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Geology topics

Michael T. Bland

Publications and source records attributed to Michael T. Bland.

At least 19 recordsLinked to original sources
USGS2026

Geodetic investigations of the Europa Clipper mission

The Europa Clipper mission will investigate the geophysical properties of Europa, one of Jupiter’s Galilean moons, to assess its habitability. Geodetic measurements will play a critical role in determining Europa’s internal structure, rotational state, and tidal deformation. The Geodesy Focus Group (GFG) coordinates cross-instrument efforts to measure Europa’s global shape, rotational parameters, gravity field, and degree-2 tidal Love numbers ( k 2 and h 2 ). The data provided by the Gravity/Radio Science (G/RS) investigation, Europa Imaging System (EIS), Radar for Europa Assessment and Sounding (REASON), and Europa Ultraviolet Spectrometer (UVS) will be used to infer geodetic constraints on the interior structure and construct a precise cartographic reference system. This combined dataset will provide new constraints on Europa’s tidal response, ice shell thickness, and the properties of its subsurface ocean. The resulting geodetic information will contribute to a deeper understanding of Europa’s internal dynamics and the potential habitability of its ocean.

Space Science Reviews↗
USGS2025

Exploring the disparate tectonic manifestations of lithospheric shortening on ocean worlds: Enceladus and Europa

The moons Europa and Enceladus both have water-ice dominated shells tens of kilometers thick overlying global oceans. Both moons are tidally heated and have a high rock mass fractions relative to other moons in their system, and both have youthful, tectonically deformed surfaces. Despite their many commonalities, the surfaces of the two moons bear little resemblance to one another. Europa’s tectonics are dominated by extensional features, including wide bands that imply extremely large surface strains. Features formed by the requisite lithospheric shortening accompanying such extension are largely absent, but long-wavelength folding may have occurred in some locations. Enceladus’ tectonics include features formed in both extension and contraction. Unlike Europa, folds, fold and thrust belts, and isolated thrust faults are plentiful there. In this investigation we use numerical simulations of the shortening of ice lithospheres to bring new insight into why two moons with so much in common appear so different from each other. We find that the temperature structure of the lithosphere controls the style of deformation that results from shortening. Warm conditions resulting from higher surface temperatures and heat fluxes produce folding, which is essentially a viscous response to shortening. Cold temperatures resulting from lower surface temperatures and heat fluxes results in faulting, which is a brittle response. Intermediate conditions can lead to complex combinations of folding and faulting. We hypothesize that Europa’s surface is generally warm enough for shortening to be accommodated by subtle folds that are difficult to detect in the sparse topography data currently available, whereas Enceladus’ colder temperatures favor a combination of folding and faulting. We argue that the dependence of tectonic style on lithospheric temperature self-consistently explains the pattern of tectonics observed across Enceladus, and we emphasize that non-rigid deformation (folding) is not inconsistent with the emerging paradigm of plate tectonics on Europa.

Icarus↗
USGS2024

The Europa Imaging System (EIS) investigation

The Europa Imaging System (EIS) consists of a Narrow-Angle Camera (NAC) and a Wide-Angle Camera (WAC) that are designed to work together to address high-priority science objectives regarding Europa’s geology, composition, and the nature of its ice shell. EIS accommodates variable geometry and illumination during rapid, low-altitude flybys with both framing and pushbroom imaging capability using rapid-readout, 8-megapixel (4k × 2k) detectors. Color observations are acquired using pushbroom imaging with up to six broadband filters. The data processing units (DPUs) perform digital time delay integration (TDI) to enhance signal-to-noise ratios and use readout strategies to measure and correct spacecraft jitter. The NAC has a 2.3° × 1.2° field of view (FOV) with a 10-μrad instantaneous FOV (IFOV), thus achieving 0.5-m pixel scale over a swath that is 2 km wide and several km long from a range of 50 km. The NAC is mounted on a 2-axis gimbal, ±30° cross- and along-track, that enables independent targeting and near-global (≥90%) mapping of Europa at ≤100-m pixel scale (to date, only ∼15% of Europa has been imaged at ≤900 m/pixel), as well as stereo imaging from as close as 50-km altitude to generate digital terrain models (DTMs) with ≤4-m ground sample distance (GSD) and ≤0.5-m vertical precision. The NAC will also perform observations at long range to search for potential erupting plumes, achieving 10-km pixel scale at a distance of one million kilometers. The WAC has a 48° × 24° FOV with a 218-μrad IFOV, achieving 11-m pixel scale at the center of a 44-km-wide swath from a range of 50 km, and generating DTMs with 32-m GSD and ≤4-m vertical precision. The WAC is designed to acquire three-line pushbroom stereo and color swaths along flyby ground-tracks.

