Aaron Zent - Academia.edu (original) (raw)

Papers by Aaron Zent

43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 2021

The goal of this work is to explore the history of the high-latitude subsurface in the latitude r... more The goal of this work is to explore the history of the high-latitude subsurface in the latitude range of the Phoenix landing site (65-75 deg. N). The approach is to use time-marching climate models to search for times, locations, and depths where thick films of unfrozen water might periodically occur. Thick films of unfrozen water (as distinct from ubiquitous monolayer water) are interesting for two reasons. First, multi-layer films of water may be bio-available. Second, patterned ground may require the occurrence of thick films of unfrozen water to facilitate the migration of particles and the development of excess pore ice, as reported by the Odyssey Gamma Ray Spectrometer (GRS) results. For the purposes of this work, we define conditions adequate to establish thick films of unfrozen water to be T greater than 268 K, and RH greater than 0.5. We start with the need to understand the atmospheric pressure. Because of the fact that we're looking at high latitudes, the seasonal cap...

Space Science Reviews, 2007

One of the fundamental challenges facing the scientific community as we enter this new century of... more One of the fundamental challenges facing the scientific community as we enter this new century of Mars research is to understand, in a rigorous manner, the biotic potential both past and present of this outermost terrestrial-like planet in our solar system. Urey:

Journal of Geophysical Research, 2009

A primary objective of the Phoenix mission was to examine the characteristics of high latitude gr... more A primary objective of the Phoenix mission was to examine the characteristics of high latitude ground ice on Mars. We report observations of ground ice, its depth distribution and stability characteristics, and examine its origins and history. High latitude ground ice was explored through a dozen trench complexes and landing thruster pits, over a range of polygon morphological provinces. Shallow ground ice was found to be abundant under a layer of relatively loose ice-free soil with a mean depth of 4.6 cm, which varied by more than 10x from trench to trench. These variations can be attributed mainly to slope effects and thermal inertia variations in the overburden soil affecting ground temperatures. The presence of ice at this depth is consistent with vapor-diffusive equilibrium with respect to a mean atmospheric water content of 3.4 Â 10 19 m À3 , consistent with the present-day climate. Significant ice heterogeneity was observed, with two major forms: ice-cemented soil and relatively pure light toned ice. Ice-cemented soils, which comprised about 90% of the icy material exposed by trenching, are best explained as vapor deposited pore ice in a matrix supported porous soil. Light toned ice deposits represent a minority of the subsurface and are thought to consist of relatively thin near surface deposits. The origin of these relatively pure ice deposits appears most consistent with the formation of excess ice by soil ice segregation, such as would occur by thin film migration and the formation of ice lenses, needle ice, or similar ice structures.

Journal of Geophysical Research, 1999

The Mars Microprobe Mission will be the second of the New Millennium Program's technology develop... more The Mars Microprobe Mission will be the second of the New Millennium Program's technology development missions to planetary bodies. The mission consists of two penetrators that weigh 2.4 kg each and are being carried as a piggyback payload on the Mars Polar Lander cruise ring. The spacecraft arrive at Mars on December 3, 1999. The two identical penetrators will impact the surface at-190 m/s and penetrate up to 0.6 m. They will land within 1 to 10 km of each other and-50 km from the Polar Lander on the south polar layered terrain. The primary objective of the mission is to demonstrate technologies that will enable future science missions and, in particular, network science missions. A secondary goal is to acquire science data. A subsurface evolved water experiment and a thermal conductivity experiment will estimate the water content and thermal properties of the regolith. The atmospheric density, pressure, and temperature will be derived using descent deceleration data. Impact accelerometer data will be used to determine the depth of penetration, the hardness of the regolith, and the presence or absence of 10 cm scale layers. taneous measurements at numerous stations distributed around a planet. It will also demonstrate subsurface sample collection and analysis. To reach this objective, a highly miniaturized penetrator-based platform for in situ science has been developed. A secondary mission goal is the collection of meaningful science data. Penetrators are instrumented devices that impact the ground at high velocity and come to rest in the subsurface due to their

