Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism

The origin of the ancient martian crustal dichotomy and the massive magmatic province of Tharsis remains an open problem. Here, we explore numerically a hypothesis for the origin of these two features involving both exogenic and endogenic processes. We propose a giant impact event during the late stage of planetary formation as the source of the southern highland crust. In a second stage, the extraction of excess heat by vigorous mantle convection on the impacted hemisphere leads to massive magmatism, forming a distinct Tharsis-like volcanic region. By coupling short-term and long-term numerical simulations, we are able to investigate both the early formation as well as the 4.5 Gyr evolution of the martian crust. We demonstrate numerically that this exogenic-endogenic hypothesis is in agreement with observational data from Mars.     

Ancient heat flow and crustal thickness at Warrego rise, Thaumasia highlands, Mars: Implications for a stratified crust

Heat flow calculations based on geological and/or geophysical indicators can help to constrain the thickness, and potentially the geochemical stratification, of the martian crust. Here we analyze the Warrego rise region, part of the ancient mountain range referred to as the Thaumasia highlands. This region has a crustal thickness much greater than the martian average, as well as estimations of the depth to the brittle-ductile transition beneath two scarps interpreted to be thrust faults. For the local crustal density (2900 kg m⁻³) favored by our analysis of the flexural state of compensation of the local topography, the crustal thickness is at least 70 and 75 km at the scarp locations. However, for one of the scarp locations our nominal model does not obtain heat flow solutions permitting a homogeneous crust as thick as required. Our results, therefore, suggest that the crust beneath the Warrego rise region is chemically stratified with a heat-producing enriched upper layer thinner than the whole crust. Moreover, if the mantle heat flow (at the time of scarp formation) was higher than 0.3 of the surface heat low, as predicted by thermal history models, then a stratified crust rise seems unavoidable for this region, even if local heat-producing element abundances lower than average or hydrostatic pore pressure are considered. This finding is consistent with a complex geological history, which includes magmatic-driven activity.     

Impact megadomes and the origin of the martian crustal dichotomy

We show that a sufficiently energetic impact can generate a melt volume which, after isostatic adjustment and differentiation, forms a spherical cap of crust with underlying depleted mantle. Depending on impact energy and initial crustal thickness, a basin may be retained or impact induced crust may be topographically elevated. Retention of a martian lowland scale impact basin at impact energies ∼3 × 10²⁸-3 × 10²⁹ J requires an initial crustal thickness greater than 10 km. Formation of impact induced crust with size comparable to the martian highlands requires a larger impact energy, ∼1-3 × 10³⁰ J, and initial crustal thickness <20 km. Furthermore, we show that the boundary of impact induced crust can be elliptical due to a spatially asymmetric impact melt volume caused by an oblique impact. We suggest the term “impact megadome” for topographically elevated, impact induced crust and propose that processes involved in megadome formation may play an important role in the origin of the martian crustal dichotomy.     

The thermal evolution of Mars as constrained by paleo-heat flows

Lithospheric strength can be used to estimate the heat flow at the time when a given region was deformed, allowing us to constrain the thermal evolution of a planetary body. In this sense, the high (>300 km) effective elastic thickness of the lithosphere deduced from the very limited deflection caused by the north polar cap of Mars indicates a low surface heat flow for this region at the present time, a finding difficult to reconcile with thermal history models. This has started a debate on the current heat flow of Mars and the implications for the thermal evolution of the planet. Here we perform refined estimates of paleo-heat flow for 22 martian regions of different periods and geological context, derived from the effective elastic thickness of the lithosphere or from faulting depth beneath large thrust faults, by considering regional radioactive element abundances and realistic thermal conductivities for the crust and mantle lithosphere. For the calculations based on the effective elastic thickness of the lithosphere we also consider the respective contributions of crust and mantle lithosphere to the total lithospheric strength. The obtained surface heat flows are in general lower than the equivalent radioactive heat production of Mars at the corresponding times, suggesting a limited contribution from secular cooling to the heat flow during the majority of the history of Mars. This is contrary to the predictions from the majority of thermal history models, but is consistent with evidence suggesting a currently fluid core, limited secular contraction for Mars, and recent extensive volcanism. Moreover, the interior of Mars could even have been heating up during part of the thermal history of the planet.     

