Insight Into Marsis Radar Data Analysis

Following completion of its deployment on 17 June, MARSIS, the Mars Advanced Radar for Subsurface and Ionospheric Sounding onboard the ESA Mars Express spacecraft, has started collecting scientific data from the surface, the subsurface and the ionosphere of Mars. The major scientific goals of this ground penetrating radar experiment are to characterise the subsurface layers of sediments and possibly detect and map underground water or ice, to characterise the radar properties of the surface and to provide data on the planet’s ionosphere.

Part of the primary objective is the search for water and ice reservoirs, aimed at addressing key issues in the hydrological, geological, and climatic evolution of Mars. These include the current and past global inventory of water, mechanisms of transport and storage of water, the role of liquid water and ice in shaping the landscape of Mars, and the stability of liquid water and ice at the surface as an indication of climatic conditions.

The MARSIS Instrument

MARSIS is a multi-frequency, coherent pulse, synthetic aperture radar sounder. Frequencies are selectable in order to adapt the experiment to the Mars environment. Lower frequencies are best suited to probe the deep subsurface and the highest frequencies are used to probe shallow subsurface depths, while all four frequency channels (1.8, 3.0, 4.0, and 5.0 MHz) are suited to study the surface and the upper atmospheric layer of Mars. The two 20-metre long antenna booms (dipole) are sending radio signals towards the Martian surface and are receiving echoes back. The secondary, receive-only, 7-metre long monopole antenna is to be used in conjunction with the MARSIS dipole, to correct for off-nadir surface roughness effects. The monopole will, therefore, find its best use during the investigations of areas where the surface roughness is higher, that is where it can minimize the effects of surface “clutter” on subsurface feature detection.

The MARSIS Scientific Data Collection

MARSIS has a unique capability of sounding the Martian environment with coherent long-wavelength wide-band pulses that allow the collection of a large amount of significant data about the subsurface, surface and ionosphere.

For subsurface probing, MARSIS must operate under 800 kilometres altitude from the Martian surface, while for ionospheric sounding MARSIS provides acceptable results from a distance of up to 2000 kilometres. The radar vertical depth resolution is 150 metres (in the free space), while echo profiles of the subsurface are acquired at a lateral spacing of about 5 to 9 km, depending on the spacecraft altitude.

The MARSIS radar is designed to operate around the pericentre of the orbit, when the spacecraft is closer to the planet’s surface. In each orbit available to MARSIS, the radar is switched on for 36 minutes around this pericentre point, dedicating the central 26 minutes to subsurface observations and the first and last five minutes of the slot to active ionosphere sounding.

Nighttime is the environmental condition favourable to subsurface sounding, as the ionospheric plasma frequency is lowest. The ionosphere is more energised during the daytime and disturbs the radio signals used for subsurface observations. Although even during the daytime, the MARSIS instrument can detect signals from the surface and subsurface after proper ionospheric corrections are made.

The radar is for the first time making measurements of the ionosphere in the sub-solar and night side regions that were not accessible to previous Mars missions. MARSIS will probe the Martian plasma environment for electron densities in the range from 30 to 3.5×106 electrons per cm3, with increased spatial resolution of vertical profiles. The active sounding ionospheric mode consists of transmitting sinusoidal pulses with a nominal duration of 91.4 ms in 160 frequency steps from 0.1 MHz to 5.5 MHz. Such measurements, with the aim of global coverage, will explore the relationships between the neutral atmosphere and ionosphere, and the interactions of the solar wind with the planet.

The MARSIS Data Processing Goals

Regarding the ground processing, the main goals of the analysis of the MARSIS scientific data sets include:

• Identify and isolate those echoes that come from the surface, subsurface, or ionosphere.
• Extract information on electrical properties of the reflecting surface for the purpose of constraining its composition.
• Estimate large-scale topography, roughness and reflectivity of the surface.
• Pick out layers of rocks interspersed with ice, which are more likely to exist close to the Martian surface than liquid water.
• Look for signatures or indications of water possibly locked into frozen or liquid underground reservoirs or aquifers, up to 5 km below ground.
• Measure the thickness of sand deposits in sand dune areas, and determine the existence of layers of sediments or volcanic flows.
• Build up a three-dimensional picture of the south polar cap and surrounding layered terrains of the upper Martian crust.
• Measure the electron density in the ionosphere and quantify the effect of charged particles streaming out from the Sun (solar wind) on the upper atmosphere.
• Produce global high-resolution ionospheric profiles for day and night times.

