Chapter 9: Sample handling and instruments for the in- situ exploration of ice-rich planets Julie C Castillo, Yoseph Bar-Cohen, Steve Vance, Mathieu Choukroun, Hyeong Jae Lee, Xiaoqi Bao, Mircea Badescu, and Stewart Sherrit, Jet Propulsion Laboratory/Caltech 4800 Oak Grove Drive, , CA 91109, USA Phone: (818) 354-4875, e-mail: [email protected] and Melissa G. Trainer and Stephanie A. Getty Mail Code: 690, NASA/GSFC, Greenbelt , MD 20771 Phone: 301-614-6104, email: [email protected] 9.1   Introduction .............................................................................................................................. 2   9.1.1   Science Rationales for the Exploration of Mars’ Polar Regions ................................................... 2   9.1.2   Enceladus Exploration .................................................................................................................. 2   9.1.3   Titan’s Surface Exploration .......................................................................................................... 3   9.1.4   Europa Exploration ....................................................................................................................... 4   9.1.5   Ceres Exploration .......................................................................................................................... 5   9.2   Geophysical Exploration Techniques ...................................................................................... 5   9.2.1 Sonar - Acoustic radar ..................................................................................................................... 6   9.2.1.1   Transducer composition, and behavior at cryogenic conditions .......................................... 7   9.2.1.2   Estimates for the operation of sonar on Titan ...................................................................... 8   9.2.1.3   Pulse-Echo versus Phase Shift Techniques ......................................................................... 11   9.2.2 Ground Penetrating Radar (GPR) .................................................................................................. 11   9.2.2.1   Dielectric properties of planetary surfaces ........................................................................ 14   9.2.3 Seismology .................................................................................................................................... 15   9.2.3.1   Identification and Modeling of Possible Seismic Sources .................................................. 16   9.2.3.2   Broadband Seismometers – Status Quo .............................................................................. 17   9.3   Chemical Measurements ........................................................................................................ 18   9.3.1   Airless Bodies ............................................................................................................................. 19   9.3.1.1   Orbital Instruments: Neutrals, Ions, Dust, and Ice ............................................................ 19   9.3.1.2   Landed Instruments: Loose Regolith, Condensed Ices, and Consolidated Rock ............... 20   9.3.2   Titan surface and liquids ............................................................................................................. 24   9.3.2.1   Sampling Lakes and seas on Titan ...................................................................................... 24   9.3.2.2   Ice and organic surfaces on Titan ....................................................................................... 31   9.3.2.3   Example of Application: Tunable Laser Spectroscopy for low temperature atmosphere on Titan 32   1 9.4   Summary/Conclusions ........................................................................................................... 34   9.5   Acknowledgement ................................................................................................................. 34   9.6   References .............................................................................................................................. 35   9.6.1   Internet references ....................................................................................................................... 42   9.1 Introduction NASA’s key science goals for the exploration of the solar system seek a better understanding the formation and evolutionary processes that have shaped planetary bodies and emphasize the search for habitable environments. Efforts are also made to detect and quantify resources that could be used for the support of human exploration. These themes call for chemistry and physical property observations that may be best approached by in situ measurements. NASA’s planetary missions have progressively evolved from remote reconnaissance to in situ exploration with the ultimate goal to return samples. This chapter focuses on the techniques, available or in development, for advanced geophysical and chemical characterization of icy bodies, especially Mars polar areas, Enceladus, Titan, Europa, and Ceres. These astrobiological targets are the objects of recent or ongoing exploration whose findings are driving the formulation of new missions that involve in situ exploration. After reviewing the overall objectives of icy body exploration (Section 9.1) we describe key techniques used for addressing these objectives from surface platforms via geophysical observations (Section 9.2) and chemical measurements (Section 9.3). 