Martha S. Gilmore

Constraints on the depth of magnetized crust on Mars from impact craters

Large (diameter greater than ∼500 km) Martian impact basins are associated with observed magnetic fields which are statistically distinct from, and smaller than, fields associated with smaller craters. We suggest that this effect arises because impacts cause shock, heating, and excavation, reducing the magnetization of previously magnetized crust. For a simple, uniformly magnetized model the magnetic field at 100 km altitude is reduced by ∼50% when a crater-shaped demagnetization zone reaches the base of the magnetized layer. By analogy with terrestrial data, we assume that in Martian craters the zone of demagnetization extends to a depth of 0.04–0.15 crater diameters. On the basis of this assumption, the data suggest that the depth to the base of the magnetized layer on Mars, if uniform, is ∼35 km, with lower and upper bounds of 10 and 100 km, respectively. These bounds imply magnetizations of 5–40 A m−1 and are consistent with likely Mars geotherms at 4 Gyr B.P.

Role of aquicludes in formation of Martian gullies

Liquid water is not currently stable on the surface of Mars, but images provided by the Mars Orbiter Camera aboard the Mars Global Surveyor spacecraft reveal erosional landforms previously interpreted to be geologically young gullies formed by groundwater seepage. We test the basic hypothesis that, as on Earth, the location of these gullies is controlled in part by the presence of an impermeable rock layer (aquiclude) and that the depths of the gully heads below the surface should thus be correlated to subsurface geology. We show that (1) gullies emanate from a specific cliff-forming layer, even if the layer is faulted, and (2) the depth of gullies below the local surface ranges from 70 to 800 m, and (3) is positively correlated to mapped geologic units. Gully formation is therefore dependent upon both favorable climatic conditions to produce and sustain liquid water and the presence of impermeable subsurface layers to collect the groundwater. Gullies may mark the distribution of subsurface impermeable layers globally, and are prime targets for the search for present water and life on Mars.

Style and sequence of extensional structures in tessera terrain, Venus

Recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation, specifically, if tessera are formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within tesserae as graben (bounded by faults ∼60°) or, alternatively, open tension fractures (dipping ∼90°). We document and assess the origin of extensional structures in tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (∼1 km width) of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km−1 (heat flows of 800 to 3000 mW m−2) are required for a range of diabase rheologies and strain rates thought typical of Venus during tessera formation. Such a thermal structure would favor partial melting at depths <1 km. Dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism, mitigating against the preservation of the closely associated tension fractures; this volcanism is not observed. Both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model. On the basis of our analysis of tesserae, we conclude that the ridges formed first in response to large-scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on Earth such as Tibet and the Altiplano.

Felsic highland crust on Venus suggested by Galileo Near‐Infrared Mapping Spectrometer data

Received 2 March 2008; revised 29 July 2008; accepted 18 September 2008; published 31 December 2008. [1] We evaluated the spatial variation of Venusian surface emissivity at 1.18 mm wavelength and that of near-surface atmospheric temperature using multispectral images obtained by the Near-Infrared Mapping Spectrometer (NIMS) on board the Galileo spacecraft. The Galileo NIMS observed the nightside thermal emission from the surface and the deep atmosphere of Venus, which is attenuated by scattering from the overlying clouds. To analyze the NIMS data, we used a radiative transfer model based on the adding method. Although there is still an uncertainty in the results owing to the not well known parameters of the atmosphere, our analysis revealed that the horizontal temperature variation in the near-surface atmosphere is no more than ±2 K on the Venusian nightside and also suggests that the majority of lowlands likely has higher emissivity compared to the majority of highlands. One interpretation for the latter result is that highland materials are generally composed of felsic rocks. Since formation of a large body of granitic magmas requires water, the presence of granitic terrains would imply that Venus may have had an ocean and a mechanism to recycle water into the mantle in the past.

Duration of tessera deformation on Venus

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of craters fractured by late-stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.

Strategies for autonomous rovers at Mars

The science return from future robotic exploration of the Martian surface can be enhanced by performing routine processing using onboard computers. This can be accomplished by using software that recognizes scientifically relevant surface features from imaging and other data and prioritizes the data for return transmission to Earth. Two algorithms have been designed and evaluated with field data to identify the properties of the environment that can be reliably detected with onboard imaging and multispectral observation. One algorithm identifies variations in surface textures in images and successfully distinguishes between rocks and soil and between differences in grain size in a rock of a single composition. A second algorithm utilizes a neural net to recognize selected carbonate minerals from spectral reflectance data and successfully identifies carbonates from a set of spectra collected in the field. These types of algorithms will contribute to the efficiency of a landed instrument suite given the limited resources of time, data storage, and available communications opportunities.

Metric learning for hyperspectral image segmentation

We present a metric learning approach to improve the performance of unsupervised hyperspectral image segmentation. Unsupervised spatial segmentation can assist both user visualization and automatic recognition of surface features. Analysts can use spatially-continuous segments to decrease noise levels and/or localize feature boundaries. However, existing segmentation methods use task-agnostic measures of similarity. Here we learn task-specific similarity measures from training data, improving segment fidelity to classes of interest. Multiclass Linear Discriminant Analysis produces a linear transform that optimally separates a labeled set of training classes. This defines a distance metric that generalizes to new scenes, enabling graph-based segmentations that emphasizes key spectral features. We describe tests based on data from the Compact Reconnaissance Imaging Spectrometer (CRISM) in which learned metrics improve segment homogeneity with respect to mineralogical classes.

