Publications

Geological remote sensing techniques can be used to investigate structural, depositional, and shock metamorphic effects associated with hypervelocity impact structures, some of which may be linked to global Earth system catastrophies. Although detailed laboratory and field investigations are necessary to establish conclusive evidence of an impact origin for suspected crater landforms, the synoptic perspective provided by various remote sensing systems can often serve as a pathfinder to key deposits which can then be targetted for intensive field study. In addition, remote sensing imagery can be used as a tool in the search for impact and other catastrophic explosion landforms on the basis of localized disruption and anomaly patterns. In order to reconstruct original dimensions of large, complex impact features in isolated, inaccessible regions, remote sensing imagery can be used to make preliminary estimates in the absence of field geophysical surveys. The experienced gained from two decades of planetary remote sensing of impact craters on the terrestrial planets, as well as the techniques developed for recognizing stages of degradation and initial crater morphology, can now be applied to the problem of discovering and studying eroded impact landforms on Earth. Preliminary results of remote sensing analyses of a set of terrestrial impact features in various states of degradation, geologic settings, and for a broad range of diameters and hence energies of formation are summarized. The intention is to develop a database of remote sensing signatures for catastrophic impact landforms which can then be used in EOS-era global surveys as the basis for locating the possibly hundreds of missing impact structures. In addition, refinement of initial dimensions of extremely recent structures such as Zhamanshin and Bosumtwi is an important objective in order to permit re-evaluation of global Earth system responses associated with these types of events.

The potential is described of a candidate Mars Observer altimeter for determining dielectric properties of Mars regolith. It is pointed out that it is straightforward to use the time between altimeter pulse trains for passive radiometry (hence dielectric properties) and roughness can be derived. Given the mission plan the whole surface can be mapped at least three times, yielding data on seasonal variability.

Understanding the divergent evolution of Venus and Earth is a fundamental problem in planetary science. Although Venus today has a hot, dry atmosphere, recent modeling suggests that Venus may have had a clement surface with liquid water until less than 1 billion years ago [1]. Venus today has a nearly stagnant lithosphere. However, Ishtar Terra’s folded mountain belts, 8-11 km high, morphologically resemble Tibet and the Himalaya mountains on Earth and apparently require several thousand kilometers of surface motion at some time in Venus’s past. Loss of liquid surface water increases the coefficient of friction in fault zones, favoring a transition from an early mobile lithosphere to a present-day stagnant lithosphere [2]. Solar-driven climate evolution could contribute to a prolonged epoch of water loss on Venus and may be the ultimate cause of the divergent evolution of both the climate history and the interior dynamics of Venus and Earth. In addition to being a problem of first-order importance for Solar System evolution, understanding the divergent evolution of Venus and Earth is also important for understanding the temporal and spatial distribution of habitable environments in the Solar System. Understanding the evolution of Venus is also a key test for models that interpret Earth-sized exoplanets. Testing evolutionary hypotheses requires interpreting clues that were left behind in both the isotopic composition of the Venus atmosphere and in the rock record of the Venus surface. Although several mission concepts are currently competing for possible flights to Venus, only the Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus (DAVINCI+) mission [3] can examine both the atmospheric isotopic record and the rock record of Venus. DAVINCI+ is therefore a compelling choice for selection in the current NASA Discovery Program Phase A competition. DAVINCI+ includes an atmospheric entry probe and a carrier spacecraft (Figure 1). The probe measures atmospheric composition using a mass spectrometer and tunable laser spectrometer, performs descent imaging, and measures atmospheric structure. Following completion of the probe mission, the carrier spacecraft enters Venus orbit and images the Venus surface in the 1 micron atmospheric window. This payload is ideally suited for testing models of Venus evolution. Figure 1: The DAVINCI+ entry probe studies the atmosphere while imaging the Alpha Regio landing site. Afterward, the carrier probe performs infrared imaging of selected targets from orbit. The History of Water: Isotopic Record Pioneer Venus measured the D/H value of an H2SO4 cloud droplet at ~55 km as 157±30 times the terrestrial value, which was interpreted as the signature of escape of water from the Venus atmosphere [4]. Terrestrial spectroscopy produced a similar range, whereas Venus Express measured a value up to three times larger between 70-90 km [5]. The large value relative to Earth shows that Venus lost a substantial amount of water, but the large uncertainty and the lack of data below the clouds makes it difficult to quantitatively model the history of water loss [6]. DAVINCI+ will measure D/H to high precision from above the clouds down to the surface, greatly improving our ability to interpret the history of water loss on Venus. In addition, the abundance and isotopic ratios of Xe and Kr, together with the Kr/Ar and Xe/Ar ratios, which DAVINCI+ will measure with high precision, will be instrumental in revealing whether Venus and Earth formed in the same way and how their climates diverged.  The History of Water: Rock Record  A key but poorly answered question is the extent to which Venus has produced granitic or felsic (SiO2-rich) volcanism. Small amounts of felsic magma can be generated by lithospheric processes [7], but large amounts of felsic material requires the presence of water in the melting zone [8], as in terrestrial subduction zones. Tessera, which are regions of old, thick, highly tectonized crust, are widely accepted as the most likely location for felsic material on Venus. Venus Express observations suggest that tessera in Alpha Regio has a felsic composition [9]. DAVINCI+ will explore the presence and distribution of felsic rock in two ways. Comparison of the reflectivity at 1 micron and in panchromatic descent images at the Alpha Regio landing site will test the presence of felsic rock at patch sizes much smaller than can be observed from orbit. Descent imaging will also explore the landing zone geomorphology, and stereo topography will enable quantitative modelling of faulting and folding. Orbital imaging in the 1 micron atmospheric window will test for the presence of felsic rock in other tessera, including Tellus Regio, Fortuna Tessera, Maxwell Montes, Ovda Regio, and Thetis Regio. Our approach is similar to VIRTIS on Venus Express [9] but focuses on regions in the northern hemisphere and near the equator that were not imaged by VIRTIS. The History of Volcanism: Isotopic Record Volcanic outgassing releases radioactive decay products such as 40Ar and 4He to the atmosphere. DAVINCI+ measurements of their atmospheric abundance can be used to estimate volcanic outgassing over time. 40Ar provides an integrated record of volcanism over Venus history. Because 4He escapes from the atmosphere to space, its atmospheric abundance constrains geologically recent (last billion years) volcanism. Existing measurements of 40Ar and 4He are too imprecise to strongly constrain the volcanic history of Venus [10] but will be measured with much greater accuracy by DAVINCI+. Because DAVINCI+ will constrain the history of both water and volcanism on Venus, it will provide new insights into the feedbacks that shaped the divergent evolution of Venus and Earth. References [1] Way and Del Genio, JGR 125, e2019JE006276, 2020. [2] Weller and Kiefer, JGR 125, e2019JE005960, 2020. [3] Garvin et al., LPSC 51, abstract 2599, 2020. [4] Donahue et al., Science 216, 630-633, 1982. [5] Bertaux et al., Nature 450, 646-649, 2007. [6] Donahue et al., Venus II, 385-414, 1997. [7] Elkins-Tanton et al., JGR 112, E04S06, 2007. [8] Campbell and Taylor, GRL 10, 1061-1064, 1983. [9] Gilmore et al., Icarus 254, 350-361, 2015. [10] Namiki and Solomon, JGR 103, 3655-3677, 1998.    

