About

Ralph Milliken is a Professor in the Department of Earth, Environmental & Planetary Sciences at Brown University. His research focuses on exploring the geology of planetary surfaces and utilizing remote sensing methods, such as satellite and rover-based data, to address geologic problems. His specific interests include understanding the distribution and role of water in the evolution of the solar system, quantifying mineral and volatile abundances in geologic materials through reflectance spectroscopy, and investigating the sedimentary rock record of Mars. His group integrates experiments, theory, and field/spacecraft data to advance these research areas. Professor Milliken is a science team member for the Mars Curiosity rover and participates in projects mapping water on the Moon, conducting laboratory studies of meteorites, and performing field studies of potential Mars analogs. He has contributed to NASA-funded research, including analyzing samples from the asteroid Bennu as part of the OSIRIS-REx mission, and is involved in efforts to understand the origins of life on Earth through space-related sample analysis. Additionally, he engages in educational outreach, supporting students in environmental science and space-related projects.

Research topics

• Astrobiology
• Geochemistry
• Mineralogy
• Geology

Selected publications

• Amapari Marker Band Metal‐Enrichments: Potential Mechanisms and Implications for Surface and Subsurface Water and Weathering in Gale Crater

  Journal of Geophysical Research Planets · 2026-04-01

  article

  Abstract NASA's Curiosity rover is exploring a 5 km tall sedimentary mound that is hypothesized to record the transition from a warm and wet (phyllosilicate‐rich) to a cold and drier (sulfate‐rich) Mars. Evidence of magnesium sulfate‐bearing rock has shown that Curiosity has crossed through this phyllosilicate‐sulfate transition. Recently, Curiosity arrived at the Amapari Marker Band, a darker, indurated unit that can be traced laterally for tens of kilometers in orbiter images. Here, Curiosity found evidence for a very broad lake, and bedforms interpreted as wave‐ripple laminated sedimentary rock that likely was deposited in shallow water in the explored location, before becoming a deeper lake. These rocks are enriched in Fe, Mn, and Zn which has major implications for groundwater paleohydrology in Gale crater. Three formation hypotheses are considered: concretion formation during early diagenetic alteration of shallow lake sediments, laterization or leaching of the sediments, and addition of Fe, Mn, and Zn by a mildly acidic and reducing groundwater interacting with a redox and/or pH front in a stratified lake. The preferred interpretation of the metal enrichments within the Amapari Marker band sedimentary rocks is that they formed in a shallow water environment at a redox and/or pH front within the ripple unit, which drove precipitation and concentration of metals. If the enrichments are due to groundwater alteration, these processes could link subsurface and surface environments. Water and the presence of high amounts of redox sensitive elements and other metals are favorable indicators for habitability.

  PublisherDOI

• Remote Compositional Analyses of Space-weathered Lunar Maria

  The Planetary Science Journal · 2026-01-01

  articleOpen access

  Abstract Visible-to-shortwave infrared (VSWIR) reflectance spectroscopy has revolutionized our understanding of planetary surface compositions. However, space-weathering processes on airless bodies complicate quantitative compositional analyses. Here, we present a framework to isolate the signatures of space weathering in VSWIR spectra of lunar maria by leveraging radiative transfer modeling under the assumptions that (i) a space-weathered target can be expressed as a mixture of fresh and fully space-weathered components and (ii) remaining signatures can be modeled by including agglutinates as an end-member component. We first validate this approach against laboratory spectra of space-weathered Apollo mare soils of known mineral compositions using a probabilistic Markov Chain Monte Carlo implementation of the Hapke radiative transfer model. Second, we illustrate how this approach can be applied to orbital Moon Mineralogy Mapper data. The proposed space-weathering correction workflow for lunar maria could be expanded to other lunar lithologies and applied to existing and future data sets.

