined by the Fresnel reflection coefficients of the surface material, the area-averaged dielectric constant of the surface material is in the range 4–20. These values are rock-like, and are unlike the powdered-regolith surface of the Moon.
Ground-based observations have shown that Jupiter is a two-component source of microwave radio emission: thermal atmospheric emission and synchrotron emission from energetic electrons spiralling in ...Jupiter's magnetic field. Later in situ measurements confirmed the existence of Jupiter's high-energy electron-radiation belts, with evidence for electrons at energies up to 20 MeV. Although most radiation belt models predict electrons at higher energies, adiabatic diffusion theory can account only for energies up to around 20 MeV. Unambiguous evidence for more energetic electrons is lacking. Here we report observations of 13.8 GHz synchrotron emission that confirm the presence of electrons with energies up to 50 MeV; the data were collected during the Cassini fly-by of Jupiter. These energetic electrons may be repeatedly accelerated through an interaction with plasma waves, which can transfer energy into the electrons. Preliminary comparison of our data with model results suggests that electrons with energies of less than 20 MeV are more numerous than previously believed.
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