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Scientific Instruments

Energetic Neutral Atoms Analyser (ASPERA)

The general scientific objective of ASPERA-3 is to study the solar wind - atmosphere interaction and to characterise the plasma and neutral gas environment in the near-Mars space through energetic neutral atom (ENA) imaging and in-situ plasma measurements. ENAs are produced via charge exchange process between singly-charged ions and neutral gases of the exosphere / upper atmosphere. Due to their neutrality, ENAs are decoupled from the electromagnetic fields and propagate straight forward like photons. Directional detection of the ENAs therefore yields a global image of the interaction region.

The main scientific objectives can be subdivided into specific tasks:

The studies to be performed address the fundamental question: How strongly do the interplanetary plasma and electromagnetic fields affect the Martian atmosphere? This question is directly related to the problem of Martian dehydration. Where is the water? Is it lost or frozen? If it is the former, what could produce such an effective escape mechanism? If it is the latter, where is the tremendous amount of water stored? Since liquid water is the fundamental requirement for life, a clear understanding of the fate of the Martian water is a crucial issue in resolving the problem whether or not life existed on Mars in the past.

No instruments with similar scientific objectives and capabilities have been or are planned to be flown to Mars.

High Resolution Stereo Color Imager (HRSC)

The HRSC will provide simultaneous high-resolution, stereo, color and multiple phase angle coverage and will acquire imaging data of unprecedented quality of the Martian surface, of atmospheric phenomena, and of the Martian moons. The scientific objectives of the HRSC will focus on:

The camera is a pushbroom-scanning instrument with nine CCD line detectors mounted in parallel behind one optical unit. Its unique feature is the ability to obtain nearly simultaneously imaging data of a specific site at high resolution, with along-track triple stereo, with four colors, and at five different viewing geometries, thus avoiding any time-dependent variations of the observing conditions (illumination, atmospheric properties, variable features).

The unique instrument combines high precision optics and modern CCDs controlled by high-speed electronics including on-line data compression up to a ratio of 20:1. During the nominal mission of two earth years the HRSC will cover 50% of the Martian surface simultaneously at a spatial resolution of 12-15 m/pixel. Also, more than 70% of the surface will be observed at a spatial resolution of better than 30 m/pixel. With these resolutions the HRSC will close the gap between medium to low-resolution and the very high resolution images provided by NASA Mars missions and will thus complete the orbital high-resolution optical reconnaissance of Mars.

The image size from the nominal pericenter altitude of 300 km (i.e. the closest distance to Mars in orbit) is 62 km wide and at least 330 km long.

The along-track stereo capability will provide accurate topographic data with height accuracies on the order of the image pixel size. The multi-phase capability allows to reconstruct the photometric surface behavior and provides information on the physical properties (e.g porosity, grain size distribution) of the uppermost surface layers as well as the scattering characteristics of the Martian atmosphere. The color capability in the visible and near-infrared of the HRSC will be used to define and map geologic surface units and to separate atmospheric features from surface characteristics.

IR Mapping Spectrometer (OMEGA)

OMEGA is a visible and near-infrared mapping spectrometer combining imagery and spectrometry to provide the mineralogical and molecular composition of the surface and the atmosphere of Mars through the spectral analysis of the re-diffused solar light and surface thermal emissions. The instrument addresses major questions associated with the internal structure, geologic and chemical evolution, past activity and present surface variation. It will greatly contribute to the understanding of the evolution of Mars from geological time scales to seasonal variations. It will in particular give unique clues for the understanding of the H2O and CO2 cycles over the Martian life.

OMEGA will provide a global coverage at medium resolution (1 to 5 km) of the entire surface of Mars from altitudes between 1000 and 4000 km. It will take snapshots of selected areas, amounting to at least a few percent of the surface, with a resolution of a few hundred meters when observed from an altitude of about 300 km. More specifically, with the given spatial resolution, OMEGA will make it possible to:

The optical part of OMEGA consists of two co-aligned units, each including a telescope. A visible channel analyses the light from 0.5 to 1.5 mm using a two-dimensional CCD matrix. The near-infrared channel disperses the light through two spectrometers onto two detecting arrays that are cooled to 70K by a mechanical cooling system. A scanning mirror in front of the infrared channel permits to acquire cross-track swaths consistent with the maximum field of view of 8.8 degrees.