  PublisherDOI

• Carbonate formation and fluctuating habitability on Mars

  Nature · 2025-07-02 · 6 citations

  articleOpen access

  Abstract The cause of Mars’s loss of surface habitability is unclear, with isotopic data suggesting a ‘missing sink’ of carbonate 1 . Past climates with surface and shallow-subsurface liquid water are recorded by Mars’s sedimentary rocks, including strata in the approximately 4-km-thick record at Gale Crater 2 . Those waters were intermittent, spatially patchy and discontinuous, and continued remarkably late in Mars’s history 3 —attributes that can be understood if, as on Earth, sedimentary-rock formation sequestered carbon dioxide as abundant carbonate (recently confirmed in situ at Gale 4 ). Here we show that a negative feedback among solar luminosity, liquid water and carbonate formation can explain the existence of intermittent Martian oases. In our model, increasing solar luminosity promoted the stability of liquid water, which in turn formed carbonate, reduced the partial pressure of atmospheric carbon dioxide and limited liquid water 5 . Chaotic orbital forcing modulated wet–dry cycles. The negative feedback restricted liquid water to oases and Mars self-regulated as a desert planet. We model snowmelt as the water source, but the feedback can also work with groundwater as the water source. Model output suggests that Gale faithfully records the expected primary episodes of liquid water stability in the surface and near-surface environment. Eventually, atmospheric thickness approaches water’s triple point, curtailing the sustained stability of liquid water and thus habitability in the surface environment. We assume that the carbonate content found at Gale is representative, and as a result we present a testable idea rather than definitive evidence.

  PublisherOA PDFDOI

• Near‐ and mid‐infrared spectral diversity in the Aguas Zarcas carbonaceous chondrite and implications for inferring aqueous processes on primitive asteroids using remote sensing

  Meteoritics and Planetary Science · 2025-03-21 · 2 citations

  articleOpen access

  Abstract CM carbonaceous chondrites are complex brecciated meteorites that exhibit significant chemical, mineralogic, and petrographic diversity both between and within individual samples. As most reflectance spectroscopy studies of carbonaceous chondrites are performed on bulk powders, important questions remain about the true spectral diversity of these complex breccias and the degree to which lab‐based meteorite spectra can be reliably related to remotely acquired spectra of primitive asteroids. The Aguas Zarcas meteorite is a unique CM chondrite in that it has been found to exhibit at least five chemically and isotopically distinct lithologies that are all associated with a single fall event. Here, we describe a coordinated petrographic and spectroscopic study to further investigate the thermochemical and collisional history of the Aguas Zarcas parent body and to better understand how to interpret remotely acquired spectra of primitive asteroids. Four intact sections of the Aguas Zarcas meteorite, which together represent at least three to four distinct lithologies, were analyzed using microscope FT‐IR (μFT‐IR) spectroscopy and electron probe microanalysis (EPMA) elemental mapping. Our study found significant variations in spectral features, particularly in the mid‐infrared (MIR) wavelength region, that can be linked to petrographic diversity between lithologies. The relative abundance of matrix phyllosilicates and pyroxene appears to have the strongest influence on the shape, position, and strength of MIR spectral features. Linear spectral unmixing models as a method for compositional interpretation showed varying accuracy when compared to EPMA‐based estimates, with integrated μFT‐IR spectral maps showing better results compared to unmixing of bulk (larger spot size) FT‐IR spectra. A notable discovery in two sections of the Aguas Zarcas meteorite was the presence of carbonate veins along the boundary of chemically and petrographically separate lithologies, which provide important constraints on the nature and timing of pre‐ and post‐brecciation aqueous alteration.

  PublisherOA PDFDOI

• Highly Efficient Compositional and Compound Specific Isotopic Analysis of Volatile Primary Amines and Ammonia in the Murchison Meteorite Using SPME On‐Fiber Derivatization: Optimization for Bennu Sample Analyses

  Rapid Communications in Mass Spectrometry · 2025-01-17 · 1 citations

  articleOpen access

  RATIONALE: Extraterrestrial amines and ammonia are critical ingredients for the formation of astrobiologically important compounds such as amino acids and nucleobases. However, conventional methods for analyzing the composition and isotopic ratios of volatile amines suffer from lengthy derivatization and purification procedures, high sample mass consumption, and chromatographic interferences from derivatization reagents and non-target compounds. METHODS: amines as well as ammonia based on solid phase micro-extraction (SPME) on-fiber derivatization. 2,3,4,5,6-pentafluorobenzyl chloroformate (PFBCF) adsorbed on a solid phase SPME fiber is subsequently exposed to the headspace of the water extract of the Murchison meteorite to selectively extract, derivatize and concentrate volatile amines and ammonia. PFBCF does not directly contact the aqueous solution containing other soluble organics. RESULTS: An aliquot of volatile amines and ammonia in the headspace are selectively derivatized on the SPME fiber and subsequently thermally desorbed onto the GC injector for analysis. Only the amounts of amines required for either compositional or isotopic analysis are derivatized and consumed in the process, preserving the bulk fraction of amines and ammonia for other analyses, and the process does not affect other volatile compound classes. Carbon and hydrogen isotopic ratios of amines are obtained by isotopic mass balance. CONCLUSIONS: The exceptional selectivity and sensitivity of SPME on-fiber derivatization of volatile amines in carbonaceous chondrite extracts allow minimization of sample consumption. Carbon and hydrogen isotopic values of individual amines in the Murchison meteorite are consistent with their extraterrestrial origin, with a substantial fraction inherited from interstellar molecular clouds. SPME on-fiber derivatization is well suited for analyzing extraterrestrial materials, especially precious asteroid return samples.

