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Pappalardo, R. T.

Publications and source records attributed to Pappalardo, R. T..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

The Europa Clipper Gravity / Radio Science Investigation

Introduction: The Jovian moon Europa has long been a fascinating planetary body because of its young icy surface and subsurface ocean. Its high appeal for astrobiology and habitability implications in the solar system and beyond have made it the subject of intense scientific inquiry and of many past mission studies. The NASA flagship Europa Clipper mission [1] is currently in development and slated to launch in 2024. After insertion into the Jupiter system, it will conduct more than 40 flybys of Europa over the course of several years [2]. The suite of nine science instruments is uniquely suited to this flyby tour strategy, providing remote sensing and in situ observations of Europa and its relationship to Jupiter [3]. The primary objective of Europa Clipper is to assess the habitability of Europa, an overarching goal that rests on improving our understanding of Europa’s interior structure, composition, and geologic activity. Here we describe the Gravity/Radio Science (G/RS) investigation of Europa Clipper, which will make important observations to address key science goals.

Pappalardo, R. T.↗
NASA NTRS2014

Science Goals and Mission Concept for the Future Exploration of Titan and Enceladus

Saturn׳s moons, Titan and Enceladus, are two of the Solar System׳s most enigmatic bodies and are prime targets for future space exploration. Titan provides an analogue for many processes relevant to the Earth, more generally to outer Solar System bodies, and a growing host of newly discovered icy exoplanets. Processes represented include atmospheric dynamics, complex organic chemistry, meteorological cycles (with methane as a working fluid), astrobiology, surface liquids and lakes, geology, fluvial and aeolian erosion, and interactions with an external plasma environment. In addition, exploring Enceladus over multiple targeted flybys will give us a unique opportunity to further study the most active icy moon in our Solar System as revealed by Cassini and to analyse in situ its active plume with highly capable instrumentation addressing its complex chemistry and dynamics. Enceladus׳ plume likely represents the most accessible samples from an extra-terrestrial liquid water environment in the Solar system, which has far reaching implications for many areas of planetary and biological science. Titan with its massive atmosphere and Enceladus with its active plume are prime planetary objects in the Outer Solar System to perform in situ investigations. In the present paper, we describe the science goals and key measurements to be performed by a future exploration mission involving a Saturn-Titan orbiter and a Titan balloon, which was proposed to ESA in response to the call for definition of the science themes of the next Large-class mission in 2013. The mission scenario is built around three complementary science goals: (A) Titan as an Earth-like system; (B) Enceladus as an active cryovolcanic moon; and (C) Chemistry of Titan and Enceladus - clues for the origin of life. The proposed measurements would provide a step change in our understanding of planetary processes and evolution, with many orders of magnitude improvement in temporal, spatial, and chemical resolution over that which is possible with Cassini-Huygens. This mission concept builds upon the successes of Cassini-Huygens and takes advantage of previous mission heritage in both remote sensing and in situ measurement technologies.

future missions↗
NASA NTRS2010

Tectonics of the Outer Planet Satellites

Tectonic features on the satellites of the outer planets range from the familiar, such as clearly recognizable graben on many satellites, to the bizarre, such as the ubiquitous double ridges on Europa, the twisting sets of ridges on Triton, or the isolated giant mountains rising from Io's surface. All of the large and middle-sized outer planet satellites except Io are dominated by water ice near their surfaces. Though ice is a brittle material at the cold temperatures found in the outer solar system, the amount of energy it takes to bring it close to its melting point is lower than for a rocky body. Therefore, some unique features of icy satellite tectonics may be influenced by a near-surface ductile layer beneath the brittle surface material, and several of the icy satellites may possess subsurface oceans. Sources of stress to drive tectonism are commonly dominated by the tides that deform these satellites as they orbit their primary giant planets. On several satellites, the observed tectonic features may be the result of changes in their tidal figures, or motions of their solid surfaces with respect to their tidal figures. Other driving mechanisms for tectonics include volume changes due to ice or water phase changes in the interior, thermoelastic stress, deformation of the surface above rising diapirs of warm ice, and motion of subsurface material toward large impact basins as they fill in and relax. Most satellites exhibit evidence for extensional deformation, and some exhibit strike-slip faulting, whereas contractional tectonism appears to be rare. Io s surface is unique, exhibiting huge isolated mountains that may be blocks of crust tilting and foundering into the rapidly emptying interior as the surface is constantly buried by deposits from hyperactive volcanoes. Of the satellites, diminutive Enceladus is spectacularly active; its south polar terrain is a site of young tectonism, copious heat flow, and tall plumes venting into space. Europa's surface is pervasively tectonized, covered with a diverse array of exotic and incompletely understood tectonic features. The paucity of impact craters on Europa suggests that its tectonic activity is ongoing. Geysers on Triton show that some degree of current activity, while tectonic features that cross sparsely cratered terrain indicate that it may also be tectonically active. Ganymede and Miranda both exhibit ancient terrains that have been pulled apart by normal faulting. On Ganymede these faults form a global network, while they are confined to regional provinces on Miranda. Ariel, Dione, Tethys, Rhea, and Titania all have systems of normal faults cutting across their surfaces, though the rifting is less pronounced than it is on Ganymede and Miranda. Iapetus exhibits a giant equatorial ridge that has defied simple explanation. The rest of the large and middle-sized satellites show very little evidence for tectonic features on their surfaces, though the exploration of Titan's surface has just begun.