9.1.1 Science Rationales for the Exploration of Mars’ Polar Regions Mars is the 4th planet from the Sun, the 2nd smallest planet in the Solar System, after Mercury, and it is a neighboring body to Earth. It was named after the Roman god of war, and it is also known as the "Red Planet" because of its reddish appearance resulting from form the iron oxide that is prevalent on its surface. Polar temperature ranges from −143°C to -125°C and operating on its surface requires the use of mechanisms that can tolerate low temperatures. Its surface contains thin atmosphere with a pressure of about one-hundredth of Earth’s atmosphere and it is filled with impact craters, and the volcanoes, valleys, deserts, and polar ice caps. Its rotational period and seasonal cycles are somewhat similar to those of Earth, as is the axial tilt that produces the seasons. For its potential of harboring life in some form including extinct, Mars has been one of the most attractive bodies for planetary exploration. This includes the five orbiting spacecraft: 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN and Mars Orbiter Mission; and the landed missions: Mars Exploration Rovers Spirit and Opportunity, Phoenix lander, and the Mars Science Laboratory Curiosity. In particular the Phoenix mission deployed a lander at high-latitude where ice was found as well as perchlorates [Hecht et al. 2009]. Several missions [e.g., Ice Breaker by McKay et al. 2015; Cryobot by Aharonson et al. [http://web.gps.caltech.edu/~oa/simulations.shtml] have been proposed to gain further insight into the structure of Mars polar caps and their isotopic makeup. The latter can help constrain the origin of Mars’ ice, which bears implications to constrain the origin of Earth’s water and organics. 9.1.2 Enceladus Exploration Saturn’s Moon Enceladus was discovered in 1789 by William Herschel. Enceladus is only about 500 kilometers (310 mi) in diameter and has been extensively studied, first by the two Voyager 2 spacecraft that passed nearby in the early 1980s, and more recently, since 2004, by the Cassini- Huygens Mission. Enceladus’ surface reflects almost all the sunlight that strikes it and it is mostly covered by clean ice with a surface temperature as low as −198 °C. In 2005 the Cassini orbiter discovered a water-rich plume venting from Enceladus's South Polar Region. Large rifts located around the South pole, called “Tiger Stripes” shoot geyser-like jets of water vapor, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, at approximately 200 kilograms (440 lb) per second [Hansen et al. 2006]. More than 100 geysers have been identified [Porco et al., 2015] whose activity is driven by the tidal stress exerted by Saturn. Some of the water vapor falls back as "snow", whereas the rest escapes, and may be the source of most of the material making up Saturn's E ring [Kempf et al., 2008]. The heat production inferred from surface temperature measurements at the Tiger Stripes is about 15 GW [Spencer and Nimmo 2013] has not been explained yet. Further, in 2014, Cassini data provided evidence for a large south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km [Iess et al., 2014]. These observations make Enceladus a major target for in situ exploration with focus on the South Polar Region and sampling of the plumes by geochemistry instruments during high-velocity flybys [Lunine et al. 2015]. Landers for Enceladus exploration have been proposed [e.g., Konstantinidis et al. 2015] that could help further the assessment of Enceladus’ habitability potential. 9.1.3 Titan’s Surface Exploration Saturn’s largest satellite Titan was discovered by Christiaan Huygens in 1655 and the fact that it had an atmosphere was established by Kuiper in 1944, who detected absorption bands due to methane at infrared wavelengths from ground-based observations. Sixty years later the Cassini- Huygens mission unveiled Titan’s complex world very much akin to Earth in many regard but shaped by the cycle of hydrocarbons and especially methane. Decades of modeling efforts, validated by ground-based, and spacecraft observations (Voyager I, 1980, and Cassini-Huygens, since 2004), elucidated the complex chain of UV- induced photochemical reactions that take place in the atmosphere and the rates at which they proceed [e.g. Yung et al., 1984; Coustenis et al., 1991; Coustenis et al., 2003; Wilson and Atreya, 2004; Lavvas et al., 2008]. Using these constraints, Lavvas et al. [2008] established a refined photochemistry model of the atmosphere, and Cordier et al. [2009] used the outputs to estimate the fluxes of