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MARSIS: Subsurface Sounding Radar/Altimeter
for the Mars Express Mission
Marsis is one among a number of experiments on the ESA Mars Express
orbiter. It is a radar system designed to penetrate the upper surface
of Mars in order to search for discontinuities indicative of
subsurface ice or water. Scientists in the Institute participate in the
preparation of experiments with groups from JPL and the University of
Rome and in the planning of analysis and interpretation of the data to
be collected.
Science objectives of MARSIS
Marsis is a radar instrument carried on Mars Express designed for the
primary task of searching for water, water-ice or permafrost layers
believed to exist at some depth under the visible surface of
Mars. There is much evidence that water once was plentiful on Mars.
There are stream lined islands formed by flowing water, flow patterns
reminiscent of wadis in Earth deserts, and outflow channels thought to
have been formed by sudden out-rush of subterranean water. Secondary
tasks are the measurement of the scattering properties of the surface
of Mars at the long wavelengths required for penetration into the
surface yet short enough to pass through the martian ionosphere. The
electron density and temperature in the topside ionosphere may also be
studied as a secondary task by using the radar as a topside sounder or
by using the radar antenna system for in situ impedance
measurements. Estimates of Martian water ranges from a 50 to a 500 m
deep planet-wide ocean. No obvious mechanism for the escape of water
from the planet has been devised. Jean's escape of water via the
atmosphere is very slow (of the order of 3 m over 5 Gy). Assuming that
Mars was formed with approximately the same relative amount of water
as the Earth, it must be assumed that a substantial fraction of this
water remains on Mars in one form or another. It is commonly believed
to be bound as ice in the polar caps and, in the ground, as ice, icy
permafrost or even as water. There is indirect evidence for
widespread presence of ice, permafrost or liquid water through the
existence of rampart craters, terrain softening, chaotic terrain and
thermokarst. Marsis will attempt to directly confirm the presence of
sub-surface water.
A brief summary of the questions addressed by SURPRISE and the
measurements which must be made in order to answer them is given in
the following table:
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Questions addressed
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Observations bearing on these questio
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| Is there subsurface ice/permafrost on Mars? |
Presence of discrete echo components would be evidence of icy layers.
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What is the depth of the layers and the nature of the layer transition?
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Determination of echo power versus time delay. Search for several echo
components..
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| Does the depth vary with latitude? |
Echo delay as the satellite covers a wide a range of latitudes |
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Do the layers and the surface echoes correlate with visible and radar
surface features?
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Comparison of properties of radar echoes with changes in terrain and
radar surface echo properties.
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| What is the density and temperature along the orbit of the satellite? |
Measurement of the impedance versus frequency of the radar antenna used as
a probe.
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What is the electron density at the top of the ionosphere? Is there
evidence of a global magnetic field?
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Invert range frequency data to obtain topside electron density profiles
and interpret in terms of scale height. Compare with riometer and total
electron content observations.
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MARSIS instrument description
The proposed 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 radar can be effectively operated at any altitude
lower than 800 km. 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. Maximum penetration depths are achieved at the lowest
frequencies. On the dayside of Mars, the ionosphere does not allow
the use of frequencies < ~3.5 MHz. To optimize subsurface probing
depths operations on the nightside of Mars are desirable. Four
frequency bands are centered at 1.9, 2.8, 3.8 and 4.8 MHz. In night
side operations it will be possible to use 1.9 MHz and 2.8 MHz bands
to estimate the dielectric properties of the subsurface detected
interfaces and the 3.8 MHz and /or 4.8 MHz for further reduction of
the surface clutter. On day side operations the 3.8 MHz and 4.8 MHz
frequencies will be able to penetrate the ionosphere and will be used
to estimate the dielectric properties. Up to four interleaved
channels of data can be processed and recorded simultaneously. Under
nominal operations, these channels will consist of main antenna and
secondary antenna receive streams, each at two frequencies. A
"chirp" signal, with a bandwidth of 1 MHz, will be generated and
transmitted at each operating frequency for a period of about 500
microseconds. The instrument then switches to a receive mode and
records the echoes from the surface and subsurface for the expected
duration. The total transmit-receive cycle lasts on the order of a
few milliseconds, depending on altitude. The received signals are
down-converted, passed to a digital-to-analogue converter, and
compressed in range and azimuth. The azimuth integration accumulates
about 1 second of pulses, resulting in an along-track footprint size
of 5 km. The cross-track footprint size is on the order of 10 km.
