Impact Crater Densities on Volcanoes and Coronae on Venus: Implications for Volcanic Resurfacing

Comparison between the geological features of Venus and Earth based on gravity aspects

We probe the gravitational properties of two neighboring planets, Earth and Venus. To justify a comparison between gravity models of the two planets, spherical harmonic series were considered up to a degree and order of 100. The topography and gravity aspects, including [Formula: see text] (vertical derivative of the vertical component of the gravity field), strike alignment (SA), comb factor (CF), and I₂ invariant derived from the Marussi tensor, were calculated for the two planets at specifically selected zones that provided sufficient resolution. From Γ_zz we discovered that the N-NW edge of Lakshmi Planum does not show any subduction-like features. Its Γ_zz signature resembles passive continental margins on Earth, like those surrounding the Indian Peninsula. Moreover, according to SA and CF, the Pacific and Philippine-North American Contact Zone on Earth indicates significantly higher level of deformation due to convergent motion of the plates, whereas the deformation level on Venus is significantly smaller and local, when considering an equatorial rifting zone (ERZ) of Venus (between Atla-Beta Regios) as diverging boundaries. The strain mode on the East African Rift system is smaller in comparison with ERZ as its Venusian analog. The topography-I₂ analysis suggests a complicated nature of the topographic rise on Beta Regio. We show that specific regions in this volcanic rise are in incipient stages of upward motion, with denser mantle material approaching the surface and thinning the crust, whereas some risen districts show molten and less dense underlying crustal materials. Other elevated districts appear to be due to mantle plumes and local volcanic activities with large density of underlying material.

Global tectonic evolution of Venus, from exogenic to endogenic over time, and implications for early Earth processes

Venus provides a rich arena in which to stretch one's tectonic imagination with respect to non-plate tectonic processes of heat transfer on an Earth-like planet. Venus is similar to Earth in density, size, inferred composition and heat budget. However, Venus' lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non-plate tectonomagmatic processes. In this paper, I explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time, divided respectively into temporal eras. Impactors played a prominent role in the ancient era, characterized by thin global lithosphere. The Artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume. The fracture zone complex era, marked by broad zones of tectonomagmatic activity, witnessed coupled spreading and underthrusting, since arrested. These three tectonic regimes provide possible analogue models for terrestrial Archaean craton formation, continent formation without plate tectonics, and mechanisms underlying the emergence of plate tectonics. A bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of Archaean crust, and recycling of significant Archaean crust to the mantle.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Venus reconsidered

The Magellan imagery shows that Venus has a crater abundance equivalent to a surface age of 300 million to 500 million years and a crater distribution close to random. Hence, the tectonics of Venus must be quiescent compared to those of Earth in the last few 100 million years. The main debate is whether the decline in tectonic activity on Venus is closer to monotonic or episodic, with enhanced tectonism and volcanism yet to come. The former hypothesis implies that most radioactive heat sources have been differentiated upward; the latter, that they have remained at depth. The low level of activity in the last few 100 million years inferred from imagery favors the monotonic hypothesis; some chemical evidence, particularly the low abundance of radiogenic argon, favors the episodic. A problem for both hypotheses is the rapid decline of thermal and tectonic activity some 300 million to 500 million years ago. The nature of the convective instabilities that caused the decline, and their propagation, are unclear.