Omega Spectrometer Research Articles

Vertical distribution of dust in the martian atmosphere: OMEGA/MEx limb observations

The OMEGA spectrometer on board Mars Express acquired several observations of the Martian limb that are still largely unexploited. Here, we explore the information content of these data in terms of abundance and size of the Martian atmospheric dust. We approach the dust retrievals by applying a full spherical multiple scattering Monte Carlo 1D radiative transfer code to model the 0.5–2.5 μm spectral range (VNIR and SWIR OMEGA channels), in order to derive dust effective radius and number density variation with altitude, between approx. 8 and 50 km. This is the first time this approach is applied to OMEGA limb data; therefore, we only present three study cases where water ice is below the detectability level, in order to focus on the methodological issues, assumptions, and performances before future more extensive applications. The model fully includes multiple scattering effects, which are known to be responsible for the coupling between the limb spectra taken at different altitudes and the surface. An extended three-dimensional modelling of the surface reflectance, acting as surface-shine for limb spectra, is developed. The VNIR channel is found useful in reducing the degeneracy of the radiative transfer solutions. The derived dust vertical distributions, yielding dust effective radii of the order of 0.85 ± 0.15 μm (corresponding to a modal radius rm of ~0.3 μm) between 15 and 30 km are quite in agreement with Global Circulation Models (GCMs), but the abundances lie about one order of magnitude above model predictions, as previously reported from comparison between models and Mars Climate Sounder (MCS) data. An overall agreement is in fact found with MCS data, and in one case the OMEGA-retrieved dust is compatible with a local storm rising from the Hellas basin. Such storms are not well represented in the Mars Climate Database, which provides monthly mean statistics. Our results demonstrate the capability of OMEGA limb data to quantitatively contribute to Martian dust studies, despite the complex and slow radiative transfer computation scheme required to accurately model multiple scattering in the probed spectral range. The desirable application of the retrieval method to the whole OMEGA limb dataset, also expected to help assess the occurrence of local dust storms, requires further work aimed to include water ice aerosols and possibly thermal emission. Being the first attempt to use a Monte Carlo modelling approach to OMEGA limb data, this work represents a useful benchmark for validating faster, although less accurate, radiative transfer models.

Acidic weathering of basalt and basaltic glass: 1. Near‐infrared spectra, thermal infrared spectra, and implications for Mars

AbstractAcid‐leached rinds and coatings occur in volcanic environments on Earth and have been identified using orbital spectroscopy on Mars, but their development is poorly understood. We simulated long‐term open‐system acidic weathering in a laboratory by repeatedly rinsing and submerging crystalline and glassy basalts in pH ~ 1 and pH ~ 3 acidic solutions for 213 days and compared their visible/near‐infrared (0.3–2.5 µm) and thermal infrared (5–50 µm) spectral characteristics to their microscopic physical and chemical properties from scanning electron microscopy (SEM). We find that while alteration at moderately low pH (~3) can produce mineral precipitates from solution, it has very little spectral or physical effect on the underlying parent material. In contrast, alteration at very low pH (~1) results in clear silica spectral signatures for all crystalline samples while glasses exhibit strong blue concave‐up near‐infrared slopes. SEM indicates that these spectral differences correspond to different modes of alteration. In glass, alteration occurs only at the surface and produces a silica‐enriched leached rind, while in more crystalline samples, alteration penetrates the interior to cause dissolution and replacement by silica. We confirm that glass is more stable than crystalline basalt under long‐term acidic leaching, suggesting that glass could be enriched and common in terrains on Mars that have been exposed to acidic weathering. Leached glasses are consistent with both OMEGA and Thermal Emission Spectrometer (TES) spectra of the Martian northern lowlands and may contribute to the high‐silica phases detected globally in TES Surface Type 2. Thus, both glass‐rich deposits and acidic weathering may have been widespread on Mars.

A review of central production experiments at the CERN Omega spectrometer

The non-Abelian nature of QCD suggests that particles that have a gluon constituent, such as glueballs or hybrids, should exist. This paper presents a study of central meson production in the fixed target experiments WA76, WA91 and WA102 at the CERN Omega spectrometer at centre-of-mass energies of [Formula: see text], 23.8 and 29 GeV. A study of the resonance production cross-section as a function of [Formula: see text] shows which states are compatible with being produced by Double Pomeron Exchange (DPE). In these DPE processes, the difference in the transverse momentum between the exchange particles (dPT) can be used to select out known [Formula: see text] states from non-[Formula: see text] candidates. The distribution of the azimuthal angle (ϕ) between the two exchange particles suggests that the Pomeron transforms like a nonconserved vector current. Finally there is evidence from an analysis of the decay modes of the scalar states observed, that the lightest scalar glueball manifests itself through the mixing with nearby [Formula: see text] states.

