Geology topics

D.M. Burr

Publications and source records attributed to D.M. Burr.

6 recordsLinked to original sources

USGS2009

Pingos on Earth and Mars

Pingos are massive ice-cored mounds that develop through pressurized groundwater flow mechanisms. Pingos and their collapsed forms are found in periglacial and paleoperiglacial terrains on Earth, and have been hypothesized for a wide variety of locations on Mars. This literature review of pingos on Earth and Mars first summarizes the morphology of terrestrial pingos and their geologic contexts. That information is then used to asses hypothesized pingos on Mars. Pingo-like forms (PLFs) in Utopia Planitia are the most viable candidates for pingos or collapsed pingos. Other PLFs hypothesized in the literature to be pingos may be better explained with other mechanisms than those associated with terrestrial-style pingos. ?? 2008 Elsevier Ltd.

Planetary and Space Science↗

USGS2009

Mesoscale raised rim depressions (MRRDs) on Earth: A review of the characteristics, processes, and spatial distributions of analogs for Mars

Fields of mesoscale raised rim depressions (MRRDs) of various origins are found on Earth and Mars. Examples include rootless cones, mud volcanoes, collapsed pingos, rimmed kettle holes, and basaltic ring structures. Correct identification of MRRDs on Mars is valuable because different MRRD types have different geologic and/or climatic implications and are often associated with volcanism and/or water, which may provide locales for biotic or prebiotic activity. In order to facilitate correct identification of fields of MRRDs on Mars and their implications, this work provides a review of common terrestrial MRRD types that occur in fields. In this review, MRRDs by formation mechanism, including hydrovolcanic (phreatomagmatic cones, basaltic ring structures), sedimentological (mud volcanoes), and ice-related (pingos, volatile ice-block forms) mechanisms. For each broad mechanism, we present a comparative synopsis of (i) morphology and observations, (ii) physical formation processes, and (iii) published hypothesized locations on Mars. Because the morphology for MRRDs may be ambiguous, an additional tool is provided for distinguishing fields of MRRDs by origin on Mars, namely, spatial distribution analyses for MRRDs within fields on Earth. We find that MRRDs have both distinguishing and similar characteristics, and observation that applies both to their mesoscale morphology and to their spatial distribution statistics. Thus, this review provides tools for distinguishing between various MRRDs, while highlighting the utility of the multiple working hypotheses approach. ?? 2008 Elsevier Ltd.

Planetary and Space Science↗

USGS2005

Near-infrared (0.8-4.0 m) spectroscopy of mimas, enceladus, tethys, and rhea

Spectral measurements from the ground in the time leading up to the Cassini mission at Saturn provide important context for the interpretation of the forthcoming spacecraft data. Whereas ground-based observations cannot begin to approach the spatial scales Cassini will achieve, they do possess the benefits of better spectral resolution, a broader possible time baseline, and unique veiewing geometries not obtained by spacecraft (i.e., opposition). In this spirit, we present recent NIR reflectance spectra of four icy satellites of Saturn measured with the SpeX instrument at the IRTF. These measurements cover the range 0.8-4.0 m of both the leading and trailing sides of Tethys and the leading side of Rhea. The L -band region (2.8-4.0 m) offers new opportunities for searches of minor components on these objects. Additionally, these data include 0.8-2.5 m spectra of both the leading and trailing sides of Mimas and of the (mostly) trailing side of Enceladus. The spectrum of Enceladus shows activity near 2.25 m that we interpret as a possible signature of NH 3 ice. The presence of ammonia in the Saturn system is not unexpected, and may help explain the apparent recent geologic activity of Enceladus. Analysis of leading/trailing differences in H 2 O band depths, spectral slopes, and albedo imply a separate regime of surface modification for Mimas and Enceladus than for the more distant icy satellites (Tethys, Dione, Rhea). Aside from the potential NH 3 on Enceladus, no other minor constituents are detected in these icy surfaces.

