James J Wray

What you see at left is a visitor from another solar system. This one is 3I/ATLAS, the third interstellar object (ISO) discovered since 2017. Like 2I/Borisov before it, it is a comet, but with striking chemical differences from most comets bound to our Sun. The first known ISO, 'Oumuamua, was arguably even stranger, with no cometary tail but showing extreme brightness variations, suggesting a highly elongated or flattened object (more so than any asteroid we've imaged) tumbling through space. Each of these objects formed in a different part of our galaxy, some perhaps long before the Sun, then were ejected from their birth environments and found themselves on a chance collision course with our solar system. Visits like this have presumably always happened, but our sky surveys are just now becoming powerful enough to detect them. With some advance planning, perhaps we could send a spacecraft to intercept the next one.

This is our groups's major new research focus, for which we are seeking students. We collaborate closely with the Hawaiian scientists whose surveys discovered 3I/ATLAS (and 'Oumuamua before it) and contributed extensively to the worldwide campaign to observe 3I as it approached and then receded from the Sun. By chance, 3I passed 10x closer to Mars than to Earth, so humanity's finest-scale image of an interstellar comet to date (visible at right) was taken from Mars orbit, which we worked to plan. There is much more we can learn from these data collected already, even as we eagerly await the next ISO. Or could any Sun-orbiting bodies be former ISOs, now captured into permanent solar system residency? We have explored this theoretically and are planning telescope observations of candidate objects.

The Vera Rubin Observatory (photo at left by JW) recently began scanning the full southern sky at greater depth and cadence than ever before. It may soon discover many more ISOs. These are exoplanetary bodies conveniently delivered to our own neighborhood for detailed study! Will the oddities observed in the first few ISOs prove representative of the population as a whole? (Could that mean that our own system is unusual??) If so, this could usher in a revolution in our understanding of planet formation across space and time.

Minerals in the ancient rocks on Mars record environmental conditions in which they formed, and the stratigraphy (i.e., sequence) of these minerals reveals how environments evolved over time. We generate mineral maps from CRISM orbital spectra and combine these with HiRISE images and topography to infer mineral stratigraphy. In the layered rocks at Mawrth Vallis, we were the first to infer that Al-rich clay minerals formed relatively late after Fe/Mg-rich clays, a pattern subsequently mapped elsewhere on Mars that may reflect ancient surface weathering. We also found deeper layers containing the Ca-sulfate bassanite. Mawrth Vallis has been a highly ranked candidate landing site for all the recent NASA and ESA rovers.

The stratigraphy of sulfates vs. clays is a key issue for understanding ancient Mars. Curiosity has been exploring this in Gale crater, and previously Opportunity spent its final years traversing a sulfate-to-clay transition at the rim of Endeavour crater. Our work with CRISM data first identified clays in this crater's rim hills, and polyhydrated sulfates in the brighter foreground rocks visible at left. As the rovers acquire "ground truth" on these outcrops, our continued analysis of nearby orbital data provides valuable context.

Another part of our stratigraphic work involves measurement of stratal (layer) geometries. We use meter-scale digital elevation models derived from HiRISE images to fit planes to strata visible in these images. The magnitude, direction, and variability of stratal dips constrain their depositional processes and subsequent deformation. Do the sediments predate or postdate the local topography (e.g., crater or valley walls)? Were they deposited in low- or high-energy settings? Do internal unconformities suggest changing environments? These questions are answerable by our methods, which are also suitable for rover-based imagery.

Numerous Martian craters appear to have once hosted lakes. The Spirit rover was sent to Gusev crater because a large valley emptied into it, suggesting water might have ponded inside. Yet Spirit found no clear lake sediments, and for many years the orbital evidence for aqueous minerals in other putative paleolakes was missing. CRISM is changing this story, with both clays and hydrated salts now being found within paleolakes such as Columbus crater. The video at right shows bright sulfate outcrops that we identified on the crater's northeast floor and in a "bathtub ring" halfway up the crater wall. These salts transition from more to less hydrated varieties with increasing depth within the crater, and may have formed when a lake evaporated or froze. Columbus crater has no major inlet valleys, but is a location in which elevated groundwater upwelling and evaporation were predicted. Such groundwater-fed lakes may represent a new class of habitable environment on ancient Mars.

We have identified polyhydrated sulfate in another Martian paleolake in Shalbatana Vallis. However, yet another paleolake, Jezero crater (the landing site of Perseverance), instead contains carbonates along with Fe/Mg-smectite clays. This mineralogy implies a different chemistry from the sulfate-rich deposits found in Columbus and Shalbatana, but is it merely a result of erosion and transport of carbonate- and smectite-rich materials known to exist in Jezero's watershed? Or do these differing salt compositions reflect local differences in water chemistry, groundwater vs. meteoric inputs, or possibly a change in atmospheric composition over time? We are studying additional paleolakes with CRISM, attempting to narrow down these hypotheses by finding more candidate lake evaporites. Earth analog studies, including by our group, are also key for probing what we can learn about ancient lakes on Mars from the salts they leave behind.