Journal of Geophysical Research, 2001

Icarus, 1986

A thermal/diffusive model of H20 kinetics and equilibrium was developed to investigate the longte... more A thermal/diffusive model of H20 kinetics and equilibrium was developed to investigate the longterm evolution and depth distribution of subsurface ice on Mars. The model quantitatively takes into account (1) obliquity variations; (2) eccentricity variations; (3) long-term changes in the solar luminosity; (4) variations in the argument of subsolar meridian (in planetocentric equatorial coordinates); (5) albedo changes at higher latitudes due to seasonal phase changes of CO, and the varying extent of CO2 ice cover; (6) planetary internal heat flow; (7) temperature variations in the regolith as a function of depth, time, and latitude due to the above factors; (8) atmospheric pressure variations over a 104-year time scale; (9) the effects of factors (l) through (5) on seasonal polar cap temperatures; and (10) Knudsen and molecular diffusion of H20 through the regolith. The migration of H20 into or out of the regolith is determined by two boundary conditions, the H20 vapor pressure at the subsurface ice boundary and the annual average H,O concentration at the base of the atmosphere. These are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap. Starting from an arbitrary initial uniform depth distribution of subsurface ice, H20 fluxes into or out of the regolith are calculated for 100 selected obliquity cycles, each representing a different epoch in Mars' history. The H20 fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time. The results show that obliquity variations influence annual average regolith temperatures in varying degrees, depending on latitude, with the greatest effect at the poles and almost no effect at 40 ° lat. Insolation changes at the pole, due to obliquity, argument of subsolar meridian, and eccentricity variations can produce enormous atmospheric H.,O concentration variations of ~6 orders of magnitude over an obliquity cycle. Superimposed on these cyclic variations is a slow, monotonic change due to the increasing solar luminosity. AIbedo changes at the polar cap due to seasonal phase changes of CO_, and the varying thickness of the CO, ice cover are critically important in determining annual average atmospheric H,O concentrations. Despite the strongly oscillating character of the boundary conditions, only small amounts of H20 are exchanged between the regolith and the atmosphere per obliquity cycle (<10 g/cruZ). The net result of H.,O migration is that the regolith below 30-40 ° lat is depleted of subsurface ice, while the regolith above 30-40 ° lat contains permanent ice due to the depth of penetration of the annual thermal wave. This result is supported by recent morphological studies. The rate of migration of H,O is strongly dependent on average pore/capillary radius for which we have assumed values of I and i0/zm. We estimate that the H20 ice removed from the regolith would produce a permanent ice cap with a volume between 2 × l06 and 6 × 106 km 3. This generally agrees with estimates deduced from deflationary features at lower latitudes, depositional features at higher latitudes, and the mass of the polar caps.

Introduction: We are performing laboratory experiments to determine the concentrations and rates ... more Introduction: We are performing laboratory experiments to determine the concentrations and rates of dissolution of ions that could occur in closed, juvenile groundwater systems on Mars. Our approach is to incubate unaltered Mars-analog minerals in initially pure liquid water in contact with a Mars gas mixture for one year. At exponentially increasing time intervals, aliquots of the solutions at three different temperatures are extracted and analyzed using standard terrestrial laboratory geochemical techniques. Ultimately, our experiments will produce Mars analog brines which will be freeze dried to create evaporites. The physical and chemical properties of these evaporites will be compared with spacecraft remote sensing and in situ compositional and physical data. Evaporite deposits may represent significant sinks of mobile cations (e.g. Ca 2+ , Na + , Mg 2+ , Fe 2+) and anions (e.g. CO3 2-, NO3 2-, NO2-, SO4 2-, SO3-, Cl-) among the materials composing the martian surface and upper crust. Carbon and nitrogen are especially interesting because of their role as atmospheric gases which can become incorporated into crustal rocks. The nature of evaporite-precursor brines formed under martian conditions is poorly understood. Our laboratory studies investigating the formation of brines will greatly aid in improving our understanding of both the fluids and their precipitates (evaporites). The modeling of "warm, wet Mars" alteration and precipitate mineralogies will benefit from laboratory data of the type we discuss here.

Advances in Space Research, 1997

The current Martian water cycle is extremely asymmetric, with large amounts of vapor subliming of... more The current Martian water cycle is extremely asymmetric, with large amounts of vapor subliming off a permanent north polar water ice cap in northern summer, but with no apparent major source of water vapor in the southern hemisphere. Detailed simulations of this process with a three-dimensional circulation model indicate that the summertime interhemispheric exchange (Hadley cell) is very much stronger than transport by eddies in other seasons. As a result, water ice would be distributed globally were it not for the buffering action of regolith soil adsorption which limits the net flux of water vapor off the north polar cap to amounts that are insignificant even on the scale of thousands of years. It has been suggested that the polar layered deposits are the result of exchange on these long time scales, driven by changes in Martian orbital parameters. We therefore are conducting simulations to test the effect of varied orbital parameters on the Martian water cycle. We find that when the perihelion summer pole is charged with a polar water ice cap, large quantities of water are quickly transfered to the aphelion summer pole, setting up an annual cycle that resembles the present one. Thus, the adsorptivity of the Martian regolith may be in the narrow range where it can limit net transport from the aphelion but not the perihelion pole.

J. Geophysical Research, 2001

Cabrol, NA; Chong-Diaz, G.; Dohm, JM; Pereira Arredondo, M.; Dunfield, G.; Gulick, VC; Jensen-Igl... more Cabrol, NA; Chong-Diaz, G.; Dohm, JM; Pereira Arredondo, M.; Dunfield, G.; Gulick, VC; Jensen-Iglesia, A.; Keaten, R.; Herrera Lamelli, C.; Landheim, R.; Lee, PC; Pederson, L.; Roush, T.; Schwehr, K.; Stoker, CR; Zent, A. ... 29th Annual Lunar and Planetary Science Conference, March 16-20, 1998, Houston, TX, abstract no. 1013.