The present-day thermal state of Mars

The present-day thermal state of the martian interior is a very important issue for understanding the internal evolution of the planet. Here, in order to obtain an improved upper limit for the heat flow at the north polar region, we use the lower limit of the effective elastic thickness of the lithosphere loaded by the north polar cap, crustal heat-producing elements (HPE) abundances based on martian geochemistry, and a temperature-dependent thermal conductivity for the upper mantle. We also perform similar calculations for the south polar region, although uncertainties in lithospheric flexure make the results less robust. Our results show that the present-day surface and sublithospheric heat flows cannot be higher than 19 and 12 mW m⁻², respectively, in the north polar region, and similar values might be representative of the south polar region (although with a somewhat higher surface heat flow due to the radioactive contribution from a thicker crust). These values, if representative of martian averages, do not necessarily imply sub-chondritic HPE bulk abundances for Mars (as previously suggested), since (1) chondritic composition models produce a present-day total heat power equivalent to an average surface heat flow of 14-22 mW m⁻² and (2) some convective models obtain similar heat flows for the present time. Regions of low heat flow may even have existed during the last billions of years, in accordance with several surface heat flow estimates of ∼20 mW m⁻² or less for terrains loaded during Hesperian or Amazonian times. On the other hand, there are some evidences suggesting the current existence of regions of enhanced heat flow, and therefore average heat flows could be higher than those obtained for the north (and maybe the south) polar region.     

Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO₂) environment in the presence of a strong (?10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.     

The influence of mantle melting on the evolution of Mars

We present a parameterized convection model of Mars by incorporating a new heat-flow scaling law for stagnant-lid convection, to better understand how the evolution of Mars may be affected by mantle melting. Melting in the mantle during convection leads to the formation of a compositionally buoyant lithosphere, which may also be intrinsically more viscous by dehydration. The consequences of these melting effects on the evolution of terrestrial planets have not been explored before. The temporal evolution of crust and lithospheric mantle is modeled in a self-consistent manner considering mantle melting, convective instability, and the rewetting of dehydrated lithosphere from below by hydrogen diffusion. Though the effect of compositional buoyancy turns out to be minimal, the introduction of viscosity contrast between wet and dry mantle can considerably slow mantle cooling and sometimes lead to non-monotonic core cooling. Furthermore, with or without dehydration stiffening, our model predicts that the martian mantle must have been degassed more extensively (>80%) than previously suggested (<10%); the loss of such a large amount of water from the mantle to surface has significant implications about the role of water in the early surface and climate evolution of Mars.     

Viscosity of the Martian mantle and its initial temperature: Constraints from crust formation history and the evolution of the magnetic field

The rheology of the Martian mantle and the planet's initial temperature is constrained with thermal evolution models that include crust growth and test the conditions for magnetic field generation in the core. As observations we use the present-day average crustal thickness of 50-120 km as estimated from the Mars Global Surveyor gravity and topography data, the evidence for the crust being produced mostly early, with a rate declining from the Noachian to the Hesperian, and the evidence for an early magnetic field that likely existed for less than a billion years. We use the fact that the rate of crust growth is a function of temperature, which must be above the solidus in the sub-lithosphere mantle, and the mantle convection speed because the latter determines the rate at which melt can be replenished. The convection speed is a strong function of viscosity which, in turn, is a strong function of temperature and also of the water content of the mantle. We use a viscosity parameterization with a reference viscosity evaluated at 1600 K the value of which can be characteristic of either a dry or a wet mantle. We further consider the Fe-FeS phase diagram for the core and compare the core liquidus estimated for a sulphur content of 14% as suggested by the SNC meteorite compositions with the core temperatures calculated for our cooling models. Two data sets of the Fe-FeS eutectic temperature have been used that differ by about 200 K [Böhler, R., 1996. Fe-FeS eutectic temperatures at 620 kbar. Phys. Earth Planet. Inter. 96, 181-186; Fei, Y., Bertka, C.M., Finger, L.W., 1997. High-pressure iron-sulphur compound, Fe₃S₂, and melting relations in the Fe-FeS system. Science 275, 1621-1623] at Martian core-mantle boundary pressure and in the eutectic composition by 5 wt%. The differences in eutectic temperature and composition translate into a difference of about 400 K in liquidus temperature for 14 wt% sulphur.We find it premature to rule out specific mantle rheologies on the basis of the presently available crustal thickness and crust growth evidence. Rather a trade-off exists between the initial mantle temperature and the reference viscosity. Both a wet mantle rheology with a reference viscosity less than 10²⁰ Pas and a dry mantle rheology with a reference viscosity of 10²¹ Pas or more can be acceptable if initial mantle temperatures between roughly 1700 and 2000 K are allowed. To explain the magnetic field history, the differences in liquidus temperatures matter. For a liquidus temperature of about 1900 K at the Martian core-mantle boundary as calculated from the Böhler et al. eutectic, a dry mantle rheology can best explain the lack of a present-day dynamo. For a liquidus temperature of about 1500 K at the core-mantle boundary as calculated from the Fei et al. eutectic all models are consistent with the observed lack of dynamo action. The reason lies with the fact that at 14 wt% S the Martian core would be close to the eutectic composition if the Fei et al. data are correct. As inner core growth is unlikely for an almost eutectic core, the early field would have been generated by a thermally driven dynamo. Together with the measured strength of the Martian crustal magnetization this would prove the feasibility of a strong thermally driven dynamo.     