McKinnon, W. B.↗
NASA NTRS2004

Domes on Europa: The Role of Thermally Induced Compositional Diapirism

The surface of Europa is peppered by topographic domes, interpreted as sites of intrusion and extrusion. Diapirism is consistent with dome morphology, but thermal buoyancy alone cannot produce sufficient driving pressures to create the observed dome elevations. Instead, we suggest that diapirs may initiate by thermal convection that induces compositional segregation within Europa's ice shell. This double-diffusive convection scenario allows sufficient buoyancy for icy plumes to create the observed surface topography, if the ice shell has a very small effective elastic thickness (approximately 0.1 to 0.5 km) and contains low-eutectic-point impurities at the percent level. Thermal buoyancy, compositional buoyancy and double-diffusive convection are discussed.

Pappalardo, R. T.↗
NASA NTRS2004

Brine Pockets in the Icy Shell on Europa: Distribution, Chemistry, and Habitability

On Earth, sea ice is rich in brine, salt, and gas inclusions that form through capturing of seawater during ice formation. Cooling of the ice over time leads to sequential freezing of captured sea-water, precipitation of salts, exsolution of gases, and formation of brine channels and pockets. Distribution and composition of brines in sea ice depend on the rate of ice formation, vertical temperature gradient, and the age of the ice. With aging, the abundance of brine pockets decreases through downward migration. De- spite low temperatures and elevated salinities, brines in sea ice provide a habitat for photosynthetic and chemosynthetic organisms. On Europa, brine pockets and channels could exist in the icy shell that may be from a few km to a few tens of km thick and is probably underlain by a water ocean. If the icy shell is relatively thick, convection could develop, affecting the temperature pattern in the ice. To predict the distribution and chemistry of brine pockets in the icy shell we have combined numerical models of the temperature distribution within a convecting shell, a model for oceanic chemistry, and a model for freezing of Europan oceanic water. Possible effects of brine and gas inclusions on ice rheology and tectonics are discussed.

Zolotov, M. Yu↗
NASA NTRS2004

Numerical Implementation of Ice Rheology for Europa's Shell

We present a discussion of approximations to the temperature dependent part of the rheology of ice. We have constructed deformation maps using the superplastic rheology of Goldsby & Kohlstedt and find that the rheologies that control convective flow in the Europa's are likely grain boundary sliding and basal slip for a range of grain sizes 0.1 mm < d < 1 cm. We compare the relative merits of two different approximations to the temperature dependence of viscosity and argue that for temperature ranges appropriate to Europa, implementing the non-Newtonian, lab-derived flow law directly is required to accurately judge the onset of convection in the ice shell and temperature gradient in the near-surface ice.

Barr, A. C.↗
NASA NTRS2004

Constraints on the Opening Rate of Bands on Europa

The opening rates of two bands on Europa, inferred to be sites of spreading of the icy lithosphere, are constrained based on a mid-ocean ridge analog model. Estimates of brittle-ductile transition depth combined with a conductive cooling model limit active band lifetimes to 0.24 - 35 Myr and strain rates of 8.1 x 10(exp -13) - 8.2 x 10(exp -15)/s. These values suggest tensile strengths for ice on Europa of 0.46 - 2.3 MPa, consistent with nonsynchronous rotation as the dominant driving mechanism for band opening.

Stempel, M. M.↗
NASA NTRS2004

Non-Newtonian Convection and Compositional Buoyancy: Advances in Modeling Convection and Dome Formation on Europa

Numerical modeling of non-Newtonian convection in ice shows that convection controlled by grain boundary sliding rheology may occur in Europa. This modeling confirms that thermal convection alone cannot produce significant dome elevations. Domes may instead be produced by diapirs initiated by thermal convection that in turn induces compositional segregation. Exclusion of impurities from warm upwellings would allow sufficient buoyancy for icy plumes to account for the observed approximately 100 m topography of domes, provided the ice shell has a small effective elastic thickness (approximately 0.2 to 0.5 km) and contains low eutectic-point impurities at the few percent level.