Space Science Reviews↗
USGS2024

Geologic constraints on the formation and evolution of Saturn’s mid-sized moons

Saturn’s mid-sized icy moons have complex relationships with Saturn’s interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn’s mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn’s mid-sized moons provide critical benchmarks for the development of formation theories.

Space Science Reviews↗
USGS2024

Increasing the usability and accessibility of Voyager 2 images of Triton

Much of what we know about Neptune’s moon Triton was inferred from the analysis of images returned by the Voyager 2 mission, the only spacecraft to have visited that putative ocean world. Unfortunately, the highest-resolution images (scales < 2 km pixel −1 ) are difficult to use because they are only available in nonstandard formats, and the locations of the images on Triton’s surface are incorrect by up to 200 km. Although image mosaics of Triton are publicly available, these do not include the highest-resolution data. Here we describe our effort to improve the usability and accessibility of Voyager 2 images of Triton. We used the USGS’s ISIS software to process 41 Triton images, including geometric calibration, radiometric calibration, and reseau removal. We improved the image locations using a photogrammetric control network with 958 points and 3910 image measurements. Least-squares bundle adjustment of the network yielded rms uncertainty of 0.50, 0.52, and 0.51 pixels in latitude, longitude, and radius, respectively, and maximum residuals of −4.21 and +3.20 pixels, respectively. Image-to-image alignment is therefore vastly improved. We have released these processed images as cloud-optimized GeoTIFFs in orthographic projection at the original pixel scale of each image. Associated mosaics have also been created and released to provide geologic context for the individual images. These products provide the science community with analysis-ready data that enable new investigations of Triton, increase accessibility to this unique data set, and continue to enhance the scientific return from the Voyager 2 mission.

Planetary Science Journal↗
USGS2024

Planned geological investigations of the Europa Clipper mission

Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.

Space Science Reviews↗
USGS2024

Current status of the community sensor model standard for the generation of planetary digital terrain models

The creation of accurate elevation models (topography) from stereo images are critical for a large variety of geospatial activities, including the production of digital orthomosaics, change detection, landing site analysis, geologic mapping, rover traverse planning, and spectral analysis. The United Stated Geological Survey, Astrogeology Science Center, continues to transition the supported planetary sensor models to the Community Sensor Model (CSM) standard. This paper describes the current state of use for this photogrammetric standard, supported sensor model types, and qualitatively compares derived topography between SOCET SET and SOCET GXP ( ® BAE Systems) using HiRISE stereo images of Mars. Our transition to the CSM standard will ensure an uninterrupted capability to make these valuable products for Mars and many other extraterrestrial planets and moons.

Remote Sensing↗
USGS2024

The evolution of a young ocean within Mimas

The fractured, young surfaces on confirmed ocean worlds such as Europa and Enceladus suggest that ocean-bearing moons with relatively thin overlying ice shells should be easy to identify. Hence, the discovery that Mimas’ rotation state is best explained by an internal ocean seems challenging to reconcile with its heavily cratered surface. Previous studies have shown that an internal ocean is compatible with Mimas’ geology as long as the ice shell has been thinning throughout the surface age. This scenario has yet to be placed into context within Mimas’ thermal-orbital history, in particular, the link between tidal dissipation and orbit circularization. Here, we model Mimas’ coupled thermal-orbital evolution, and the implications of an emerging ocean on Mimas’ geology, to determine whether a thinning ice shell is feasible. We find that the ice shell can thin – and the ocean grow – even as the eccentricity decays due to tidal dissipation as long as the ocean formed within the past 10 – 15 million years and the onset of melting occurred when Mimas’ eccentricity was between 2.5 and 3 times the present-day value. As Mimas’ eccentricity continues to decay, the ocean will likely enter an epoch of freezing, leading to fractures within the ice shell and eruptions of ocean material at the surface, before freezing completely. These results suggest that Mimas, Enceladus, and Tethys may represent three points along a continuum of ocean initiation, growth, and loss.

