Abstract
The Viking Mars mission raised intriguing questions about Mars’ surface chemistry. More than three decades later, an array of small electrochemical sensors included on the Phoenix Mars lander provided a key insight: Perchlorate, chlorine’s most highly oxidized form, was present at surprisingly high concentrations in the regolith. This has implications for Mars’ geochemistry, habitability, potential to support microbial life, and human exploration; as a strong oxidant, it might also help explain the destruction of organic compounds on the martian surface. Here, we examine the role of chemical sensors in the exploration of Mars and a critical allied enabling technology, microfluidics, from Phoenix to the present day and beyond. Enormous technological advances in microtechnologies, targeting terrestrial applications from everyday consumer electronics to wearable medical diagnostic devices, are just now beginning to be adapted and harnessed to support planetary science and discovery. These advances are poised to revolutionize how much can be learned using robust systems with unimaginably small requirements for size, weight, and power, making them compatible with small, potentially lower-cost delivery to the martian surface on hard landers, impactors, penetrators, and even rotorcraft.
Introduction
Around 3–4 billion years ago, Mars briefly possessed a warm and wet environment, likely similar to that of Earth at that time, with potentially large bodies of stable water. During that period, life emerged on Earth and could also have reasonably done so on Mars as well. In the ensuing billions of years, the absence of a strong magnetic field and plate tectonics led to the loss of the martian atmosphere and surface waters, as well as a drop to subzero temperatures. Today, we find a harsh hyperarid martian environment with a surface bombarded by solar energetic particles and galactic cosmic rays. Its thin (600 Pa) CO2 atmosphere allows UV radiation in the 190–400 nm wavelength range to reach the surface, which presents a major survival challenge to any putative martian organisms.
Viking and Its Successors: Seeking Answers, Raising Questions
It became evident in the early 1960s that to understand whether microbial life was ever present on Mars and enable future human exploration and habitation, knowledge of the martian aqueous regolith chemistry and the potential habitability of its surface was required. Given these goals, the 1975 Viking 1 and 2 Mars missions both included an X-ray fluorimeter, three life-detection instruments, and a gas chromatograph–mass spectrometer (GC-MS) capable of detecting organic compounds at parts-per-billion (ppb) concentrations (Clark, 2026; MicKinnon and Naz, 2026). However, it was reported that neither lander had found any indigenous organic compounds in the martian regolith they sampled (Biemann et al., 1977). A variety of hypotheses were advanced as to why the expected organic compounds had not been detected. These included: the presence of strong oxidants and/or other minerals and/or processes in the regolith that destroyed them; degradation by UV and/or other radiation; and the Viking GC-MSs failed to detect them even though they were present. The most broadly accepted view for several decades was that the martian soil contained oxidants, such as metal oxides or oxygen radicals, which were activated by interaction of the martian surface materials with UV or cosmic radiation (Zent and McKay, 1994; Yen et al., 2000).
In the ensuing years, two ill-fated Mars lander missions were launched; the Roscosmos Mars-96 lander and penetrator and the NASA 1998 Mars Polar Lander with two penetrators. The former carried aboard each of its identical hard landers fiber-optic-based chemical sensor arrays to investigate the nature of the martian surface material, particularly its oxidative character. The Mars Oxidant instrument’s fiber-optic technology was designed to monitor real-time physicochemical changes in a suite of “micromirrors” made from a diverse range of chemically sensitive thin-film materials, some contacting the martian surface, others exposed only to its atmosphere (Grunthaner et al., 1995; McKay et al., 1998). A successor, the Mars Atmospheric Oxidant Sensor instrument, used a similar approach for a chemometric thin-film sensor array, which weighed in at just 55 g (Zent et al., 2003), although it was never flown. The Mars Polar Lander carried the Mars Microprobe Mission (aka Deep Space 2), a pair of 2.4 kg penetrator probes designed to impact the surface at ∼190 m/s and penetrate to as much as 60 cm, where they would characterize regolith physical properties, including ice, if present (Smrekar et al., 1999). Successful surface missions in 1997 (Pathfinder/Sojourner Rover) and 2004 (Opportunity and Spirit rovers) mainly included X-ray spectrometers and imagers, their science focused predominantly on geology.
