Earth_Mars

Martian volcanism is extensive but not uniformly distributed and includes a diversity of volcanic landforms (Neukum & Hiller 1981; Neukum et al. 2004). Many volcanic constructs are associated with regional tectonics or local deformational features, but no landforms are convincingly associated to ancient plate tectonics. The two topographically dominating volcanic provinces are the Tharsis and the Elysium regions (Fig. 2), situated close to the equator on the dichotomy boundary between the cratered (older) highlands and the northern lowlands (c. 120° apart). The regions are characterized by volcanoes whose morphologies are strongly analogous to volcanic landforms on Earth and the huge volcanoes in the Tharsis region (Olympus Mons, Ascraeus Mons, Pavonis Mons & Arsia Mons) are prime examples resembling many characteristics of Hawaiian shield volcanoes. The main difference between the Martian and terrestrial volcanoes are their size and the length of the flows, mainly due to higher eruption rates, the "stationary" character of the source (no plate tectonics) and possibly the lower gravity.

Based on new spacecraft imagery and crater-count statistics (Werner & Neukum 2005) it is evident that volcanism was planet-wide in the Early Martian history and most volcanic regions were emplaced before c. 3.5 Ga. Importantly, however, we can demonstrate that the Elysium and Tharsis (e.g. Olympus Mons & Tharsis Montes) regions experienced volcanic activity until very recently (i.e. 200 to 100 Ma ago) and this suggests that the most recent activities (over the last 500 Ma) are more wide-spread than earlier believed. In this joint venture we will investigate the relation between volcanism and areoid on Mars, guided by insights into this topic from Earth. In addition to supplementary age-determinations dating (crater-count statistics based on image data from the Mars Express High Resolution Stereo Camera) in order to further improve the Martian volcanic record we will address the following fundamental questions:

1. Is it possible that a large part of the large-scale areoid is due to density anomalies in the Martian lower mantle, with areoid highs corresponding to low density? We see considerable similarities in the patterns of geoid and lowermost density heterogeneities on Earth (Figure 1), particularly in regions around Africa without recent subduction, as on Mars. On the other hand, recent work has suggested that areoid anomalies have rather shallow origin (Zhong, 2002). On Earth, we can combine rheological models based on mineral physics with density models based on seismic tomography to explain a large part of the geoid. We will adapt these rheological models to Mars and test whether they are compatible with the correlation between areoid and density anomalies proposed here.

2. Can the areoid spectrum yield further insights into Martian mantle dynamics? On Earth it is possible to explain the geoid spectrum, even in the absence of any knowledge from seismic tomography, by making some rather simple assumptions about the statistical nature of density heterogeneities, in combination with mantle rheology models based on mineral physics (Steinberger & Holme 2002). We will test whether, after adapting these rheological models to Mars, the areoid spectrum can be explained with the same simple assumptions. By obtaining a good fit between model and observation we expect to better constrain some of the assumptions that went into the model.

3. Do claims of Martian true polar wander indicate a change in viscosity or density structure with time? The above two points concern the present-day situation on Mars, and true polar wander can help us gain insight about its past: Arkani-Hamed & Boutin (2004) argue that at one time the Tharsis region was close to the Martian pole. If correct, this implies that, at that time there was no areoid high in that region. We will test within the framework of our mineral physics model, whether it is possible that Martian rheology has changed with time such that, large-scale negative density anomalies in the Martian lower mantle have corresponded to geoid lows rather than highs at some time in the past, and hence whether true polar wander was possible without changing large-scale density structure.

Answering these questions will help us to better understand a possible relation between lowermost mantle density anomalies and volcanism on Mars. On Earth, such a proposed relation (Burke & Torsvik 2004) has been used to argue for deep mantle plume origin, a subject of much recent debate. With a similar relationship made plausible on Mars, the deep plume hypothesis could gain additional support for yet another planet. Ultimately our results will contribute to our knowledge of similarities and differences between the two planets, thus improve understanding how each one of them works.

Figure 1 LEFT: SMEAN shear wave velocity anomaly model for Earth at 2800 km depth (Becker & Boschi 2002). RIGHT: Earth geoid. Open white symbols are reconstructed Large Igneous Provinces (LIPs) updated from Burke & Torsvik (2004) using a revised palaeomagnetic reference model.

Figure 2 Mars Laser Altimeter Topographic data (shaded relief) with a superimposed areoid (red is high, blue low). Volcanic regions are marked. A correlation of possibly still active volcanic provinces and a high areoid is obvious in the Tharsis region, and probably a local high in the Elysium region. Olympus Mons and Alba Patera are situated along the margin of the areoid high as seen from many LIPs from Earth.

Arkani-Hamed, J. and Boutin, D., 2004. Paleomagnetic poles of Mars: Revisited. J. Geophys. Res. 109, E03001.

Becker, T.W. & Boschi, L., 2002. A comparison of tomographic and geodynamic mantle models, Geochem. Geophys. Geosys. 3, 2001GC000168.

Burke, K. & Torsvik, T.H. 2004. Derivation of large igneous provinces of the past 200 million years from long-term heterogeneities in the deep mantle. Earth Planet Sci. Lett., 227, 531-538.

Neukum, G., Jaumann, R., Hoffmann, H., Hauber, E., Head, J.W., Basilevsky, A.T., Ivanov, B.A., Werner, S.C., van Gasselt, S., Murray, J.B., McCord, T., and the HRSC Co-Investigator, 2004. Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera, Nature, 432, 346—351.

Steinberger, B. & Holme, R. 2002. An explanation for the shape of Earth's gravity spectrum based on viscous mantle flow models. Geophys. Res. Lett., 29(21), 2019, doi:10.1029/2002GL015476

Werner, S.C., Neukum, G. and the HRSC Co-Investigator Team, 2004, Major Volcanic Constructs Seen from Mars Express HRSC - New Insights into Their Evolutionary History, 36th LPSC , 1766.

Zhong, S. Effects of lithosphere on the long-wavelength gravity anomalies and their implications for the formation of the Tarsis rise on Mars, J. Geophys. Res., 107, 10.1029/2001JE001589, 2002.