Earth and Planetary Science Letters↗
USGS2024

The inevitability of large shallow craters on Callisto and Ganymede: Implications for crater depth-diameter trends

Complex craters with diameters ( D ) ≥ 40 km on Callisto and Ganymede are shallower than would be expected from simply extrapolating the depth-diameter trend from smaller ( D ≤ 40 km) craters. This unusual depth-diameter ( d-D ) trend, and associated changes in crater morphology, have been hypothesized to result from rheological transitions, including the existence of an ocean, within the moons' ice shell. Simulations of impact crater formation can reproduce the observed shallow depths but require heat fluxes roughly twice the maximum radiogenic flux to do so. Here we demonstrate that the d-D trends on Callisto and Ganymede can instead be explained as a direct consequence of viscous relaxation under radiogenic heating. We use numerical simulations of viscous relaxation to show that if craters form at the depth expected from an extrapolation of the complex crater d-D trend, they will evolve to the observed depths over timescales of 200 Myrs to 1 Gyrs. Large craters (e.g., D ≥ 80 km) younger than 200 Myrs, which would retain greater depths, should be relatively rare. If we instead assume that the craters formed at their observed depths, as proposed by previous impact modeling, they quickly become much shallower than observed. We find excellent agreement between observed crater depths on Ganymede and our simulated crater depths by assuming a pure-water ice composition and a diurnally averaged surface temperature of 120 K, but require either larger-grained or “dirty” ice with a modestly higher viscosity to match observations at Callisto, where the surface temperature is warmer (130 K). We favor the latter explanation because it is consistent with the existence of a dusty lag on Callisto's surface and the absence of a similar lag on Ganymede. Our results predict that, for a given crater diameter, post-relaxation crater depth should increase with increasing latitude, a hypothesis best tested on Callisto, whose relatively quiescent geologic history best preserves the signature of viscous relaxation under radiogenic heating.

Icarus↗
USGS2023

Viscous relaxation of Oort and Edgeworth craters on Pluto: Possible indicators of an epoch of early high heat flow

Impact craters, with their well-defined initial shapes, have proven useful as heat flow probes of a number of icy bodies, provided characteristics of viscous relaxation can be identified. For Pluto's numerous craters, such identifications are hampered/complicated by infilling and erosion by mobile volatile ices, but not in every case. Large craters offer relatively deep probes of rheological structure, and on Pluto two large old craters in a major dark (volatile-ice free) region are probably the best examples for possible viscous relaxation: Oort (115-km diameter) and Edgeworth (140-km diameter). They are similar in size, location, and apparent age (morphological preservation), but may or may not be coeval. Edgeworth is particularly shallow and its floor appears bowed up above the original ground plane, a classic hallmark of viscous relaxation in which viscosity decreases sharply with depth. Oort is less relaxed, but may be somewhat younger and less affected by an early epoch of high heat flow. Finite element calculations show that this heat flow would have to have been substantial to explain Edgeworth's upbowed floor by viscous relaxation, several times steady-state radiogenic values for present-day surface temperatures. We expect Pluto's brittle ice lithosphere to be fractured and porous, however, markedly reducing thermal conductivity and increasing temperatures at depth and relaxation for a given heat flow. We find that most relaxation occurs well within 100 Myr after impact for Edgeworth and Oort, and focus attention on a temporal (and/or regional) epoch of elevated heat flow, possibly tied to the serpentinization of Pluto's rocky core.

