Geoscience Reference
In-Depth Information
The spacecraft altitude varies approximately between 250 km at the pericenter,
placed over the south pole, and 320 km at the apocenter. This characteristic, which
precludes the opportunity to observe Phobos, allows, for each orbit, to acquire
observations on both Martian hemispheres.
MRO was launched from Cape Canaveral Air Force Station on 12th August 2005,
with an Atlas V-Centaur launch vehicle, and reached Mars orbit on 10th March
2006.
SHARAD transmits pulses through a 10 m dipole antenna operating exclusively
as a subsurface sounder with a carrier frequency of 20 MHz and a bandwidth of
10 MHz, parameters which allow a vertical resolution varying between 10 and 20 m.
SHARAD uses, as MARSIS, synthetic aperture radar (SAR) techniques; but, while
MARSIS is an unfocused SAR with a best along-track resolution of 2 km, SHARAD
data through a processing with a focusing algorithm (Chirp Scaling Algorithm) can
achieve a best along-track resolution of 300 m.
5.4.3
MARSIS and SHARAD Performances Comparison
One of the main quests in the exploration of the Red Planet is the identification of
water deposits on or below the surface of Mars, which are supposed to last as a relic
of ancient Martian oceans. The capability of penetrating deep into a planet surface
makes low-frequency radars the perfect candidates for detecting such deposits.
The operating principle of a subsurface sounder is extremely simple: the radar
antenna transmits a pulse of electromagnetic energy which, as it reaches the planet
surface, produces a first reflection echo propagating back to the radar. Due to the
low frequencies adopted, a large fraction of the electromagnetic energy propagates
through the crust generating additional reflections as it encounters subsurface
discontinuities. Also those secondary echoes, much weaker than the front surface
signal, propagate back to the receiver. The radar, through SAR processing, produces
sets of synthetic apertures, called frames, adjacent to each other, which altogether
form a radargram, which basically is a cross section of the planet surface.
MARSIS and SHARAD are complementary instruments: MARSIS is able to
detect subsurface interfaces at depth of more than 3 km, but due to the limited
bandwidth adopted, it has a limited range resolution. SHARAD, on the contrary, has
a penetration capability of approximately 1,500 m, but can discriminate subsurface
interfaces close to the surface, allowing to analyze the upper portions of the Martian
crust.
Figure 5.3 shows two radargrams produced with data acquired over Martian
south pole: MARSIS is capable to detect the basal reflector at a depth of 3.7 km,
while SHARAD, which signal cannot reach the basal unit, manages to generate a
radargram with a finer resolution, both horizontal and vertical, providing unique
information on Martian stratigraphy.
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