Annual survey of water vapor behavior from the OMEGA mapping spectrometer onboard Mars Express

We present here the annual behavior of atmospheric water vapor on Mars, as observed by the OMEGA spectrometer on board Mars Express during its first martian year. We consider all the different features of the cycle of water vapor: temporal evolution, both at a seasonal and at a diurnal scale; longitudinal distribution; and the vertical profile, through the variations in the saturation height. We put our results into the context of the current knowledge on the water cycle through a systematic comparison with the already published datasets. The seasonal behavior is in very good agreement with past and simultaneous retrievals both qualitatively and quantitatively, within the uncertainties. The average water vapor abundance during the year is ∼10 pr. μm, with an imbalance between northern and southern hemisphere, in favor of the first. The maximum of activity, up to 60 pr. μm, occurs at high northern latitudes during local summer and shows the dominance of the northern polar cap within the driving processes of the water cycle. A corresponding maximum at southern polar latitudes during the local summer is present, but less structured and intense. It reaches ∼25 pr. μm at its peak. Global circulation has some influence in shaping the water cycle, but it is less prominent than the results from previous instruments suggest. No significant correlation between water vapor column density and local hour is detected. We can constrain the amount of water vapor exchanged between the surface and the atmosphere to few pr. μm. This is consistent with recent results by OMEGA and PFS-LW. The action of the regolith layer on the global water cycle seems to be minor, but it cannot be precisely constrained. The distribution of water vapor on the planet, after removing the topography, shows the already known two-maxima system, over Tharsis and Arabia Terra. However, the Arabia Terra increase is quite fragmented compared with previous observations. A deep zone of minimum separates the two regions. The saturation height of water vapor is mainly governed by the variations of insolation during the year. It is confined within 5–15 km from the surface at aphelion, while in the perihelion season it stretches up to 55 km of altitude.

A study of the bound water, water ice, and frost distribution over the Martian surface: Treatment and correcting of the data of observations with the OMEGA spectrometer onboard Mars Express

The results of the analysis of the spectral observations of Mars carried out with the OMEGA spectrometer onboard the Mars Express spacecraft are presented. The data from one of the spectrometer’s channels working in the near-IR spectral range (0.93–2.69 µm) were analyzed. This range includes the characteristic absorption bands of both condensed water phases (ice and frost) and bound water contained in hydrated minerals of the Martial soil. From the 1.93-µm band indicating the presence of these minerals, global maps of the bound-water index have been made. They show a noticeable latitude dependence of the index: the largest values refer to high latitudes (>60°), while they gradually diminish toward the equator. Seasonal variations of the spectral index obtained by the 1.93-µm band are connected with the hydration-dehydration processes occurring in hydrogenous minerals when the temperature of the soil and the relative humidity in the near-surface atmospheric layer are changing. The evolution of the spectral absorption bands of water ice (1.2 and 1.5 µm) dependent on the season testifies to the changes in the microstructure of the surface layer in the North ploar cap caused by the sublimate re-crystallization processes in the ice sheet. The spatial pattern of the location of the areas where the microstructure most quickly grows could be formed under the influence of the stationary atmospheric waves.

The retrieval of altitude profiles of the Martian aerosol microphysical characteristics from the limb measurements of the Mars Express OMEGA spectrometer

Parameterization of the spectral dependence of the optical characteristics of Martian aerosols has been proposed for processing the results of measurements of outgoing radiation. A method for retrieving the altitude profiles of the microphysical characteristics of Martian aerosols from the limb spectrometry of the OMEGA instrument of the Mars Express mission have been developed. For one of the observational sessions, the altitude profiles of the concentration and the modal radius of the size distribution function of aerosol particles retrieved with the proposed parameterization are presented. For the purpose of the interpretation of the data acquired from the optical remote sensing of planetary atmospheres, we consider how the spectral dependence of aerosol optical characteristics, in particular the volume coefficients of aerosol scattering and absorption and the phase function, can be parameterized in a specified, probably wide, spectral range. The method of such a parameterization has been proposed for the cases of a fixed chemical composition of the aerosol materials dominating in the atmosphere. It has been shown that these cases allow the required spectral dependences to be presented as a function of a small number of parameters, for which the parameters of the size distribution function of aerosol particles can be successfully used. However, since such direct calculations of the aerosol characteristics require an inadmissibly long period of time for the tasks of interpretation of the remote sensing data, an algorithm for organizing the parameterization function as a special, preliminary generated computer database has been suggested. This database provides the continuity in the dependence on the parameters, the specified computation accuracy, and the required output speed of the results. As a specific application, the parameterization of the spectral dependence of the optical characteristics of the Martian aerosols has been proposed for the tasks of processing the results of measurements of outgoing radiation. As a result, the method for retrieving the altitude profiles of the microphysical characteristics of Martian aerosols from the limb spectrometric measurements of the OMEGA instrument of the Mars Express mission has been developed. For one of the observational sessions, the altitude profiles of the concentration and the modal radius of the size distribution function of aerosol particles retrieved with the proposed parameterization are presented.

