Beyer, R., K. Stack, K. Griffes, R. Milliken, K. Herkenhoff, S. Byrne, J. W. Holt, and J. Grotzinger, An atlas of Mars sedimentary rocks as seen by HiRISE, in , (in press), 2011, #2424
Karlsson, N. B., J. W. Holt, and R. C. A. Hindmarsh, Testing for flow in the north polar layered deposits of Mars using radar stratigraphy and a simple 3D ice-flow model, Geophys. Res. Lett., 38, L24204, 2011, doi:10.1029/2011GL049630, #2458 
The water-ice-rich north polar layered deposits (NPLD) of Mars play a key role in the Martian climate through an active exchange of water vapor with the atmosphere. Conditions are not currently amenable for flow of the NPLD; however, gross morphological evidence for past flow suggests the possibility of a warmer climate in the past. Here we present the first comparison of internal stratigraphy predicted by a flow model with that observed by an orbital radar sounder. We have analyzed radar data from Gemina Lingula, the southernmost tongue of the NPLD, acquired by the Shallow Radar on board NASAââ¬â¢s Mars Reconnaissance Orbiter. The data shows extensive internal reflections and several radar reflectors were mapped to create gridded surfaces of this part of the NPLD. All the mapped radar reflectors were smooth with no sudden dips towards the surface or the bedrock. The internal radar reflectors were then compared with modeled isochrones in two different areas of Gemina Lingula under the assumption of flow occurring. Results indicate that flow of ice is unlikely to have occurred between the main dome and Gemina Lingula. Furthermore, we found no evidence for the existence of a prior ablation zone in Gemina Lingula as predicted in another study.
Phillips, R. J., B. J. Davis, K. L. Tanaka, S. Bryne, M. T. Mellon, N. E. Putzig, R. M. Haberle, M. A. Kahre, B. A. Cambell, L. M. Carter, I. B. Smith, J. W. Holt, S. E. Smrekar, G. T. Nunes, J. J. Plaut, A. F. Egan, T. N. Titus, and R. R. Seu, Massive CO2 ice deposits sequestered in the South Pole layered deposits of Mars, Science, 332, 838-841, 2011, 3 citations, doi:10.1126/science.1203091, #2381
Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO2) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO2 volatile release. If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.
Young, D. A., A. P. Wright, J. L. Roberts, R. C. Warner, N. W. Young, J. S. Greenbaum, D. M. Schroeder, J. W. Holt, D. E. Sugden, D. D. Blankenship, T. D. van Ommen, and M. J. Siegert, A dynamic early East Antarctic Ice Sheet suggested by ice covered fjord landscape, Nature, 474, 72-75, 2011, 3 citations, doi:10.1038/nature10114, #2344
The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains1 and other highlands as a result of rapid global cooling ~34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes
Holt, J. W., K. E. Fishbaugh, S. Byrne, S. Christian, K. L. Tanaka, P. Russell, K. Herkenhoff, A. Safaeinili, N. E. Putzig, and R. J. Phillips, The construction of Chasma Boreale on Mars, Nature, 465, 446-449, 2010, 3 citations, doi:10.1038/nature09050, #2186
The polar layered deposits of Mars contain the planet's largest known reservoir of water ice and the prospect of revealing a detailed Martian palaeoclimate record, but the mechanisms responsible for the formation of the dominant features of the north polar layered deposits (NPLD) are unclear, despite decades of debate. Stratigraphic analyses of the exposed portions of Chasma Boreale, a large canyon 500 km long, up to 100 km wide, and nearly 2 km deep, have led most researchers to favour an erosional process for its formation following initial NPLD accumulation. Candidate mechanisms include the catastrophic outburst of water, protracted basal melting, erosional undercutting, aeolian downcutting and a combination of these processes. Here we use new data from the Mars Reconnaissance Orbiter to show that Chasma Boreale is instead a long-lived, complex feature resulting primarily from non-uniform accumulation of the NPLD. The initial valley that later became Chasma Boreale was matched by a second, equally large valley that was completely filled in by subsequent deposition, leaving no evidence on the surface to indicate its former presence. We further demonstrate that topography existing before the NPLD began accumulating influenced successive episodes of deposition and erosion, resulting in most of the present-day topography. Long-term and large-scale patterns of mass balance achieved through sedimentary processes, rather than catastrophic events, ice flow or highly focused erosion, have produced the largest geomorphic anomaly in the north polar ice of Mars.
