E. P. Turtle

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (257)496.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a radar map of the Titan’s seas, with bathymetry estimated as proportional to distance from the nearest shore. This naïve analytic bathymetry, scaled to a recent radar sounding of Ligeia Mare, suggests a total liquid volume of ∼32,000 km3, at the low end of estimates made in 2008 when mapping coverage was incomplete. We note that Kraken Mare has two principal basins, separated by a narrow (∼17 km wide, ∼40 km long) strait we refer to as the ‘throat’. Tidal currents in this strait may be dramatic (∼0.5 m/s), generating observable effects such as dynamic topography, whirlpools, and acoustic noise, much like tidal races on Earth such as the Corryvreckan off Scotland. If tidal flow through this strait is the dominant mixing process, the two basins take ∼20 Earth years to exchange their liquid inventory. Thus compositional differences over seasonal timescales may exist, but the composition of solutes (and thus evaporites) over Croll–Milankovich timescales should be homogenized.
    Icarus 07/2014; 237:9–15. · 3.16 Impact Factor
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    ABSTRACT: We present a map of the distribution and liquid volume in lakes/seas using a combination of images acquired using the Cassini RADAR, VIMS, and ISS instruments.
    02/2014;
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    ABSTRACT: The IVO mission would make multiple close encounters with Io while orbiting Jupiter in an inclined elliptical orbit. The payload includes narrow-angle and wide-angle cameras (NAC and WAC), dual fluxgate magnetometers (FGM), a thermal mapper (ThM), dual ion and neutral mass spectrometers (INMS), and dual plasma ion analyzers (PIA). The mission is designed to answer key outstanding questions about Io, especially the nature of the intense active volcanism and internal processes that drive the volcanism. IVO can collect and return 20 Gb of compressed science data per Io encounter, 100 times the total Io data return from the 8yr Galileo tour.
    Acta Astronautica 01/2014; · 0.70 Impact Factor
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    ABSTRACT: Instruments on the Cassini spacecraft discovered new phenomena related to the (presumably) seasonal behavior of photochemical haze and formation of the winter polar vortex. West et al. 2011 (Geophys. Res. Lett. , 380 , L06204. doi: 10.1029/2011GL046843) described a ‘detached’ haze layer that dropped in altitude from about 500 km in 2005 to about 360 km by late 2010. New images from the Cassini ISS camera show that the appearance of a detached layer is produced by a gap in the haze vertical profile and it is the gap rather than a haze layer that drops in altitude. Intensity profiles from different epochs form an envelope when plotted on top of each other, and the downward movement of the gap can be most easily seen when plotted that way. The movement of a gap rather than movement of a layer of enhanced haze density was suspected in the earlier publication but now it is more apparent. In recent months the gap became very shallow and the limb intensity profiles at a pixel scale ~10 km/pixel evolved from one local maximum/minimum into two local minima/maxima of smaller amplitude and appear to be trending toward the disappearance of relative maxima and minima, leaving a smooth envelope. These observations will require new developments in coupled dynamical and haze microphysical models as none of the current models account for this behavior. Titan’s south polar vortex cloud was detected concurrently by the ISS, VIMS, and CIRS instruments on Cassini in May of 2012. It has an unusual color (more yellow than Titan’s main haze in ISS images), morphology and texture (suggestive of a condensate cloud experiencing open cell convection) and displays a spectral feature at 220 cm-1 (Jennings et al., 2012, Astrophys. J. Lett. 761, L15 DOI: 10.1088/2041-8205/761/1/L15). These attributes point to a condensate of unknown composition. The haze patch is seen in images up to the present (July, 2013), but the latest images suggest a ‘softening’ or more diffuse edge than the earlier images. The feature is being engulfed by shadow as the season progresses, eventually preventing future observations in reflected sunlight. Acknowledgement: Part of this work was performed by the Jet Propulsion Lab, Calif. Inst. Of Technology.
    10/2013;
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    ABSTRACT: The longevity of the Cassini mission, which has been orbiting the Saturn system since 2004, has started to permit the generation of novel data products that utilize overlapping radar observations of Titan. Repeat observations allow investigations of temporal change, surface properties via microwave backscatter modelling at SAR resolution, and the generation of digital terrain models (DTMs). We will utilize these capabilities to discuss constraints on the evolution of Titan's North Polar Landscape. Discussion will include (1) implications of the absence of observed temporal change in Northern lakes, (2) morphologic evidence for dynamic base level changes separated by intermittent periods of quiescence, (3) topographically closed depressions that imply karstic collapse and/or dissolution processes, and (4) the identification of a regionally common elevation amongst the floors of paleolake basins and shorelines of Kraken, Ligeia, and Punga Mare.
