Experimental constraints on the rheology, eruption and emplacement dynamics of analog lavas comparable to Mercurys northern volcanic plains

We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569-1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s 1). Melt viscosity shows moderate variations (4-16 Pa s) in the temperature range of 1736-1600 K. Experiments performed below the liquidus temperature show an increase in viscosity as shear rate decreases from 5 to 0.1 s 1, resulting in a shear thinning behavior, with a decrease in viscosity of 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometers in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5, and 10 m are likely to have velocities of 4.8, 6.5, and 7.2 m/s, respectively, on a 5 degree ground slope. Numerical modeling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (>10,000 m3/s) are necessary to cover the large distances indicated by satellite data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft.

💡 Research Summary

The study addresses the rheology, eruption dynamics, and emplacement of lava flows that formed Mercury’s Northern Volcanic Plains (NVP), the planet’s largest and possibly the Solar System’s longest lava flows. To simulate NVP lava, the authors prepared a synthetic silicate melt whose bulk chemistry matches the compositional estimates derived from MESSENGER spectroscopy. High‑temperature viscosity measurements were carried out in two regimes: (1) super‑liquidus (up to 1736 K) where the melt is completely liquid, and (2) sub‑liquidus (1569–1502 K) where crystals develop. Viscosity was measured over a range of shear rates (0.1–5 s⁻¹) and crystallinities (0–28 %).

In the super‑liquidus regime the melt behaved as a Newtonian fluid with a modest temperature dependence: viscosity increased from ~4 Pa·s at 1736 K to ~16 Pa·s at 1600 K. These values are one to two orders of magnitude lower than typical terrestrial basaltic lavas (10²–10⁴ Pa·s), reflecting the combined effect of Mercury’s high surface temperature and low gravity on melt dynamics.

Below the liquidus, the presence of crystals introduced non‑Newtonian behavior. As shear rate decreased from 5 s⁻¹ to 0.1 s⁻¹, viscosity dropped by roughly one logarithmic unit (a factor of ten), indicating pronounced shear‑thinning. The authors attribute this to the alignment and rearrangement of crystal aggregates under low shear, which reduces flow resistance. The degree of shear‑thinning intensified with increasing crystal fraction, highlighting the importance of crystallization in controlling flow rheology during emplacement.

Using the experimentally derived viscosity–temperature–shear relationships, the authors estimated flow velocities for idealized planar channels of 1 m, 5 m, and 10 m thickness on a 5° slope. The calculated mean velocities are 4.8 m s⁻¹, 6.5 m s⁻¹, and 7.2 m s⁻¹, respectively. Such speeds are far higher than those of terrestrial basaltic flows (typically <1 m s⁻¹) and are sufficient to transport lava over hundreds of kilometres before cooling terminates the flow.

To test whether these velocities can be sustained over the distances inferred from orbital imagery, the authors performed one‑dimensional numerical simulations that couple heat loss (conduction and radiation) with crystallization‑induced viscosity increase. The model shows that, for a flow to remain turbulent and travel >100 km, an effusion rate exceeding 10 000 m³ s⁻¹ is required. Lower effusion rates lead to rapid cooling, crystallization, and a transition to laminar flow, which would arrest the lava within a few tens of kilometres.

The paper acknowledges several limitations. Laboratory experiments cannot fully replicate Mercury’s surface pressure, gravity, and vacuum conditions, which may affect bubble nucleation and degassing behavior. The crystallinity was quantified only by bulk volume fraction, without detailed analysis of crystal size, shape, or distribution, all of which influence non‑Newtonian response. Moreover, the flow model assumes a simple planar geometry and neglects topographic curvature, channel confinement, and possible interaction with pre‑existing crustal structures.

Future work suggested includes high‑pressure, high‑temperature experiments to explore volatile effects, microstructural characterization of crystal aggregates, and three‑dimensional computational fluid dynamics that incorporate realistic topography and variable slope.

In summary, the research provides the first quantitative rheological dataset for a Mercury‑analog melt, demonstrates that NVP lavas were extremely low‑viscosity, shear‑thinning fluids capable of turbulent flow, and shows that only exceptionally high effusion rates (>10⁴ m³ s⁻¹) can account for the observed kilometre‑scale flow lengths. These findings refine our understanding of planetary volcanism under low‑gravity, high‑temperature conditions and set a benchmark for interpreting future Mercury missions and comparative planetology studies.