Jordan, T. A., F. Ferraccioli, D. G. Vaughan, J. W. Holt, H. F. J. Corr, D. D. Blankenship, and T. M. Diehl, Aerogravity evidence for major crustal thinning under the Pine Island Glacier region (west Antarctica), Geol. Soc. Amer. Bull., 122, 714-726, 2010, 1 citation, doi:10.1130/B26417.1, #2188
The West Antarctic Rift System provides critical geological boundary conditions for the overlying West Antarctic Ice Sheet. Previous geophysical surveys have traced the West Antarctic Rift System and addressed the controls that it exerts on the West Antarctic Ice Sheet in the Ross Sea Embayment. However, much less is known about the rift system under the Amundsen Sea Embayment, a key sector of the West Antarctic Ice Sheet, which is thinning significantly today. New aerogravity data over the Pine Island Glacier region, one of the fastest flowing glaciers within the Amundsen Sea Embayment, sheds new light into the crustal structure under this dynamic part of the West Antarctic Ice Sheet. Three-dimensional (3-D) inversion of terrain-decorrelated free-air and Bouguer gravity anomaly data reveal significant crustal thinning beneath the catchment of Pine Island Glacier. Under the Byrd Subglacial Basin and the newly identified Pine Island Rift, Moho depth is estimated to be 19 ñ 1 km. This is the thinnest crust observed beneath the West Antarctic Ice Sheet. Estimates of lithosphere rigidity (Te), based on isostatic models, yield a Te of 5 ñ 5 km, which is comparable to values from modern rift systems such as the Basin and Range Province. Major crustal thinning, coupled with low lithosphere rigidity, attest to the considerable impact of continental rifting beneath this part of the West Antarctic Ice Sheet. In analogy with the better known Ross Sea segment of the West Antarctic Rift System we suggest that the Amundsen Sea Embayment was affected by distributed Cretaceous rifting, followed by Cenozoic narrow-mode rifting. Narrow-mode rifting within the Pine Island Rift is particularly important as it may serve as a geological template for enhanced glacial flow associated with Pine Island Glacier.
Nunes, G. T., S. E. Smrekar, A. Safaeinili, J. W. Holt, R. J. Phillips, R. R. Seu, and B. A. Cambell, Examination of gully sites on Mars with the shallow radar, J. Geophys. Res., 115, E10004, 2010, doi:10.1029/2009JE003509, #2189
Martian gullies, found on steep slopes along broad midââ¬Âlatitudinal bands, have morphologies resembling those of waterââ¬Âcarved gullies on Earth and have been dated to <10 Ma. As such, one of the leading hypotheses, though not unique, is that martian gullies formed by the flow of liquid water in the very recent geologic past. Since the permittivity of liquid water is about one order of magnitude higher than that of most silicates, it is plausible that subsurface geologic interfaces involving liquid water may be detected via ground penetrating radar. We have surveyed a substantive portion of the martian gully population with data from the Shallow Radar (SHARAD) instrument, on board the Mars Reconnaissance Orbiter (MRO), in search of strong subsurface radar reflections indicative of the presence of liquid water reservoirs, which would serve as sources to the flows occurring within gullies. No such reflections are found at most of the locations surveyed, suggesting that either liquid water is not likely present in detectable amounts or that the shallow martian subsurface is unusually electrically conductive (i.e., lossy) at all of the locations examined. Strong subsurface reflections occur in the vicinity of gullies at two locations in the northern lowlands: Arcadia and southeastern Utopia Planitiae. In both cases, the reflectors occur at a range in depth of 45 to 90 m, considering a range in permittivity of 3 to 10, and âËâ20 to âËâ30 dB weaker than the surface reflection. In the case or Arcadia, the reflector corresponds to the eastern edge of Plaut et al.'s (2009) extensive radar subsurface unit; in both Arcadia and Utopia we interpret the reflectors as ground ice. Though our results offer a general assessment of the gully population, SHARAD is continuing its survey of gully rich locations.
Quidelleur, X., J. W. Holt, T. Salvany, and H. Bouquerel, New K-Ar ages from La Montagne massif, Reunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2.2-2.0 Ma, Geophys. J. Int., 182, 699-710, 2010, 2 citations, doi:10.1111/j.1365-246X.2010.04651.x, #2351
We present new radiometric ages obtained at the type locality in La RÃÆéunion Island where palaeomagnetic records of the RÃÆéunion events have first been identified. Seven dated lava flows from the RiviÃÆère Saint Denis section, which recorded a reverse-to-normal transition, display ages from 2.12 Ãâñ 0.03 to 2.17 Ãâñ 0.03 Ma, with a mean age of 2.15 Ãâñ 0.02 Ma. Two significantly younger flows from this section, interpreted here as valley fill flows from trace elements compositions and Pb isotopic data, have been dated at 2.06 Ãâñ 0.03 and 2.08 Ãâñ 0.03 Ma. Within the Grande Chaloupe section, where a normal-to-reverse transition is recorded, two coherent ages of 2.05 Ãâñ 0.03 and 2.03 Ãâñ 0.03 Ma have been obtained. When a direct comparison was possible, our new K-Ar ages performed on separated groundmass show a rather good coherence with previous ages from La RÃÆéunion Island. When considered with continuous palaeomagnetic sedimentary records in the 2.2âââ‰â¬Å2.0 Ma interval, these new results suggest that two distinct events are recorded in La Montagne lava flows at La RÃÆéunion Island, with ages of 2.15 Ãâñ 0.02 and 2.04 Ãâñ 0.02 Ma. Following recent nomenclature, the former is the RÃÆéunion event s.s., while the latter can be related to the Huckleberry Ridge event. Globally distributed volcanic and sedimentary records show that the first (RÃÆéunion s.s.; RU-1) is associated with a large dipole intensity decrease at 2.15 Ãâñ 0.02 Ma, and hence is recorded in many sequences. On the other hand, the dipole intensity decrease was not as pronounced at 2.04 Ãâñ 0.02 Ma, when the Huckleberry Ridge (RU-2) palaeomagnetic event occurred. Consequently, it is not present as a full directional change in many sections worldwide, but rather appears as a geomagnetic excursion during an episode of increased secular variation. Finally, the use of the RÃÆéunion event for magnetostratigraphic studies is recommended, while the clear identification of the Huckleberry Ridge excursion might often be difficult, preventing its use as a reliable time marker.
