Research
My research is focused on water-rock interaction as reflected in a body's near-surface mineral and geological record. I use both experimental methods (laboratory and analogue studies) and observational methods (rover, returned sample analysis, and orbital data analysis) to quantitatively address geologic questions on Earth, Mars, asteroids, and other rocky and icy bodies. Descriptions of my past and ongoing projects can be found below.
High-pressure salty ices relevant to Europa
2025-present
I am collecting NIR-MIR spectra of high-pressure Mg-sulfate-bearing water ice relevant to the upcoming exploration of Europa by the MAJIS instrument on board JUICE. This work is being done at the Soleil Synchrotron in Orsay, France.
Phosphate geochemistry
2024-present
I am collecting detailed chemical, mineralogical, and Vis-NIR-MIR spectral data of a wide variety of natural and synthetic phosphates to understand the details of phosphate spectral features being found on Mars and within asteroids. This includes using the flight spare version of SuperCam located in Toulouse.
Spectral heterogeneity in rubble pile asteroids
2022-present
I am part of the MicrOmega team that spans IAS (Orsay, France) and ISAS/JAXA (Sagamihara, Japan) studying samples returned from the rubble pile asteroids Bennu (OSIRIS-REx) and Ryugu (Hayabusa2). I am particularly interested in spectral variation at multiple spatial scales, from between sampling sites to within a single grain. My work also compares the origin and abundances of phosphates in the two asteroids. (Pilorget et al., 2025; Fukai et al., 2025)
Revisiting martian chaos terrains
2023-present
As part of an MDAP I am a Co-I on, we are revisiting orbital spectra of martian chaos terrains and comparing the mineral stratigraphies to generate an environmental model of the area. I am focusing on the Iani Chaos region.
Continuous flow experiments to track the incorporation of Li into Mars-relevant clays
2021-2025
We used continuous flow laboratory experiments at JPL to study which clay minerals can incorporate Li into their structure from Li-bearing groundwater exposure. This was based on a hypothesis proposed for co-occurring elevated Li and clay content in Gale crater's Vera Rubin ridge. We found that Li can be efficiently pulled out of groundwater by talc and illite. Curiosity's drill samples Aberlady (AL) and Kilmarie (KM) have the highest Li measured by ChemCam (shown right) as well as XRD peaks that have been attributed to talc. (Sheppard et al., 2026; Weber et al., 2026)
Spectral masking: when one mineral can hide another
2019-present
I am interested in how certain surface minerals can obscure the spectral evidence of others. Using a spectrometer with an atmospheric chamber at RELAB, we found that the spectral evidence of dehydrated Mg sulfates is easily obscured if clay minerals are present in the same sample (Sheppard et al., 2020). In a second project using the MicrOmega imaging spectrometer at IAS, we found that the presence of Mg sulfates can obscure evidence of carbonates, which may explain the lack of orbital signatures of carbonate in Gale crater where carbonate has been detected by the Curiosity rover (Sheppard et al., 2025).
Mineralogy and stratigraphy of Mt. Sharp
2018-present
I mapped the mineral stratigraphy of Mt. Sharp beyond the Curiosity rover traverse to understand how mineralogy changes across morphological contacts. These stratigraphic relationships and how they vary laterally can help constrain the crater's history, both during its lacustrine phase and long after. My students have also documented the diagenetic fluid events superimposed on the primary mineral stratigraphy. I am continuing this work as a Participating Scientist on MSL. (Sheppard et al., 2020; Sheppard et al., 2021; Manelski, Sheppard, et al., 2023; Ando, Sheppard, et al., 2025)
Persistence of amorphous Fe in Lake Towuti: a terrestrial analogue for Gale crater
2015-2020
Lake Towuti is a siderite-rich, redox-stratified lake in ultramafic bedrock in Indonesia. I characterized the crystalline and amorphous mineralogy of the catchment, lake basin, and a 100 m long drill core to understand what signals can be uniquely linked to lake conditions. I found that amorphous Fe-rich phases are created rapidly in the water column, and that they persist in >1 million year old sediment, two observations also seen in Gale crater sediment on Mars. Lake Towuti provides an interesting analogue that is complementary to many other cold-weather and basaltic Mars analogues because of its long-lived amorphous Fe. (Sheppard et al., 2019; Russell et al., 2020; Sheppard et al., 2021)
Diamond color origin using FTIR and Raman spectroscopy
2013-2015
Spectroscopy is a main tool to investigate whether diamonds are natural, lab-grown, and/or have had their color temporarily or permanently enhanced. My expertise at the Gemological Institute of America was in the spectral analysis of very small gems, or melee. I developed new analytical techniques for quickly determining the mineralogy and color origin of melee using FTIR spectroscopy, publishing results and methods in Gems & Gemology. This job between my Bachelor's and PhD was how I first fell in love with spectroscopy. (Wang et al., 2014; Sheppard et al., 2015a; Sheppard et al., 2015b)
Organic molecule biomarkers record earthquake fault heating conditions
2011-2014
Methylphenanthrenes are organic molecules that break down under different heating conditions. We calibrated a technique to use methylphenanthrenes as a paleothermometer to understand past fault slip rates using fault samples with pseudotachylite, visible melt that records the most intensely heated part of the rock. To understand the kinetics of these reactions we also conducted rapid heating experiments to see how the molecules broke down under different time and temperature regimes. This was part of my Bachelor's research done at Lamont-Doherty Earth Observatory. (Savage, Polissar, Sheppard, et al., 2014; Sheppard et al., 2015)