To ensure the greatest possible penetration of the electromagnetic
waves into the ground the wavelength must be chosen as long as
possible, only limited by the requirement that the waves penetrate the
ionosphere without appreciable distortion to reach the ground. A
secondary task of the radar system is to examine changes in
reflectivity and scattering properties of local areas on the surface,
and to relate them to optical images and other data in order to
understand the nature of different terrain forms.
Another secondary task which may also be assigned to the radar system
is to probe the top- side ionosphere. The topside of the ionosphere
is now known only from occultation experiments, which are limited by
geometry to near the morning or evening terminators. The study of the
ionosphere is of considerable interest for the understanding of the
interaction of the solar wind with a weakly magnetized planet,
particularly in view of the recent discovery of localized magnetic
field structures. These studies were a major task in the Mars96
mission which Mars Express is intended to partially replace. In
addition it is intended to use the radar antenna as an impedance probe
to measure the local electron density and temperature in the AIM
experiment.
The radar system proposed will operate in the frequency range 0.2 to
7.5 MHz into a dipole antenna which will be shorter than a half wave
dipole except near the upper end of the band. The radar will operate
in a stepped frequency mode where the reactive part of the antenna
impedance is tuned out instantaneously with an active network to
insure as good antenna match as possible. The resistive part of the
antenna will be matched in four bands only. The stepping through any
one of the three bands in some 100 steps, depending on altitude, will
take 50 to 100 msec. The received signals will be sampled at each
frequency, and the sampled data for each step cycle (frequency sweep)
will be stored for transmission to Earth where it will be processed
for removal of surface clutter and for decoding the signal into a
power versus depth profile. The transmitted signals in subsequent
frequency step cycles must be so related that coherent combinations of
the samples can be made. As the duty cycle of the radar will be held
close to 50%, it is necessary to also include a special mode of
operation, to determine the approximate time delay of the surface
echo, and to adjust the repetition rate of the radar to place the
echoes appropriately in the transmitter-off intervals. In addition
orbit information is expected to be available in the spacecraft to
help adjust repetition rate and pulse lengths along the orbit. The
radar transmitter, the receiver and the tunable antenna systems are
available as spare flight models from the Mars96 Long Wavelength Radar
(LWR) with some modifications. The control and the data storage
system must be reprogrammed or modified in order to adjust it to the
new modes of operation of the radar described in Section 3.
A special mode to determine the time delay versus frequency in the
lower frequency bands will be used to detect the echoes from the
topside of the ionosphere, and will be converted to topside electron
density profiles. The data thus obtained will be complementary to the
measurement of electron content from ground stations through the total
electron content method, to the observations with a riometer (relative
ionospheric opacity meter) of the ionospheric absorption from the
ground, both measurements proposed for landers in the Mars Express
mission, and to the measurements in situ of the electron density and
temperature using the radar antenna as an impedance probe. The
impedance probe measurements are included as a part of the current
proposal. The measurement of atmospheric water vapor content by
microwave emission observations proposed for the mission will also
bear on the water budget of the planet.
MPS contribution and MARSIS investigators
The responsibility for the development of the radar system and for the
coordination of the modifications of the radar system and its
operation will rest with MPS, with key partners in IRE/RAS and
IKI/RAS (Russia), CNRS CEPHAG and SA (France) and ESA/SSD (the
Netherlands).
Related links
Institute of Space Sensor Technology and Planetary Exploration
at the DLR, in Berlin
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