Mars Astrobiological Cave and Internal habitability Explorer (MACIE): A New Frontiers Mission Concept

Martian subsurface habitability and astrobiology can be evaluated via a lava tube cave, without drilling. MACIE addresses two key goals of the Decadal Survey (2013-2022) and three MEPAG goals. New advances in robotic architectures, autonomous navigation, target sample selection, and analysis will enable MACIE to explore the Martian subsurface.


NASA's Strategic Plan (2020), Planetary Decadal Survey (2013-2022), NASEM's Astrobiology Strategy (2019), and MEPAG goals (Bandfield et al., 2020)
. MACIE may yield bonus science including insight into future human exploration (MEPAG Goal 4). Goal 1: Assess the present and past habitability of a Martian lava tube Understanding the present and past habitability of a lava tube will be essential in providing the context for evidence of life and/or life-related processes if they are observed. If no evidence of present or past life is detected, the habitability assessment would help explain why evidence of life was not observed. MACIE's habitability assessment addresses the Planetary Decadal Survey Goals 4 and 5, NASEM's Astrobiology Strategy (2019), and multiple MEPAG Goals. Objective 1A Determine whether brines or water ice are present: Water is essential to habitability of environments as we know them. MACIE would quantify and characterize the distribution of liquid brines and water ice that may be present in the cave using Raman and visible and infrared reflectance (VISIR) spectroscopy and a camera with appropriate lighting. MACIE would also assess the thickness of the overburden, temperature, and relative humidity along a transect within the cave using a meteorological suite, which can aid in determining why brines or ice are or are not detected during MACIE's mission. Such an endeavor would also help to ascertain the structural integrity and approachability of a particular lava tube or cave for sustained future human exploration. These measurements address MEPAG Goal 2 "Assess the processes and climate of Mars" and Goal 4 "Prepare for human exploration." Objective 1B Determine whether aqueous alteration occurred now or in the past: Evidence of water:rock interaction will determine whether liquid water was present in the cave over geologic time. Using a multispectral instrument, we would determine the presence of alteration minerals, including clays, Feoxyhydroxides, sulfates, and other minerals indicative of aqueous alteration of the primary igneous rock substrate. Using Raman and VISIR spectroscopy, MACIE is designed to assess the mineralogy and chemistry of the cave ceilings, walls, and floors and enable MACIE to determine whether oxidant rich dust has been transported from the surface. Martian dust, which contains salts (i.e. chlorides, perchlorate, and sulfates), may influence the sublimation of ice and the development of brines, which would influence the development of redox gradients and long-term maintenance of ice that could support or sustain life. MACIE addresses "the spatial and temporal distribution of potentially habitable environments…in the subsurface," (NASEM, 2019) and MEPAG Goals 2 and 3). Objective 1C Determine the presence of nutrients and chemical disequilibria necessary to support life: The distribution and availability of CHNOPS+Fe elements in the solid phase has been shown to influence microbial colonization in terrestrial lava tubes (Popa et al., 2012; Phillips-Lander et al., 2020). Nutrients required for life are accessible from both the aqueous (condensation, liquid water) and solid phases (rocks, minerals, and water ice). A camer paired with Raman and laser-induced breakdown spectroscopy (LIBS) will identify and quantify the chemistry of ices, primary rock substrates, and alteration phases to determine the availability of nutrients and energy, in the form of chemical disequilibria, available within the cave to promote and sustain life in the absence of light (NASEM, 2019; MEPAG Goal 3). Objective 1D Radiation Flux in the Subsurface: While some microorganisms survive exposure to high radiation levels (Battista, 1997), many microorganisms cannot. Cave roof thickness determines whether the radiation environment is within the cave. We anticipate cosmic rays and neutrons will penetrate a Martian cave with an overburden up to 500 g cm −2 (~3 m; Turner and Kunkel, 2017). Quantifying radiation levels using a radiation sensor would serve to explain microbial habitat suitability (MEPAG Goal 1).