Evidence for Mg-rich carbonates on Mars from a 3.9 μm absorption feature

The origin and nature of the early atmosphere of Mars is still debated. The discovery of sulfate deposits on the surface, coupled with the evidence that there are not large abundances of carbonates detectable on Mars in the optically accessible part of the regolith, leaves open different paleoclimatic evolutionary pathways. Even if carbonates are responsible for the feature observed by TES and Mini-TES at 6.76 μm, alternative hypotheses suggest that it could be due to the presence of Hydrated Iron Sulfates (HIS). Carbonates can be discerned from HIS by investigating the spectral region in which a strong overtone carbonate band is present. The Planetary Fourier Spectrometer on board the Mars Express spacecraft has acquired several thousand martian spectra in the range 1.2-45 μm since January 2004, most of which show a weak absorption feature between 3.8 and 4 μm. A similar feature was observed previously from the Earth, but its origin could not be straightforwardly ascribed to surface materials, and specifically to carbonates. Here we show the surficial nature of this band that can be ascribed to carbonate mixed with the martian soil materials. The materials that best reproduce the detected feature are Mg-rich carbonates (huntite [CaMg₃(CO₃)₄] and/or magnesite [MgCO₃]). The presence of carbonates is demonstrated in both bright and dark martian regions. An evaluation of the likeliest abundance gives an upper limit of ∼10 wt%. The widespread distribution of carbonates supports scenarios that suggest carbonate formation occurred not by precipitation in a water-rich environment but by weathering processes.     

A cold and wet Mars

Water on Mars has been explained by invoking controversial and mutually exclusive solutions based on warming the atmosphere with greenhouse gases (the “warm and wet” Mars) or on local thermal energy sources acting in a global freezing climate (the “cold and dry” Mars). Both have critical limitations and none has been definitively accepted as a compelling explanation for the presence of liquid water on Mars. Here is considered the hypothesis that cold, saline and acidic liquid solutions have been stable on the sub-zero surface of Mars for relatively extended periods of time, completing a hydrogeological cycle in a water-enriched but cold planet. Computer simulations have been developed to analyze the evaporation processes of a hypothetical martian fluid with a composition resulting from the acid weathering of basalt. This model is based on orbiter- and lander-observed surface mineralogy of Mars, and is consistent with the sequence and time of deposition of the different mineralogical units. The hydrological cycle would have been active only in periods of dense atmosphere, as having a minimum atmospheric pressure is essential for water to flow, and relatively high temperatures (over ∼245 K) are required to trigger evaporation and snowfall; minor episodes of limited liquid water on the surface could have occurred at lower temperatures (over ∼225 K). During times with a thin atmosphere and even lesser temperatures (under ∼225 K), only transient liquid water can potentially exist on most of the martian surface. Assuming that surface temperatures have always been maintained below 273 K, Mars can be considered a “cold and wet” planet for a substantial part of its geological history.     

