Distributed-style volcanism is an end member of terrestrial volcanism that produces clusters of small volcanoes when isolated magma bodies ascend from broad magma source regions. Volcano clusters can develop over millions of years, one volcano at a time, and can be used to infer unobserved geologic phenomena, including subsurface stresses and cracks during eruption periods. The Tharsis Volcanic Province covers approximately one-quarter of the martian surface and hosts a large concentration of small volcanoes that formed from distributed volcanism. We present a catalog of 1,106 small volcanic vents identified within Tharsis Volcanic Province. This catalog includes morphologic measurements for each cataloged vent. Vent lengths range from 71 m to 51 km, widths range from 40 m to 3.1 km, and 90% of vents have lengths at least 1.5 times their widths. Additionally, 90% of edifices associated with vents have topographic prominences <100 m. Vents are found throughout Tharsis, though they generally form clusters near large volcanoes or among large graben sets. Older regions with volcanic eruption ages of >1 Ga are found at the Tharsis periphery in the Tempe-Mareotis region and Syria Planum. Vents in the Tharsis interior have reported ages <500 Ma. Regional trends in vent orientation and intervent alignment are dependent on nearby central volcanoes and fossae. We use these findings to hypothesize that within the most recent 500 Ma, magma was present under and to the east of the Tharsis Montes and that some of this magma erupted and built hundreds of small volcanoes in this region.

PROSPECTING OF SUBSURFACE FEATURES E. Bell1, N. Schmerr1, K. Young2, J. Bleacher2, S. Esmaeili3, W. Brent Garry2, S. Jazayeri3, S. Kruse3, R. Porter4, J. Richardson5, P. Whelley5. 1University of Maryland, Department of Geology, 8000 Regents Dr., College Park, MD 20742 ebell1@umd.edu, nschmerr@umd.edu; 2NASA Goddard Space Flight Center; 3University of South Florida; 4Northern Arizona University; 5University of Maryland.

Supplemental code and data for Richardson et al. "Small volcanic vents of the Tharsis Volcanic Province, Mars". Data provided here include an database of volcanic vent locations within the Tharsis Volcanic Province of Mars and associated morphologic measurements.

Introduction: Volcanism is a fundamental process involved throughout Mars’ history [Fig 1] and in nearly every aspect of Mars’ evolution [1-3]. Our understanding of explosive volcanism (i.e., violent expulsions of ash, pumice and rock fragments) on Mars continues to evolve as numerous, small (10s km diameter) and dispersed volcanic centers are recognized throughout the Tharsis region [4-9] and degraded, ancient volcanic centers are recognized in the southern highlands [10-13]. While volcanic deposits have been suggested to exist in Arabia Terra [e.g., 14], few vents are identified in the region. The fretted terrain is a landform type within Arabia Terra defined as smooth low-lying plains separated from complex plateaus by steep uniform cliffs [15] forming a polygonal network of slot canyons [Fig.2]. It is an enigmatic geological unit composed of altered, fine-grained, layered, clayand sulfate-bearing sediments suggested to be eroded ancient explosive volcanic deposits [e.g., 14, 16, 17]. However, this explanation remains unconfirmed because it has two important deficiencies: (1) a logical source [18] and (2) direct morphologic evidence of volcanic deposits. Michalski and Bleacher [19] proposed that several large and irregularly shaped depressions in Arabia Terra [Fig. 2] are calderas that produced colossal explosive eruptions (i.e., supereruptions) that could have been the largest to ever occur on Mars. Consistent with past mapping [20], if these features are indeed explosive calderas, ash dispersion modeling [e.g., 21; 18] suggests extensive ash deposits, carried by ancient westerlies, should be common throughout the region. The km-deep canyon walls within the fretted terrain should therefore expose evidence for Noachian-Hesperian era volcanic deposits. This work is cataloging the layered deposits exposed in these canyon walls to study their mineralogy using CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data, morphology using HiRISE (High Resolution Imaging Science Experiment) and CTX (Context Camera) data, thermal inertia using THEMIS (Thermal Emissions imaging system) data and distribution using a GIS (geographic information system). Through this work we are assessing if Arabia Terra could have been the site of early martain supereruptions. Even if this hypothesis is not supported, our team is collecting a new set of observations relevant to understanding the origin of the layered material in Arabia Terra. In addition, the Mars 2020 Rover landing site (Jezero) is within the potential ash dispersion reach of the suggested calderas [Fig. 2], giving this work the potential to comment directly on the geology of the selected landing site. Methods: To test the caldera hypothesis we have assembled a team of planetary geologists, volcanic geomorphologists, modelers, GIS scientists and remote sensing experts familiar with identifying, cataloging, and describing explosive volcanic deposits. The project is enabled by recent increases in data coverage of Arabia Terra (particularly of CRISM and HiRISE data). GIS Survey: Our team is performing a regional sur-

We present the design of a portable version of our miniaturized laser heterodyne radiometer (mini-LHR) that simultaneously measures methane (CH4) and carbon dioxide (CO2) in the atmospheric column. The mini-LHR fits on a backpack frame, operates autonomously, and requires no infrastructure because it is powered by batteries charged by a folding 30 W solar panel. Similar to our earlier instruments, the mini-LHR is a passive laser heterodyne radiometer that operates by collecting sunlight that has undergone absorption by CH4 and CO2. Within the mini-LHR, sunlight is mixed with light from a distributive feedback (DFB) laser centered at approximately 1.64 μm where both gases have absorption features. The laser scans across these absorption features roughly every minute and the resulting beat signal is collected in the radio frequency (RF). Scans are averaged into half hour and hour data products and analyzed using the Planetary Spectrum Generator (PSG) retrieval to extract column mole fractions. Instrument performance is demonstrated through two deployments at significantly different sites in interior Alaska and Hawaii. The resolving power (λ/∆λ) is greater than 500,000 at 1.64 μm with precisions of better than 20 ppb and 1 ppm for CH4 and CO2, respectively. Because mini-LHR instruments are portable and can be co-located, they can be used to characterize bias between larger, stationary, column observing instruments. In addition, mini-LHRs can be deployed quickly to respond to transient events such as methane leaks or can be used for field studies targeting geographical regions.