Ancient Volcanism Of Southern Highlands On Mars

Mars has a unique position in solar system exploration:many important planetary scientific questions can be answered by systematic and relatively short-period exploration. The main scientific objectives of the Mars exploration are to decipher the geological evolution of the planet, to understand the interaction of tectonics and climate, and to search for a habitable environment for life.41Mars exploration missions have been carried out:the first spacecraft flew by the red planet in1965, followed by orbiters, landers, and rovers on the surface. These systematic exploration missions have shown us many important characteristics of Mars, including the global topography, geological tectonic and process, mineralogical and elemental composition of the surface, distribution of near-surface water, remnant magnetic field, gravity field and crustal structure, and time-varying atmospheric composition. The study of martian geology and comparative planetary geology started late in China, and the foundation is weak. However, with the constant enhancement of China’s national strength, the country has developed a medium-and long-term deep space exploration plan that will continue to carry out a series of deep space exploration projects. With the success of the Chang’E lunar missions and the space station project, independent missions to Mars and other celestial bodies in the solar system will be launched in the next5-10years. Meanwhile, the construction of planetary science programs also continues to receive attention. Therefore, using the existing data to study the morphology, topography, chronology, mineralogy, and volcanology of Mars is not only a forefront research topic, but it also provides a necessary knowledge basis for China’s future exploration of Mars. It is in line with national strategic requirements and of scientific significance.Terrestrial planets have experienced similar formation and early evolution history (>3Ga), but their subsequent geological evolution differed greatly and resulted in variations of the surface morphology, composition, and internal structure. For example, the Earth’s tectonics have been so active that the average surface age is about100Ma, and the early (>2.5Ga in Archean and Proterozoic) volcanic magma records rarely survive. Mars has a large amount of volcanic features (volcanoes and lava plains), and it is the planet with both the best-preserved ancient volcanic features and the most prominent volcanic processes. It is very important to study the geomorphology, composition, and modification of these volcanic features to reveal the early thermal evolution and subsequent geologic processes. In addition, it is important to understand the relationship between terrestrial planets and comparative planetary studies.In this paper, we summarized the volcanic features on the surface of Mars, reported a new method and its application for the semi-quantitative determination of major rock-forming minerals, presented the morphology and chronology of newly identified ancient volcanoes on the southern highlands, showed a newly discovered dike in Thaumasia planum, and revealed the surface composition and thermophysical properties of lava plains in Syria-Thaumasia Block.We have developed a new method (LRISMA:Least Residual Iterative Spectral Mixture Analysis) to semi-quantitatively determine major rock forming minerals (feldspar, pyroxene, olivine, high-silica phases and quartz) with multispectral data. Sub-libraries of minerals, generated from a master library of minerals based on prior knowledge, are used to produce every possible mineral end-member combination to fit the target spectra. Mineral abundances that correspond to the least root-mean-square (RMS) errors (best fit) generally agree best with previous petrographic and hyperspectral studies, given the greatly reduced spectral range and resolution. The accuracy and reproducibility of LRISMA is～4-16%and～5-20%respectively, while the accuracy of petrographic and previous hyperspectral studies is～5-15%. LRISMA can be applied to semi-quantitatively characterize the bulk surface mineralogy of small-scale geologic features with high quality Thermal Emission Imaging System (THEMIS) spectral data (high surface temperature, low atmospheric opacity) with the ultimate goal of better understanding regional geologic processes.Volcanism plays an important role in the formation and thermal evolution of the crusts of all terrestrial planets. Martian volcanoes have been extensively studied, and it has been suggested that the volcanism on Mars that created the visible volcanic features was initiated in the Noachian (>3.7Ga) and continued to the Late Amazonian (<0.1Ga). However, styles of ancient volcanism, their links with the earliest volcanic constructions, and the thermal evolution of the planet are still not well understood. Here we show that numerous Early Noachian (>4.0Ga) volcanoes are preserved in the heavily cratered southern highlands. Most of these are central volcanoes with diameters ranging from50to100km and heights of2-3km. Most of them are spatially adjacent to and temporally continuous with the Tharsis and circum-Hellas volcanic provinces, suggesting that these two volcanic provinces have experienced more extensive and longer duration volcanism than previously thought. These edifices are heavily cut by radial channels, suggesting that an early phase of aqueous erosion initiated and ended prior to the emplacement of the encircling Hesperian lava fields.Tharsis is the most prominent volcanic province on Mars, yet the composition of its lava flows and the relationship of composition to the development of Tharsis are poorly known. Most of Tharsis is covered with air-fall dust, which inhibits spectroscopic determination of lava mineralogy. The Syria-Thaumasia block (STB) is a complex tectono-volcanic province closely related to the Tharsis bulge. The lava plains of STB have different emplacement ages, which provide an opportunity to examine whether magma composition changed with the evolution of Tharsis. In this study, we assessed the lava plains using Thermal Emission Spectrometer (TES) data. Using derived physical properties, we targeted dust-free regions from four different-aged surfaces. We determined the mineralogical composition by modeling the average TES surface spectrum from each of the four surfaces. All units have similar mineralogy, but the younger two units have more high-SiO2phases. We also identified long distance lava channels/tubes in this region for the first time using data of the THEMIS instrument, Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE). They provided an efficient mechanism for observing long-distance and widely distributed lava emplacement. The spatial distribution of wrinkle ridges indicates that lava emplacement in the lava plains units HNr (older ridged plains material) and Hr (younger ridged plains material) happened before the rise of Tharsis. Finally, lava was emplaced in the lava plains of units Hsl (flows of lower member) and Hsu (Upper members). We showed the magma composition changed in the lava plains of STB before and after the uplift of Tharsis. This helps to characterize both the composition and evolution of the early martian magma as well as to provide insight into the mechanism of emplacement of lava plains and early volcanism.Finally, we have identified several exposed dikes in Thaumasia Planum Mars using THEMIS, CTX, HiRISE and CRISM data. These dikes extend from tens to～100kilometers in length with average widths of～50m. They display classic’en echelon’ patterns while cross-cutting preexisting geologic features, including extensive wrinkle ridges. Both the dikes and associated fissure eruption products have very blocky morphologies with～38%higher thermal inertia than the surrounding regions. The dikes are all enriched in Mg-rich olivine relative to the surrounding terrain, while a subset also contains elevated high-calcium pyroxene, both of which indicate relatively primitive magma compositions. We propose that these dikes might have served as feeders for the olivine-enriched flood basalts in this region, and may be derived from the Tharsis plume. These observations provide further evidence that the opening of Valles Marineris was facilitated by tectonic stresses following paths of preferential weakness along preexisting structures—such as fractures and faults like those indicated by these dikes.In summary, the results in this paper can improve our understanding about ancient volcanism on Mars (including the morphology, composition, thermophysics, mechanism of emplacement, and evolution of magma), and they provide geologic evidence for various geophysical and numerical models. They are important in learning the early history of volcanism in the solar system and comparative planetary geology.