Atmospheric Fourier Spectrometer (PFS)

The Planetary Fourier Spectrometer (PFS) for the Mars Express mission is an infrared spectrometer optimised for atmospheric studies able to cover the wavelength range from 1.2 to 45 mm divided in two channels (Short Wavelength and Long Wavelength) with a boundary at 5 mm. The spectral resolution is 2 cm-1. The spatial resolution is 10 km for the SW and 20 km for the LW when Mars is observed from a height of 300 km (nominal height of the pericentre). The scientific objectives of the PFS experiment can be summarised as it follows:
  1. Atmospheric studies (Primary)
    1. global long time monitoring of the three-dimensional temperature field in the lower atmosphere (from the surface up to 40-60 km);
    2. measurements of the minor constituents variations (water vapor and carbon monoxide);
    3. search for possible other small components of the atmosphere;
    4. new determination of the D/H ratio;
    5. study of the optical properties of the atmospheric aerosols: dust clouds ice clouds hazes; determination of the size distribution and chemical composition;
    6. investigation of radiance balance of the atmosphere and the influence of aerosols on energetics of the atmosphere.
    7. study of global circulation, mesoscale dynamics and wave phenomena.

  2. Surface studies (Supplemental):
    1. monitoring of the surface temperature;
    2. determination of the thermal inertia obtained from the daily surface temperature variations;
    3. determination of the restrictions on the mineralogical composition of the surface layer;
    4. determination of the nature of the surface condensate and seasonal variations of its composition;
    5. measurements of the scattering phase function for selected places of the surface;
    6. pressure and height local determination (CO2 altimetry) for selected regions;
    7. surface-atmosphere exchange processes.

Radio Science Experiment (RSE)

The Mars Express Orbiter Radio Science experiment is proposed:
  1. to perform radio sounding of the neutral Martian atmosphere (occultation experiment) to derive vertical density, pressure and temperature profiles as a function of height (height resolution better than 100 meter)
  2. to perform radio sounding of the ionosphere (occultation experiment) to derive vertical ionospheric electron density profiles
  3. to determine the dielectric and scattering properties of the Martian surface in specific target areas by a bistatic radar experiment
  4. to determine gravity anomalies in conjunction with simultaneous observations by the stereo camera as a base for three-dimensional (3D) topography, for the investigation of a high resolved structure and evolution of the local Martian crust and lithosphere
  5. to perform radio sounding of the solar corona during the superior conjunction of the planet Mars with the Sun. In the case that the Mars Netlander will be launched in 2005, RSE can support the Netlander Geodesy Experiment for the derivation of the rotational state and internal structure of the planet.

The radio links of the spacecraft TT&C subsystem between the orbiter and the Earth will be used for these investigations. An Ultra Stable Oscillator (USO) added to the TT&C subsystem would considerably improve the atmospheric sounding investigations because regular transponder oscillators have frequency stabilities typically six orders of magnitude poorer than an USO and are not useable for this kind of radio science investigation. Atmospheric sounding using a two-way radio link is principally feasible but would result in a loss of 50% of the data and shows further disadvantages with respect to profile accuracy. A simultaneous and coherent dual-frequency downlink via the High Gain Antenna (HGA) is required to separate the contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively.