Journal of Geophysical Research: Planets↗
USGS2023

Exploring the interior of Europa with the Europa Clipper

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa’s interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

Space Science Reviews↗
USGS2023

Constraints on the composition and thermal structure of Ariel’s icy crust as inferred from its largest observed impact crater

The large graben-like troughs and smooth plains visible on the surface of Ariel are indicative of a period of high heat flow in the Uranian moon's past. High heat flows on icy moons like Ariel can also enable viscous flow that removes impact crater topography, a process called viscous relaxation. Here we use numerical modeling to investigate the conditions necessary to viscously relax Ariel's largest impact crater, Yangoor, which is 80 km in diameter and unusually shallow. If we assume that Ariel's crust consists of non-porous water ice, heat fluxes ≥60 mW m −2 are required to reduce an initially deep Yangoor-like crater to its current observed depth. Lower fluxes are required if a high-porosity (30%), low-conductivity surface layer several kilometers thick is assumed to exist, but in any case, fluxes in excess of 30 mW m −2 are necessary to substantially reduce Yangoor's topography. The inclusion of ammonia dihydrate has a negligible effect on our results despite decreasing the viscosity of Ariel's deep ice. Our results are consistent with previous inferences of high heat fluxes on Ariel, but exceed both expected radiogenic heat fluxes and known equilibrium tidal heat fluxes by an order of magnitude. If Yangoor's shallow depth is the result of tidal heating, then short-lived non-equilibrium tidal dissipation or some other source of energy is required. Notably, although our results do not require the presence of an ocean within Ariel, the thermal conditions necessary to viscously relax Yangoor also imply a relatively thin ice shell (∼10-km thick) if conductive heat transport is assumed.

Icarus↗
USGS2022

Silicate volcanism on Europa’s seafloor and implications for habitability

Habitable ocean environments on Europa require an influx of reactants to maintain chemical disequilibrium. One possible source of reactants is seafloor volcanism. Modeling has shown that dissipation of tidal energy in Europa's asthenosphere can generate melt, but melt formation cannot be equated with volcanism. Melt must also be transported through Europa's cold lithosphere to erupt at the seafloor. Here, we use two models of dike propagation to show that dikes can only traverse the lithosphere if either the fracture toughness of the lithosphere or the flux into the dike is large (>500 MPa m 1/2 or ∼1 m 2 s −1 , respectively). We conclude that cyclic volcanic episodes might provide reactants to Europa's ocean if magma accumulates at the base of the lithosphere for several thousand years. However, if dikes form too frequently, or are too numerous, the magma flux into each will be insufficient, and volcanism cannot support a habitable ocean environment.

Geophysical Research Letters↗
USGS2021

How well do we know Europa’s topography? An evaluation of the variability in digital terrain models of Europa.

Jupiter’s moon Europa harbors one of the most likely environments for extant extraterrestrial life. Determining whether Europa is truly habitable requires understanding the structure and thickness of its ice shell, including the existence of perched water or brines. Stereo-derived topography from images acquired by NASA Galileo’s Solid State Imager (SSI) of Europa are often used as a constraint on ice shell structure and heat flow, but the uncertainty in such topography has, to date, not been rigorously assessed. To evaluate the current uncertainty in Europa’s topography we generated and compared digital terrain models (DTMs) of Europa from SSI images using both the open-source Ames Stereo Pipeline (ASP) software and the commercial SOCET SET ® software. After first describing the criteria for assessing stereo quality in detail, we qualitatively and quantitatively describe both the horizontal resolution and vertical precision of the DTMs. We find that the horizontal resolution of the SOCET SET ® DTMs is typically 8–11× the root mean square (RMS) pixel scale of the images, whereas the resolution of the ASP DTMs is 9–13× the maximum pixel scale of the images. We calculate the RMS difference between the ASP and SOCET SET ® DTMs as a proxy for the expected vertical precision (EP), which is a function of the matching accuracy and stereo geometry. We consistently find that the matching accuracy is ~0.5 pixels, which is larger than well-established “rules of thumb” that state that the matching accuracy is 0.2–0.3 pixels. The true EP is therefore ~1.7× larger than might otherwise be assumed. In most cases, DTM errors are approximately normally distributed, and errors that are several times the derived EP occur as expected. However, in two DTMs, larger errors (differences) occur and correlate with real topography. These differences primarily result from manual editing of the SOCET SET ® DTMs. The product of the DTM error and the resolution is typically 4–8 pixel 2 if calculated using the RMS image scale for SOCET SET ® DTMs and the maximum images scale for the ASP DTMs, which is consistent with recent work using martian data sets and suggests that the relationship applies more broadly. We evaluate how ASP parameters affect DTM quality and find that using a smaller subpixel refinement kernel results in DTMs with smaller (better) resolution but, in some cases, larger gaps, which are sometimes reduced by increasing the size of the correlation kernel. We conclude that users of ASP should always systematically evaluate the choice of parameters for a given dataset.