MACIE Leverages Heritage Instrumentation to Achieve Science Goals
Recent in situ investigations of Mars, including Mars Science Laboratory and Insight, include a suite of instruments to characterize the habitability, composition, and interior structure of Mars. Access to samples may be limited; many scientifically interesting samples may reside on overhangs or walls. Therefore, MACIE's instrument suite would conduct stand-off in situ experiments, allowing the mission to pursue the most interesting habitability and astrobiological targets. The baseline payload includes three instruments: a meteorological suite (e.g. ExoMars 2022 HABIT), a multi-spectral instrument to assess chemistry and mineralogy and provide hand lens quality microscopy (e.g. Mars2020 SuperCam), and a high-resolution context camera (e.g. Mars2020 Mastcam-Z). Estimates for mass, power, and data are based on these heritage instruments listed in Table 2. These instruments would address all science experiments in the STM ( Table 1). These instruments operate at high stand-off distances from science targets, and represent recently flown, technically mature payloads for a Mars environment that would require limited modifications for a planetary caves mission.

Meteorological suite
HAbitability: Brines, Irradiation and Temperature (HABIT) (Martín-Torres et al., 2020) measures temperature, relative humidity, barometric pressure, wind, dust, and radiation; these data would be incorporated into 2D and 3D cave climate models, explain the presence or absence of predicted water ice, and determine the radiation environment (Science Objectives 1A and 1C; Table 1).

Multi-spectral Instrument
A multi-spectral instrument similar to the Mars2020 SuperCam suite (with stand-off LIBS, Raman, VISIR, and fluorescence spectroscopies, and color context imaging) would provide multiple probes to assess the habitability and astrobiological potential of interesting targets (Rees et al., 2019). Primary and alteration mineralogy could be characterized using Raman spectroscopy and VISIR. LIBS provides elemental abundances of target material, with potential to yield direct measure of CHNOPS+Fe elements (Rees et al., 2019). These analyses would provide data required to address Science Objective 1B. SuperCam's remote microimaging (RMI) camera has a resolution of 60 µm at 1.5 m stand-off distance, which would allow the detection of ice (Objective 1A) and biofabrics (Objective 2C; Table 1). The camera would require active illumination; we baseline a tungsten halogen light with an optical system that projects a flat illumination of 10 o for both RMI and the camera (below). Organics could be determined with time-resolved fluorescence spectroscopy as organics fluoresce when excited by the 532 nm laser (Objective 2A; Table 1). Fluorescence from organics decays over very short timeframes (<1 ns to 200 ns), significantly shorter than fluorescence from minerals (µs-ms), allowing detection and differentiation of organic and mineral components in the sample.

Camera
Context imaging would be conducted via a camera like Mastcam-Z, which would allow for high definition and 3D cave images when paired with an illumination source. Image resolutions vary between 150 µm and 750 mm per pixel depending on distance. The camera would be able to capture a range of images to map the extent of brines or ice in the cave and provide high resolution imaging of the cave itself, which would determine cave size and extent (Objective 1A). The 3D information extracted could further be used to characterize the geomorphometric parameters such as orientation, slope, and surface roughness to enable safe rover maneuvering.

Technological Development
Key technological developments for instrumentation would further aid Mars cave exploration including autonomous sample selection and data processing. Communication between the robotic platform and Earth may be limited due to bandwidth restrictions between the rover and relay points (e.g. deployable relays or orbiting assets); therefore, mission operations may require autonomous sample selection to vet targets before measurement with resource-intensive instruments. Additionally, science data collected throughout the mission may need to be processed or interpreted onboard to prioritize downlinking the most valuable science data in cases of bandwidth-restricted mission architectures. MACIE would benefit from advances in autonomous sample selection using machinelearning algorithms currently being advanced for the Europa Lander and through DoD projects.

MACIE Mission Architecture and Concept of Operations (ConOps) Site Selection
Cave site selection would be based on expected habitability and astrobiological potential, as defined by smaller entrance opening, longer subsurface cavity lateral extent (Howarth, 1980), and persistence of water ice (Williams et al., 2010)