Smith, I. B., and J. W. Holt, Onset and migration of spiral troughs on Mars revealed by orbital radar, Nature, 465, 450-453, 2010, 4 citations, doi:10.1038/nature09049, #2187
The landscape of the north polar layered deposits of Mars (NPLD) is dominated by a pinwheel array of enigmatic spiral troughs. The troughs have intrigued planetary scientists since the Mariner 9 spacecraft returned the first close-up image in 1972, but conclusive evidence of their origin has remained elusive. Debate continues regarding all aspects of the troughs, including the possibility that they have migrated, their age in relation to the current NPLD surface, and whether they are fundamentally erosional or constructional features. The troughs are probably related to climatic processes, yet the nature of this relationship has remained a mystery. Previous data characterizing only the exposed NLPD surface were insufficient to test these hypotheses. Here we show that the central spiral troughs initiated after deposition of three-quarters of the NPLD, quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so. Our radar stratigraphy rules out hypotheses of erosional incision post-dating deposition, and instead largely validates an early hypothesis for constructional trough migration with wind transport and atmospheric deposition as dominant processes. These results provide hard constraints for palaeo-climate models and a new context for evaluating imagery, spectral data, and now radar sounding data, the better to understand the link between orbital parameters and climate, the role of climate in shaping the polar ice of Mars, and eventually, the age of the polar deposits themselves
Carter, S. P., D. D. Blankenship, D. A. Young, and J. W. Holt, Using radar-sounding data to identify the distribution and sources of subglacial water: Application to Dome C, East Antarctica, J. Glaciology, 55, 1025-1040, 2009, 4 citations, doi:10.3189/002214309790794931, #2126
Basal radar reflectivity is the most important measurement for the detection of subglacial water. However, dielectric loss in the overlying ice column complicates the determination of basal reflectivity. Dielectric attenuation is a function of ice temperature and impurity concentration. Temperature distribution is a function of climate history, basal heat flow and vertical strain rate, all of which can be partially inferred from the structure of dated internal layers. Using 11 dated layers, isotope records from Dome C, East Antarctica, and a model of the spatial variation of geothermal flux, we calculate the vertical strain rate and accumulation-rate history, allowing identification of areas where the basal melt rate exceeds 1.5 mm a-1. The accumulation-rate history and vertical strain rates are then used as inputs for a transient temperature model. The model outputs for the present-day temperature distribution are then combined with depth-dependent ionic concentrations to model dielectric loss and infer basal reflectivity. The resulting reflection coefficients are consistent (∼-5 dB) across a variety of subglacial water bodies. We also identify a high reflectivity >-15 dB in Concordia Trench and along suspected subglacial water-flow routes in Vincennes Basin. Highland areas tend to have highly variable reflection coefficients near -30 dB, consistent with an ice-bedrock interface. This combined model also identifies three areas of enhanced basal melting along Concordia Ridge, Concordia Subglacial Lake and Vincennes Basin. Melt at Concordia Subglacial Lake exceeds 5 mm a-1. The inferred basal melt at these locations is not possible without enhanced geothermal flux. We demonstrate how radar-sounding data can provide both input and verification for a self-consistent model of vertical strain, vertical temperature distribution and meltwater distribution.
