Geology topics

A.S. McEwen

Publications and source records attributed to A.S. McEwen.

7 recordsLinked to original sources

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↗

USGS2021

Solar-system-wide significance of Mars polar science

1. The North Polar Layered Deposits contain thousands of ice layers that record accumulation and climate history for at least several million years, making the most accessible and most complete climate record aside from the Earth’s – and the only one to record the impact of large obliquity shifts. 2. Mars Polar Science is a diverse and integrated system spanning much of the planet, above and below the surface: geographic poles, mid- and high-latitudes, numerous active volatile processes that interact with the near-surface, and the lower and middle atmosphere circulations, even influencing the upper atmosphere and atmospheric escape. 3. Mars polar processes serves as an analogue and scientific link to numerous solar system bodies. Mars has two volatile cycles that influence surface processes, atmospheric processes, and climate cycles. Mars, with many data sets and observations, serves as a proxy for outer planets and moons that undergo the same physics but with other volatile species. Additionally, technology designed for Mars polar exploration benefits exploration elsewhere. 4. Mars Polar and Atmospheric Science results are critical for future human exploration and in situ resources and the study of planetary habitability.

Bulletin of the AAS↗

USGS2019

The NASA Roadmap to Ocean Worlds

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to “identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find.” The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Astrobiology↗

USGS2018

Correcting spacecraft jitter in HiRISE images

Mechanical oscillations or vibrations on spacecraft, also called pointing jitter, cause geometric distortions and/or smear in high-resolution digital images acquired from orbit. Geometric distortion is especially a problem with pushbroom sensors, such as the High Resolution Imaging Science Experiment (HiRISE) instrument on-board the Mars Reconnaissance Orbiter (MRO). Geometric distortions occur at a range of frequencies that may not be obvious in the image products, but can cause problems with stereo image correlation in the production of digital elevation models, and in measuring surface changes in time series with orthorectified images. The HiRISE focal plane comprises a staggered array of fourteen charge-coupled devices (CCDs) with pixel instantaneous field of view (IFOV) of 1 microradian. The high spatial resolution of HiRISE makes it both sensitive to, and an excellent recorder of jitter. We present an algorithm using Fourier analysis to resolve the jitter function for a HiRISE image that is then used to update instrument pointing information to remove geometric distortions from the image. Implementation of the jitter analysis and image correction is performed on selected HiRISE images made available to the public. Results show marked reduction of geometric distortions. This work has applications to similar cameras operating now (such as the Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (LROC NAC) on-board the Lunar Reconnaissance Orbiter) and to the design of future instruments (such as the Europa Imaging System, planned for the Europa Clipper mission).

Book chapter↗

USGS2017

Correcting spacecraft jitter in HiRISE images

Mechanical oscillations or vibrations on spacecraft, also called pointing jitter, cause geometric distortions and/or smear in high resolution digital images acquired from orbit. Geometric distortion is especially a problem with pushbroom type sensors, such as the High Resolution Imaging Science Experiment (HiRISE) instrument on board the Mars Reconnaissance Orbiter (MRO). Geometric distortions occur at a range of frequencies that may not be obvious in the image products, but can cause problems with stereo image correlation in the production of digital elevation models, and in measuring surface changes over time in orthorectified images. The HiRISE focal plane comprises a staggered array of fourteen charge-coupled devices (CCDs) with pixel IFOV of 1 microradian. The high spatial resolution of HiRISE makes it both sensitive to, and an excellent recorder of jitter. We present an algorithm using Fourier analysis to resolve the jitter function for a HiRISE image that is then used to update instrument pointing information to remove geometric distortions from the image. Implementation of the jitter analysis and image correction is performed on selected HiRISE images. Resulting corrected images and updated pointing information are made available to the public. Results show marked reduction of geometric distortions. This work has applications to similar cameras operating now, and to the design of future instruments (such as the Europa Imaging System).

The International Archives of the Photogrammetry, ↗

USGS2003

Orogenic tectonism on Io

We catalog 143 Ionian mountains (montes) and mountain‐like features (mensae, tholi, plana, and small peaks) in order to investigate orogenic tectonism on Io. From this comprehensive list, we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features. Three of the 96 mountains are probably volcanoes, 92 appear to be tectonic massifs, and 1 is ambiguous. Of the 92 tectonic mountains, 38 abut paterae (volcanic or volcano‐tectonic craters with irregular or scalloped margins). This juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is ∼0.1%. We propose instead that orogenic faults may act as conduits for magma ascent, thus fueling patera formation near mountains. As resurfacing buries a shell of material from Io's surface to the base of the lithosphere, its effective radius is reduced and it heats up. We calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness. Conservation of volume dictates that this material must be uplifted at Io's surface. By estimating the total volume of the mountains, we are able to place a lower limit of 12 km on Io's lithospheric thickness. We hypothesize that, in some cases, mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere. The resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism. This model is one of several possible mechanisms for uplifting isolated mountains such as are observed on Io.

Journal of Geophysical Research E: Planets↗

USGS1990

Surface and airborne evidence for plumes and winds on Triton

Aeolian features on Triton that were imaged during the Voyager Mission have been grouped. The term "aeolian feature" is broadly defined as features produced by or blown by the wind, including surface and airborne materials. Observations of the latitudinal distributions of the features probably associated with current activity (known plumes, crescent streaks, fixed terminator clouds, and limb haze with overshoot) all occur from latitude -37° to latitude -62°. Likely indicators of previous activity (dark surface streaks) occur from latitude -5° to -70°, but are most abundant from -15° to -45°, generally north of currently active features. Those indicators which give information on wind direction and speed have been measured. Wind direction is a function of altitude. The predominant direction of the surface wind streaks is found to be between 40° and 80° measured clockwise from north. The average orientation of streaks in the northeast quadrant is 59°. Winds at 1- to 3-kilometer altitude are eastward, while those at >8 kilometers blow west.

Science↗