the competition between thermal contraction and differentiation in the stress history of the moon the scarcity of both extension and compression features on the moon strongly constrains the history of the lunar radius to variations of less than 1 km over the past 3 8 gyr this limit has traditionally been interpreted as requiring a delicate balance between thermal contraction of the near surface and expansion of a substantial cold interior region recent theories of lunar origin e g giant impact in contrast favor a hot initial state we propose that a reconciliation may be possible by taking account of the volume change v v d due to differentiation we calculate stp densities based on simplified normative mineralogies for a suite of estimates of the bulk lunar composition of primary lunar basalt and of the residuum left when the maximum amount of the latter is extracted from the former typically v v d 2 to 5 an expansion equivalent to heating by 10 3 k provided the timing of differentiation is correct one might offset the cooling of a magma ocean as much as 630 km deep by differentiation of the remainder of the moon which need not start much below the solidus temperature a large but not impossible amount of gabbroic melt production is implied 100 times the volume of mare basalts known to have been extruded we do not address the detailed genetic relationship of this melt to the basalts observed on the lunar surface but point out that it need not have reached the surface directly or even have entered the crust in order for the expansion to have occurred to assess the timing of melt formation we investigate a simple conductive lunar thermal model which takes account of both v v d and thermal contraction our initial state is characterized by a central temperature t c and a depth z 0 above which the material derived from the magma ocean is already at the solidus and is not suceptible to volume changes upon further differentiation we find a range of models satisfying the limits on radius increase and decrease the hottest has t c 1210 k z 0 400 km without v v d we would need a larger or colder or both core e g t c 700 k for z0 200 400 km in agreement with previous investigators our modeling thus lends credence to the idea that the moon could have been initially 50 molten with the remainder relatively close to the solidus and yet experienced little volume change over the last 3 8 gyr

constitution of the moon 2 composition and seismic properties of the lower mantle abstract to constrain the composition of the moon we have calculated the stable phase assemblages and their seismic properties by the method of minimization of gibbs free energy to determine whether various compositional models are consistent with the seismic observations for the lunar lower mantle at depths between 500 and 1000 km p wave and s wave velocities have been calculated for a series of proposed bulk composition models compositional models for the moon impactor composition orgueil composition etc and earth like compositions lherzolite and pyrolite have been considered it is found that impactor and orgueil models are unlikely for the lower mantle within reasonable temperature adjustment in the temperature profile all of the terrestrial compositional models come close to the seismic data below the 500 km discontinuity it is found that a composition enriched in al 2 o 3 up to about 6 wt and having pyrolite content of the other major oxides lunar pyrolite is adequate to describe the seismic properties of the lunar lower mantle within the uncertainty of calculations and resolution of seismic data an al rich model with a terrestrial ol px ratio and unusually enriched in garnet gives the best fit to the lunar lower mantle seismic properties despite the uncertainty of the seismic data it is shown that none of the analysed compositional models satisfies the seismic constraints simultaneously in the middle and lower mantle phase changes in any model considered are not able to explain the nature of the 500 km discontinuity it is concluded that the lunar mantle is chemically stratified

a mechanism for the generation of the lunar mare basalts summary according to the magma ocean or magmasphere model of lunar formation rapid accretion would result in the formation of a solid primitive core of undifferentiated material surrounded by a spherical shell of molten silicate as the outer layers of the moon cool by radiation of heat to space the deep interior will warm due to the decay of trace radionuclides this will lead to compression of the remnant magmasphere producing excess pressure in the fluid magma it is proposed that relief of this excess pressure through vents opened by the excavation of the mascon basins was sufficient to drive the major episodes of mare volcanism between 3 9 3 2 ga thermal models of the moon are constructed and a range of initial conditions based on the magmasphere model are identified which satisfy known constraints on lunar evolution the rate at which radial thermoelastic stress accumulates within the moon is calculated and from this estimates of the maximum volume of basaltic magma which could be produced by this mechanism are obtained by comparing these calculated magma generation rates with estimates of the volume of mare basalts residing on the lunar surface we are able to identify a set of initial temperature profiles which both satisfy the known constraints on lunar evolution and are able to produce the observed volume of mare basalts these models are characterised by deep magmaspheres and or initially warm central temperatures