Carter, S. P., D. D. Blankenship, D. A. Young, M. E. Peters, J. W. Holt, and M. J. Siegert, Dynamic distributed drainage implied by the flow evolution of the 1996-1998 Adventure Trench subglacial lake discharge, Earth Planet. Sci. Lett., 283, 24-37, 2009, 13 citations, doi:10.1016/j.epsl.2009.03.019, #2127
The transport of subglacial water beneath the East Antarctic Ice Sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities, and ultimately global climate. Periodic outbursts are a critical mechanism in this process. Recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km3 traveling some 260 km along the ice-bed interface of the Adventure Subglacial Trench between 1996 and 1998 (Wingham et al., 2006. Rapid discharge connects Antarctic subglacial lakes. Nature 440, 1033â1036). Using radar echo sounding data from the Adventure Subglacial Trench region in conjunction with the previously reported satellite observations, along with some basic modeling, we calculate a mass budget and infer a flow mechanism for the 1996â1998 event. The volume released from the source lake exceeded the volume received by the destination lakes by ~ 1.1 km3. This discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward. The downstream release of water from the destination lakes continued until at least 2003, several years after the 1998 cessation of surface subsidence at the source lake. By 2003 a total of 1.5 km3 or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes. The temporal evolution of discharge from the outlet can be simulated with the classic ice-walled semicircular channel model, if and only if the retreat of the source lake shoreline is taken into account. Further downstream, the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable. Along these reaches mechanical lifting of the ice roof and/or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge. A distributed system is also consistent with the 3-month delay between water release at the source lake and water arrival at the destination lake. Observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system. Ultimately the presence of large subglacial lakes along the flow path of the 1996â1998 Adventure Subglacial Trench flow path delayed the arrival of water to points downstream by approximately 12 months.
Carter, L. M., B. A. Cambell, J. W. Holt, R. J. Phillips, N. E. Putzig, S. Mattei, R. R. Seu, C. H. Okubo, and A. F. Egan, Dielectric properties of lava flows west of Ascraeus Mons, Mars, Geophys. Res. Lett., 36, L23204, 2009, 4 citations, doi:10.1029/2009GL041234, #2180
The SHARAD instrument on the Mars Reconnaissance Orbiter detects subsurface interfaces beneath lava flow fields northwest of Ascraeus Mons. The interfaces occur in two locations; a northern flow that originates south of Alba Patera, and a southern flow that originates at the rift zone between Ascraeus and Pavonis Montes. The northern flow has permittivity values, estimated from the time delay of echoes from the basal interface, between 6.2 and 17.3, with an average of 12.2. The southern flow has permittivity values of 7.0 to 14.0, with an average of 9.8. The average permittivity values for the northern and southern flows imply densities of 3.7 and 3.4 g cm−3, respectively. Loss tangent values for both flows range from 0.01 to 0.03. The measured bulk permittivity and loss tangent values are consistent with those of terrestrial and lunar basalts, and represent the first measurement of these properties for dense rock on Mars.
Joughin, I., S. Tulacyzk, J. L. Bamber, D. D. Blankenship, J. W. Holt, T. A. Scambos, and D. G. Vaughan, Basal conditions for Pine Island and Thwaites Glaciers, West Antarctica, determined using satellite and airborne data
, J. Glaciology, 55, 245-257, 2009, 18 citations, doi:10.3189/002214309788608705, #2065
We use models constrained by remotely sensed data from Pine Island and Thwaites Glaciers, West Antarctica, to infer basal properties that are difficult to observe directly. The results indicate strong basal melting in areas upstream of the grounding lines of both glaciers, where the ice flow is fast and the basal shear stress is large. Farther inland, we find that both glaciers have 'mixed' bed conditions, with extensive areas of both bedrock and weak till. In particular, there are weak areas along much of Pine Island Glacier's main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line. In agreement with earlier studies, our forward ice-stream model shows a strong sensitivity to small perturbations in the grounding line position. These results also reveal a large sensitivity to the assumed bed (sliding or deforming) model, with non-linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear-viscous till rheology. Finally, comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress.
Plaut, J. J., A. Safaeinili, J. W. Holt, R. J. Phillips, J. W. Head, R. R. Seu, N. E. Putzig, and A. Frigeri, Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars, Geophys. Res. Lett., 36, L02203, 2009, 36 citations, doi:10.1029/2008GL036379, #2064
Subsurface radar sounding data indicate that lobate debris aprons found in Deuteronilus Mensae in the mid-northern latitudes of Mars are composed predominantly of water ice. The position in time delay and the relatively low amount of signal loss of the apparent basal reflectors below the debris aprons indicate that aprons contain only a minor component of lithic material. The current presence of large ice masses at these latitudes has important implications for the climate evolution of Mars, and for future targets for in situ exploration.
Putzig, N. E., R. J. Phillips, B. A. Campbell, J. W. Holt, J. J. Plaut, L. M. Carter, A. F. Egan, F. Bernardini, A. Safaeinili, and R. R. Seu, Subsurface structure of Planum Boreum from Mars reconnaissance orbiter shallow radar soundings, Icarus, 204, 443-457, 2009, 10 citations, doi:10.1016/j.icarus.2009.07.034, #2129
We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444â474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318â358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182â1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km3), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375â377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dustâcontent variation during accumulation rather than one of lag production during periods of sublimation.
