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Composition of terrestrial planets records planetary accretion, core-mantle and crust-mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 $times$ CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disks lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earths mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt. In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.
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The ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf-W chronology of Nedagolla indicates that this mixed NC-CC isotopic composition was established relatively late, more than 7 million years after Solar System formation. The mixed NC-CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re-equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit-and-run collision between two differentiated bodies, which also provides a possible pathway for Nedagollas extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiters growth.
It is known that the so-called problem of solar power pacemaker related to possible existence of some hidden but key mechanism of energy influence of the Sun on fundamental geophysical processes is one of the principal and puzzling problems of modern climatology. The tracks of this mechanism have been shown up in different problems of solar-terrestrial physics for a long time and, in particular, in climatology, where the solar-climate variability is stably observed. However, the mechanisms by which small changes in the Suns energy (solar irradiance or insolation) output during the solar cycle can cause change in the weather and climate are still unknown. We analyze possible causes of the solar-climate variability concentrating ones attention on the physical substantiation of strong correlation between the temporal variations of magnetic flux of the solar tachocline zone and the Earth magnetic field (Y-component). We propose an effective mechanism of solar dynamo-geodynamo connection which plays the role of the solar power pacemaker of the Earth global climate.
The impactor flux early in Mars history was much higher than today, so sedimentary sequences include many buried craters. In combination with models for the impactor flux, observations of the number of buried craters can constrain sedimentation rates. Using the frequency of crater-river interactions, we find net sedimentation rate lesssim 20-300 {mu}m/yr at Aeolis Dorsa. This sets a lower bound of 1-15 Myr on the total interval spanned by fluvial activity around the Noachian-Hesperian transition. We predict that Gale Craters mound (Aeolis Mons) took at least 10-100 Myr to accumulate, which is testable by the Mars Science Laboratory.
We analyze thermal emission spectra using the 2001 Mars Odyssey Thermal Emission Imaging System (THEMIS) and the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) to characterize grain size and mineralogical composition of dunes at Hargraves crater, Mars. Thermal inertia and bulk composition of the dunes were compared to inferred provenances from the thermal infrared response of surface constituent materials. We use a Markov Chain Monte Carlo (MCMC) technique to estimate the bulk amount of mineralogy contributed by each inferred provenance to the dune field composition. An average thermal inertia value of 238+/-17 Jm-2K-1s-0.5 was found for the dunes corresponding to a surface composed of an average effective grain size of ~391+/-172 um. This effective particle size suggests the presence of mostly medium sand-sized materials mixed with fine and coarse grain sands. The dunes are likely comprised of a weakly indurated surface mixed with unconsolidated materials. Compositional analysis specifies that the dunes are comprised of a mixture of feldspar, olivine, pyroxene, and relatively low bulk-silica content. Dune materials were likely derived from physical weathering, especially eolian erosion, predominantly from the crater ejecta unit at the crater, mixed with a small amount from the crater floor and crater rim and wall lithologies - indicating the dune materials were likely sourced locally.
The dynamics of Mars obliquity are believed to be chaotic, and the historical ~3.5 Gyr (late-Hesperian onward) obliquity probability density function (PDF) is high uncertain and cannot be inferred from direct simulation alone. Obliquity is also a strong control on post-Noachian Martian climate, enhancing the potential for equatorial ice/snow melting and runoff at high obliquities (> 40{deg}) and enhancing the potential for desiccation of deep aquifers at low obliquities (< 25{deg}). We developed a new technique using the orientations of elliptical craters to constrain the true late-Hesperian-onward obliquity PDF. To do so, we developed a forward model of the effect of obliquity on elliptic crater orientations using ensembles of simulated Mars impactors and ~3.5 Gyr-long Mars obliquity simulations. In our model, the inclinations and speeds of Mars crossing objects bias the preferred orientation of elliptic craters which are formed by low-angle impacts. Comparison of our simulation predictions with a validated database of elliptic crater orientations allowed us to invert for best-fitting obliquity history. We found that since the onset of the late-Hesperian, Mars mean obliquity was likely low, between ~10{deg} and ~30{deg}, and the fraction of time spent at high obliquities > 40{deg} was likely < 20%.
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