retracted middle miocene to early pliocene contraction in the chos malal fold and thrust belt neuquen basin argentina insights from structural analysis and apatite fission tracks thermochronology neogene contraction in the chos malal ftb

the ferroan anorthositic suite the extent of primordial lunar melting and the bulk composition of the moon the geochemical bimodality among compositionally pristine lunar rocks is strong evidence that a major component of the moon s crust is fundamentally unrelated to the plagioclase flotation crust that accumulated over the primordial magma ocean the ferroan anorthositic suite fas has all the characteristics expected for a series of magma ocean plagioclase flotation cumulates but we estimate based on mass balance for al2o3 that 30 wt of the crust and 20 wt of the crust s total al2o3 or roughly 15 of the bulk moon al2o3 content assumed to be 3 4 wt was emplaced by postmagma ocean mg suite serial magmatism remelting of plagioclase bearing magma ocean cumulates cannot plausibly account for the late emplacement of al2o3 because the only plagioclase rich cumulates produced by a deep ultramafic magma ocean would have mg ratios too low to engender the mg suite conceivably remelting of garnet rich magma ocean cumulates supplied the late al2o3 but only if the bulk moon al2o3 content is substantially higher than generally assumed high pressures in the magma ocean acted to enhance the py pyroxene pyroxene olivine ratio of the mo cumulates and the al2o3 content of whatever pyroxene formed remelting of pyroxene rich mo cumulates could conceivably have supplied the late mg suite al2o3 to the crust the source region would have to encompass roughly 40 wt of the mantle another possible source for late al2o3 addition is a large portion of the deepest mantle that avoided more than a slight degree of partial melting and thus remained al rich during formation of the magma ocean the source region would have to encompass 20 30 wt of the mantle ascent of diapiric masses from the lowermost mantle would likely coincide with trigger or be triggered by a large scale overturn of the gravitationally unstable mo cumulate pile with concomitant pressure release melting in which case the mg suite parental melts would be derived from a mix of deepest mo cumulates and the even deeper primitive mantle the popular giant impact hypothesis of lunar origin implies an extremely hot beginning for the moon suggesting that the deep interior was either molten or depleted in al2o3 by differentiation in hot protomoons as it formed origin of 30 wt of the moon s crust by postmagma ocean serial magmatism is more consistent with coaccretion models that form the moon without a giant hot impact in any scenario with a large fraction of the crust originating by partial remelting of deep magma ocean cumulates mass balance for al2o3 implies that the bulk moon normative py ratio must be greater than roughly 0 5 i e significantly higher than generally estimated in this respect the moon appears more compositionally similar to chondritic silicates than to the earth s upper mantle

geochemical constraints on the seismic properties of the lunar mantle abstract thermodynamic modeling of phase relations and physical properties in the na2o tio2 cao feo mgo al2o3 sio2 fe fes system and a monte carlo inversion procedure have been used to calculate the density and velocity distribution in the mantle and core radii of the moon from the mass and moment of inertia constraints from recent lunar prospector gravity measurements for the computation of the phase diagram for a given chemical composition we have used a method of minimization of the total gibbs free energy and self consistent thermodynamic data for minerals and solid solutions for the crustal thickness and density of 60 km and 3 0 g cm 3 the largest core radii are estimated to be 530 km for an fes troilite core 4 1 of total mass 490 km for a eutectic fe fes core 3 4 445 km for an fe 10 wt s core 2 9 and 350 km for an fe core 2 the existence of an fe fes core of intermediate radius of 250 400 km is compatible with recent lunar core information and lend support to a chemically stratified lunar mantle the sensitivity of density and seismic velocities to temperature pressure and composition variations is examined the modeling results indicate that we can determine the permissible density and velocity ranges in the zoned lunar mantle with confidence the maximum and minimum density changes at depths of the lunar mantle consistent with geochemical constraints and mass and moment of inertia requirements are found to be 3 22 g cm 3 3 44 g cm 3 at depths of 60 500 km and 3 34 g cm 3 3 52 g cm 3 at depths between 500 km and the core mantle boundary core free models of the moon are possible only if the lower mantle densities are between 3 49 and 3 52 g cm 3 20 wt feo wt the calculated field of seismic velocities in the zoned lunar mantle shows that over a broad range of mantle compositions temperatures and densities the maximum variations are as follows 4 23 km s 1 v s 4 69 km s 1 and 7 41 km s 1 v p 8 24 km s 1 at depths of 60 500 km and 4 24 km s 1 v s 4 64 km s 1 and 7 63 km s 1 v p 8 39 km s 1 at depths between 500 km and the core mantle boundary these petrologically and geophysically admissible ranges of velocities and densities may constitute a significant constraint for the analysis of internal structure of the moon using the apollo seismic data