Wet Chemistry Laboratory on Phoenix: A New Understanding of Mars Surface Chemistry
In the summer of 2008, the Phoenix Mars lander successfully set down on the norther plains of Mars at 68.2°N, 125.9°W (Smith et al., 2009). As the first follow-up mission to the Viking landers that also included a mass spectrometer, the goals of Phoenix were to verify the presence of subsurface ice, understand the history of the water at the landing site, detect organic compounds if present, and assess the habitability of the landing site. To meet these goals, the Phoenix payload included a mass spectrometer (the Thermal and Evolved Gas Analyzer), an optical microscope and atomic force microscope, a Thermal and Electrical Conductivity Probe, a Stereoscopic Surface Imager, a weather station (MET), and four identical single-use Wet Chemistry Laboratory (WCL) cells (Kounaves et al., 2009). The WCLs performed the first wet chemical analyses of the martian regolith to determine its habitability, aqueous history, the availability of chemical energy sources, and the geochemistry at the landing site. It accomplished this using an array of electrochemical sensors to determine a variety of soluble salts and soil parameters (Kounaves et al., 2010a, 2010b; Quinn et al., 2011).
One of the most surprising findings from the Phoenix WCL analyses was the presence of ∼0.6 wt% perchlorate ion (ClO4–) (Hecht et al., 2009; Kounaves et al., 2010a) with parent salts identified as most likely being a 60/40 mixture of calcium and magnesium perchlorates (Ca(ClO4)2 and Mg(ClO4)2), respectively (Kounaves et al., 2014b). Subsequently, the occurrence of ClO4– and chlorate (ClO3–) on Mars was found to be widespread, as confirmed by their detection in the martian meteorites EETA79001 and Tissint (Kounaves et al., 2014a; Jaramillo et al., 2019), at Gale crater by the sample analyses at Mars instrument on Curiosity (Glavin et al., 2013; Leshin et al., 2013), at Jezero crater by the SHERLOC spectrometer on Perseverance (Scheller et al., 2022), and at multiple sites by detection of chlorinated hydrocarbons (Ming et al., 2014; Freissinet et al., 2015; Eigenbrode et al., 2018).
By identifying a high concentration of perchlorate in the soil, the Phoenix WCL results helped explain how the pyrolytic breakdown of ClO4– could have played a role in the destruction of martian organic compounds in the GC-MS during the measurements on the Viking landers and the Curiosity rover (Navarro-González et al., 2010; Glavin et al., 2013). It also led to the suggestion that the detection of chloromethane (CH3Cl) and dichloromethane (CH2Cl2) by the Viking lander’s GC-MS (Biemann et al., 1977) may have been a result of the ClO4– in the regolith oxidizing either terrestrial organic contaminants or indigenous organic compounds (Biemann and Bada, 2011).
The discovery also immediately generated a variety of hypotheses with implications for Mars’ geochemistry, habitability, liquid water, and potential for supporting microbial life (Rennó et al., 2009; Fisher et al., 2010; Stoker et al., 2010; Chevrier and Rivera-Valentin, 2012; Quinn et al., 2013; Carrier and Kounaves, 2015). The presence of perchlorate and chlorate (and potentially other oxychlorines) has multiple implications and roles: the control of water content of the martian soil and atmosphere; as a marker for the history of water in sediments/soils; as an energy source for microbial life forms; depression of the freezing point of water to temperatures in the −50°C to −70°C range; and the possibility to provide explorers with energy, fuel, and oxygen, but also potential health hazards (Mitra, 2025).
From Phoenix to Today: Big Mars Missions Fly; Small Technologies Advance
While Viking was a mission explicitly designed to seek extant life on Mars, subsequent missions had different emphases. The Phoenix mission’s instrumentation payload, ∼60 kg, was exceeded by just 15 kg by the Mars Science Laboratory (MSL) mission that landed 4 years later, yet with a massive difference: MSL is mobile, carried about on the 900 kg Curiosity rover. The Mars 2020 Perseverance rover, of analogous design, size, and mobility, arrived at Jezero Crater in 2021. Both the Phoenix and MSL missions included powerful suites of instruments whose performance rivals laboratory analogs, from MSL’s GC-MS to Perseverance’s integrated Raman-and-luminescence spectrometer. These missions have analyzed samples from shallow depths, just 6–7 cm down, delivering an improved understanding of habitability, climate, and modern surface geology and geochemistry, as well as caching samples and preparing for future human Mars missions.
Even as our understanding of Mars’s surface and geological history charged ahead due to the Viking missions, developments that should shape future Mars science missions were afoot. The search for extraterrestrial life in our solar system became an explicit, publicized target, with multiple new mission concepts defined, headlined by the outer planets’ ocean-covered icy moons, yet with Mars still included on the solar system exploration timeline, mainly in the context of sub-surface water or buried evidence of ancient life. Enabling technology development for icy-moon missions was supported via NASA’s Concepts for Ocean Worlds Life Detection Technology (COLDTech) and Instrument Concepts for Europa Exploration (ICEE) initiatives, bolstered by NASA’s Maturation of Instruments for Solar System Exploration (MatISSE) program. With similar European efforts, such programs have spurred advances in small technologies, particularly (bio)chemical sensors and fluidic systems, useful in limited-resource environments or when samples have limited mass and volume, to seek indicators of life, extant or ancient.