Spectral and geological study of the sulfate-rich region of West Candor Chasma, Mars

Sulfates have been discovered by the OMEGA spectrometer in different locations of the planet Mars. They are strongly correlated to light toned layered deposits in the equatorial regions. West Candor Chasma is the canyon with the thickest stack of layers and one with the largest area covered by sulfates. A detailed study coupling mineralogy derived from OMEGA spectral data and geology derived from HRSC imager and other datasets leads to some straightforward issues. The monohydrated sulfate kieserite is found mainly over heavily eroded scarps of light toned material. It likely corresponds to a mineral present in the initial rock formed either during formation and diagenesis of sediments, or during hydrothermal alteration at depth, because it is typically found on outcrops that are eroded and steep. Polyhydrated sulfates, that match any Ca-, Na-, Fe-, or Mg-sulfates with more than one water molecule, are preferentially present on less eroded and darker outcrops than outcrops of kieserite. These variations can be the result of a diversity in the composition and/or of the rehydration of kieserite on surfaces with longer exposure. The latter possibility of rehydration in the current, or recent, atmosphere suggests the low surface temperatures preserve sulfates from desiccation, and, also can rehydrate part of them. Strong signatures of iron oxides are present on sulfate-rich scarps and at the base of layered deposits scarps. They are correlated with TES gray hematite signature and might correspond to iron oxides present in the rock as sand-size grains, or possibly larger concretions, that are eroded and transported down by gravity at the base of the scarp. Pyroxenes are present mainly on sand dunes in the low lying terrains. Pyroxene is strongly depleted or absent in the layered deposits. When mixed with kieserite, local observations favor a spatial mixing with dunes over layered deposits. Sulfates such as those detected in the studied area require the presence of liquid water to form by precipitation, either in an intermittent lacustrine environment or by hydrothermal fluid circulation. Both possibilities require the presence of sulfur-rich groundwater to explain fluid circulation. The elevation of the uppermost sulfate signatures suggests the presence of aquifers up to 2.5 km above datum, only 1 km below the plateau surface.

Hydration state of the Martian surface as seen by Mars Express OMEGA: 1. Analysis of the 3 μm hydration feature

Global mapping by the visible/near‐infrared OMEGA spectrometer gives the first opportunity to study in detail the characteristics of the 3 μm hydration absorption on the surface of Mars. This feature is caused by bending and stretching vibrations of adsorbed or structural H2O and/or OH‐ bound to minerals. A specific data reduction scheme has been developed to remove the contribution of thermally emitted radiance from OMEGA spectra. With the derived albedo spectra, variations in strength and shape of the 3 μm feature can be accurately assessed. Calibration issues are discussed and data only within the nominal calibration level of the instrument are analyzed, which corresponds to a surface coverage of ∼30%. All OMEGA spectra exhibit the presence of this absorption feature, which can be explained by the presence of adsorbed water as well as by alteration rinds or coatings resulting from weathering. Strong variations of the band strength are observed. Correlations between hydration, albedo, and elevation are examined. Terrains enriched in phyllosilicates, sulfates, or hydroxides exhibit an increased hydration signature as well as a weaker combination overtone H2O absorption at ∼1.9 μm. Careful analysis also reveals seasonal variations in surface hydration, with soils in the northern midlatitudes decreasing in hydration between northern spring and summer. This hydration change is best explained by the presence of winter frost followed by equilibration of frost‐free soil with the atmosphere and by increased surface temperatures reducing the adsorptive capacity of the regolith.