contraction or expansion of the moon s crust during magma ocean freezing the lack of contraction features on the moon has been used to argue that the moon underwent limited secular cooling and thus had a relatively cool initial state a cool early state in turn limits the depth of the lunar magma ocean recent grail gravity measurements however suggest that dikes were emplaced in the lower crust requiring global lunar expansion starting from the magma ocean state we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently the most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle

thermo chemical evolution and global contraction of mercury the very limited amount of global contraction observed on mercury s surface poses severe constraints on models of the planet s thermo chemical evolution and current models rely on a very refractory thorium rich composition to slow planetary cooling however a refractory composition appears to be incompatible with evidence for pyroclastic eruptions which require a substantial amount of volatiles to be present in the planetary interior furthermore volcanic activity appears to have been ongoing for a considerable part of the planet s history while current models predict an early cessation of crustal production to address these inconsistencies we have reinvestigated the thermo chemical evolution of mercury using a non refractory compositional model taking the presence of a thermally insulating regolith layer into account we find that models with a stiff mantle rheology satisfy the observational constraints if the regolith layer is at least 2 km thick in these models inefficient mantle convection and thermal insulation significantly slow planetary cooling and prolong the phase of crustal production to 2 5 gyr after core formation allowing the volume increase associated with mantle differentiation to offset some of the radial contraction caused by planetary cooling models furthermore predict substantial core sulfur contents above 6 wt average crustal thicknesses between 10 and 40 km and secular cooling rates of 30 k gyr

petrogenesis of picritic mare magmas constraints on the extent of early lunar differentiation calculations of isobaric batch polybaric batch and polybaric fractional melting have been carried out on a variety of proposed lunar and terrestrial source region compositions results show that magmas with a generally tholeiitic character plagioclase and high ca pyroxene crystallize before low ca pyroxene reflecting relatively high al2o3 concentrations 12 wt are the inevitable consequence of anhydrous partial melting of source regions composed primarily of olivine and two pyroxenes with an aluminous phase on the solidus low al2o3 magmas 10 wt as typified by the green picritic glasses in the lunar maria require deep 700 1000 km low al2o3 source regions without an aluminous phase the difference between primitive and depleted mantle beneath mid ocean ridges amounts to less than 0 1 wt al2o3 whereas formation of the green glass source region requires a net loss of between 1 5 and 2 5 wt al2o3 basalt extraction cannot account for fractionations of this magnitude accumulation of olivine and pyroxene at the base of a crystallizing magma ocean is however an effective method for producing the necessary al2o3 depletions both olivine rich and pyroxene rich source regions can produce the picritic magmas but mixing calculations show that both types of source region are likely to be hybrids consisting of an early to intermediate stage cumulate olivine plus enstatite and a later stage cumulate assemblage mass balance calculations show that refractory element enriched bulk moon compositions contain too much al2o3 to allow for the deep low al2o3 source regions even after extraction of an al2o3 rich 26 30 wt crust between 50 and 70 km thick