Pappalardo, R. T.↗
NASA NTRS2004

Convective Instability in Ice I: Application to Callisto and Ganymede

Laboratory experiments measuring ice rheology suggest that it deforms under the influence of several nonNewtonian creep mechanisms, where the viscosity depends on both strain rate and temperature [Goldsby & Kohlstedt, 2001]. Whether or not a fluid with a purely temperature-dependent viscosity convects can be determined by comparing the Rayleigh number of the system to the critical Rayleigh number (Racr), which depends on rheological, thermal, and physical parameters of the fluid layer. However, in a nonNewtonian fluid where viscosity depends on the strain rate (i.e. velocity), convection can only occur if a temperature or velocity perturbation is issued to the system to lower the viscosity and permit fluid motions. Therefore, whether convection occurs in an ice I layer depends on initial conditions, in addition to rheological, thermal, and physical properties of the layer. We show new results for a scaling between the critical Rayleigh number and perturbation amplitude for grain boundary sliding rheology. This scaling can be used to determine the conditions required to initiate convection in the ice I shell of a generic icy satellite. We use this scaling to judge the convective instability of Ganymede and Callisto's ice shells in the absence of tidal dissipation.

Barr, A. C.↗
NASA NTRS2004

Combined Effects of Diurnal and Nonsynchronous Surface Stresses on Europa

To date, modeling of the surface stresses on Europa has considered tidal, nonsynchronous, and polar wander sources of stress. The results of such models can be used to match lineament orientations with candidate stress patterns. We present a rigorous surface stress model for Europa that will facilitate comparison of principal stresses to lineament orientation, and which will be available in the public domain. Nonsynchronous rotation and diurnal motion contribute to a stress pattern that deforms the surface of Europa. Over the 85-hour orbital period, the diurnal stress pattern acts on the surface, with a maximum magnitude of approximately 0.1 MPa. The nonsynchronous stress pattern sweeps over the surface due to differential rotation of the icy shell relative to the tidally locked interior of the moon. Nonsynchronous stress builds cumulatively with approximately 0.1 MPa per degree of shell rotation.

Stempel, M. M.↗
NASA NTRS2004

SILVER: Surface Imaging for Lunar Volatiles, Resources, and Exploration

The Surface Imaging for Lunar Volatiles, Exploration, and Resources (SILVER) instrument is a proposed imaging investigation for the 2008 Lunar Reconnaissance Orbiter (LRO) mission. SILVER and its experienced Measurement Team will prepare for and support future lunar human exploration activities, especially landing site identification and certification on the basis of potential resources. SILVER combines a high-resolution pushbroom visible imaging channel (SILVER-HR) and a wide-field-of-view (45 deg) framing imaging channel (SILVER-WF). SILVER-HR will obtain a single-detector 6 km imaging swath of 12,228 pixels at 0.5 m/pixel to image greater than 100 sq km target areas from 50 km altitude, imaging greater than 15% the lunar surface during a 1 year nominal mission. SILVER-HR has excellent stray-light rejection and its imaging detector has selectable time delay integration (TDI) with up to 128 stages for extreme low-light sensitivity, permitting direct imaging of permanently shadowed polar regions in scattered sunlight or earthshine. SILVER-WF will obtain geodetic framing images in a 2048 x 2048 format at 20m/pixel, with 60% along-track overlap stereo for imaging context and for derivation of a global digital elevation model of meter-scale lunar topography.

Pappalardo, R. T.↗
NASA NTRS2004

Convective Instability in Ice I with Non-Newtonian Rheology: Application to the Galilean Satellites

At the temperatures and stresses associated with the onset of convection in an ice I shell of the Galilean satellites, ice behaves as a non-Newtonian fluid with a viscosity that depends on both temperature and strain rate. The convective stability of a non-Newtonian ice shell can be judged by comparing the Rayleigh number of the shell to a critical value. Previous studies suggest that the critical Rayleigh number for a non-Newtonian fluid depends on the initial conditions in the fluid layer, in addition to the thermal, rheological, and physical properties of the fluid. We seek to extend the existing definition of the critical Rayleigh number for a non-Newtonian, basally heated fluid by quantifying the conditions required to initiate convection in an ice I layer initially in conductive equilibrium. We find that the critical Rayleigh number for the onset of convection in ice I varies as a power (-0.6 to -0.5) of the amplitude of the initial temperature perturbation issued to the layer, when the amplitude of perturbation is less than the rheological temperature scale. For larger-amplitude perturbations, the critical Rayleigh number achieves a constant value. We characterize the critical Rayleigh number as a function of surface temperature of the satellite, melting temperature of ice, and rheological parameters so that our results may be extrapolated for use with other rheologies and for a generic large icy satellite. The values of critical Rayleigh number imply that triggering convection from a conductive equilibrium in a pure ice shell less than 100 km thick in Europa, Ganymede, or Callisto requires a large, localized temperature perturbation of a few kelvins to tens of kelvins to soften the ice and therefore may require tidal dissipation in the ice shell.