Diehl, T. M., J. W. Holt, D. D. Blankenship, D. A. Young, T. A. Jordan, and F. Ferraccioli, First airborne gravity results over the Thwaites Glacier catchment, West Antarctica, Geochem., Geophys., Geosyst., 9, Q04011, 2008, 1 citation, doi:10.1029/2007GC001878, #1940
Recent satellite observations of Thwaites Glacier in the Amundsen Sea Embayment, West Antarctica, have shown that the glacier is changing rapidly. The causes of its dynamic behavior are uncertain but are of concern because this glacier has the most negative mass balance of all Antarctic glaciers. To better understand Thwaites Glacier's subglacial setting, we conducted a multi-instrumented aerogeophysical survey of its catchment and present here the first gravity results. We employed a new gravimeter, and it performed well despite extreme conditions and an unusual survey design. The unleveled free-air gravity anomalies have a 2.3 mGal RMS error and a 9 km spatial resolution. Despite slightly higher than standard noise levels, the free-air anomalies correlate well with radar-derived subglacial topography. The new airborne gravity data assist in interpreting radar-identified bedrock features and are an ideal basis for future studies of subglacial geology and its control on the dynamics of Thwaites Glacier.
Holt, J. W., A. Safaeinili, J. J. Plaut, J. W. Head, R. J. Phillips, R. R. Seu, S. D. Kempf, P. Choudhary, D. A. Young, N. E. Putzig, D. Biccari, and Y. Gim, Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars, Science, 322, 1235-1238, 2008, 51 citations, doi:10.1126/science.1164246, #2006
Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars.
Kirschvink, J. L., R. E. Kopp, T. D. Raub, C. T. Baumgartner, and J. W. Holt, Rapid, precise, and high-sensitivity acquisition of paleomagnetic and rock-magnetic data: Development of a low-noise automatic sample changing system for superconducting rock magnetometers, Geochem., Geophys., Geosyst., 9, Q05Y01, 2008, 10 citations, doi:10.1029/2007GC001856, #1941
Among Earth sciences, paleomagnetism is particularly linked to the statistics of large sample sets as a matter of historical development and logistical necessity. Because the geomagnetic field varies over timescales relevant to sedimentary deposition and igneous intrusion, while the fidelity of recorded magnetization is modulated by original properties of rock units and by alteration histories, âidealâ paleomagnetic results measure remanent magnetizations of hundreds of samples at dozens of progressive demagnetization levels, accompanied by tests of magnetic composition on representative sister specimens. We present an inexpensive, open source system for automating paleomagnetic and rock magnetic measurements. Using vacuum pick-and-place technology and a quartz-glass sample holder, the system can in 1 h measure remanent magnetizations, as weak as a few pAm2, of ∼30 specimens in two vertical orientations with measurement errors comparable to those of the best manual systems. The system reduces the number of manual manipulations required per specimen approximately eightfold.
Phillips, R. J., M. T. Zuber, S. E. Smrekar, S. E. Smrekar, M. T. Mellon, J. W. Head, K. L. Tanaka, N. E. Putzig, S. M. Milkovitch, B. A. Campbell, J. J. Plaut, A. Safaeinili, R. R. Seu, D. Biccari, L. M. Carter, G. Picardi, R. Orosei, B. A. R. Mohr, P. S. Mohit, E. Heggy, R. W. Zubek, A. F. Egan, E. Giacomoni, F. Russo, M. Cutigni, E. Pettinelli, J. W. Holt, C. J. Leuschen, and L. Marinangeli, Mars north polar deposits: Stratigraphy, age, and geodynamical response, Science, 320, 1182-1185, 2008, 59 citations, doi:10.1126/science.1157546, #2027
The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter has imaged the internal stratigraphy of the north polar layered deposits of Mars. Radar reflections within the deposits reveal a laterally continuous deposition of layers, which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice. The packet/interpacket structure can be explained by approximately million-year periodicities in Mars' obliquity or orbital eccentricity. The observed 100-meter maximum deflection of the underlying substrate in response to the ice load implies that the present-day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers. Alternatively, the response to the load may be in a transient state controlled by mantle viscosity. Both scenarios probably require that Mars has a subchondritic abundance of heat-producing elements.
Young, D. A., S. D. Kempf, D. D. Blankenship, J. W. Holt, and D. L. Morse, New airborne laser altimetry over the Thwaites glacier catchment, West Antarctica, Geochem., Geophys., Geosyst., 9, Q06006, 2008, 2 citations, doi:10.1029/2007GC001935, #1982
A new airborne altimetry data set collected over Thwaites Glacier, one of Antarctica's most active ice streams, demonstrates the improvement in publicly available digital elevation models (DEMs) of the Antarctic ice sheet. The airborne altimetry comprises 35,000 line km sampled at 20 m along track. The full data set has a relative error of ±20 cm; a reference subset has an error of ±8 cm. These data are offset from ICESat observations by +20 cm. We find that a recently released ICESat DEM provides a good model of the surface of Thwaites Glacier, despite cloud cover and wide track spacings. However, the ICESat DEM's accuracy is an order of magnitude less than that of the ICESat profile data. Our airborne data will serve as an additional temporal reference for the evolution of Thwaites Glacier's surface as well as aid the construction of future high-resolution DEMs.
