The effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size, velocity, and mass. We carried out a systematic numerical study employing more than 1000 two‐dimensional models of simple crater formation under lunar conditions in targets with varying properties. A simple numerical setup is used where targets are approximated as granular or brecciated materials, and any compression of porous materials results in permanent compaction. The results are found to be consistent with impact laboratory experiments for water, low‐strength and low‐porosity materials (e.g., wet sand), and sands. Using this assumption, we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws, i.e., the effective strength. The effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into π group scaling laws, and predict transient crater diameters within an accuracy of ±5% for targets with friction coefficients f ≥ 0.4 and porosities Φ = 0–30%. Moreover, 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction, porosity, and cohesion. The derived relationships are most robust for targets with Φ > 10–15%, applicable for a lunar environment, and could therefore yield significant insights into the influence of target properties on cratering statistics.
Venus has similar size, density and bulk composition as Earth, but has tectonically evolved clearly differently, and this divergence remains enigmatic. Surface observations such as gravity, topography and surface age constrain Venus' evolution, but interpreting these signals requires understanding of the surface-interior coupling and thus insight into the structure and evolution of the venusian mantle and lithosphere. Here, we investigate how such observables may be generated from interior dynamics using numerical forward models of global mantle convection that consistently link the thermochemical, magmatic and tectonic evolution of Venus. Venus' present surface gravity spectrum and its relation to topography is matched best by our models with a mantle viscosity profile featuring a sublithospheric minimum of ∼ 2 × 10 20 Pa s and a gradual increase by a factor of ∼ 100 down to a depth of ∼ 250 km above the core-mantle boundary. No pronounced viscosity jump around the mantle transition as inferred for Earth is favoured for Venus, which points to a relatively dry venusian upper mantle compared to Earth's as previously suggested. This holds true for both a pure stagnant-lid scenario and in the presence of episodic catastrophic overturns triggered by cumulative crustal growth due to ongoing magmatism and volcanism. Overturns strongly perturb the surface gravity spectrum up to ∼ 150 Myr after overturn cessation. Material deeply recycled by the resurfacing event annihilates the developed plume pattern, which needs much longer than those 150 Myr to recover to a state comparable to the pattern suggested by thermal emissivity anomalies observed on Venus. Moreover, overturns limit crustal thicknesses to reasonable values and are more capable than stagnant-lid evolutions in generating mean surface ages > 500 Myr. These findings seem to confirm previous suggestions that the episodic regime is more applicable to Venus than a purely stagnant-lid regime. Yet, the relatively long time span required to recycle the entire surface (∼ 150 − 200 Myr) and the presently ongoing volcanic resurfacing predicted by our models complicate the formation of a uniform surface age as indicated by Venus' crater population and may also suggest that the latest
[1] Previous mantle convection studies with continents have revealed a first-order influence of continents on mantle flow, as they affect convective wavelength and surface heat loss. In this study we present 3D spherical mantle convection models with self-consistent plate tectonics and a mobile, rheologically strong continent to gain insight into the effect of a lithospheric heterogeneity (continents vs. oceans) on plate-like behaviour. Model continents are simplified as Archaean cratons, which are thought to be mostly tectonically inactive since 2.5 Ga. Long-term stability of a craton can be achieved if viscosity and yield strength are sufficiently higher than for oceanic lithosphere, confirming results from previous 2D studies. Stable cratons affect the convective regime by thermal blanketing and stress focussing at the continental margins, which facilitates the formation of subduction zones by increasing convective stresses at the margins, which allows for plate tectonics at higher yield strength and leads to better agreement with the yield strength inferred from laboratory experiments. Depending on the lateral extent of the craton the critical strength can be increased by a factor of 2 compared to results with a homogeneous lithosphere. The resulting convective regime depends on the lateral extent of the craton and the thickness ratio of continental and oceanic lithosphere: for a given yield strength a larger ratio favours plate-like behaviour, while intermediate ratios tend towards an episodic and small ratios towards a stagnant lid regime.
The distribution of seafloor ages determines fundamental characteristics of Earth such as sea level, ocean chemistry, tectonic forces, and heat loss from the mantle. The present-day distribution suggests that subduction affects lithosphere of all ages, but this is at odds with the theory of thermal convection that predicts that subduction should happen once a critical age has been reached. We used spherical models of mantle convection to show that plate-like behavior and continents cause the seafloor area-age distribution to be representative of present-day Earth. The distribution varies in time with the creation and destruction of new plate boundaries. Our simulations suggest that the ocean floor production rate previously reached peaks that were twice the present-day value.
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