Assessing the limits of the Modified Gaussian Model for remote spectroscopic studies of pyroxenes on Mars

We investigate the ability to refine pyroxene composition and modal abundance from laboratory and remotely acquired spectra. Laboratory data including the martian meteorites, Shergotty, Zagami, MIL03346, and ALH84001 as well as additional pyroxene-rich spectra obtained from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) spectrometer for Mars are characterized using the Modified Gaussian Model (MGM), a spectral deconvolution method developed by Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S., 1990. J. Geophys. Res. 95, 6955–6966]. We develop two sensitivity tests to assess the extent to which the MGM can consistently predict (1) pyroxene composition and (2) modal abundance for a compositionally diverse suite of pyroxene spectra. Results of the sensitivity tests indicate that the MGM can be appropriately applied to remote spectroscopic measurements of extraterrestrial surfaces and can estimate pyroxene composition and relative abundance within a derived uncertainty. Deconvolved band positions for laboratory spectra of the meteorites Shergotty and Zagami are determined within ±17 nm while remotely acquired OMEGA spectra are defined within ±50 nm. These results suggest that absolute compositions can be uniquely derived from laboratory pyroxene-rich spectra and non-uniquely derived from the remote measurements of OMEGA at this time. While relative pyroxene chemistries are not assessed from OMEGA measurements at this time, relative pyroxene abundances are estimated using a normalized band strength ratio between the low-calcium (LCP) and high-calcium (HCP) endmember components and are constrained to ± 10 % . The fraction of LCP in a two-pyroxene mixture is the derived value from the normalized band strength ratio, LCP/(LCP + HCP). This calculation for relative abundance is robust in the presence of up to 10–15% olivine. Deconvolution results from the OMEGA spectra indicate that the ancient terrain in the Syrtis Major region is uniquely enriched in LCP ( 59 ± 10 % LCP) relative to HCP while the volcanics of Syrtis Major are uniquely enriched in HCP ( 39 ± 10 % LCP).

Good news, bad news, one small step

We begin this month’s column with a few remarks about earlier news items. On a happy note, the trio of Mars probes are now all on their way, after some delays. We cannot, however, uncross our fingers just yet. The Beagle 2 mission has been causing a few anxious moments. The power system has registered low voltages, but instruments on board are now said to be working well. Nevertheless, data have been received from the Omega spectrometer, which detects constituents of Earth’s atmosphere. By way of confirming the value of the mission, new photos of the Earth and Moon have been sent back. Mission controllers can point the spacecraft’s high-resolution stereo camera during instrument checks, and gave us some superb true color images, revealing the blue Pacific Ocean and white and grey clouds. It provides a taster of what we can expect once the spacecraft is in orbit around Mars — high-resolution images with brilliant true color and in 3D.

Observation of a soft photon signal in excess of QED expectations in pp interactions

Photons produced in pp interactions at 450 GeV/c were detected by reconstructing the e+e− pairs of photon conversions in a 1 mm thick lead sheet placed in front of the MWPCs of the OMEGA spectrometer at CERN. A soft photon signal 4.1±0.8 times the inner bremsstrahlung prediction was observed in the forward rapidity region (yc.m.s.⩾1.2), thus extending the domain of the anomalous soft photon effect seen already in K+p, π+p and π−p interactions.

Resonance production in central pp collisions at the CERN Omega Spectrometer

A study of resonance production in central pp collisions is presented as a function of several kinematical variables. In particular the difference in the transverse momentum ( dP T ) of the exchanged particles shows that undisputed q q ̄ mesons are suppressed at small dP T whereas glueball candidates are enhanced and in addition, the azimuthal angle φ gives information on the nature of the Pomeron.

New effects observed in central production by experiment WA102 at the CERN Omega Spectrometer

A partial wave analysis of the centrally produced KK and pipi systems shows that the fJ(1710) has J = 0. In addition, a study of central meson production as a function of the difference in transverse momentum (dPT) of the exchanged particles shows that undisputed qqbar mesons are suppressed at small dPT whereas the glueball candidates are enhanced and that the production cross section for different resonances depends strongly on the azimuthal angle between the two outgoing protons.

Comprehensive evidence for a narrow ${\mathrm{p}}\overline{\mathrm{p}}$ state of mass 2.02 GeV $/c^2$

We review earlier results, and add new evidence, on the existence of a narrow \({\mathrm{p}}\overline{\mathrm{p}}\) state at a mass of 2.02 GeV\(/c^2\), which is seen in six different final states, produced via baryon exchange in \(\pi^+{\mathrm{p}}\)\((\pi^-{\mathrm{p}})\) interactions at 20 (12) GeV\(/c\). The data come from a new analysis of the WA56 experiment done at the CERN Omega spectrometer. The 2020 peak is produced near mid-rapidity, in a kinematic region not explored before in baryon exchange processes. Agreement is found with an earlier observation of this state, and its absence in a series of unsuccessful searches is explained. Very low upper limits on mesonic decays of this state favor its interpretation as a baryonium candidate.

