SHARAD (Mars SHAllow RADar sounder) is a subsurface soundingradar embarked on theMars Reconnaissance Orbiter (MRO) probe. It complements theMARSIS radar onMars Express orbiter,[1] providing lower penetration capabilities (some hundred meters) but much finer resolution of 15 meters in free space.[2]
SHARAD was developed under the responsibility of theItalian Space Agency (ASI, Agenzia Spaziale Italiana), and provided toJPL for use on board NASA'sMars Reconnaissance Orbiterspacecraft in the frame of a NASA/ASI agreement which foresees exploitation of the data by a joint Italian/US team. The INFOCOM dept. of the University ofSapienza University of Rome is responsible for the instrument operations, whileThales Alenia Space Italia (formerly Alenia Spazio) designed and built the instruments. SHARAD operations are managed by INFOCOM from the SHARAD Operation Centre (SHOC), located within theAlcatel Alenia Space facilities in the suburbs ofRome.
SHARAD is intended to map the first kilometer below theMars surface,[3] providing images ofsubsurface scattering layers with high vertical resolution (15 m), with the intent to locate water/ice deposits and to map the vertical structure of the upper subsurface layers.
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SHARAD operates on acarrier frequency of 20 MHz, transmitting a "chirped" signal with a bandwidth of 10 MHz. Pulsewidth is 85 μs and the nominalPulse Repetition Frequency is 700.28 Hz. Transmitted power is 10 W peak. Theantenna is a 10 m dipole.Asynthetic aperture is generated on-ground to reduce the unwanted surface returns from off-nadir scatterers at the same range of the subsurface echoes.
SHARAD is physically divided into two elements:
The instrument operates at fixedPRF (700.28 Hz) and the echo is received in rank 1 (i.e., after the second transmitted pulse). Two alternate (higher and lower) PRF are available to deal with the extended mission orbit range. An open-loop tracking system, based on a prior knowledge of the surface topography, is the nominal means to position the 135 μs receive window on the expected echo position (a closed-loop tracker is available as backup).
The instrument on-boardsignal processing is minimal, and consists of a coherent presumption of the received echoes (programmable between 1 and 32 in power of 2 steps) to reduce the generated data rate, with programmable number of bits (8, 6, 4).
Thechirp signal is generated directly on the 20 MHzcarrier by a digital chirp generator and fed to the power amplifier, followed by a Transmit/Receive switch and thematching network.The receiver provides amplification, filtering and digital gain control directly at RF, and the digitised using anundersampling technique at a rate of 26.6 MHz.A singledigital signal processor provides both the control and processing function.
The instrument industrial team is composed as follows:
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While the initial studies date back to 2001, full-scale development was released only in February 2003.The Engineering Model (EM) of the instrument was delivered toLockheed Martin Space Systems inDenver (responsible for the spacecraft) in March 2004, and integrated into the Orbiter Test Bed.TheProtoFlight Model (PFM) was delivered and integrated on theMars Reconnaissance Orbiter in Denver in September 2004.Mars Reconnaissance Orbiter was launched fromCape Canaveral Air Force Station on August 12, 2005, with anAtlas V-Centaur launch vehicle, and reachedMarsorbit on March 10, 2006. Theaerobraking phase, needed to reach the operational orbit, lasted until August 30, 2006.On September 17, 2006, the SHARAD antenna was deployed, and the first in-flight test of theradar was successfully carried out on September 19. SHARAD has been operational since November 2006.
The SHARAD radar penetrated the north polar layered ice deposits of Mars and revealed a relatively small (about 100 meter) maximum deflection of the underlying rock, which suggests a stronglithosphere greater than 300 kilometers thick.[4] Radar results consistent with massive deposits of water ice in middle latitudes support a debris-covered glacier hypothesis.[5]
On November 22, 2016, NASA reported finding a large amount ofunderground ice in theUtopia Planitia region of Mars using SHARAD. The volume of water detected has been estimated to be equivalent to the volume of water inLake Superior.[6][7][8]
The calculations for the volume of water ice in the region were based on measurements from SHARAD, the ground-penetrating radar instrument on theMars Reconnaissance Orbiter (MRO).
SHARAD radar data when combined to form a 3D model reveal buried craters in the north polar cap. These may be used to date certain layers.[9]
Research, published in April 2011, described a large deposit of frozen carbon dioxide near the south pole. Most of this deposit probably enters Mars' atmosphere when the planet's tilt increases. When this occurs, the atmosphere thickens, winds get stronger, and larger areas on the surface can support liquid water.[10] After more analysis, it was discovered that if these deposits were all changed into gas, the atmospheric pressure on Mars doubles.[11] There are three layers of these deposits; each are capped with a 30-meter layer of water ice that prevents the CO2 from sublimating into the atmosphere. Insublimation a solid material goes directly into a gas phase. These three layers are linked to periods when the atmosphere collapsed when the climate changed.[12]