Astronomy and Astrophysics↗

USGS2005

Possible pingos and a periglacial landscape in northwest Utopia Planitia

Hydrostatic (closed-system) pingos are small, elongate to circular, ice-cored mounds that are perennial features of some periglacial landscapes. The growth and development of hydrostatic pingos is contingent upon the presence of surface water, freezing processes and of deep, continuous, ice-cemented permafrost. Other cold-climate landforms such as small-sized, polygonal patterned ground also may occur in the areas where pingos are found. On Mars, landscapes comprising small, elongate to circular mounds and other possible periglacial features have been identified in various areas, including Utopia Planitia, where water is thought to have played an important role in landscape evolution. Despite the importance of the martian mounds as possible markers of water, most accounts of them in the planetary science literature have been brief and/or based upon Viking imagery. We use a high-resolution Mars Orbiter Camera image (EO300299) and superposed Mars Orbiter Laser Altimeter data tracks to describe and characterise a crater-floor landscape in northwest Utopia Planitia (64.8?? N/292.7?? W). The landscape comprises an assemblage of landforms that is consistent with the past presence of water and of periglacial processes. This geomorphological assemblage may have formed as recently as the last episode of high obliquity. A similar assemblage of landforms is found in the Tuktoyaktuk peninsula of northern Canada and other terrestrial cold-climate landscapes. We point to the similarity of the two assemblages and suggest that the small, roughly circular mounds on the floor of the impact crater in northwest Utopia Planitia are hydrostatic pingos. Like the hydrostatic pingos of the Tuktoyaktuk peninsula, the origin of the crater-floor mounds could be tied to the loss of ponded, local water, permafrost aggradation and the evolution of a sub-surface ice core. ?? 2004 Elsevier Inc. All rights reserved.

Icarus↗

USGS2005

Young (late Amazonian), near-surface, ground ice features near the equator, Athabasca Valles, Mars

A suite of four feature types in a ???20 km2 area near 10?? N, 204?? W in Athabasca Valles is interpreted to have resulted from near-surface ground ice. These features include mounds, conical forms with rimmed summit depressions, flatter irregularly-shaped forms with raised rims, and polygonal terrain. Based on morphology, size, and analogy to terrestrial ground ice forms, these Athabascan features are interpreted as pingos, collapsing pingos, pingo scars, and thermal contraction polygons, respectively. Thermal Infrared Mapping Spectrometer (THEMIS) data and geological features in the area are consistent with a sedimentary substrate underlying these features. These observations lead us to favor a ground ice interpretation, although we do not rule out volcanic and especially glaciofluvial hypotheses. The hypothesized ground ice that formed the mounds and rimmed features may have been emplaced via the deposition of saturated sediment during flooding; an alternative scenario invokes magmatically cycled groundwater. The ground ice implicit in the hypothesized thermal contraction polygons may have derived either from this flooding/ground water, or from atmospheric water vapor. The lack of obvious flood modification of the mounds and rimmed features indicates that they formed after the most recent flood inundated the area. Analogy with terrestrial pingos suggests that ground ice may be still extant within the positive relief mounds. As the water that flooded down Athabasca Valles emerged via a volcanotectonic fissure from a deep aquifer, any extant pingo ice may contain evidence of a deep subsurface biosphere. ?? 2005 Elsevier Inc. All rights reserved.

Icarus↗

USGS2004

Flood-formed dunes in Athabasca Valles, Mars: Morphology, modeling, and implications

Estimates of discharge for martian outflow channels have spanned orders of magnitude due in part to uncertainties in floodwater height. A methodology of estimating discharge based on bedforms would reduce some of this uncertainty. Such a methodology based on the morphology and granulometry of flood-formed ('diluvial') dunes has been developed by Carling (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165-179) and applied to Pleistocene flood-formed dunes in Siberia. Transverse periodic dune-like bedforms in Athabasca Valles, Mars, have previously been classified both as flood-formed dunes and as antidunes. Either interpretation is important, as they both imply substantial quantities of water, but each has different hydraulic implications. We undertook photoclinometric measurements of these forms, and compared them with data from flood-formed dunes in Siberia. Our analysis of those data shows their morphology to be more consistent with dunes than antidunes, thus providing the first documentation of flood-formed dunes on Mars. Other reasoning based on context and likely hydraulics also supports the bedforms' classification as dunes. Evidence does not support the dunes being aeolian, although a conclusive determination cannot be made with present data. Given the preponderance of evidence that the features are flood-formed instead of aeolian, we applied Carling's (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165-179) dune-flow model to derive the peak discharge of the flood flow that formed them. The resultant estimate is approximately 2??106 m3/s, similar to previous estimates. The size of the Athabascan dunes' in comparison with that of terrestrial dunes suggests that these martian dunes took at least 1-2 days to grow. Their flattened morphology implies that they were formed at high subcritical flow and that the flood flow that formed them receded very quickly. ?? 2004 Elsevier Inc. All rights reserved.

Icarus↗