photochemical products to the surface. This information establishes two essential starting points about Titan: 1) the dominant products of the photochemistry are ethane C H and propane C H , which are in liquid state at Titan’s surface conditions (1.5 bar of N , 2 6 3 8 2 92-94 K); 2) the higher molecular weight compounds would be in the solid state on the surface, where they may form the observed dunes [e.g. Lorenz et al., 2008), and/or contribute to evaporitic materials tentatively detected by the VIMS instrument [e.g. Barnes et al., 2011]. These solid organic compounds are thus expected to form a blanket cover on the presumed water ice bedrock. A recent study discusses the possibility of the emergence of a form of life not based on liquid water in Titan’s hydrocarbon lakes [Stevenson et al., 2015]. Titan, and particularly its lakes, has been identified as a high-priority target by the NRC Planetary Science Decadal Survey “Visions and Voyages” for 2013-2022. Several mission concepts have been suggested for the follow-on exploration of Titan with focus on the habitability potential of its surface and subsurface. The Titan and Saturn System Mission concept developed in 2008 [Coustenis et al., 2009] introduced an architecture involving an orbiter, balloon, and surface element. More recent concepts have focused on landers and especially lake 3 landers [e.g., Stofan et al., 2013; Mitri et al. 2014], and even submarines [http://www.gizmag.com/nasa-titan-submarine-concept/35960/]. 9.1.4 Europa Exploration Europa is the smallest of Jupiter’s Galilean satellites and the 6th largest moon in the Solar System. Its average surface temperature is about −171°C. It was discovered by Galileo Galilei in 1610. Ground-based observations (e.g., radar) and flybys by the Voyager spacecrafts have revealed a complex surface dominated by ice and salt compounds associated with tectonic features (e.g., ridges). The Galileo mission, launched in 1989, provided the majority of the current data on Europa, revealing a geologically young surface, as indicated by the variety of tectonic features and the scarcity of impact craters. Galileo also uncovered the presence of a deep ocean from the detection of an induced (time-variable) magnetic field. That same technique also led to the discovery of deep oceans in the other Galilean moons Ganymede and Callisto. Because of its active geology and expected high tidal heat input, Europa’s ocean is believed to be relatively close to the surface (<25 km, Pappalardo 2010) and in contact with a rocky core, while the thick hydrospheres of Ganymede (~800 km) [Vance et al. 2014] and Callisto (~200-400 km) [Schubert et al. 2004] suggest the presence of high-pressure ice phases at the interface with the rocky core. Ongoing analyses of Galileo datasets over the past 20 years continue to reveal surprises at Europa, including evidence for subduction of the brittle upper crust of Europa’s ice, a key feature of plate tectonics on Earth [Prockter and Kattenhorn 2014]. Also recently, Hubble Space Telescope observations of Jupiter’s magnetosphere in the vicinity of Europa have revealed the evidence for transient water vapor plumes [Roth et al. 2014]; these have yet to be confirmed by subsequent observations (Roth et al. 2015). Because of the strong likelihood of having a global ocean in contact with a rocky mantle, Europa is a primary target for future exploration, as illustrated by the numerous mission concepts developed since even before the end of the Galileo mission (Figure 1). These missions all focus on getting a better understanding of Europa’s internal structure and surface composition. Several concepts have considered some form of in situ platform, such as penetrators [Gowen et al. 2010] and legged landers [Pappalardo et al. 2013]. 4 Figure 1: Europa’s hydrosphere structure based on the current state of knowledge inferred from observations by the Galileo Mission. Image credit: NASA/JPL-Caltech. 9.1.5 Ceres Exploration Ceres is the largest body in the asteroid belt. It was also the first asteroid to be discovered when it was observed by Giuseppe Piazzi in 1801. Ceres’ density suggests it contains about 50% of ice in volume [McCord et al. 2011]. Most of our knowledge of Ceres to date comes from ground- based and Earth-orbiting telescopes, especially the NASA's Hubble Space Telescope [Thomas et al. 2005]. An extended vapor cloud at Ceres was detected with the Herschel Space Telescope [Kueppers et al. 2014]. This is evidence for the presence of ice in Ceres’ subsurface that was predicted by Thomas et al. [2005] Ground-based infrared observations enabled the discovery of carbonates and possibly of brucite [Rivkin 2006], which are signatures of formation in a hydrothermal environment. The Dawn mission achieved rendezvous at Ceres early March 2015 and will carry out in-depth investigations of Ceres’ chemistry and geology that will help answer questions on Ceres’ origin and habitability potential. The prospect of shallow ice tables or even cold traps in shadowed craters have already prompted interest for surface exploration as a possible precursor to a Europa lander [Poncy et al. 2011]. While the Dawn mission is in its early stage at Ceres it is expected that it will obtain critical data needed to formulate a follow on in situ mission. 