Carter, S. P., D. D. Blankenship, M. E. Peters, D. A. Young, J. W. Holt, and D. L. Morse, Radar-based subglacial lake classification in Antarctica, Geochem., Geophys., Geosyst., 8, Q03016, 2007, 17 citations, doi:10.1029/2006GC001408, #1887
Subglacial lakes in East Antarctica can be separated into four categories specified by radar reflection properties. Definite lakes are brighter than their surroundings by at least 2 dB (relatively bright) and both are consistently reflective (specular) and have a reflection coefficient greater than −10 dB (absolutely bright). Dim lakes are relatively bright and specular but not absolutely bright, indicating nonsteady ice dynamics. Fuzzy lakes are both relatively and absolutely bright, but not specular, and may indicate saturated sediments or ââ¬Åswamps.ââ¬Â Indistinct lakes are absolutely bright and specular but no brighter than their surroundings. Lakes themselves and the different classes of lakes are not arranged randomly throughout Antarctica but are clustered around ice divides, ice stream onsets, and prominent bedrock troughs, with each cluster demonstrating a different characteristic lake classification distribution. The lake classification algorithm expands on previous studies and demonstrates a novel way to characterize ice-water interactions in East Antarctica.
Holt, J. W., D. D. Blankenship, F. Ferraccioli, D. G. Vaughan, D. A. Young, S. D. Kempf, and T. M. Diehl, New aeromagnetic results from the Thwaites glacier catchment, West Antarctica, in Antarctica: A Keystone in a Changing World Proc. Tenth Int. Symp. Antarctic Earth Sciences, edited by A. K. Cooper, C. R. Raymond et al, USGS Open-File Rept. 2007-1047, abstract 153, 3 p., 2007, #2028
Peters, M. E., D. D. Blankenship, D. E. Smith, J. W. Holt, and S. D. Kempf, The distribution and classification of bottom crevasses from radar sounding of a large tabular iceberg, IEEE Geoscience and Remote Sensing Lett., 4, 142-146, 2007, 7 citations, doi:10.1109/LGRS.2006.887057, #1835
Bottom crevasses at the base of an iceberg or ice shelf are identified in radar sounding observations from their long echo tails. In November 2001, a radar sounding survey was conducted over iceberg B15A, which calved off from the Ross Ice Shelf, Antarctica, in March 2000. Pervasive basal cracking was observed, and the distribution of bottom crevasses along the flight lines is presented. The echo tails were quantitatively analyzed using a physically based model for backscattering from bottom crevasses. The identified crevasses are classified as either major water-filled crevasses or incipient/freezing crevasses, and estimates for crevasse heights are given
Peters, M. E., D. D. Blankenship, S. P. Carter, S. D. Kempf, D. A. Young, and J. W. Holt, Along-track focusing of airborne radar sounding data from West Antarctica for improving basal reflection analysis and layer detection, IEEE Geoscience and Remote Sensing Lett., 45, 2725-2736, 2007, 10 citations, doi:10.1109/TGRS.2007.897416, #1886
This paper presents focused synthetic aperture radar (SAR) processing of airborne radar sounding data acquired with the High-Capability Radar Sounder system at 60 MHz. The motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking. The processing and reflection analyses are applied to data from Kamb Ice Stream, West Antarctica. The SAR processor correlates the radar data with reference echoes from subsurface point targets. The references are 1-D responses limited by the pulse nadir footprint or 2-D responses that include echo tails. Unfocused SAR and incoherent integration are included for comparison. Echoes are accurately preserved from along-track slopes up to about 0.5deg for unfocused SAR, 3deg for 1-D correlations, and 10deg for 2-D correlations. The noise/clutter levels increase from unfocused SAR to 1-D and 2-D correlations, but additional gain compensates at the basal interface. The basal echo signal-to-noise ratio improvement is typically about 5 dB, and up to 10 dB for 2-D correlations in rough regions. The increased noise degrades the clarity of internal layers in the 2-D correlations, but detection of layers with slopes greater than 3deg is improved. Reflection coefficients are computed for basal water detection, and the results are compared for the different processing methods. There is a significant increase in the detected water from unfocused SAR to 1-D correlations, indicating that substantial basal water exists on moderately sloped interfaces. Very little additional water is detected from the 2-D correlations. The results from incoherent integration are close to the focused SAR results, but the noise/clutter levels are much greater.