Mars is a volcanic world. Despite most surface missions there specifically seeking sedimentary rocks, almost all found igneous rocks instead, at least at first. This includes NASA's newest rover, Perseverance in Jezero crater. We characterized a mountain with summit crater (right) rising from Jezero's southeast corner---from which material appears to extend onto the crater floor---finding its properties consistent with explosive volcanoes elsewhere on Mars and on Earth. If some materials sampled by Perseverance are indeed from this 'Jezero Mons', and if the Perseverance samples are eventually returned to laboratories on Earth as planned, then this could become the first volcano of precisely known age (among other properties) on the surface of another planet. Yet it was never even discussed during Perseverance landing site selection! If such a significant landform went essentially unnoticed at one of the most-studied sites on Mars until recently, then how many other key features (including volcanoes) still await discovery in our burgeoning Mars data sets?

We have contributed to studies of potential volcanic features spread widely across Mars. One focus has been to distinguish igneous volcanoes from mud volcanoes, a morphologically similar landform but for which the materials, processes, environmental and even astrobiological implications are quite different. We have characterized potential martian mud volcanoes, which are intriguing future mission targets, using morphological, hyperspectral, and multispectral imaging methods.

Within igneous volcanism, recent discoveries have expanded the range of known eruptive styles and compositions. We reported the first exceptionally feldspar-rich rocks (granite? or anorthosite??) on a martian volcano, shortly before similar compositions were found in situ by the Curiosity rover. As such evidence for complex magmatic processes on Mars continues to grow, we have been finding them in more and more locations across the planet. The more we explore Mars, the more it resembles Earth in the myriad ways that magmatic and sedimentary processes have interacted over time.

Modern Mars is an active world. Several types of hillslope features have been observed to change via repeat spacecraft monitoring, fueling speculation on the role of liquid water in each case. Warm-season recurring slope lineae such as those at left are arguably the most consistent with flow of liquid brines. We have used CRISM to search for independent spectral confirmation of liquid water or salts in these flows, finding some tantalizing hints but whose interpretation continues to be debated. In situ measurements of these features may be needed to better understand them. Certainly there is water on Mars, even if only as ice ... which can also flow!

Additional evidence for brine activity in recent Martian history comes from the morphology and chemistry of dust-mantled regions. We analyzed a thick dust mantle west of the Tharsis volcanoes in a study emphasizing gamma spectroscopy data. Like other dust mantles, this region shows a honeycomb-like texture with characteristic spacing ~5 meters. The dust maintains steep cliffs where penetrated by impact craters or other topography, implying cemented rather than loose dust. Remote chemical data suggest a sulfate cement. Intriguingly, older Hesperian outcrops with monohydrated sulfates (e.g., kieserite) have similar textures, suggesting that sulfate-cemented "duststone" may have formed over much of Martian history, perhaps at regional to planetary scales.

One provocative and enduring martian mystery is the possible identification of methane in its atmosphere that appears to vary over space and time. Methane is unstable on Mars, so would likely imply ongoing geologic or biological activity. If future observations confirm discrete methane plumes, then the geologic properties of plume regions should help to constrain their origin and release mechanisms. We have begun studying these regions with orbital remote sensing, and this was a prime objective for the CaSSIS camera launched in 2016 aboard ESA's ExoMars Trace Gas Orbiter. Although the orbiter has not detected methane, CaSSIS is now acquiring the best-ever color images of Mars from orbit; Co-Investigator Wray leads the prioritization of CaSSIS images targeted with the goal of mineral classification and mapping. CaSSIS can image at various times of day, giving it unique ability to constrain active processes on modern Mars.

Geologic evidence points to a wetter Martian surface in the distant past. If liquid water was ever stable at the surface, then both the temperature and pressure there must have been higher. A formerly thicker CO₂ atmosphere has been a long-hypothesized solution. But where did the bulk of the CO₂ go? It might have escaped to space, but if some remains on Mars, then carbonate minerals are the largest potential reservoir. Hence the search for carbonates on Mars has long been a key thrust in the effort to understand early planetary habitability.

We contributed to the first identification of carbonate-bearing rocks on Mars, specifically Mg-rich carbonates dating to the Late Noachian. More recently, we have used CRISM to identify a spectrally distinct phase consistent with Fe- and/or Ca-rich carbonates, especially in and around the Early Noachian Huygens basin. These Fe/Ca-carbonates may indicate a higher water activity or distinctive chemistry of the precursor rocks. One challenge to carbonate identification is the ability of smectite clays to "mask" the presence of carbonates in the 2.3 micron spectral region. Thousands of clay outcrops have been reported on Mars, but what fraction of these might also contain carbonate? To address this, we can use the oft-neglected 3-4 micron portion of CRISM spectra, where many laboratory carbonates have their strongest spectral features.

The Curiosity rover has advanced these efforts through its unprecedented sensitivity to carbonates. The SAM instrument suite includes a mass spectrometer that can measure carbonate abundances as low as tens of parts per million, and constrain carbonate chemistry via the temperature at which CO₂ evolves upon heating. SAM's Tunable Laser Spectrometer can measure even lower abundances, potentially down to parts per billion, and for higher abundances can measure isotope ratios that may reflect alteration processes, atmospheric escape, volcanic outgassing, biology, or all of the above. We contributed to the analysis of early SAM data from Mars, and more recent findings confirm that carbonates are more widespread than can be sensed remotely.