9.2 Geophysical Exploration Techniques This section reviews key geophysical techniques that have been developed with the objective to characterize the physical properties of outer planet satellites with in situ platforms. Such techniques complement global scale observations of the gravity and magnetic fields. These techniques are: acoustic radar with application to the characterization of Titan’s lake; seismometry with application to Europa’s deep interior; and ground-penetrator radar with application to Enceladus in particular. Other techniques are notable, such as electric fields, induction, etc. 5 9.2.1 Sonar - Acoustic radar Conceived at a time when it was widely thought that Titan had a global ocean, the Surface Science Package (SSP) carried by the Huygens probe, and deployed by the Cassini Orbiter in January 2004, had the ability to measure the sound velocity (Zarnecki et al. 2000). This measurement could have been used to determine the relative amounts of methane and ethane in the ocean. It also had a limited ability to do sonar measurements. The key challenges to using sonar, also known as Acoustic Radar, is to employ piezoelectric transducers that are operational at such extremely low temperatures (~90K), that is specifically applicable to Titan’s lakes. The issues involve a very large thermal expansion mismatch that may be associated with the construction materials and the composition of the liquids (methane/ethane) in these lakes. To address the challenges, one needs to take advantage of the piezoelectric materials that can potentially be used to serve as transmitters and receivers of acoustic waves. The required transducer needs to perform optimally at the temperature of 90K. Also, the operation in liquid methane/ethane requires addressing the related physics that affects the response and performance of the sonar as an analyzer. Combining the use of transducer array and phase control, one may be able to scan the terrain without physically moving the transducer. Combing the use of an array and phase control as well as vehicle movement, 3D mapping can be made, which is scientifically desirable. Sonars can be used to perform ultrasonic analysis for investigating Titan’s wet subsurface beneath and around its hydrocarbon lakes [e.g. Stofan et al., 2007; Hayes et al., 2008]. Such a scientific instrument, based on the transmission and reception of acoustic waves by one or more piezoelectric transducers, can be installed on an in-situ platform (lander, floater, submarine, rover, etc.) and measure the bathymetry of the lakes. This ultrasonic analyzer may be essential to measure directly the structure of the subsurface, and its interactions with the lakes: depth of liquid percolation, stratigraphy (porosity gradients and/or discontinuities between subsurface materials), existence and thickness of the organic deposits blanket, location of bedrock beneath, connectivity of the lakes, existence and extent of a deep-seated aquifer. One sonar for Titan exploration has been reported in the literature. It was part the Meteorology and Physical Properties Package (MP3), conceived at John Hopkins University / Applied Physics Laboratory [Lorenz et al., 2012], and proposed as part of the scientific payload for the Titan Mare Explorer (TiME), a Discovery 12 Step 2 mission concept. This sonar of the MP3 package was solely dedicated to measuring bathymetry of Ligeia Mare, using standard (low-efficiency) PZT transducers. Alternatively proposed by JPL, and described herein, is the use of recent developments of high-efficiency piezoelectric transducer technology to achieve similar capabilities with less power. Such an instrument would enhance the scientific return of an ultrasonic analyzer by enabling the capability of sounding the subsurface beneath the hydrocarbon lakes (Figure 2). The terrain beneath the piezoelectric transducer can be scanned without physically moving the transducer. This can be accomplished by the use of a transducer array and phase control. The image of the lake bottom and the subsurface can be made in 3D by combining the operation of the sounding device with the vehicle movement. 