Seu, R. R., J. D. Phillips, G. Alberti, D. Biccari, F. Bonaventura, M. Bortone, D. Calabrese, B. A. Campbell, M. Cartacci, L. M. Carter, C. Catallo, A. Croce, R. Croci, M. Cutigni, A. Di Placido, S. Dinardo, C. Federico, E. Flamini, F. Fois, A. Frigeri, O. Fuga, E. Giacomoni, Y. Gim, M. Guelfi, J. W. Holt, W. Kofman, C. J. Leuschen, L. Marinangeli, P. Marras, A. Masdea, S. Mattei, R. Mecozzi, S. M. Milkovitch, A. Morlupi, J. Mouginot, R. Orosei, C. Papa, T. Paterno, P. P. Del Marmo, E. Pettinelli, G. Pica, G. Picardi, J. J. Plaut, M. Provenziani, N. E. Putzig, F. Russo, A. Safaeinili, G. Salzillo, M. R. Santovito, S. E. Smrekar, B. Tattarletti, and D. Vicari, Accumulation and erosion of Mars' south polar layered deposits, Science, 317, 1715-1718, 2007, 20 citations, doi:10.1126/science.1144120, #1888
Mars' polar regions are covered with ice-rich layered deposits that potentially contain a record of climate variations. The sounding radar SHARAD on the Mars Reconnaissance Orbiter mapped detailed subsurface stratigraphy in the Promethei Lingula region of the south polar plateau, Planum Australe. Radar reflections interpreted as layers are correlated across adjacent orbits and are continuous for up to 150 kilometers along spacecraft orbital tracks. The reflectors are often separated into discrete reflector sequences, and strong echoes are seen as deep as 1 kilometer. In some cases, the sequences are dipping with respect to each other, suggesting an interdepositional period of erosion. In Australe Sulci, layers are exhumed, indicating recent erosion.
Vaughan, D. G., J. W. Holt, and D. D. Blankenship, West Antarctic links to sea level estimation, Eos, Trans. Amer. Geophys. Un., 88, 485-486, 2007, doi:10.1029/2007EO460001, #1986
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Filina, I. Y., D. D. Blankenship, L. Roy, M. K. Sen, T. G. Richter, and J. W. Holt, Inversion of airborne gravity data acquired over subglacial lakes in East Antarctica, Antarctica: Contributions to Global Earth Sciences, edited by D. K. Futterer, D. Damaske, G. Kleinschmidt,H. Miller, and F. Tessensohn, Springer-Verlag, Heidelberg, Germany, 129-133, 2006, 5 citations, #1714
Holt, J. W., M. E. Peters, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Echo source discrimination in single-pass radar sounding data from the dry valleys, Antarctica: Implications for the orbital sounding of Mars, J. Geophys. Res., 111, E06S24, 2006, 9 citations, doi:10.1029/2005JE002525, #1761
The interpretation of radar sounding data from Mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface. This can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes. We have developed general techniques for this using airborne radar data from the Dry Valleys of Antarctica. These data were collected in a single pass, including Taylor Glacier, ice-covered Lake Bonney, and an ice-free area of Taylor Valley. The pulsed radar (52.5â67.5 MHz) was coherently recorded. Our echo discrimination techniques included a radar simulator using a digital elevation model (DEM) to predict the location and shape of surface echoes in the radar data. Real and simulated echo strengths were used to calculate a signal-to-clutter ratio. This was complemented by the cross-track migration of radar echoes onto the surface. These migrated echoes were superimposed on the DEM and imagery in order to correlate with surface features. Using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes, including a continuous reflector under the main trunk of Taylor Glacier and multiple reflectors beneath the terminus of Taylor Glacier. Surface-based radar confirms the thickness of the glacier at three crossing points. The results illustrate the importance of using complementary techniques, the usefulness of a DEM, and the limitations of single-pass radar sounding data.
Holt, J. W., T. G. Richter, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Airborne gravity over Lake Vostok and adjacent highlands, East Antarctia, Geochem., Geophys., Geosyst., 7, Q11012, 2006, 6 citations, doi:10.1029/2005GC001177, #1802
Lake Vostok and a 1200 km transect were the targets of aerogeophysical surveys in East Antarctica during the austral summer of 2000/2001. The measurement of gravity anomalies for geologic studies was the primary goal. A total of 24,459 line-km of data were acquired. Favorable weather, aircraft navigation, and instrument performance contributed to excellent data quality. Multiple carrier-phase GPS solutions to determine aircraft-induced accelerations were available for each flight. Raw gravity and GPS position solutions were initially filtered to compensate for hardware filtering within the gravity meter. Filtering of remaining high-frequency noise was accomplished with a spatial, moving average smoother. Due to upward continuation effects imposed by the ice cover, the theoretically estimated minimum resolvable gravity feature size for the Lake Vostok survey is 8 km, consistent with an analysis of power spectra comparing the gravity signal to noise calculated from geographically repeated lines. Comparison of gravity results with subice topography indicates that the gravity data are sensitive to real features including the existence of major crustal structures. Repeated lines and crossovers were analyzed to estimate uncertainties for the Lake Vostok data set, with both of these repeatability measures indicating relative accuracy in the 2 mGal range for the unleveled data and 1 mGal after leveling.