In order to effectively capture a realistic terrain applicable to studies of cratering processes and landing hazards on Mars, we have obtained high resolution digital elevation models of several pristine explosion craters at the Nevada Test Site. We used the Airborne Terrain Mapper (ATM), operated by NASA's Wallops Flight Facility to obtain DEMs with 1 m spacing and 10 cm vertical errors of 4 main craters and many other craters and collapse pits. The main craters that were mapped are Sedan, Scooter, Schooner, and Danny Boy. The 370 m diameter Sedan crater, located on Yucca Flat, is the largest and freshest explosion crater on Earth that was formed under conditions similar to hypervelocity impact cratering. As such, it is effectively pristine, having been formed in 1962 as a result of a controlled detonation of a 100 kiloton thermonuclear device, buried at the appropriate equivalent depth of burst required to make a simple crater. Sedan was formed in alluvium of mixed lithology and subsequently studied using a variety of field-based methods. Nearby secondary craters were also formed at the time and were also mapped by ATM. Adjacent to Sedan and also in alluvium is Scooter, about 90 m in diameter and formed by a high-explosive event. Schooner (240 m) and Danny Boy (80 m) craters were also important targets for ATM as they were excavated in hard basalt and therefore have much rougher ejecta. This will allow study of ejecta patterns in hard rock as well as engineering tests of crater and rock avoidance and rover trafficability. In addition to the high resolution DEMs, crater geometric characteristics, RMS roughness maps, and other higher-order derived data products will be generated using these data. These will provide constraints for models of landing hazards on Mars and for rover trafficability. Other planned studies will include ejecta size-frequency distribution at the resolution of the DEM and at finer resolution through air photography and field measurements, correlation of ejecta size and composition with radar and visible-thermal IR remote sensing signatures, and comparison of these results with similar measurements of Mars. The final DEMs, ancillary data sets, and derived data products will be made available to the community.

This study combines MOLA altimetry with photographic imagery to begin assessing the extent to which sedimentary and volcanic processes have affected impact crater morphology in the Arrhenius region of Mars.