    EPSC; 09/2013
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    ABSTRACT: Cassini RADAR SARtopo and altimetry data are used to construct a global gridded 1 × 1° elevation map, for use in Global Circulation Models, hydrological models and correlative studies. The data are sparse, and so most of the map domain (∼90%) is populated with interpolated values using a spline algorithm. The highest (∼+520 m) gridded point observed is at 48°S, 12°W. The lowest point observed (∼1700 m below a 2575 km sphere) is at 59°S, 317°W: this may be a basin where liquids presently in the north could have resided in the past. If the deepest point were once a sea with the areal extent of present-day Ligeia Mare, it would be ∼1000 m deep. We find four prominent topographic rises, each ∼200 km wide, radar-bright and heavily dissected, distributed over a ∼3000 km arc in the southeastern quadrant of Titan (∼40–60°S, 15–150°W).
    Icarus 07/2013; 225(1):367–377. · 3.16 Impact Factor
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    ABSTRACT: Titan's Soi crater / Barely makes a surface dent / Filled by sediments?
    03/2013;
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    ABSTRACT: We analyzed dunes coverage of Titan's surface and the correlation between the dunes imaged by the RADAR/SAR with the two "brown" and "blue" units given by VIMS.
    Icarus 03/2013; · 3.16 Impact Factor
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    ABSTRACT: We present a comprehensive review of available crater topography measurements for Saturn’s moon Titan. In general, the depths of Titan’s craters are within the range of depths observed for similarly sized fresh craters on Ganymede, but several hundreds of meters shallower than Ganymede’s average depth vs. diameter trend. Depth-to-diameter ratios are between 0.0012 ± 0.0003 (for the largest crater studied, Menrva, D ∼ 425 km) and 0.017 ± 0.004 (for the smallest crater studied, Ksa, D ∼ 39 km). When we evaluate the Anderson–Darling goodness-of-fit parameter, we find that there is less than a 10% probability that Titan’s craters have a current depth distribution that is consistent with the depth distribution of fresh craters on Ganymede. There is, however, a much higher probability that the relative depths are uniformly distributed between 0 (fresh) and 1 (completely infilled). This distribution is consistent with an infilling process that is relatively constant with time, such as aeolian deposition. Assuming that Ganymede represents a close ‘airless’ analogue to Titan, the difference in depths represents the first quantitative measure of the amount of modification that has shaped Titan’s surface, the only body in the outer Solar System with extensive surface–atmosphere exchange.
    Icarus 03/2013; 223(1):82–90. · 3.16 Impact Factor
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    ABSTRACT: The total organic inventory from dunes on Saturn’s moon, Titan, measured in SAR and HiSAR images is ~150,000-300,000 km^3 or ~14% global coverage.
    LPI Contributions. 03/2013;
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    ABSTRACT: We will present an examination of Titan's polar landscapes through an examination of the relationships between lacustrine, fluvial, and hillslope morphologies.
    LPSC; 03/2013
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    ABSTRACT: Observations from Cassini VIMS and ISS show localized but extensive surface brightenings in the wake of the 2010 September cloudburst. Four separate areas, all at similar latitude, show similar changes: Yalaing Terra, Hetpet Regio, Concordia Regio, and Adiri. Our analysis shows a general pattern to the time-sequence of surface changes: after the cloudburst the areas darken for months, then brighten for a year before reverting to their original spectrum. From the rapid reversion timescale we infer that the process driving the brightening owes to a fine-grained solidified surface layer. The specific chemical composition of such solid layer remains unknown. Evaporative cooling of wetted terrain may play a role in the generation of the layer, or it may result from a physical grain-sorting process.
    Planetary Science. 01/2013; 2(1).
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    ABSTRACT: Northern spring equinox on Titan occurred on August 11, 2009. In March of 2012 the Imaging Science Subsystem (ISS) on the Cassini spacecraft saw the first evidence for the formation of a polar hood in the atmosphere above Titan’s south pole. Views of the limb showed an optical thickening primarily at about 360 km altitude across a few degrees of latitude centered on the pole. Images of Titan in front of Saturn provide a nearly direct measure of the line-of-sight optical depth as a function of latitude and altitude from about 250 km and higher. Two or more distinct layers are seen, both near the pole and at other latitudes. The highest of these, near 360 km altitude, hosts the embryonic polar hood. On June 27, 2012 ISS observed the pole from high latitude. These images show a distinct and unusual cloudy patch, elongated and not centered on the pole and with an elevated perimeter. The morphology and color indicate an unfamiliar (for Titan) composition and dynamical regime. The interior of the feature consists of concentrations of cloud/haze organized on spatial scales of tens of kilometers. Its morphology is reminiscent of the open cellular convection sometimes seen in the atmospheric boundary layer over Earth’s oceans under conditions of large-scale subsidence. Unlike Earth, where such convection is forced by large surface heat fluxes or the onset of drizzle, convection at 360 km on Titan is more likely to be driven from above by radiative cooling. During the 9 hours we observed Titan, this feature completed a little over one rotation around the pole, providing direct evidence for a polar vortex rotating at a rate roughly consistent with angular-momentum-conserving flow for air displaced from the equator. Part of this work was performed by the Jet Propulsion Laboratory, California Institute of Technology.