Holt, J. W., D. D. Blankenship, D. L. Morse, D. A. Young, M. E. Peters, S. D. Kempf, T. G. Richter, D. G. Vaughan, and H. F. J. Corr, New boundary conditions for the West Antarctic Ice Sheet: Subglacial topography of the Thwaites and Smith glacier catchments, Geophys. Res. Lett., 33, L09502, 2006, 36 citations, doi:10.1029/2005GL025561, #1809
Airborne radar sounding over the Thwaites Glacier (TG) catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the West Antarctic Ice Sheet (WAIS) and reveals that TG is underlain by a single, broad basin fed by a dendritic pattern of valleys, while Smith Glacier lies within an extremely deep, narrow trench. Subglacial topography in the TG catchment slopes inland from a broad, low-relief coastal sill to the thickest ice of the WAIS and makes deep connections to both Pine Island Glacier and the Ross Sea Embayment enabling dynamic interactions across the WAIS during deglaciation. Simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation, aside from an island chain near the present-day Ross-Amundsen ice divide. The lack of topographic confinement along TG's eastern margin implies that it may continue to widen in response to grounding line retreat.
Holt, J. W., M. E. Peters, D. L. Morse, D. D. Blankenship, L. E. Lindzey, J. L. Kavanaugh, and K. M. Kuffey, Identifying and characterizing subsurface echoes in airborne radar sounding data from a high-clutter environment in Taylor Valley, Antarctica, Proc. 11th Int. Conf. Ground Penetrating Radar, 5, 2006, #1847
Vaughan, D. G., H. F. J. Corr, F. Ferraccioli, N. Frearson, A. O'Hare, D. Mach, J. W. Holt, D. D. Blankenship, D. L. Morse, and D. A. Young, New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island glacier, Geophys. Res. Lett., 33, L09501, 2006, 45 citations, doi:10.1029/2005GL025588, #1816
Predictions about future changes in the Amundsen Sea sector of the West Antarctic ice sheet (WAIS) have been hampered by poorly known subglacial topography. Extensive airborne survey has allowed us to derive improved subglacial topography for the Pine Island Glacier basin. The trunk of this glacier lies in a narrow, 250-km long, 500-m deep sub-glacial trough, suggesting a long-lived and constrained ice stream. Two tributaries lie in similar troughs, others lie in less defined, shallower troughs. The lower basin of the glacier is surrounded by bedrock, which, after deglaciation and isostatic rebound, could rise above sea level. This feature would impede ice-sheet collapse initiated near the grounding line of this glacier, and prevent its progress into the deepest portions of WAIS. The inland-slope of the bed beneath the trunk of the glacier, however, confirms potential instability of the lower basin, containing sufficient ice to raise global sea by ∼24 cm.
Studinger, M., R. E. Bell, G. D. Karner, A. A. Tikku, J. W. Holt, D. L. Morse, T. G. Richter, S. D. Kempf, M. E. Peters, D. D. Blankenship, R. E. Sweeney, and V. Rystrom, Ice cover, landscape setting, and geological framework of Lake Vostok, East Antarctica, Earth Planet. Sci. Lett., 205, 195-210, 2003, 65 citations, doi:10.1016/S0012-821X(02)01041-5, #1594
Lake Vostok, located beneath more than 4 km of ice in the middle of East Antarctica, is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment. Melting and freezing at the base of the ice sheet, which slowly flows across the lake, controls the flux of water, biota and sediment particles through the lake. The influx of thermal energy, however, is limited to contributions from below. Thus the geological origin of Lake Vostok is a critical boundary condition for the subglacial ecosystem. We present the first comprehensive maps of ice surface, ice thickness and subglacial topography around Lake Vostok. The ice flow across the lake and the landscape setting are closely linked to the geological origin of Lake Vostok. Our data show that Lake Vostok is located along a major geological boundary. Magnetic and gravity data are distinct east and west of the lake, as is the roughness of the subglacial topography. The physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lakeâs circulation. Lake Vostok is a tectonically controlled subglacial lake. The tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake.
Holt, J. W., Airborne surveys conducted by SOAR for geologic studies in Antarctica, Eos, Trans. Amer. Geophys. Un., 82, 20, 2001, #1566
Holt, J. W., T. G. Richter, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Airborne gravity over Lake Vostok and advancent highlands of East Antarctica, KIS 2001: Proc. Int. Symp. Kinematic Systems in Geodesy, Geomatics and Navigation, 576-585, 2001, #1611
Richter, T. G., J. W. Holt, and D. D. Blankenship, Airborne gravity over East Antarctica, in KIS 2001: Proc. Int. Symp. Kinematic Systems in Geodesy, Geomatics, and Navigation, Banff, Alberta, 576-585, 2001, #2224
Holt, J. W., E. W. Holt, and J. M. Stock, An age constraint on Gulf of California rifting from the Santa Rosalia basin, Baja California Sur, Mexico, Geol. Soc. Amer. Bull., 112, 540-549, 2000, 33 citations, doi:10.1130/0016-7606(2000)112<540:AACOGO>2.0.CO;2, #1967