the al 2 o 3 contents of enstatite in equilibrium with garnet in the system mgo al 2 o 3 sio 2 at 15 40 kbar and 900 1 600 c forty six reversed determinations of the al2o3content of enstatite in equilibrium with garnet were made in the p t range 15 40 kbar 900 1 600 c in the mgo al2o3 sio2 system starting materials were mixtures of synthetic pyrope al free enstatite and pyrope enstatite 5 12 al2o3 al2o3 contents in reversal run pairs closely approached common values from both the high and low al sides most experiments were done in a piston cylinder device using a nacl medium some runs at very high temperatures were made in pyrex nacl or pyrex talc assemblies the measured enstatite compositions expressed as mole fractions of mg2 mgal alsi3 o12 x opy en were fitted by a monte carlo method to the equilibrium condition begin gathered delta h 970 0 970 delta s 970 0 hfill mathop smallint limits 1 p delta v 970 0 dp mathop smallint limits 970 t delta s t 0 dt rt ln x opy en 0 hfill end gathered where the best fit parameters of h s and v 1 bar 970 k for the reaction pyrope opy are 2 040 cal mol 2 12 eu and 9 55 cc mol in addition to the determination of al2o3 contents of enstatite the univariant reaction pyrope forsterite enstatite spinel was reversibly located in the range 1 100 1 400 c a best fit line passes through 22 22 5 and 25 kbar at 1 040 1 255 and 1 415 c respectively our results for the univariant reaction are in agreement with previous studies of macgregor 1974 and haselton 1979 however comparison of the experimentally determined curve with thermochemical calculations suggests that there may be a small error in the tabulated h f 970 1 0 value for enstatite a value of 8 32 rather than 8 81 kcal mole charlu et al 1975 is consistent with the present data application of garnet enstatite spinel forsterite equilibria to natural materials is fraught with difficulties the effects of nonternary components are poorly understood and the low solubilities of al2o3 in enstatite under most geologically reasonable conditions make barometric or thermometric calculations highly sensitive more detailed studies including reversed determinations in low friction assemblies are sorely needed before the effects of important diluents such as fe ca and cr can be fully understood

the procellarum kreep terrane implications for mare volcanism and lunar evolution geophysical remote sensing and sample data demonstrate that the procellarum and imbrium regions of the moon make up a unique geochemical crustal province here dubbed the procellarum kreep terrane geochemical studies of imbrium s ejecta and the crustal structure of the imbrium and serenitatis basins both suggest that a large portion of the lunar crust in this locale is composed of a material similar in composition to apollo 15 kreep basalt kreep basalt has about 300 times more uranium and thorium than chondrites so this implies that a large portion of moon s heat producing elements is located within this single crustal province the spatial distribution of mare volcanism closely parallels the confines of the procellarum kreep terrane and this suggests a causal relationship between the two phenomena we have modeled the moon s thermal evolution using a simple thermal conduction model and show that as a result of the high abundance of heat producing elements that are found in the procellarum kreep terrane partial melting of the underlying mantle is an inevitable outcome specifically by placing a 10 km kreep basalt layer at the base of the crust there our model predicts that mare volcanism should span most of the moon s history and that the depth of melting should increase with time to a maximum depth of about 600 km we suggest that the 500 km seismic discontinuity that is observed in the apollo seismic data may represent this maximum depth of melting our model also predicts that the kreep basalt layer should remain partially molten for a few billion years thus the imbrium impact event most likely excavated into a partially molten kreep basalt magma chamber we postulate that the kreep basalt composition is a by product of mixing urkreep with shallow partial melts of the underlying mantle since mg suite rocks are likely derived from crystallizing kreep basalt the provenance of these plutonic rocks is likely to be unique to this region of the moon

fault dislocation modeled structure of lobate scarps from lunar reconnaissance orbiter camera digital terrain models 1 before the launch of the lunar reconnaissance orbiter known characteristics of lobate scarps on the moon were limited to studies of only a few dozen scarps revealed in apollo era photographs within 20 of the equator the lunar reconnaissance orbiter camera now provides meter scale images of more than 100 lobate scarps as well as stereo derived topography of about a dozen scarps high resolution digital terrain models dtms provide unprecedented insight into scarp morphology and dimensions here we analyze images and dtms of the slipher racah x 1 mandel shtam 1 feoktistov simpelius 1 and oppenheimer f lobate scarps parameters in fault dislocation models are iteratively varied to provide best fits to dtm topographic profiles to test previous interpretations that the observed landforms are the result of shallow low angle thrust faults results suggest that these faults occur from the surface down to depths of hundreds of meters have dip angles of 35 40 and have typical maximum slips of tens of meters these lunar scarp models are comparable to modeled geometries of lobate scarps on mercury mars and asteroid 433 eros but are shallower and 10 steeper than geometries determined in studies with limited apollo era data frictional and rock mass strength criteria constrain the state of global differential stress between 3 5 and 18 6 mpa at the modeled maximum depths of faulting our results are consistent with thermal history models that predict relatively small compressional stresses that likely arise from cooling of a magma ocean