Remote Sensing↗
USGS2021

Improving the usability of Galileo and Voyager images of Jupiter’s moon, Europa

NASA's Voyager 1, Voyager 2, and Galileo spacecraft acquired hundreds of images of Jupiter's moon Europa. These images provide the only moderate- to high-resolution views of the moon's surface and are therefore a critical resource for scientific analysis and future mission planning. Unfortunately, uncertain knowledge of the spacecraft's position and pointing during image acquisition resulted in significant errors in the location of the images on the surface. The result is that adjacent images are poorly aligned, with some images displaced by more than 100 km from their correct location. These errors severely degrade the usability of the Voyager and Galileo imaging data sets. To improve the usability of these data sets, we used the U.S. Geological Survey Integrated Software for Imagers and Spectrometers to build a nearly global image tie-point network with more than 50,000 tie points and 135,000 image measurements on 481 Galileo and 221 Voyager images. A global least-squares bundle adjustment of our final Europa tie-point network calculated latitude, longitude, and radius values for each point by minimizing residuals globally, and resulted in root mean square (RMS) uncertainties of 246.6 m, 307.0 m, and 70.5 m in latitude, longitude, and radius, respectively. The total RMS uncertainty was 0.32 pixels. This work enables direct use of nearly the entire Galileo and Voyager image data sets for Europa. We are providing the community with updated NASA Navigation and Ancillary Information Facility Spacecraft, Planet, Instrument, C-matrix (pointing), and Events kernels, mosaics of Galileo images acquired during each observation sequence, and individual processed and projected level 2 images.

Earth and Space Science↗
USGS2020

A global shape model for Saturn's moon Enceladus from a dense photogrammetric control network

A planetary bodys global shape provides both insight into its geologic evolution, and a key element of any Planetary Spatial Data Infrastructure (PSDI). NASAs Cassini mission to Saturn acquired more than 600 moderate- to high-resolution images (< 500 m/pixel) of the small, geologically active moon Enceladus. The moons internal global ocean and intriguing geology mark it as a candidate for future exploration and motivates the development of a PSDI. Recently, Bland et al. (2018) provided two foundational elements of this PSDI: geodetic control and orthoimages. To provide the third foundational data set we generate a new shape model for Enceladus from Cassini images and a dense photogrammetric control network (nearly 1 million tie points) using the United States Geological Surveys Integrated Software for Imagers and Spectrometers (ISIS) and the Ames Stereo Pipeline (ASP). The new shape model is near-global in extent and gridded to 2.2 km/pixel, ~50 times better resolution than previous global models. Our calculated triaxial shape, rotation rate, and pole orientation for Enceladus is consistent with current IAU values to within the error; however, we determined a new prime meridian offset (Wo) of 7.063o. We calculate Enceladus long-wavelength topography by subtracting the best-fit triaxial ellipsoid from our shape model. The result is comparable to previous global models but can resolve topographic features as small as 5-7 km across in certain areas. To evaluate the spatially varying quality of the model we calculate the point density (variable from 5 to more than 50 per pixel), normalized median absolute deviation of the points within each pixel (typically less than 100 m), and the minimum expected vertical precision of each point (ranging from 2 km to 29 m).

Conference Paper↗
USGS2020

Planetary science decadal survey planetary mission concept study report: Ceres: Exploration of Ceres’ habitability

Dwarf planet Ceres is a compelling target as an evolved ocean world with, at least, regional brine reservoirs and potentially ongoing geological activity. As the most water-rich body in the inner solar system (in relative abundance), it is a representative of the population of planetesimals that brought volatiles and organics to the inner solar system. Situated in the Main Belt of asteroids, Ceres is accessible enough for a sample return with the resources of a typical medium-class (New Frontiers) NASA mission. Under the Discovery program, Dawn explored Ceres from 2015 to 2018. The extensive dataset revealed the presence of liquid, brine-driven activity, organic matter, and a rich salt chemistry. With this evidence, the overarching goals of the mission concept presented herein are to quantify Ceres’ current habitability potential and origin.

Cooperator Report↗