Subsurface and Surface Data Investigation

An essential objective of the data processing is to detect, map and characterize subsurface material distribution and dielectric discontinuities in the upper portions of the crust of Mars. These features may include boundaries of liquid water-bearing zones, icy layers, geologic units and geologic structures.

The process used to obtain the surface returns is standard to any terrestrial ground penetrating radar, but the application to Mars is unique with respect to subsurface investigations. The first surface reflection echoes of MARSIS operating as a sounder are processed to give estimates of the average height, roughness and reflection coefficient of the surface layer.

The processing scheme of the MARSIS data sets involves the in-depth analysis, at each frequency, of a set of parameters that are consolidated into a database for interpretation of local and regional behaviour and comparison with other data sets and simulations:

• The sounding parameters, which can be tuned in response to variations in topography and solar illumination conditions, and be used in the planning of future measurements.
• Confidence criteria pointing to subsurface interface detection. The MARSIS team has started screening all data received on ground in order to make sure that signals that could be interpreted as coming from different underground layers are not actually produced by surface irregularities. The presence of weaker signals after the first strong surface return enables the detection of subsurface interfaces. Also, echo profiles collected at different frequencies can be processed to enhance the discrimination of subsurface reflections, which are strongly dependent on the frequency, from the surface reflections, which are less dependent on the radar frequency.
• The intensity of surface and subsurface reflection(s). The peak value of the average echo waveform can be used to estimate the backscattering coefficient. In conjunction with the roughness value, the determination of the Fresnel reflection coefficient of the surface can be made.
• The echo dispersion at the surface. This gives indications of surface roughness.
• The dielectric constants. Observed discontinuities may indicate the presence of water/ice or dry/ice interfaces.
• The time delay to subsurface reflector(s) and related surface elevation. The time delay between the first strong echo and secondary subsequent signals allows approximate measurement of the depth of the interfaces assuming an average dielectric property. Measurement of the time delay of the echo leads to estimates of the average distance of the radar from a reference flat surface level, while the duration of the waveform leading edge is proportional to the large-scale surface roughness.

Subsurface and surface MARSIS data analysis is expected to yield significant new insights into the subsurface structure and lithology of the Martian crust, including the nature of the polar-layered deposits. Among the potential outcomes of radar sounding data processing is the detection of shallow reservoirs of liquid water, perhaps associated with thermal anomalies or an insulating upper stratigraphy. The discrimination of ground-ice boundaries should be within reach of the MARSIS detection capabilities. Other stratigraphic and structural boundaries could be identified, providing a view of the vertical dimension of the Martian geology. For a number of regions of interest, modelling of the electrical properties of the layers and interfaces will be performed to estimate the thickness of layers, the depth to interfaces, the composition and dielectric properties of the materials.

With respect to instrument performance, one challenge that must be overcome is the presence of radar scattering from the surface of Mars, which will be simultaneously detected with the echoes from subsurface interfaces. To solve this problem, the monopole signal will be used in conjunction with predictions of the scattering behaviour that may be expected from Martian surface topography, through simulations and comparisons with existing theoretical scattering models.

Ionospheric Sounding Data Investigation

MARSIS ionospheric measurements are performed in both passive and active mode. In passive mode, the thermal emission line at the local electron plasma frequency is used to measure the local electron density. The active technique uses radar soundings to measure the vertical range to the ionospheric reflection point as a function of frequency.

The various parameters and factors to be investigated as part of the ionospheric sounding data processing are:

• Vertical profiles of the plasma electron frequency (hence, the electron density).
• Temperature profiles.
• Effects associated with magnetic merging, plasma clouds, and plasma streams.
• Effects from dust storms and precipitations of solar wind ions in cusp-like magnetic regions.
• Effects from the excitation of hydrodynamic waves by the interaction of the ionosphere with the solar wind.

Notes

The MARSIS instrument was developed within the framework of a Memorandum of Understanding between the Italian Space Agency (ASI) and NASA. Alenia Spazio developed it under ASI management and the scientific supervision of the University of Rome La Sapienza, in partnership with the Jet Propulsion Laboratory (JPL) in Pasadena, California, and the University of Iowa. JPL provided the antenna manufactured by Astro Aerospace. It is the first instrument designed to actually look below the surface of Mars.

The Italian-American MARSIS team is also largely involved on the SHARAD radar, a facility instrument provided by ASI for NASA’s Mars Reconnaissance Orbiter (MRO), launched on 12 August 2005. MARSIS and SHARAD are two radars designed to provide complementary information about the Martian subsurface. MARSIS can penetrate to an average depth of five kilometres, while SHARAD will concentrate on layers closer to the surface.