lunar cryptomaria physical characteristics distribution and implications for ancient volcanism cryptomaria lunar volcanic deposits obscured by crater and basin impact ejecta can provide important information about the thermal and volcanic history of the moon the timing of cryptomare deposition has implications for the duration and flux of mare basalt volcanism in addition knowing the distribution of cryptomaria can provide information about mantle convection and lunar magma ocean solidification here we use multiple datasets e g m3 lola lroc diviner to undertake a global analysis to identify the general characteristics e g topography surface roughness rock abundance albedo etc of lunar light plains in order to better distinguish between ancient volcanic deposits cryptomaria and impact basin and crater ejecta deposits we find 20 discrete regions of cryptomaria covering approximately 2 of the moon which increase the total area covered by mare volcanism to 18 of the lunar surface comparisons of light plains deposits indicate that the two deposit types volcanic and impact produced are best distinguished by mineralogic data on the basis of cryptomaria locations the distribution of mare volcanism does not appear to have changed in the time prior to its exposed mare basalt distribution there are several hypotheses explaining the distribution of mare basalts which include the influence of crustal thickness mantle convection patterns asymmetric distribution of source regions kreep distribution and the influence of a proposed procellarum impact basin the paucity of farside mare basalts means that multiple factors such as crustal thickness variations and mantle convection are likely to play a role in mare basalt emplacement

influence of flow variability on floodplain formation and destruction little missouri river north dakota resolving observations of channel change into separate planimetric measurements of floodplain formation and destruction reveals distinct relations between these processes and the flow regime we analyzed a time sequence of eight bottomland images from 1939 to 2003 along the little missouri river north dakota to relate geomorphic floodplain change to flow along this largely unregulated river at the decadal scale floodplain formation and destruction varied independently destruction was strongly positively correlated with the magnitude of infrequent high flows that recur every 5 10 yr whereas floodplain formation was negatively correlated with the magnitude of frequent low flows exceeded 80 of the time at the century scale however a climatically induced decrease in peak flows has reduced the destruction rate limiting the area made available for floodplain formation the rate of destruction was not uniform across the floodplain younger surfaces were consistently destroyed at a higher rate than older surfaces suggesting that throughput of contaminants would have occurred more rapidly than predicted by models that assume uniform residence time of sediment across the floodplain maps of floodplain ages produced by analysis of sequential floodplain images are similar to maps of forest ages produced through dendrochronology confirming the assumption of dendrogeomorphic studies that riparian tree establishment in this system is limited to recent channel locations

investigation of newly discovered lobate scarps implications for the tectonic and thermal evolution of the moon abstract using observations of lunar scarps in apollo panoramic camera photos binder and gunga 1985 tested competing models for the initial thermal state of the moon i e whether it was initially completely molten or if the molten portion was limited to a global magma ocean binder and gunga 1985 favored the concept of an initially molten moon that had entered into a late stage epoch of global tectonism since the start of the lunar reconnaissance orbiter mission thousands of new small lobate scarps have been identified across the lunar surface with high resolution images from the lunar reconnaissance orbiter camera lroc as such we selected spatially random scarps and reevaluated the fault dynamical calculations presented by binder and gunga 1985 additionally we examined the geometry and properties of these fault scarps and place better constraints on the amount of scarp related crustal shortening we found that these low angle thrust faults 23 have an average relief of 40 m and average depths of 951 m using crater size frequency distribution csfd measurements we derived absolute model ages for the scarp surfaces proximal to the trace of the fault and found that the last slip event occurred in the last 132 ma along with young model ages lunar lobate scarps exhibit a youthful appearance with their crisp morphologies which is indicative of late stage horizontal shortening in conclusion interior secular cooling and tidal stresses cause global contraction of the moon