  PublisherOA PDFDOI

• Opportunities for Planetary Analog Studies in Rochechouart Impactites

  Abstracts with programs - Geological Society of America · 2025-01-01

  article
  PublisherDOI

• Remote Determination of Martian Chloride Salt Abundances

  Journal of Geophysical Research Planets · 2025-03-01 · 1 citations

  articleOpen access

  Abstract Chloride salt‐bearing deposits are widely distributed across the southern highlands of Mars. Because chloride salts are highly water‐soluble, these deposits may be representative of the last significant period of stable liquid water at the Martian surface. Therefore, these deposits are key to understanding the fate and evolution of surface waters on Mars. However, little consensus exists about the formation conditions of these deposits, and their origins remain enigmatic. This is due in part because remote spectroscopic detection and quantification of many anhydrous chlorides is hampered by a lack of easily discernible diagnostic absorption features. To address this issue, we present a novel Hapke radiative transfer model‐based method to estimate hydration states and salt abundances of Martian chloride salt‐bearing deposits using visible/near‐infrared (VNIR) reflectance spectra. VNIR laboratory spectra are used to derive water abundances of analog chloride‐bearing materials, establishing an experimental basis for application of these methods to Mars. These methods are then applied to orbital Compact Reconnaissance Imaging Spectrometer for Mars data to create maps of the hydration state and modeled salt abundance of chloride‐bearing deposits. When overlain onto high resolution 3D digital terrain models, these methods produce the highest resolution site‐specific salt abundance maps currently available, enhancing our understanding of chloride deposit geologic context. As an example, deposits in the Terra Sirenum region are observed to have higher estimated salt abundances than previously recognized, exhibiting spatial variations in both abundance and surface morphology.

  PublisherOA PDFDOI

• Asteroid (101955) Bennu in the laboratory: Properties of the sample collected by <scp>OSIRIS</scp>‐<scp>REx</scp>

  Meteoritics and Planetary Science · 2024-06-26 · 166 citations

  articleOpen access

  Abstract On September 24, 2023, NASA's OSIRIS‐REx mission dropped a capsule to Earth containing ~120 g of pristine carbonaceous regolith from Bennu. We describe the delivery and initial allocation of this asteroid sample and introduce its bulk physical, chemical, and mineralogical properties from early analyses. The regolith is very dark overall, with higher‐reflectance inclusions and particles interspersed. Particle sizes range from submicron dust to a stone ~3.5 cm long. Millimeter‐scale and larger stones typically have hummocky or angular morphologies. Some stones appear mottled by brighter material that occurs as veins and crusts. Hummocky stones have the lowest densities and mottled stones have the highest. Remote sensing of Bennu's surface detected hydrated phyllosilicates, magnetite, organic compounds, carbonates, and scarce anhydrous silicates, all of which the sample confirms. We also find sulfides, presolar grains, and, less expectedly, Mg,Na‐rich phosphates, as well as other trace phases. The sample's composition and mineralogy indicate substantial aqueous alteration and resemble those of Ryugu and the most chemically primitive, low‐petrologic‐type carbonaceous chondrites. Nevertheless, we find distinct hydrogen, nitrogen, and oxygen isotopic compositions, and some of the material we analyzed is enriched in fluid‐mobile elements. Our findings underscore the value of sample return—especially for low‐density material that may not readily survive atmospheric entry—and lay the groundwork for more comprehensive analyses.

  PublisherOA PDFDOI

• The curious case of the missing mantle: How carbonaceous chondrites may confound the spectral identification of partially differentiated asteroids

  Icarus · 2024-12-24 · 5 citations

  articleSenior author
  PublisherDOI

• Reflectance spectroscopy (0.250–2.500 µm) of an unsorted particulate sample from asteroid (101955) Bennu

  2024-01-01

  articleOpen access
  PublisherOA PDFDOI

Frequent coauthors

• B. L. Ehlmann

  California Institute of Technology

  192 shared

• John F. Mustard

  Brown University

  186 shared

• F. Poulet

  Université Paris-Saclay

  178 shared

• S. L. Murchie

  177 shared

• J. L. Bishop

  Search for Extraterrestrial Intelligence

  146 shared

• Gregg A. Swayze

  United States Geological Survey

  135 shared

• L. H. Roach

  88 shared

• J. J. Wray

  86 shared