In parallel, commercial microelectronic devices and technologies—for five decades the foundation and occasional inspiration for, many chemical sensors and supporting measurement systems—have rocketed forward, not only in their well-publicized energy efficiency and critical feature size (Schaller, 1997), but through advances that range from robust hermetic packaging and hybridization of multiple device classes, to radiation tolerance (a coincidental benefit of small feature size and thin dielectrics), to a versatile range of high-efficiency light sources that span the ultraviolet to near infrared, to waveguide technologies and arrays of micromirrors to direct and pattern light, to sophisticated, robust application-specific integrated circuits and field-programmable gate arrays, to ubiquitous low-cost memory and massively parallel video processors that make high-speed math a snap and have spun up the runaway AI maelstrom. Shadowing these mainstream developments, microelectromechanical systems (MEMS)—started as a niche research topic over 40 years ago (Terry et al., 1979; Petersen, 1982; Howe, 1988)—combine silicon micromachining, additive construction using thin-film microstructures, novel wafer- and chip-scale bonding methods, and incorporation of “non-standard” materials from shape-memory alloys and magnetic elements to micro-optical components. Now the core of such devices as the accelerometers and pressure sensors found in vehicles as diverse as automobiles, cargo ships, and planetary landers, MEMS showed promise from its beginnings as a revolutionary analytical enabler, exemplified by the seminal silicon-wafer-based GC in 1979 (Terry et al., 1979) (further discussed below).
Less visible but quite impactful for fieldable analytical systems, microfluidics today enable everything from high-speed digital printing to point-of-care medical diagnostics and wearables, to high-throughput drug discovery and massively parallel sequencing of DNA and RNA. Nearly all microelectronics, MEMS, and microfluidics advances have been driven by purely terrestrial, predominantly commercial applications, yet their implications for space exploration are major: they deliver new measurement capabilities, more performance, and greater robustness while demanding less size, weight, and power (SWaP) to function.
Tying together the relevant outputs of the technological revolution and the newfound emphasis of space agencies on life-search is the need to do more with less—mainly, less money (Roy, 1998) meaning that small yet highly capable micro- and miniature systems could be poised to replace 1 ton rovers as a mainstay of planetary science missions—thankfully, without sacrificing the capacity to explore multiple locations during one mission. The term “low-cost Mars”, including multiple copies of small hard landers, penetrators, or impactors, has become a stimulating talking point if not (yet) reality (Curry et al., 2022).
An illustrative evolution of planetary analytical technology, featuring reduced SWaP, more extensive integration, and promising improved performance, is embodied by a succession of electroanalytical sensor arrays based on the flight heritage of the Phoenix WCL, which have alternately, and at times simultaneously, targeted both Mars and the icy moons. Figure 1 shows the chronological progression of the WCL concept, from its initial implementation for the canceled 2001 Mars Surveyor Program lander (Kounaves et al., 2003) and successful 2007 Phoenix Mars Lander mission (Kounaves et al., 2009) to the Microfluidic Icy-world Chemical Analyzer in 2026.
Technological progression of electrochemical sensor arrays for planetary science from the Wet Chemistry Laboratory, initiated in 1996, to the Microfluidic Icy-world Chemical Analyzer in 2026.
The single-use concept of the WCL was driven by the desire to ensure that a sample that contained a sensor-poisoning substance would not affect latter analyses. Thus, two NASA Planetary Instrument Definition and Development Program (PIDDP)-funded instrument developments over the following decade expanded on that concept. The Robotic Chemical Analysis Lab increased the number of samples and automated sample delivery while maintaining the single-use concept with individual tubes preloaded with water and electrolytes on a rotating carrousel (NASA Astrobiology Institute, 2012). The second, also NASA PIDDIP-funded, development known as the In-Situ Wet Chemical Analysis Laboratory & Sensor Array (CHEMSENS) halved the total size of the WCL cell and included a platform where ∼100 “mini-WCLs” could be deployed (Kounaves et al., 2012).