basaltic magmatism on the moon a perspective from volcanic picritic glass beads abstract it is widely accepted that basaltic magmas are products of partial fusion of periodotite within planetary mantles as such they provide valuable insights into the composition structure and processes of planetary interiors those compositions which approach primary melt compositions provide the most direct information about planetary interiors and serve as a starting point to understand basaltic evolution within the collection of lunar samples returned by the apollo and luna missions are homogeneous picritic glass beads of volcanic origin these picritic glasses are our closest approximations to primary magmas as such these glass beads provide a unique perspective concerning the origin of mare basalts the characteristics of the lunar interior and processes in the early differentiation of the moon we have obtained trace element data for these picritic glasses using sims techniques these data and literature isotopic and experimental data on the picritic glasses are placed within the framework of mare basaltic magmatism the volcanic glasses are very diverse in their trace element characteristics for example they have a wide range of ree pattern shapes and concentrations like the crystalline mare basalts all picritic glasses have a negative eu anomaly unlike the crystalline mare basalts there is little correlation between the size of the eu anomaly and overall ree concentrations trace element differences among the various glasses suggests that a kreep component was incorporated into their mantle source this implies large scale mixing of the lunar magma ocean derived cumulate pile subtle differences among glasses suggest that local mixing of sources may also have been an important process preservation of subtle chemical differences in the picritic glasses and crystalline basalts may be interpreted as indicating that they were produced by small to moderate degrees of partial melting and that the lunar mantle did not experience extensive melting during episodes of mare volcanism several lines of evidence are consistent with the view that the picritic glasses were derived from mantle sources that were compositionally distinct from the sources for crystalline mare basalts these are parallel but no common liquid lines of descent chemical differences between picritic glasses and the more primitive crystalline mare basalts experimental studies indicating that the picritic glasses are multiply saturated at depths greater than that of the mare basalts differences in lead isotopic data and the mode of eruption i e fire fountaining for glass beads these data also provide circumstantial evidence that suggests that the picritic glasses were derived from a source somewhat more volatile rich than that of the mare basalts several petrogenetic models are suggested by the trace element characteristics of the picritic glasses 1 1 partial melting of heterogeneous lunar mantle at depths greater than 300 km to produce the parental magmas picritic for both the mare basalts and picritic glasses picritic magmas represented by glass beads were erupted to the surface with small degrees of fractional crystallization while mare basalts were produced by larger degrees of fractional crystallization 15 30 of similar but not identical picritic magmas 2 2 picritic magmas represented by the glass beads were generated at depths greater than 400 km in a volatile enriched relative to the mare basalt source heterogeneous mantle while mare basalts are fractional crystallization products of picritic magmas generated at depths of less than 400 km 3 3 the picritic magmas represented by the glass beads represent polybaric melting that initiated at depths of at least 1000 km a primitive mantle component or less processed cumulate mantle components may have been involved in the generation of the picritic glasses in any of these models

evidence for recent and ancient faulting at mare frigoris and implications for lunar tectonic evolution abstract nearside basin related extensional tectonism on the moon was thought to have ended by about 3 6 billion years ago and mare basin localized contractional deformation ended by about 1 2 billion years ago lunar reconnaissance orbiter camera lroc high resolution 50 200 cm pixel images show the moon s surface in unprecedented detail and have enabled us to find many previously unidentified tectonic landforms forcing a re assessment of our views of tectonism in the maria the morphology and stratigraphic relationships of these newly identified populations of tectonic landforms imply a more complex and longer lasting history of deformation we selected mare frigoris as an ideal location to perform a mapping survey where excellent imaging conditions abundant tectonic landforms and restricted mascon allow us to unravel the mare s tectonic evolution similar to previous surveys we find that tectonism in the eastern portion of mare frigoris was controlled by ancient mascon induced flexure in the western portion however we identify a parallel set of ancient compressional wrinkle ridges across the mare that is inconsistent with an origin by mare basin centric mascon flexure or influence from the mare imbrium mascon instead our results imply an ancient regional non isotropic stress field over western mare frigoris we also identify young wrinkle ridges and show that they have likely been active within the last 1 billion years and some ridges as recently as within 40 million years finally we identify a 300 km long series of lobate scarps coincident with one of the shallow moonquakes recorded during apollo and use geodetic strain from the mapped global population of young lobate scarps to predict a level of seismicity consistent with the shallow moonquakes recorded during apollo in tandem with similarly young lobate scarps and small graben as well as recorded shallow moonquakes these young wrinkle ridges imply that some tectonism in and around mare frigoris has occurred in the geologically recent past and likely still continues today