Multiple Labeling for EM using Particles of Different Shape and Metal Composition

Abstract Multiple labeling for electron microscopy (EM) is typically accomplished by using colloidal gold (cAu) particles of different sizes. The size distribution of each cAu preparation, which may vary by up to 15%, is the principal factor that limits the number of labels which can be used simultaneously. Furthermore, there is no effective way to use multiple labeling for quantitative, high resolution EM studies. Such analysis requires the use of a single Fab antibody fragment conjugated to a single cAu particle in order to ensure that each particle corresponds to only one antigenic site. Whole antibody molecules cannot be used for quantitative analysis because they are at least divalent, and some, such as dimeric IgA or pentameric IgM, have even more antigen-binding sites. Consequently, it is impossible to deduce whether the presence of one whole antibody molecule corresponds to the presence of one, two, or more targets. Particle diameters ranging from 3 to 5nm are optimal for quantitation because one Fab fragment adsorbs to one particle, more than one Fab fragment may adsorb to a single particle larger than 5nm, and, when smaller than 3nm, several particles may bind to a single Fab fragment. We are evaluating parameters apart from particle size variation to accomplish multiple labeling for both qualitative and quantitative analyses. One method relies on electron energy loss spectroscopy (EELS) to distinguish particles of several metallic compositions, including Au, Ag, Pt, Pd, Rh, and Ru. EELS is performed using a LEO 912 energy filtering transmission electron microscope (EFTEM) with an in-column Omega spectrometer.

The Omega RICH in the CERN hyperon beam experiment

The Omega RICH, a large-aperture detector for identification of secondary pions, kaons, and (anti) protons was in operation at the CERN Omega spectrometer facility between 1984 and 1994. Cherenkov photons from a 5 m long radiator were detected in drift chambers with quartz windows, using TMAE-loaded counting gases. The RICH was used by experiments WA69 and WA82, until 1988. It was then equipped with new drift chambers and mirrors and was in use since 1990 in experiments WA89 and WA94. The setup in the WA89 hyperon beam experiment is described in more detail and efficiencies, resolutions, and physics results are discussed.

Comprehensive evidence for a narrow

We review earlier results, and add new evidence, on the existence of a narrow ${\mathrm{p}}\overline{\mathrm{p}}$ state at a mass of 2.02 GeV $/c^2$ , which is seen in six different final states, produced via baryon exchange in $\pi^+{\mathrm{p}}$ $(\pi^-{\mathrm{p}})$ interactions at 20 (12) GeV $/c$ . The data come from a new analysis of the WA56 experiment done at the CERN Omega spectrometer. The 2020 peak is produced near mid-rapidity, in a kinematic region not explored before in baryon exchange processes. Agreement is found with an earlier observation of this state, and its absence in a series of unsuccessful searches is explained. Very low upper limits on mesonic decays of this state favor its interpretation as a baryonium candidate.

Production of hyperons and anti-hyperons in heavy-ion collisions at SPS: Experiments at the CERN Omega spectrometer

Recent results from the CERN Omega spectrometer on the enhancement of strange and multiply strange particle production in nucleus-nucleus with respect to proton-nucleus collisions are summarized. In particular, the new data from the WA97 experiment on Ω, Ξ and Λ production in PbPb and p-Pb collisions are presented and discussed.

Strange and multistrange baryon production in PbPb collisions at 158 A GeV/ c

The series of experiments (WA85, WA94 and WA97) at the Omega spectrometer at CERN SPS have systematically studied the production of different strange particles. Strangeness enhancements at central rapidities have been reported by WA85 and WA94 in S-W and SS with respect to p-A collisions. For the investigation of PbPb interactions in the WA97 experiment a new detector suitable for high particle densities, the Pixel Tracking Chamber, has been developed based on silicon pixel detectors. The results on production ratios and cross sections of Λ, Λ , Ξ −, Ξ + , Ω − and Ω + detected by this experiment are presented. A significant enhancement of the Ω over Ξ ratio is observed when comparing p-Pb to PbPb data (a factor of about 3).

Charged particle production in S-S collisions at 200 GeV/c per nucleon

Charged particle production in central S-S collisions at 200 GeV/ c per nucleon has been studied by the WA94 experiment at the CERN-SPS. Particle identification has been provided by the Omega RICH, while a silicon telescope in the Omega spectrometer and an array of MultiWire Proportional Chambers have been used to trace particles through the RICH detector. Production ratios and transverse mass spectra for π ± , K ± and p( p ) at central rapidity and p T > 1.3 GeV/ c are presented.