A geomorphic analysis of Hale crater, Mars: The effects of impact into ice-rich crust

Hale crater, a 125 × 150 km impact crater located near the intersection of Uzboi Vallis and the northern rim of Argyre basin at 35.7°S, 323.6°E, is surrounded by channels that radiate from, incise, and transport material within Hale’s ejecta. The spatial and temporal relationship between the channels and Hale’s ejecta strongly suggests the impact event created or modified the channels and emplaced fluidized debris flow lobes over an extensive area (>200,000 km²). We estimate ∼10¹⁰ m³ of liquid water was required to form some of Hale’s smaller channels, a volume we propose was supplied by subsurface ice melted and mobilized by the Hale-forming impact. If 10% of the subsurface volume was ice, based on a conservative porosity estimate for the upper martian crust, 10¹² m³ of liquid water could have been present in the ejecta. We determine a crater-retention age of 1 Ga inside the primary cavity, providing a minimum age for Hale and a time at which we propose the subsurface was volatile-rich. Hale crater demonstrates the important role impacts may play in supplying liquid water to the martian surface: they are capable of producing fluvially-modified terrains that may be analogous to some landforms of Noachian Mars.     

Towards self-consistent modeling of the martian dichotomy: The influence of one-ridge convection on crustal thickness distribution

In order to find an explanation for the origin of the martian crustal dichotomy, a number of recent papers have examined the effect of layered viscosity on the evolution of a degree-1 mantle convection, e.g. Roberts and Zhong [Roberts, J.H., Zhong, S., 2006. J. Geophys. Res. 111. E06013] and Yoshida and Kageyama [Yoshida, M., Kageyama, A., 2006. J. Geophys. Res. 111, doi:10.1029/2005JB003905. B03412]. It was found that a mid-mantle viscosity jump, combined with highly temperature- and depth-dependent rheology, are effective in developing a degree-1 convection within a short timescale. Such a layered viscosity profile could be justified by martian mineralogy. However, the effect of a degree-1 convective planform on the crustal thickness distribution has not yet been demonstrated. It is not obvious whether a thinner crust, due to sublithospheric erosion and crustal thinning, or a thicker crust, due to enhanced crustal production, would form above the hemisphere of mantle upwelling. Also, the general shape of the dichotomy, which is not strictly hemispherical, has not yet been fully investigated. Here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection, using the finite-volume multigrid code StagYY [Tackley, P.J., 2008. Phys. Earth Planet. Int. 107, 7-18, doi:10.1016/j.pepi.2008.08.005] A self-consistent treatment of melting, crustal formation and chemical differentiation has been added to models of three-dimensional thermal convection. This allows us to obtain global maps of the crustal thickness distribution as it evolves with time. The obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree-1 mantle convection early in Mars' history. We find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy. In all models, the region of thick crust is located over the region of mantle upwelling, which itself is a ridge-like structure spread over roughly one half of the planet, a planform we describe as “one-ridge convection.”     

Geodesy constraints on the interior structure and composition of Mars

Knowledge of the interior structure of Mars is of fundamental importance to the understanding of its past and present state as well as its future evolution. The most prominent interior structure properties are the state of the core, solid or liquid, its radius, and its composition in terms of light elements, the thickness of the mantle, its composition, the presence of a lower mantle, and the density of the crust. In the absence of seismic sounding only geodesy data allow reliably constraining the deep interior of Mars. Those data are the mass, moment of inertia, and tides. They are related to Mars’ composition, to its internal mass distribution, and to its deformational response to principally the tidal forcing of the Sun. Here we use the most recent estimates of the moment of inertia and tidal Love number k₂ in order to infer knowledge about the interior structure of the Mars.We have built precise models of the interior structure of Mars that are parameterized by the crust density and thickness, the volume fractions of upper mantle mineral phases, the bulk mantle iron concentration, and the size and the sulfur concentration of the core. From the bulk mantle iron concentration and from the volume fractions of the upper mantle mineral phases, the depth dependent mineralogy is deduced by using experimentally determined phase diagrams. The thermoelastic properties at each depth inside the mantle are calculated by using equations of state. Since it is difficult to determine the temperature inside the mantle of Mars we here use two end-member temperature profiles that have been deduced from studies dedicated to the thermal evolution of Mars. We calculate the pressure and temperature dependent thermoelastic properties of the core constituents by using equations state and recent data about reference thermoelastic properties of liquid iron, liquid iron-sulfur, and solid iron. To determine the size of a possible inner core we use recent data on the melting temperature of iron-sulfur.Within our model assumptions the geodesy data imply that Mars has no solid inner core and that the liquid core contains a large fraction of sulfur. The absence of a solid inner is in agreement with the absence of a global magnetic field. We estimate the radius of the core to be 1794 ± 65 km and its core sulfur concentration to be 16 ± 2 wt%. We also show that it is possible for Mars to have a thin layer of perovskite at the bottom of the mantle if it has a hot mantle temperature. Moreover a chondritic Fe/Si ratio is shown to be consistent with the geodesy data, although significantly different value are also possible. Our results demonstrate that geodesy data alone, even if a mantle temperature is assumed, can almost not constrain the mineralogy of the mantle and the crust. In order to obtain stronger constraints on the mantle mineralogy bulk properties, like a fixed Fe/Si ratio, have to be assumed.     

