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Methods and Methods and Data Systems Data Systems Discussions Open Access Open Access Geoscientific Geoscientific Model Development Model Development Discussions Muon radiography for exploration of Mars geology Open Access Open Access S. Kedar1, H. K. M. Tanaka2, C. J. Naudet1, C. E. Jones1, J. P. Plaut1, and F. H. WebbHydrology1 and Hydrology and 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena,Earth CA, System USA Earth System 2Earthquake Research Institute, University of Tokyo, Tokyo, Japan Sciences Sciences Correspondence to: S. Kedar ([email protected]) Discussions Open Access Open Access Received: 31 August 2012 – Published in Geosci. Instrum. Method. Data Syst. Discuss.: 18 October 2012 Revised: 29 March 2013 – Accepted: 7 April 2013 – Published: 17 June 2013 Ocean Science Ocean Science Discussions Abstract. Muon radiography is a technique that uses natu- 1 Introduction rally occurring showers of muons (penetrating particles gen- Open Access erated by cosmic rays) to image the interior of large-scale ge- Open Access Muon radiography was first applied for practical purposes as ological structures in much the same way as standard X-ray Solid Earth radiography is used to image the interior of smaller objects. an imaging technique in the 1950sSolid by Earth E. P. George to measure Discussions Recent developments and application of the technique to ter- the overburden over a tunnel in Australia (George, 1955), restrial volcanoes have demonstrated that a low-power, pas- and in the 1960s by Luis Alvarez in his famous attempt to sive muon detector can peer deep into geological structures discover hidden chambers in the Second Pyramid of Chep- up to several kilometers in size, and provide crisp density hren in Giza (Alvarez et al., 1970). Although theOpen Access concept has Open Access been around for decades, recent advances in detector tech- profile images of their interior at ten meter scale resolution. The Cryosphere Preliminary estimates of muon production on Mars indicate nology and data processingThe Cryosphere have brought the technology to Discussions that the near horizontal Martian muon flux, which could be a point where it can be used reliably for subsurface imaging used for muon radiography, is as strong or stronger than that of geological structures. This was recently demonstrated in on Earth, making the technique suitable for exploration of several terrestrial volcano imaging campaigns (Tanaka et al., numerous high priority geological targets on Mars. The high 2010, 2009a,b, 2008) that provided subsurface density pro- spatial resolution of muon radiography also makes the tech- files of unprecedented resolution (Fig. 1). The detected vari- nique particularly suited for the discovery and delineation of ations are of the order of a few percent in density contrast, Martian caverns, the most likely planetary environment for making it a powerful tool for discriminating between hot rock biological activity. and cold rock, rock and voids, and rock and water. Applying As a passive imaging technique, muon radiography uses this technique to Martian geology is attractive for several rea- the perpetually present background cosmic ray radiation as sons: (1) cosmic rays are ubiquitous and generate secondary the energy source for probing the interior of structures from muons in any atmosphere; (2) a preliminary Monte Carlo the surface of the planet. The passive nature of the measure- simulation of the cosmic ray muon production on the Mar- ments provides an opportunity for a low power and low data tian surface by Tanaka (2007) shows that the technique is ripe rate instrument for planetary exploration that could operate for application on planetary bodies other than the Earth; (3) as a scientifically valuable primary or secondary instrument high energy muons are extremely penetrating, easily passing in a variety of settings, with minimal impact on the mission’s through hundreds of meters of rock; (4) detectors, as collec- other instruments and operation. tors of muons radiation, are passive in the sense that they use nature’s cosmic rays and thus have significantly lower mass and power requirements than active imagers, such as radar, sonar or X-ray; (5) successful application of this tech- nology on planetary bodies would bring new capabilities to future Solar System exploration missions by enabling high- resolution imaging of the interior of planetary surface fea- tures with passive sensing of secondary cosmic ray muons, Published by Copernicus Publications on behalf of the European Geosciences Union. 1 7 Figures 158 S.1 Kedar F etigure al.: Muon 2 radiography for exploration of Mars geology 2 Figure 1 3 4 Fig. 1. Multiple examples of the resolving power of muon radiog- 5 Figure 1: Multipleraphy. examples Top left: of density the resolving profile ofpower a terrestrial of muon volcano radiography. obtained Top with left: Density 6 profile of a terrestrialsecondary volcano cosmic obtained ray muons with secondary (Tanaka etcosmic al., 2009a). ray muons Bottom [Tanaka left: et al, 2009a]. 7 Bottom left: Detecteddetected mass mass change change inside inside a volc a volcanoano using using cosmic cosmic-ray-ray muon muon radiography ra- Figure 2: The muon range in standard rock underground on Earth as a function of muon 8 [Tanaka et al., 2009b]. Right: Vertical slices showing a 3D profile of the interior of Mt. Fig.Asama 2. The muon range in standard rock underground on Earth as diography (Tanaka et al., 2009b). Right: vertical slices showing a aenergy, function of expressed muon energy, expressedin meters in meters of water of water equivalent equivalent [Reichenbacher et al, 2007]. The depth 9 constructed by 3-DTanaka profile et al of. [2010] the interior using ofdata Mt. from Asama a muon constructed radiography by Tanakasurvey at et multiple 10 locations around the dome. (Reichenbacheraccessible andto muon De Jong, radiography 2007). The depth depends accessible to upon muon the energy of the muons, the composition of al. (2010) using data from a muon radiography survey at multiple radiographythe rock dependsin question, upon the the energy surface of the muons, muon the flux, composi- target/detector geometry, detector size and 11 locations around the dome. tion of the rock in question, the surface muon flux, target/detector geometry,efficiency, detector image size and resolution efficiency, image and resolution measurement and mea- integration time. surement integration time. thereby providing new information about the planet’s evolu- tionary state and history. 2 Cosmic rays are particles generated in outer space, usu- ally of extra-solar origin. The primary cosmic rays reaching3 objectives and duration. To determine whether the science re- the outer edge of the Earth’s atmosphere, and which create quirements can be met, the horizontal muon flux at the Mar- showers of particles that reach Earth’s surface, are composed tian surface must be estimated. Particle production rates in of 90 % protons, 9 % alpha particles and 1 % heavier nu- any given atmosphere are simulated using a Monte Carlo ap- clei (Gaisser, 1990). Many are very high energy, but each proach accounting for the initial cosmic ray particles flux and of these primary constituents undergoes strong nuclear inter- energy and the dynamics of their interaction with molecules actions and therefore none are sufficiently penetrating to be in the atmosphere. used directly for radiography of planetary surfaces. However, The calculated terrestrial vertical muon flux is in excellent as the primary cosmic rays interact with matter in the atmo- agreement with measurements. On Earth, the atmosphere is sphere, they generate showers of secondary particles, which sufficiently thick that a cosmic ray shower maximum (the in turn can decay or create more19 showers. These extensive altitude at which the shower is at a maximum in terms of air showers (EAS) in the upper atmosphere generate many the number of secondary particles) is reached after 1/10 of unstable mesons (hadronic subatomic particles including pi- the atmosphere is traversed, or at ∼ 15 km altitude. Below ons and kaons) that decay into muons. Muons have a long this altitude, the net number and energy of the secondary lifetime and a small cross section for interactions, so those particles decreases. The rate of generation of muons within with high energy and relativistic momentum can travel long the Earth’s atmosphere is roughly 1000 per meter squared distances before decaying. Most importantly, muons pene- per second, with the maximum muon flux coinciding