Magmatic history of Venus and implications for habitability

Kurzfassung

Similar in size, mass and composition to the Earth, Venus remains one of the most enigmatic terrestrial planets. Traces of the early history of Venus and whether it might once have had clement conditions, similar to the Earth, are difficult to reconstruct, given its young surface age. Some climate models suggest that as recently as less than 1 Ga, Venus may have had milder surface conditions and potential liquid water [1]. However, other studies discuss that large-scale volcanic eruptions and continuous degassing of CO2 from Venus' mantle into the atmosphere may have ended a potential early habitable period [2]. Plate tectonics is thought to be an important factor for Earth’s long-term habitability [3], but the geodynamic regime on Venus is still debated. Whether Venus once had some sort of surface mobilization and the tesserae plateaus, the oldest regions on Venus, are similar to the continents on the Earth, is poorly known. Several scenarios have been proposed being one of the most recent ones the “plutonic-squishy” lid regime [4] that involves the presence of magmatic intrusions. All the aforementioned suggests that the magmatism style and the resulting volcanic and outgassing processes on Venus are determinant for understanding its habitability. Surface expressions, from the ~85000 volcanoes [5] to other geological features like corona associated to mantle plume origin [6], bear witness of the magmatic processes that happened on Venus. Moreover, there is evidence that Venus is still volcanically active: Venus Express's VIRTIS instrument observed high emissivity regions interpreted as recent, unweathered basalt possibly linked to magmatic activity, SO2 variability recorded in Venus Express data is likely due to volcanic outgassing [7, 8], and recent analysis of synthetic aperture radar data [9] and images [10] from the NASA's Magellan mission suggest volcanism is still ongoing on Venus. In this study, we model the thermal history of Venus until present day and account for magmatic processes that seem to have played and still play an important role for this planet. Partial melting occurs in the interior of a planet when the temperature of the mantle rocks is higher than the solidus (i.e., melting temperature), for which we assume values analogous to those proposed for the Earth's interior [11]. We test different magmatic styles: from extrusive magmatism (or volcanism), where melt produced deep in the interior can reach the surface, to intrusive magmatism (or plutonism), where part of the melt remains trapped in the crust and the lithosphere. Since the ratio between extrusive and intrusive melt, as well as the depth of magmatic intrusions are poorly constrained, we vary these in our models between 0% (fully intrusive) and 100% (fully extrusive), and the depth of intrusions between 10 km and 90 km. In Fig. 1 we present a summary of our results. Our models show that depending on the percentage of extrusive melt and the depth of magmatic intrusions, the present thermal gradient varies from a few K/km up to almost 40 K/km, with higher values obtained for higher percentages of intrusive melt and shallower magmatic intrusions (Fig. 3a). Thermal gradients from studies that analyze the gravity and topography signatures on Venus [12, 13, 14] have a median around 14 K/km with only a few local values above 40 K/km. Our models with less than 40% extrusive magmatism have a thermal gradients distribution that explains most of the local thermal gradients found in the observations. A recent study [15] suggested that Venus, similar to the Earth, possesses a low viscosity layer (LVL) in the shallow mantle. While on the Earth this layer (i.e., the asthenosphere) is possibly related to the presence of water [16], on Venus the presence of this layer may be attributed to the presence of partial melt. The LVL starts beneath the lithosphere at depths shallower than 200 km. This places constraints on the depth of melting that we can use to select successful models. Models that are compatible with partial melting starting at a depth of 200 km or less beneath the surface require less than 40% extrusive magmatism and magmatic intrusions located strictly deeper than 10 km (Fig. 3b). The growth of the lithosphere, i.e. the immobile layer (stagnant lid) that forms at the top of the convective mantle due to the strong temperature-dependent viscosity is controlled by the depth of magmatic intrusions. We observe in Fig. 3c that the more intrusive magmatism, the thinner the lithosphere is at present day. The thickness of the lithosphere is also influencing the melt properties, with highly extrusive cases showing hotter melts from greater depths than cases with high percentage of intrusions (Fig. 3d). The temperature and depth where melt forms in our models will serve to inform laboratory experiments currently carried out at the University of Münster in a research of the mantle compositions of Venus that are able to explain the Venera and Vega data [17]. Our findings support recent observations of a magmatically active planet primarily governed by highly intrusive processes. This leads to a geodynamic regime on Venus that involves the presence of magmatic intrusions such as the proposed plutonic-squishy lid. The future Venus missions VERITAS and EnVision will provide relevant data to unravel the Venusian past and present conditions, which will help, among other topics, to understand how geodynamic processes operate on this planet. This will provide a better understanding of the similarities between Venus and Earth, and when the two planets have started to diverge in their history, which represent key aspects for a potential early habitable period on Venus. This knowledge could be extrapolated to Venus-like exoplanets to study the likelihood of their habitability.