Journal of Geophysical Research, 1995

The transection and superposition relationships among channels, chaos, surface materials units, a... more The transection and superposition relationships among channels, chaos, surface materials units, and other features in the circum-Chryse region of Mars were used to evaluate relative age relationships and evolution of flood events. Channels and chaos in contact (with one another) were treated as single discrete flood-carved systems. Some outflow channel systems form networks and are inferred to have been created by multiple flood events. Within some outflow channel networks, several separate individual channel systems can be traced to a specific chaos which acted as flood-source area to that specific flood channel. Individual flood-carved systems were related to widespread materials units or other surface features that served as stratigraphic horizons. Chryse outflow channels are inferred to have formed over most of the perceivable history of Mars. Outflow channels are inferred to become younger with increasing Proximity to the Chryse basin. In addition, outflow channels closer to the basin show a greater diversity in age. The relationship of subsequent outflow channel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile nearsurface water supply. Instead, we propose the circum-Chryse region as a candidate site of past hydrological recycling. The discharge rates necessary to carve the circum-Chryse outflow channels would have inevitably formed temporary standing bodies of H20 on the Martian surface where the flood-waters stagnated and pooled (the Chryse basin is topographically enclosed). These observations and inferences have led us to formulate and evaluate two hypotheses: (1) large amounts of the sublimated H20 off the Chryse basin flood lakes precipitated (snowed) onto the flood-source highlands and this H20 was incorporated into the near surface, recharging the H20 sources, making possible subsequent deluges; and (2) ponded flood-water in Chryse basin drained back down an anti basinward dipping subsurface layer accessed along the southern edge of the lake, recharging the flood-source aquifers. H20 not redeposited in the flood-source region was largely lost to the hydrologic cycle. This loss progressively lowered the vitality of the cycle, probably by now killing it. Our numerical evaluations indicate that of the two hypotheses we formulated, the groundwater seep cycle seems by far the more viable. Optimally, ~ 3/4 of the original mass of an ice-covered cylindrical lake (albedo 0.5, 1 km deep, 100-km radius, draining along its rim for one quarter of its circumference into substrata with a permeability of 3000 darcies) can be modeled to have moved underground (on timescales of the order of 103 years) before the competing mechanisms of sublimation and freeze down choked off further water removal. Once underground, this water can travel distances equal to the separation between Chryse basin and flood-source sites in geologically short (~106 year-scale) times. Conversely, we calculate that optima!ly only ~40% of the H20 carried from Chryse can condense at the highlands, and most of the precipitate would either collect at the base of the highlands/lowlands scarp or sublimate at rates greater than it would accumulate over the flood-source sites. Further observations from forthcoming missions may permit the determination of which mechanisms may have operated to recycle the Chryse flood-waters.

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Searching for evidence of life on Mars will probably require access to the subsurface. The Martia... more Searching for evidence of life on Mars will probably require access to the subsurface. The Martian surface is bathed in ultraviolet radiation which decomposes organic compounds, destroying possible evidence for life. Also, experiments performed by the Viking Landers imply the presence of several strongly oxidizing compounds at the Martian surface that may also play a role in destroying organic compounds near the surface. While liquid water is unstable on the Martian surface, and ice is unstable at the surface at low latitudes, recent results from the Mars Odyssey Gamma Ray Spectrometer experiment indicate that water ice is widely distributed near the surface under a thin cover of dry soil. Organic compounds created by an ancient Martian biosphere might be preserved in such ice-rich layers. Furthermore, accessing the subsurface provides a way to identify unique stratigraphy such as small-scale layering associated with lacustrine sediments. Subsurface access might also provide new ins...

A. S. Ichimura, A. P. Zent, R. C. Quinn, L. A. Taylor, Department of Chemistry and Biochemistry, ... more A. S. Ichimura, A. P. Zent, R. C. Quinn, L. A. Taylor, Department of Chemistry and Biochemistry, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, ichimura@sfsu.edu, MS 245-3, NASA Ames Research Center, Moffett Field, CA 94035 aaron.zent@nasa.gov, MS 239-4, NASA Ames Research Center, Moffett Field, CA 94035 richard.c.quinn@nasa.gov, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, lataylor@utk.edu.

Introduction: Excess ground ice, or ice that exceeds the pore voume of its host soil, has been ob... more Introduction: Excess ground ice, or ice that exceeds the pore voume of its host soil, has been observed at several locations on Mars. Data from the Mars Odyssey Gamma Ray Spectrometer (GRS) indicates that ice occupies >90% of the regolith by volume over large regions of the high latitudes (>50) in both hemispheres [1]. Thermal and optical observations of fresh impact craters also indicate the presence of relatively pure sub-surface ice at mid-latitudes [2]. At the Phoenix landing site (68 N), trenching activies primarily revealed ice that was pore-filling. However, excess ice (98-99% water by volume) was found in the Dodo/Goldilocks trench complex [3]. The origin of excess ice at its various locations is not well understood. Excess ice cannot be coldtrapped from atmospheric water vapor. Its presence implies either bulk deposition or in situ segregation of pre-existing pore ice. Mellon et al. [3] examined the properties of the Dodo/Goldilocks ice and discussed evidence supporti...