Evidence for a differentiated crust in Solis Planum, Mars, from lithospheric strength and heat flow

Two independent sets of heat flow estimates provide constraints on the Hesperian-era surface and mantle heat flows, and the thickness of the heat-producing elements (HPE)-enriched upper crust, in the Solis Planum region of Mars. The calculations, which use the concentration of uppermost crust heat sources deduced from orbital gamma ray spectroscopy and soils geochemistry, are based on the effective elastic thickness of the lithosphere and the minimum depth of faults underlying winkle ridges. We find that, for the majority of analyzed settings, the HPE-enriched crust is thinner than the whole crust thickness in this region (∼65 km). Thus, our results strongly support a differentiated martian crust.     

Sorted clastic stripes, lobes and associated gullies in high-latitude craters on Mars: Landforms indicative of very recent, polycyclic ground-ice thaw and liquid flows

Self-organised patterns of stone stripes, polygons, circles and clastic solifluction lobes form by the sorting of clasts from fine-grained sediments in freeze-thaw cycles. We present new High Resolution Imaging Science Experiment (HiRISE) images of Mars which demonstrate that the slopes of high-latitude craters, including Heimdal crater - just 25 km east of the Phoenix Landing Site - are patterned by all of these landforms. The order of magnitude improvement in imaging data resolution afforded by HiRISE over previous datasets allows not only the reliable identification of these periglacial landforms but also shows that high-latitude fluviatile gullies both pre- and post-date periglacial patterned ground in several high-latitude settings on Mars. Because thaw is inherent to the sorting processes that create these periglacial landforms, and from the association of this landform assemblage with fluviatile gullies, we infer the action of liquid water in a fluvio-periglacial context. We conclude that these observations are evidence of the protracted, widespread action of thaw liquids on and within the martian regolith. Moreover, the size frequency statistics of superposed impact craters demonstrate that this freeze-thaw environment is, at least in Heimdal crater, less than a few million years old. Although the current martian climate does not favour prolonged thaw of water ice, observations of possible liquid droplets on the strut of the Phoenix Lander may imply significant freezing point depression of liquids sourced in the regolith, probably driven by the presence of perchlorates in the soil. Because perchlorates have eutectic temperatures below 240 K and can remain liquid at temperatures far below the freezing point of water we speculate that freeze-thaw involving perchlorate brines provides an alternative “low-temperature” hypothesis to the freeze-thaw of more pure water ice and might drive significant geomorphological work in some areas of Mars. Considering the proximity of Heimdal crater to the Phoenix Landing Site, the presence of such hydrated minerals might therefore explain the landforms described here. If this is the case then the geographical distribution of martian freeze-thaw landforms might reflect relatively high temperatures (but still below 273 K) and the locally elevated concentration of salts in the regolith.     

Magnitude of global contraction on Mars from analysis of surface faults: Implications for martian thermal history

Faults provide a record of a planet’s crustal stress state and interior dynamics, including volumetric changes related to long-term cooling. Previous work has suggested that Mars experienced a pulse of large-scale global contraction during Hesperian time. Here we evaluate the evidence for martian global contraction using a recent compilation of thrust faults. Fault-related strains were calculated for wrinkle ridges and lobate scarps to provide lower and upper bounds, respectively, on the magnitude of global contraction from contractional structures observed on the surface of Mars. During the hypothesized pulse of global contraction, contractional strain of −0.007% to −0.13% is indicated by the structures, corresponding to decreases in planetary radius of 112 m to 2.24 km, respectively. By contrast, consideration of all recognized thrust faults regardless of age produces a globally averaged contractional strain of −0.011% to −0.22%, corresponding to a radius decrease of 188 m to 3.77 km since the Early Noachian. The amount of global contraction predicted by thermal models is larger than what is recorded by the faults at the surface, paralleling similar studies for Mercury and the Moon, which suggests that observations of fault populations at the surface may provide tighter bounds on planetary thermal evolution than models alone.     