gas driven water volcanism and the resurfacing of europa we have analyzed the problem of how pathways are created for the resurfacing by water or volatiles for a europa model in which there is an ocean underlying a thin ice shell 10 km thickness open cracks propagating downward cannot reach the ocean unless the relative tension in the ice is both very high 100 bar and applied very rapidly 10 4 10 5sec there is no known way to satisfy these criteria water filled cracks propagating upward can approach the surface for relative tensions of 5 10 bar comparable to the stresses induced by the eccentricity tide exsolution of plausible gas species co 2 co ch 4 so 2 can then occur at the crack tip the application of linear elastic fracture mechanics shows that the gas filled portion of the crack pinches off from the water filled portion and may rise rapidly to the surface entraining some water many of these cracks may end up as intrusions rather than extrusions because of partial freeze out of co 2 and water on the walls of the crack the resulting eruption is initially gas dominated but may include a subsequent less extended eruption of foam there may not be a direct relationship between the resurfacing process described here and the currently known geologic features observed on the surface the main problem with this or any model for tapping the ocean is the propagation of cracks through the soft ice immediately overlying the ocean

remelting mechanisms for shallow source regions of mare basalts abstract two mechanisms are studied which could induce remelting at shallow depths within the moon a radioactive heating model in which remelting is the result of lateral variations of the heat source concentration emplaced by the end of the initial differentiation event and a thermal insulation model in which remelting is induced by lateral variations of near surface conductivity as a result of a high porosity ejecta blanket created by basin forming impacts the effects of these perturbations are studied by evaluating the thermal evolution of an otherwise radially symmetric lunar model the radioactive heating models predict extensive pre basin volcanism which would probably remove from the source region many of the heat producing elements potentially responsible for melting therefore localized high concentrations of heat producing elements emplaced early would most probably not be heat sources for the basin filling mare basalts however they are plausible heat sources for pre basin mare basalts the thermal insulation model predicts a delay of about 200 m y between impact and volcanism thermal insulation may also allow for the persistence of source regions for young basalts at shallow depths if greater basin ages are correct from 4 2 to 3 9 ga and most volcanism occurred between 3 8 and 3 6 ga then the thermal insulation model may explain the delay between impact and basin filling if the basins formed during a terminal cataclysmic event at about 3 9 ga then the earliest stages of basin filling at about 3 8 ga may have been a result of melting beneath the basin as a result of mantle uplift basin filling may have been characterized by a migration of the source region from beneath the basin to beneath the adjacent highlands cooling of the top 200 km beneath the basin after 200 m y because of its high surface conductivity is consistent with the mascon support requirement

channel slope reversal near the martian dichotomy boundary testing tectonic hypotheses abstract faults along the martian dichotomy boundary are evidence of tectonic activity which by analogy with terrestrial tectonism may cause changes in or even reversal of fluvial longitudinal profiles in the eastern hemisphere between 30 e and 150 e this tectonic activity has been hypothesized to result from lower crustal flow or from lithospheric flexure for which loading e g by material deposition of the northern lowlands is a possible cause watters 2003a nimmo 2005 the topographic slope changes resulting from these two different mechanisms are distinct and can provide a means for distinguishing between them although other processes may complicate interpretations of reversed longitudinal profiles two fan shaped networks of inverted channels are located near 150 e in the aeolis dorsa region just north of the dichotomy boundary their original flow directions inferred from planform morphology suggest flow to the northeast in contrast to their current longitudinal profiles sloping down to the southwest this contrast indicates slope reversal we investigate the lower crustal flow and flexure mechanisms for slope reversal by testing three different hypotheses 1 lower crustal flow 2 flexure caused by material erosion from the highlands and deposition in the lowlands and 3 flexure caused by highland erosion and deposition in the lowlands plus deposition of the medusae fossae formation in the lowlands we test these three hypotheses by comparing the inferred magnitudes of the slope reversals with predicted slope changes from geophysical models for these processes taking the possibility of non tectonic i e collapse processes into account our results suggest that among these three models the slope reversal is most consistent with the predicted tectonic response to erosion and deposition of highland material in conjunction with deposition of the medusae fossae formation contrary to previous findings our results do not support the mechanism of lateral crustal flow although they do not rule it out because lower crustal flow and erosion may both have been operating but on different time scales the result of this hypothesis testing provides insight into the evolution of the dichotomy boundary and martian crust in this location