Barr, A. C.↗
NASA NTRS2004

Origin of Domes on Europa: The Role of Thermally Induced Compositional Buoyancy,

The surface of Jupiter's moon Europa is peppered by topographic domes, interpreted as sites of intrusion and extrusion. Diapirism is consistent with dome morphology, but thermal buoyancy alone cannot produce sufficient driving pressures to create the observed dome elevations. Instead, diapirs may initiate by thermal convection that induces compositional segregation. Exclusion of impurities from warm upwellings allows sufficient buoyancy for icy plumes to create the observed surface topography, provided the ice shell has a small effective elastic thickness (0.2 to 0.5 km) and contains low-eutectic point impurities at the few percent level. This model suggests that the ice shell may be depleted in impurities over time.

Pappalardo, R. T.↗
NASA NTRS2004

Geology of Europa

Europa is a rocky object of radius 1565 km (slightly smaller than Earth s moon) and has an outer shell of water composition estimated to be of order 100 km thick, the surface of which is frozen. The total volume of water is about 3 x 10(exp 9) cubic kilometers, or twice the amount of water on Earth. Moreover, like its neighbor Io, Europa experiences internal heating generated from tidal flexing during its eccentric orbit around Jupiter. This raises the possibility that some of the water beneath the icy crust is liquid. The proportion of rock to ice, the generation of internal heat, and the possibility of liquid water make Europa unique in the Solar System. In this chapter, we outline the sources of data available for Europa (with a focus on the Galileo mission), review previous and on-going research on its surface geology, discuss the astrobiological potential of Europa, and consider plans for future exploration.

Greeley, R.↗
NASA NTRS2004

Onset of Convection in Ice I with Composite Newtonian and Non-Newtonian Rheology: Application to the Icy Galilean Satellites

Ice I exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice I shells of Europa, Ganymede, and Callisto. We use numerical methods and existing parameterizations of the critical Rayleigh number to determine the conditions required to trigger convection in an ice I shell with the stress-, temperature- and grain size- dependent rheology measured in laboratory experiments by Goldsby and Kohlstedt [2001]. The critical Rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice I shell. If the shells have an assumed uniform grain size less than 0.4 mm, deformation during initial plume growth is accommodated by Newtonian volume diffusion. If the ice grain size is between 0.4 mm and 3 cm, deformation during plume growth is accommodated by weakly non-Newtonian grain boundary sliding, where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the _0.5 power. If the ice grain size exceeds 2 cm, convection can not occur in the ice I shells of the Galilean satellites regardless of the amplitude or wavelength of temperature perturbation. If the grain size in a convecting ice I shell evolves to effective values greater than 2 cm, convection will cease. If the ice shell has a grain size large enough to permit flow by dislocation creep, the ice is too stiff to permit convection, even in the thickest possible ice I shell. Consideration of the composite rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice I shells.

Barr, A. C.↗
NASA NTRS2004

Convection in Icy Satellites: Implications for Habitability and Planetary Protection

Solid-state convection and endogenic resurfacing in the outer ice shells of the icy Galilean satellites (especially Europa) may contribute to the habitability of their internal oceans and to the detectability of any biospheres by spacecraft. If convection occurs in an ice I layer, fluid motions are confined beneath a thick stagnant lid of cold, immobile ice that is too stiff to participate in convection. The thickness of the stagnant lid varies from 30 to 50% of the total thickness of the ice shell, depending on the grain size of ice. Upward convective motions deliver approximately 10(exp 9) to 10(exp 13) kg yr(sup -1) of ice to the base of the stagnant lid, where resurfacing events driven by compositional or tidal effects (such as the formation of domes or ridges on Europa, or formation of grooved terrain on Ganymede) may deliver materials from the stagnant lid onto the surface. Conversely, downward convective motions deliver the same mass of ice from the base of the stagnant lid to the bottom of the satellites ice shells. Materials from the satellites surfaces may be delivered to their oceans by downward convective motions if material from the surface can reach the base of the stagnant lid during resurfacing events. Triggering convection from an initially conductive ice shell requires modest amplitude (a few to tens of kelvins) temperature anomalies to soften the ice to permit convection, which may require tidal heating. Therefore, tidal heating, compositional buoyancy, and solid-state convection in combination may be required to permit mass transport between the surfaces and oceans of icy satellites. Callisto and probably Ganymede have thick stagnant lids with geologically inactive surfaces today, so forward contamination of their surfaces is not a significant issue. Active convection and breaching of the stagnant lid is a possibility on Europa today, so is of relevance to planetary protection policy.

Barr, A. C.↗