Three continuous-flow analysis (CFA) systems have also been developed. The NASA Astrobiology Science and Technology Instrument Development funded instrument the Next Generation Wet Chemistry Laboratory, a lab-on-a-chip implementation of WCL (McElhoney et al., 2014), the NASA COLDTech funded microfluidic-WCL (mWCL) (Kounaves et al., 2019), and the most recent CFA, the NASA ICEE/MatISSE-funded Microfluidic Icy-world Chemical Analyzer (MICA) (Ricco et al., 2022). MICA was developed initially for the analyses of samples of melted ice (potentially including sand/clay particles) to be provided by a Europa lander—an approach readily adapted to Mars sampling scenarios—then adapted to the smaller liquid volumes anticipated from the icy plume particles of Saturn’s moon Enceladus. Its purpose is to conduct a chemical survey, quantifying key chemical components and electrochemically measurable properties of aqueous samples. By providing fundamental understanding of energy and redox gradients, along with aqueous geochemistry, this approach enables direct evaluation of habitability and offers contextual evidence for potential biosignature discoveries, both critical to interpreting and validating results from other life-search payload instruments.
The detection of molecular bio/chemical signatures and morphological clues that will support future search-for-life Mars efforts requires three distinct functions: (1) sample acquisition; (2) sample processing; and (3) sample analysis. Acquisition and analysis are the most mature of these, with decades of development, functional prototypes, a substantial publication record, and impressive performance demonstrations in laboratory, field, and mission settings. Fluidic sample processing for wet chemical analysis, however, remains a technological gap, and relevant technological advances warrant a fresh look at analytical methods, particularly their miniaturized implementations.
Indeed, technological evolution and revolution continue to (re)shape the capabilities of the miniature- and microscale sensors, sample processors, and analytical (micro)systems poised to enable future Mars exploration. Sensor and microsystem technology choices, and their customization for a particular Mars mission or instrument, must respond directly to its science objectives and measurement requirements. Here, we examine promising extant and emerging technologies to support or directly implement many of the science measurements described in the other articles of this collection. We focus on microfluidics, chemical sensors, and microanalytical systems: The first are effective for handling, pre-processing, and distributing standards and samples for analysis by many classes of instrument; the latter two can provide in situ, “low-overhead” (see below) analytical results for chemical reactivity, energy sources and gradients, biological habitability, and the presence of specific molecules or entire classes of (bio)chemical molecular indicators of life, whether modern, recent, or ancient.
Chemical (micro)sensors
Although the flagship-class missions described above often support relatively complex analytical instruments whose performance can approach that of laboratory instrumentation, like the Curiosity GC-MS or the Perseverance spectroscopy suite, chemical sensors and their arrays [for gas-phase analysis, sometimes called “electronic noses” (Li et al., 2023), and for aqueous measurements, “electronic tongues”] can complement more sophisticated instruments, enhancing overall science return. Small chemical sensors are particularly well suited to signaling the local presence and nature of oxidants and other chemically reactive species in or on regolith and dust particles (Grunthaner et al., 1995; McKay et al., 1998; Zent et al., 2003), or in the local atmosphere, of importance for understanding fundamental planetary processes, human hazards, and challenges for the materials and machines of exploration. They also can detect methane and other volatilizable hydrocarbons and lipids, whether from spontaneous surface emissions or generated by thermal processing of a regolith sample, relevant to seeking evidence of fossilized ancient life or identifying potential natural chemical resources. Local humidity and ice/frost can be precisely mapped across diurnal and seasonal cycles. As described above, the ion-sensitive electrodes at the heart of Phoenix’s WCL have been miniaturized, integrated with fluidics, and can increasingly be made with solid-state materials (Hu et al., 2016; Jaramillo and Noell, 2020), adding specific ions and electron-transfer-amenable species to the small-and-robust-sensor-measurable chemical inventory.
Chemical sensors can also assist with one of the most challenging choices for costly “one-shot” Mars (sub)surface missions: where to land. By deploying comparatively low-cost, low-SWaP sensors on precursor missions to multiple locations—determined to be promising sites based on their geological characteristics and data from remote images—the optimal location can be selected using site-specific ground-truth data for key regolith characteristics, including chemical composition and reactivity. Indeed, sensors can be expressly designed as “chemical reactivity detectors” (Grunthaner et al., 1995; Zent et al., 2003), an advantage over spectroscopic analyses, from which reactivity must be inferred. Because of their robustness and small size and mass, such sensors can probe the sub-surface, providing information not accessible to remote (e.g., spectroscopic) techniques which are limited to the upper few micrometers to millimeters of material, subsequent to delivery by small hard landers, impactors, or penetrators, or making multiple hops and stops aboard small rotorcraft. Even after a massive lander- or rover-supported scientific laboratory (Mahaffy et al., 2012; Farley et al., 2020) begins surface operations, limited reagents and mission duration constrain the total number of analyses performed; smarter decisions about where to drill, scoop, or scrape a sample can be informed by local chemical sensor results. Looking to the future, “chemical sensor scouting” reports could be assessed using local autonomous expert systems and machine-learning methods to decide, for example, to travel and assess conditions elsewhere or initiate (or recommend mission controllers initiate) sample collection at the current location.