6 Figure 2: The envisioned marine lander and the ultrasonic analyzer for the exploration of Titan (left) and illustration of the instrument operation as compared to conventional sonars (right). The sonar as an analytical instrument can be used to emit and receive sound waves, where the reflected or backscattered echoes from acoustic interfaces are used to measure the distance of objects by analyzing the travel time between the transducer and the objects/layers. This principle is also used in Navy sonars but the focus in this application is beyond mapping the lake bottom surface and immersed objects. It is interesting to note that the same principle is also widely used in diagnostic medical imaging and nondestructive testing. The main differences between medical imaging and underwater sonar are the operating frequency and the power level. In general, megahertz ultrasound (1-10 MHz) is used for non-destructive testing and medical imaging for high resolution, and this type of ultrasonic test does not require high power level (typically much less than 1 watt). In contrast, for great distance range finding and imaging sonar transducers use higher power with low to moderate frequencies in the Hz - kHz. This is also a requirement for an ultrasonic instrument, that may be used as an analyzer for the subsurface of Titan’s lakes but the frequency needs to be focused on the ~ 1 - 100 kHz range. 9.2.1.1 Transducer composition, and behavior at cryogenic conditions Piezoelectric transducers convert applied electrical signals into acoustic radiation and they are designated as projectors. Further, transducers that convert received acoustic radiation into electrical signals are designated as hydrophones. The performance criteria for projectors and hydrophones are quite different; for example, the major concern of projectors is high power output, while that of hydrophones is high sensitivity (signal-to-noise ratio). Piezoelectric transducers can serve as both projectors and hydrophones, in which the performance is greatly dependent on the properties of the piezoelectric materials such as mechanical Q (inverse of m mechanical loss) and electromechanical k coupling. High values of these coefficients allow generating broad bandwidth signals and provide high sensitivity and increased power efficiency. Currently, the majority of piezoelectric materials for such transducers are ferroelectric materials due to their high electromechanical properties, which arise from the two types of contributions; the intrinsic (lattice effects) and extrinsic contributions (the motion of ferroelectric–ferroelastic domain walls) in ferroelectric materials. One of the most important characteristics of this kind of materials is the morphotropic phase boundary (MPB), which refers to the boundary between two compositions where the two phases are present in equivalent energy states. MPB is an important 7 concept for ferroelectric materials as MPB compositions offer enormously high dielectric and piezoelectric properties as a result of enhanced intrinsic contributions. Lead Zirconate Titanate PZT is one of the most widely used piezoelectric materials because of the MPB characteristics [Jaffe et al., 1971]. The piezoelectric response contains not only the intrinsic contribution, but also an extrinsic contribution caused by movement of non-180˚ domain walls, which is strongly temperature dependent. MPB-based PZTs are generally tailored with dopant ions, which impede or facilitate domain wall movements. Importantly, in PZT ceramics, more than 50% of the net piezoelectric responses arises from these extrinsic contributions; therefore, when PZT materials are used at cryogenic temperatures, most of the extrinsic contributions are frozen out, consequently, the materials lose their piezoelectric performance; for example, the piezoelectric d coefficient was reported to decrease from 760 pC/N to 220 pC/N when the operating temperature was decreased from 300K to 30K (Park et al., 1999; Hackenberger et al., 2008). This indicates the necessity for appropriate piezoelectric materials to be used to make ultrasonic analyzer for operation at cryogenic temperatures. It is interesting to note that the transducer of the JHU/APL sonar is made of PZT-5A (Lorenz et al., 2012). Recently, domain engineered <001> relaxor-PT single crystal family, such as PZN-PT, PMN-PT and PIN-PMN-PT, has been studied extensively due to their extremely high piezoelectric responses, strain over 1.7%, piezoelectric constant d over 2000 pC/N, 33 electromechanical coupling factor k over 90%, with almost non-hysteretic strain-field behavior 33 (Park et al., 1999). Of particular significance is that, in contrast to PZT ceramics, the mechanical Q values can be tailored by the crystallographic orientation, being on the order of 200 and >800 m for <001> and <011> oriented PMN-PT crystals, respectively, without sacrificing the electromechanical k coupling. Since the origin of such high electromechanical properties of relaxor-PT single crystals is polarization rotation effect, (i.e., intrinsic contributions), the property degradation at cryogenic temperatures is much lower than in PZT ceramics, making them promising candidates for cryogenic transducers from the perspective of bandwidth and power efficiency of transducers (Fu and Cohen, 2000). The relaxor-PT single crystal transducers, specifically <110> oriented binary PMN-PT or ternary PIN-PMN-PT, can be used to produce a probe that can potentially sustain the very cold conditions, be inert to potential chemical reactions and constructed of materials with minimal thermal mismatch. 