Global distribution, structure, and solar wind control of low altitude current sheets at Mars

We present the results of the first systematic survey of current sheets encountered by Mars Global Surveyor in its ∼400 km mapping orbit. We utilize an automated procedure to identify over 10,000 current sheet crossings during the ∼8 year mapping mission. The majority of these lie on the nightside and in the polar regions, but we also observe over 1800 current sheets at solar zenith angle <60°. The distribution and orientation of current sheets and their dependence on solar wind drivers suggests that most magnetotail current sheets have a local induced magnetospheric origin caused by magnetic field draping. On the other hand, most current sheets observed on the day side likely result from solar wind discontinuities advected through the martian system. However, the clustering of low altitude dayside current sheet crossings around the perimeters of strongly magnetized crustal regions, and the smaller than expected rotations in the IMF draping direction, suggest that crustal magnetic fields may also play an indirect role in their formation. The apparent thicknesses of martian current sheets, and the characteristics of electrons observed in and around the current sheets, suggest one of two possibilities. Martian current sheets at low altitudes are either stationary, with thicknesses of a few hundred km and currents carried by low energy (<10 eV) electrons, or they move at tens of km/s, with thicknesses of a few thousand km and currents carried by ions.     

How rapidly did Mars accrete? Uncertainties in the Hf-W timing of core formation

Estimates for the martian core formation timescale based on the hafnium-tungsten (Hf-W) isotopic system have varied by almost an order of magnitude, because of uncertainties in the martian mantle Hf/W ratio. Here we argue that the Hf/W ratio is ∼4 but is uncertain by ∼25%, resulting in (instantaneous) martian core formation timescales ranging from 0 to 10 Myr; accordingly, Hf-W isotope observations currently have limited utility in distinguishing between scenarios in which Mars formed as a stranded embryo and scenarios in which Mars suffered a prolonged accretion history.     

A top-down origin for martian mantle plumes

The two main volcanic centers on Mars, Tharsis and Elysium, are often interpreted in terms of mantle plume hotspots, even though there are several problems with the plume hypothesis for Mars. We present results of 2D cylindrical shell numerical mantle convection experiments in which we try to ascertain whether flushing of the hot lower mantle could provide a mechanism for the generation of a small number of plume-like features, i.e., localized upwelling of hot material. In this scenario the formation of hot upwellings is driven from the top by cold downwellings rather than from a hot thermal boundary layer at the CMB. First we construct a range of Mars interior structure models consistent with observations in order to demonstrate that the presence of a thin lower mantle in the martian interior is a viable scenario. Then we use a series of numerical convection experiments to investigate the effects of solid-state phase transitions, different stratified and temperature-dependent viscosity models, and the presence of a thick southern hemisphere crust on the operation of such a mechanism. Our results show that it is possible to generate hot strong localized upwellings from top-down dynamics if the lithosphere is thin or actively involved in the convective pattern. The presence of a thick, immobile, insulating southern hemisphere crust reduces the number of upwellings, and the perovskite phase transition causes a focusing of the upwellings. Further experiments demonstrate that an initial 500 Myr phase of mobile lid is sufficient to start this process create an upwelling which is stable for billions of years.     

An Origin for the Linear Magnetic Anomalies on Mars through Accretion of Terranes: Implications for Dynamo Timing

The proposed existence of magnetic lineations in the Terra Cimmeria and Terra Sirenum regions of Mars was initially explained by Earth-like sea-floor spreading. Here we argue instead that these lineations could have been formed at a convergent plate margin through collision and accretion of terranes. A similar process produced banded magnetic anomalies, similar in geometry and even in size to those in Earth's North American Cordillera. Because only sparse and generally weak anomalies have been detected in the martian northern lowlands, which could constitute an analog to the terrestrial oceanic crust, it is possible that the magnetic field stopped its activity while crustal recycling was still active in Mars.