Sample processing and microfluidics: Function drives form
Future flagship-class Mars exploration missions, including ExoMars (Vago et al., 2017) and the proposed Mars Life Explorer (Williams et al., 2021), embrace increased sampling depths (Tosi et al., 2026), promising to provide samples better protected from long-term radiation and thermal effects while obtaining them from geological strata well suited to harboring (fossilized) molecular and morphological evidence of past life, such as ancient alluvial and sedimentary formations. Yet individual sample volumes will still be measured, at best, in cubic centimeters. While some instruments can work directly with “raw” regolith samples, greater information content, better limits of detection, and more reliable results will often be attainable by processing samples in a manner bespoke to the strengths and limitations of the science measurement objective, the specific analytical method, and the characteristics of the sample itself.
To find molecular and morphological indicators of extraterrestrial life with optimal use of constrained mass and power requires “preparing” the sample for its analysis in order to:
maximize the performance of each analytical instrument by presenting the sample in a form supporting the best limits of detection with the greatest certainty and accuracy of result; optimize sample collection performance, e.g., by consolidating and handling small-volume samples; provide reagents, standards and calibrants; distribute multiple measured aliquots of appropriate volumes to each instrument or sensor for statistically robust measurement outputs; and leverage sample-preparation commonalities across instruments to minimize technology development cost, mass, volume, and energy consumption.
An exemplary system, the Sample Processor for Life on Icy worlds, was developed around capabilities targeting sample-input requirements for a range of analytical systems (Ricco et al., 2024), typified by those shown in Figure 2. Such processors can perform one or several operations on each sample, often commencing by delivering water, buffer, or solvent to a particulate/icy sample collector to solubilize, extract, or gather small particles and transfer the resultant solution or suspension to the analytical system. Figure 2 shows a litany of operations that can be implemented in situ—singly or in specified sequences—using fluidically enabled and integrated analytical systems that support the input requirements of each measurement and instrument. To improve the efficacy and accuracy of the measurement process, the processor can also provide calibration standards, controls, and blanks; deliver metered aliquots of (processed) sample to one or multiple instruments; and provide electrical isolation or closely controlled temperatures or pressures, as required.
A Mars sample is retrieved by a microfluidic processor, which then prepares aliquots for analysis via one or more of the processor functions listed according to the requirements for each instrument in an analytical suite (examples shown at right).
The 45-year-old wafer-based GC (see above) has since been supplanted by more capable and complex GC-on-a-chip technologies that combine tiny etched columns with micromachined valves, integrated detectors, even wireless communication (Collin et al., 2016; Regmi and Agah, 2018; Wang et al., 2019). Such systems are not restricted to gaseous analytes: microfluidics also can enable single-purpose small analytical systems. Designed to reprise one of Viking’s key experiments, the microfluidic microbial activity microassay (μMAMA) system was developed to detect viable microbes, culturable or not, even in the absence of cellular replication, via colorimetric assays that rely on highly sensitive redox dye chemistry in martian (or icy moon) samples of regolith or ice to detect electron-donor moieties (intracellular or excreted), common to all branches of terrestrial life and widely expected to exist in any live biological entity that enables life to utilize energy (Touchette et al., 2022). A different sort of analysis, the characterization of unknown charged polymers and nanoparticles that could signal life (the former potential information carriers), is the domain of the proposed instrument Charged Information-Polymer Preparation System (ChIPPS), a front-end for nanopore-based sizing or (crude) sequencing of charged multimers (Bywaters et al., 2021).
Microanalytical systems (Krakos, 2023), whether for aqueous assays or gas-phase detection, are sufficiently small and robust to be built into payloads weighing hundreds of grams or incorporated into drill strings. Even as the line blurs between analytical microsystems such as simple stand-alone chemical microsensors, which weigh milligrams to one or two kilograms, and larger planetary instruments, with masses of several to tens of kilograms, both classes of technology should be considered for their complementarity, not as a succession plan. Analytical microsystems can go directly to places—down holes, into cracks, or onto small landers, impactors, and rotorcraft—that larger instruments cannot, and they can be affordable enough for so-called low-cost Mars missions, or to map surface chemical conditions, via many parallel deployments, in support of larger landed laboratories and, someday, human exploration.
Footnotes
Author Disclosure Statement
No competing financial interests exist.
Funding Information
No funding was received for this article.
Associate Editor: Michael A. Meyer