9.2.1.2 Estimates for the operation of sonar on Titan The estimation of detection range for sonar transducer for a given input power is one of the most important considerations for the success of research related to potential missions to Titan. In order to accurately estimate the detection range for mapping the topography of Titan’s lake, the elastic properties of propagating media need to be known. Table 1 shows reference properties of various media from several sources, which allow for the estimation of the transmission range and/or transmission loss of acoustic waves from sonar using a given input power. Table 1: Material properties used for assessing the ability of the proposed ultrasonic analyzer to detect subsurface interfaces. Material Vp (km/s) Vs (km/s) elastic modulus (GPa) Attenuation Methane (94 1.520 (94 K – [Singer 1969] --- 0.9-1.9 GPa [Marx Alpha/f^2x 10^17: K) 1.490 (96 K – [Straty 1974] 1984, Shimizu 1996]) 5.6-6.2 cm-1/s2 [Singer 1969, Straty 1974] Ethane (95 1.974 [Tsumura & --- K) Straty1977] 8 H2O ice (90 4.2 [Proctor 1966] 1.98 [Proctor 5-9 GPa Young ~ 1 db/100 m at -25°C for K) 1966] modulus for 35 and 60 MHz polycrystalline ice. [Johari & Charrette 1975] Benzene ~2.8 - 5.72 GPa. Linear ~ 2-4 db/cm at 255 K. (solid, 170 [Heseltine et al., 1964] extrapolation yields [Heseltine et al., 1964]; K) 6.5 at 90 K [Heseltine 3.1x10^5 cm/s at 273 K et al., 1964] [Liebermann 1959] For the detection of acoustic wave from a sonar, the signal-to-noise (SNR) ratio should be higher than the detection threshold (DT), i.e., SNR > DT, where SNR can be written as follows: SNR = L -L =(SL−2TL)−(NL−DI) (1) S N where SL is source level, DI is the directivity index, in the case of omni-directional, DI=1, TL is one way transmission loss and NL is background noise level. The value of detection threshold of sonar transducer is dependent on the transducer performance and signal processing method. The source level (SL) is defined as the intensity of the radiated acoustic wave relative to the intensity in medium referenced to 1 micropascal at 1 m, given in the Eq (1). I SL=10log( S)=10log(Wa)+168.9dB re:1µPaat1m (2) I 0 where W is acoustic output power. I is the reference sound intensity of methane, referenced to 1 a o micropascal at 1 m. The transmission loss (TL) includes all the effects of the energy losses, such as geometric losses (spreading, spherical or cylindrical), and attenuation due to scattering, viscosity, and adsorption. The main source of attenuation is generally associated with absorption, where acoustic energy is converted to heat energy. In this estimation, only spreading and absorption losses are considered and these are the main causes of transmission loss of acoustic waves. Transmission loss due to the effects of spreading and absorption can be expressed as follows: TL=20log(x)+αx (3)   where x is distance and α is attenuation coefficient. Note that the attenuation coefficient has a strong frequency dependence, much greater losses at higher frequency. In addition, the attenuation is also temperature dependent; generally increasing with decreasing temperature. Unfortunately, the attenuation coefficients of most materials at 90K are not available; thus, from this estimation we used the attenuation values given in Figure 3. 9 Figure 3: Diagrams showing transmission loss (top) and sound pressure level (bottom) of sound wave through our nominal subsurface model for an ultrasonic analyzer using 10 W input power. The materials presented in Table 1 are used as reference to estimate one-way sound pressure level (SPL) at distance through the structural model presented in Figure 4. Using a given 10W input power, the transmission loss and pressure level of the acoustic wave (predicted by Eq. (2) and (3), respectively) can be estimated as a function of distance with different frequencies (see Figure 2). From the figure, it can be seen that the most dominant factor for the transmission loss is spreading loss for sonar range (<10 kHz); however, with increasing frequency (ultrasound >20kHz), the attenuation coefficient factor becomes a significant factor in figuring the transmission range. Although this is a rough estimation, it is promising that if we design the sonar below 20 kHz, it is possible for sounding the subsurface beneath Titan’s lakes with 10 W. If we assume that the transducer can detect the sound when DT>0, we can also estimate the detection range with Eq. (1) for a given input power, assuming that NL-DI is 0, which is shown in Figure 4. From the figure, it can be seen that the sonar transducer can detect the sound up to 50 kHz for a given 10 W. Therefore, using a frequency of a few tens of KHz, the sonar as an ultrasonic analyzer can enable to transmit sound through the various layers of Titan subsurface, down to several hundreds of meters, providing detailed information on its structure. The use of acoustic radar to perform analysis on Titan may enable detection of various anomalies and discontinuities as well as the characterization of material properties. Generally, the detection requires acoustic properties mismatch between the propagating medium and the presence of other materials such as cryo liquids, sediments, run-off, etc. 10