geophysical constraints on lunar bulk composition and structure a reassessment theoretical lunar mantle seismic velocity and density profiles are calculated for a series of proposed bulk compositions present day selenotherms and models for early differentiation and evolution the seismic velocity profiles are compared to the most recent observational model derived from the complete apollo data set to evaluate compositional implications the mantle density profiles are compared to mean density moment of inertia and crustal density thickness constraints to evaluate the need for a metallic core the calculated seismic velocity profiles are in qualitative agreement with the observational model in two general respects 1 a quasicontinuous velocity decrease occurs in the upper mantle due to the large thermal gradients that characterize all three of the considered thermal models 2 a discontinuous velocity increase occurs at the spinel to garnet transition approximately 500 km depth due mainly to the higher garnet velocities as compared to olivine and orthopyroxene in the upper mantle all four of the considered bulk compositions spanning a range of plausible feo and al2o3 contents are allowed when uncertainties in the temperature profile and depth of differentiation are taken into account however in the middle mantle depths 500 km the most aluminous models those with larger garnet abundances consistently produce larger velocity increases that are most nearly in accord with the observational model in general models that assume differentiation of the upper mantle only that are more aluminous in bulk composition and that have strong thermal gradients in the upper mantle but smaller gradients in the middle mantle result in calculated velocity profiles that most nearly agree with the observational velocity model an alternate differentiation model in which the middle mantle is dominated by mgo rich mafic silicates is also in accord with the observational seismic velocity model but results in density decreases that appear to violate stability arguments in every case calculated mantle density increases are insufficient to match the one standard deviation upper limit on the moment of inertia even when the most favorable combinations of allowed crustal thickness and density are assumed agreement with the moment of inertia upper limit is obtained if metallic cores representing 1 4 of the lunar mass are added to the bulk composition such cores are larger than needed to explain the observed depletions of lunar siderophile elements if the moon formed entirely from terrestrial mantle material

a 3 d numerical study of the thermal evolution of the moon after cumulate mantle overturn the importance of rheology and core solidification 1 models in which the mantle of the moon evolves from an initially stratified state following magma ocean solidification and overturn have been applied to address important features of long term thermal evolution of the moon including convective instability of overturned ilmenite bearing cumulates ibc at the lunar core mantle boundary generation of mare basalts core sulfur content and inner core radius paleomagnetism and the present day mantle structure whether a dense overturned ibc rich layer at the bottom of the mantle can become thermally unstable to generate a single upwelling is controlled largely by the temperature dependence of viscosity the activation energy convective instability of the ibc rich layer controls the heat flux out the core and the presence of an internally generated magnetic field a long period of 700 ma high positive core mantle boundary cmb heat flux after the instability of the ibc rich layer is expected from our models present day deep mantle temperatures inferred from seismic and gravitational inversion constrain the magnitude of mantle viscosity from 5 1019 to 1 1021 pa s the cmb temperature and solidified inner core radius inferred from seismic reflection constrain the core sulfur content our evolution models with 5 10 wt sulfur content can produce the observed 240 km radius inner core at the present day the asymmetrical distribution of the deep moonquakes only in the nearside mantle could be explained as the remnant structure of the single chemical upwelling generated from ibc rich layer our evolution model after the overturn results in an early 0 55 km expansion in radius for 1000 ma due to the radiogenic heating associated with ibc in the deep mantle and may provide a simple explanation for the early expansion inferred from the gravity recovery and interior laboratory mission

seismic velocity structure of the lunar mantle i have inverted the recently completed set of seismic arrival times from the apollo lunar seismic network to estimate the average seismic velocities in three sections of the lunar mantle two for the upper mantle and one for the middle mantle the method used is a variation of the linearized least squares inversion where the inversion is accomplished in steps the estimated average velocities in the upper mantle decrease from vp 7 74 km s and vs 4 49 km s in the section above 270 km depth to vp 7 46 km s and vs 4 25 km s in the section between 270 and 500 km depth confirming the earlier finding of negative gradients based on seismic amplitude variations the average velocities in the middle mantle between the depths of 500 km and 1000 km of vp 8 26 km s and vs 4 65 km s are significantly higher than those in the upper mantle contradicting earlier estimates based on more limited data the higher velocities may suggest initial melting of the moon down to at least 1000 km depth