    10/2012;
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    ABSTRACT: A camera concept is presented that can accomplish science goals for several candidate missions to the high-radiation environment of Jupiter. Included are new radiation test results.
    LPI Contributions. 10/2012;
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    ABSTRACT: Turtle et al. (2011) previously announced large-scale surface changes in Titan's tropics following a 2010 September cloudburst event. Those changes were areas that had darkened, and the darkening was attributed to surface wetting by rain. Here we will discuss the results of continued monitoring of the darkened areas by Cassini VIMS and ISS. These new observations show that instead of reverting to their previous state, the rain-darkened areas instead brightened beyond their original albedos starting a few months after the cloudburst event. The brightening was unexpected, and spectra show that it occurs in each of Titan's atmospheric wavelength windows. The brightened spectra show some similarity to he heretofore unique signature of Xanadu. The areas slowly revert to their original spectra over a period of a year. The two hypotheses that we have not eliminated involve (1) volatile frosts resulting from evaporative cooling of rain-derived surface methane that later sublime, (2) deposition of a thin surface layer of very-fine-grained particles (similar to terrestrial playa lakes) that degrade due to aeolean erosion. Future calculations and observations will serve to constrain the mechanism that drives the brightening.
    10/2012;
  • Elizabeth Turtle, Alfred McEwen
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    ABSTRACT: Post-equinox changes in Titan's atmospheric circulation brought clouds and extensive methane rain to low latitudes [1,2]. Observations by Cassini ISS over the ensuing ~1.5 yr revealed surface changes to be short-lived; few rain-darkened areas persisted through 2011. In an unsaturated permeable medium, infiltration rates are >20 mm/week [3], so persistence of surface liquids over several months suggests that either an impermeable layer or the local methane table lies close to the surface. Evaporation rates >1 mm/week are predicted at low latitudes [4] and 20 mm/week has been documented at Titan's poles [5], thus areas where darkening persisted must be saturated ground at the level of a methane table or have had ponded liquid 2.5-50 cm deep. Several smaller areas of surface brightening were also observed, a phenomenon that is less well understood. Cassini VIMS spectra of these regions do not match clouds or other surface units [6]. Interpretations include cleaning by runoff [2] or deposition of a fine-grained volatile solid as the result of evaporative cooling of the surface [6]. In general, brightening has persisted longer than darkening, but these areas are also reverting to their original appearance, possibly due to evaporation/sublimation of the bright material or re-deposition of darker hydrocarbons by aeolian transport or precipitation from the atmosphere. Cassini and Earth-based observers monitor Titan frequently, but few clouds have been observed since Fall 2010, which may indicate that enough methane was removed from the atmosphere and the lapse rate stabilized sufficiently that activity will not resume until the onset of convection at mid-northern latitudes later in northern spring. A similar lapse followed a 2004 outburst of south-polar clouds [7], which also appeared to produce significant rainfall [8]. [1] Turtle et al., GRL 38, L03203, doi:10.1029/2010GL046266, 2011. [2] Turtle et al., Science 331, 10.1126/science.1201063. 2011. [3] Hayes et al., GRL 35, L09204, 2008. [4] Schneider et al., Nature 481, doi:10.1038/nature10666, 2012. [5] Hayes et al., Icarus 211, 2011. [6] Barnes et al., LPSC XXXXIII, 2012. [7] Schaller et al., Icarus 184, 2006. [8] Turtle et al., GRL 36, L02204, doi:10.1029/2008GL036186, 2009.
    07/2012;
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    ABSTRACT: Lakes and seas on Titan provide the first evidence for an extraterrestrial active liquid cycle and play a key role in its climate. Constraints on Titan's methane cycle, analogous to Earth’s hydrologic cycle, can be made through in situ measurements.
    LPI Contributions. 06/2012;
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    ABSTRACT: Cassini ISS saw large-scale surface darkenings in the wake of a tropical cloudburst event in 2010 September. In concert with the abstract by Turtle et al., in this presentation we show that weeks to months after darkening the surfaces did not revert to their pre-cloudburst brightness, but rather became brighter. VIMS observations of four distinct areas show these brightenings: Yalaing Terra, Hetpet Regio, Concordia Regio, and Adiri. Each study area brightened within each near-infrared atmospheric window, though not equally. In each case the brightened areas fade to their original spectra over a timescale of about a year. This rapid reversion time is inconsistent with chemical alteration of the surface - haze fallout would take hundreds to tens of thousands of years to recover an altered surface. Instead the deposition and removal of a volatile layer is more consistent with the observed evolution. Different scenarios for the production and removal of such a layer are possible. We will discuss these scenarios, which include evaporative cooled frost that later sublimates, and dissolution and reprecipitation of surface organics that may later be eroded by wind.