Creation and testing of an artificial neural network based carbonate detector for Mars rovers

We have developed an artificial neural network (ANN) based carbonate detector capable of running on current and future rover hardware. The detector can identify calcite in visible/NIR (350-2500 nm) spectra of both laboratory specimens covered by ferric dust and rocks in Mars analogue field environments. The ANN was trained using the backpropagation algorithm with sigmoid activation neurons. For the training dataset, we chose nine carbonate and eight noncarbonate representative mineral spectra from the USGS spectral library. Using these spectra as seeds, we generated 10,000 variants with up to 2% Gaussian noise in each reflectance measurement. We cross-validated several ANN architectures, training on 9,900 spectra and testing on the remaining 100. The best performing ANN correctly detected, with perfect accuracy, the presence (or absence) of carbonate in spectral data taken on field samples from the Mojave desert and clean, pure marbles from CT. Sensitivity experiments with JSC Mars-1 simulant dust suggest the carbonate detector would perform well in aeolian Martian environments.

Geologic evolution of the Akna Montes-Atropos Tessera region, Venus

The investigated area comprises an arcuate mountain belt, Akna Montes, in Western Ishtar Terra, associated with an outboard plateau, Atropos Tessera, to the west and a volcanic plateau, Lakshmi Planum, to the east. Eight geologic units have been recognized on the basis of their geomorphic and structural characteristics as they appear on Magellan radar images. Our stratigraphic analysis shows that the geological evolution of the study area can be explained by four main steps: (1) formation of the older substrata of Atropos Tessera and Lakshmi, (2) extensive plains emplacement, (3) an orogenic phase including the formation of Akna Montes, and (4) local emplacement of younger plains. The tectonic evolution shows a deformational sequence characterized by contraction, shear, and topographic relaxation. This sequence is interpreted to be a consequence of the variation of crustal stresses and crustal thickening during orogenic events as observed for terrestrial high plateaus associated with a mountain belt (i.e., Himalaya and Tibet, Andes and Altiplano). In order to estimate the amount of crustal shortening associated with the Akna Montes, we considered two end-members for structural style of the mountain belt: a symmetric fold model and a fault-bend fold model. The models are theoretical because terrestrial orogenic belts are often formed by a combination of different compressional structures. However, symmetric and fault-bend faults represent the minimum and maximum crustal shortening, respectively, and thus they do place bounds on the amount of strain recorded by Akna Montes. The first model yields a shortening value less than 1%, whereas a range of 17–34% is derived for the second model. The large difference between these values underscores the importance of fold geometries for estimating strain and to place constraints on geodynamic models for mountain belt formation. On the basis of our study we think that a combination of mantle downwelling and horizontal convergence may provide a good explanation of the geology and tectonics we observed in the Akna Montes-Atropos Tessera region.

Acid Rivers and Lakes at Caviahue-Copahue Volcano as Potential Terrestrial Analogues for Aqueous Paleo-Environments on Mars

Mars carries primary rock with patchy occurrences of sulfates and sheet silicates. Both Mg- and Fe- sulfates have been documented, the former being rather uncommon on Earth. To what extent can a natural acidic river system on Earth be a terrestrial analog for early Mars environments? Copahue volcano (Argentina) has an active acid hydrothermal system that has precipitated a suite of minerals in its hydrothermal reservoir (silica, anhydrite, alunite, jarosite). Leakage from this subterranean system through hot springs and into the crater lake have formed a strongly acidified watershed (Rio Agrio), which precipitates a host of minerals during cooling and dilution downstream. A suite of more than 100 minerals has been found and conditions for precipitation of the main phases are simulated with speciation/saturation routines. The lower part of the watershed (Lake Caviahue and the Lower Rio Agrio) have abundant deposits of ferricrete since 2003: hydrous ferric oxides and schwertmannite occur, their precipitation being mediated by Fe-oxidizing bacteria and photochemical processes. Further downstream, at greater degrees of dilution, hydrous aluminum oxides and sulfates form and create ‘alcretes’ lining the river bed. The watershed carries among others jarosite, hematite, anhydrite, gypsum and silica minerals and the origin of all these minerals could be modeled through cooling/dilution of the primary hot spring fluids. Single evolution (acidification through capture of volcanic gases, water rock interaction to acquire the dissolved cations) through cooling of the primary fluids could explain most of the Fe-bearing minerals, but to precipitate Mg-sulfates, evaporation and renewed interaction with olivine-rich rocks is needed to saturate some common Mg-sulfates (e.g., epsomite). The schwertmannite beds formed through processes involving Fe-oxidizing bacteria, which may be significant if this mineral was common on Mars in the past. Photochemical processes on Mars are commonly discussed in terms of photo-oxidation of Fe, but photo-reduction may be a common process as well, as was found to be the case in the Rio Agrio watershed. A model of waters acidified by the capture of S-rich volcanic gases that have reacted with basaltic rocks, and then evaporated or were neutralized by higher alkalinity surface fluids may explain the origin of the sulfate mineral suites on Mars quite well.