The high signal-to-noise ratio of a downlink radio carrier at X-band will allow to sound the atmosphere to an altitude of 50 km. Derived atmospheric density, pressure and temperature will have a height resolution better than 100 meter which is far superior than what can be achieved by other instruments which are limited to typically one scale height (order of kilometers). Local heating of the atmosphere during a dust storm or entrained dust will clearly be visible in the derived temperature profiles. An S-band downlink frequency, most sensitive to the ionospheric plasma, will provide electron density profiles from 90 km to 300 km altitude with a resolution of 100 el/cm3. This allows to investigate in particular the most interesting night time ionosphere of Mars and to address the question of the height of the Martian ionopause. Combined observations by radio science and the stereo camera during pericenter passes will yield first time correlations of 3D high resolution topography (order of 10 m) and gravity accelerations (order of several mgal). The bistatic radar experiment will address open questions in surface composition e.g. "Stealth area" south of Tharsis and the composition of polar ice caps. The superior solar conjunction of Mars in fall 2004 will allow a dual-frequency sounding of the solar corona for the first time since the Ulysses conjunctions. There is also potential of a first dual-spacecraft radio sounding since Rosetta will also be at superior solar conjunction at the same time.

UV Atmospheric Spectrometer (SPICAM)

The atmosphere of Mars contains ozone and water vapor, as does the Earth's atmosphere. However, the quantity of ozone is much smaller than on the Earth, and as a consequence the solar UV radiation reaches the ground. In addition, ozone on Mars is a strong source of oxidation at ground level, destroying very fast all organic molecules (together with the action of OH radicals produced by chemical reactions between O3 and H2O. When looking at nadir along track, the SPICAM UV spectrometer (118 - 320 nm on an intensified CCD, 3.8 kg) is essentially an Ozone detector, where its strongest UV absorption band at 250 nm is imprinted in the spectrum of the solar light scattered back from the ground. This very technique allowed the discovery of Ozone on Mars with Mariner 9, and is heavily used to map the total Ozone content of the Earth atmosphere from space. On Mars Express, the correlated study of ozone and water vapor will allow to compute the quantities of ozone and other oxidants, and solar UV reaching the ground. This Mars environment needs to be understood for a better understanding of conditions in which life could have developed on Mars (or not), and the possible transformation of the some rocks by oxidation.

The second major objective of the SPICAM UV is to determine the vertical profile of CO2, temperature, O3 and clouds and aerosols by the technique of stellar occultation. The instrument is oriented toward a star setting behind the horizon. From the atmospheric absorption imprinted progressively on the star spectrum, the density of CO2, O3 and dust are retrieved as a function of altitude, and also the temperature from scale height. Several (3-5) occultations per orbit are foreseen.

Finally, spectroscopic UV observations of the upper atmosphere will allow to study the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind, variable with solar activity. Also, it will allow a better understanding of escape mechanisms and magnitude estimates, crucial for the long-term evolution of the atmosphere.

Sub-surface Sounding Radar/Altimeter (SSRA)

The primary scientific objective of the SSRA is to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Detection of such reservoirs of water will address key issues in the hydrologic, geologic, climatic and possible biologic evolution of Mars. Three secondary objectives are also defined: subsurface geologic probing, surface characterisation, and ionospheric sounding. An additional secondary scientific objective is to characterise the properties of the surface of Mars by operating the SSRA as an altimeter. This will generate a new topographic data set for Mars at a resolution better than 10 km, and will also measure the roughness and Fresnel reflection coefficient of the surface. The final objective of the investigation is to use radar soundings to study the electron density in the Martian ionosphere. Since the ionosphere strongly affects the propagation of low-frequency radar signals, ionospheric measurements play a crucial role in selecting an operational strategy for subsurface sounding and in interpreting the resulting data. Ionospheric sounding measurements will also be used to study the interaction of the solar wind with the ionosphere and upper atmosphere of Mars.

The instrument is a multi-frequency nadir-looking pulse limited radar sounder and altimeter, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections. The SSRA can be effectively operated at any altitude lower than 800 km with full performance. The multi frequency observation will allow the estimation of the material attenuation in the crust and will give significant indications on the dielectric properties of the detected interfaces. The instrument consists of two antenna assemblies and an electronics assembly. The antenna assembly consists of a primary dipole antenna, parallel to the surface and perpendicular to the direction of motion, used to receive echoes reflected by the Martian surface and subsurface, and a secondary monopole antenna, oriented along the nadir, used to receive only off-nadir surface returns.

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