    04/2012;
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    ABSTRACT: Post-equinox changes in Titan's atmospheric circulation brought clouds and extensive methane rain to Titan's low latitudes [1,2]. Observations by Cassini ISS over the ~1.5 years since the storm revealed most of the changes to be short-lived; only a few darkened patches persisted through Fall 2011. In an unsaturated permeable medium, infiltration rates are >20 mm/week [3], so persistence of surface liquids over several months suggests either a shallow impermeable layer or that the local methane table lies close to the surface. Evaporation rates >1 mm/week are predicted in equatorial regions [4] and rates of 20 mm/week have been documented at the poles [5], thus areas where darkening persisted must be saturated ground at the level of a methane table or have had liquid ponded to depths of 2.5-50 cm. Several smaller areas of surface brightening were also observed, a phenomenon that is less well understood. Cassini VIMS spectra of these regions do not match those of clouds or other surface units [6, 7]. Interpretations include cleaning by runoff [2] or deposition of fresh methane ice [6, 7]. In general, brightening has persisted longer than darkening, but these areas are also reverting to their original appearance, which could constrain the rate of re-deposition of darker hydrocarbon materials by aeolian transport or possibly precipitation of aerosols from the atmosphere. Although we monitor Titan frequently (at least a few times per month), little cloud activity has been observed since Fall 2010. This lack of clouds may indicate that the outbreak removed enough methane from the atmosphere and the lapse rate stabilized sufficiently that activity will not resume until the onset of convection at mid-northern latitudes later in northern spring. A similar lapse followed a large outbreak of south-polar clouds in Fall 2004 [8], which also appeared to produce significant rainfall [9]. References: [1] Turtle et al., GRL 38, L03203, doi: 10.1029/2010GL046266, 2011. [2] Turtle et al., Science 331, p. 1414, 10.1126/science.1201063. 2011. [3] Hayes et al., GRL 35, L09204, 2008. [4] Schneider et al., Nature 481, doi:10.1038/nature10666, 2012. [5] Hayes et al., Icarus 211, p. 655, 2011. [6] Barnes et al., LPSC XXXXIII, 2012. [7] Barnes et al., Titan Through Time, 3-5 April 2012. [8] Schaller et al., Icarus 184, p. 517, 2006. [9] Turtle et al., GRL 36, L02204, doi:10.1029/ 2008GL036186, 2009.
    04/2012;
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    ABSTRACT: Titan exhibits an active weather cycle involving methane. Equatorial and mid-latitude clouds can be organized into fascinating morphologies on scales exceeding 1,000 km. Observations include an arrow-shaped equatorial cloud that produced detectable surface accumulation, probably from the precipitation of liquid methane. An analysis of an earlier cloud outburst indicated an interplay between high- and low-latitude cloud activity, mediated by planetary-scale atmospheric waves. We present a combined analysis of cloud observations and simulations with a three-dimensional general circulation model of Titan's atmosphere providing a physical interpretation of observed storms, their relation to atmosphere dynamics and their aggregate effect on surface erosion. We find that planetary-scale Kelvin waves arise naturally in our simulations, and robustly organize convection into chevron-shaped storms at the equator during the equinoctial season. A second and much slower wave mode organizes convection into southern-hemisphere streaks oriented in a northwest-southeast direction, similar to observations. As a result of the phasing of these modes, precipitation rates can be as high as twenty times the local average in our simulations, possibly playing a crucial role in fluvial erosion of Titan's surface. We also present a forecast of the weather expected in the coming season on Titan.
    04/2012;

Publication Stats

2k Citations
496.02 Total Impact Points

Institutions

  • 2008–2014
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, Maryland, United States
  • 1996–2011
    • The University of Arizona
      • Department of Planetary Sciences
      Tucson, Arizona, United States
  • 2009
    • Laboratoire d'Etudes en Géophysique et Óceanographie Spatiales
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2007–2008
    • Cornell University
      • • Department of Astronomy
      • • Center for Radiophysics and Space Research (CRSR)
      Ithaca, NY, United States
    • Laurel University
      North Carolina, United States
  • 2006
    • Freie Universität Berlin
      • Institute of Geological Sciences
      Berlin, Land Berlin, Germany
  • 2004–2005
    • Planetary Science Institute
      Los Angeles, California, United States