4:00 PM - 4:15 PM
[MIS20-08] Low-temperature friction experiments of ice and salt; implication for the strength of plate boundary on Europa
The observations of tectonic features on Europa imply the existence of icy plate motions and tectonic resurfacing, otherwise locally known as "subsumption" (e.g., Kattenhorn and Prockter, 2014). This is the key to understanding the possibility of life in the Europan ocean, because subsumption can carry oxidants from the icy surface to the subsurface ocean, possibly causing reduction-oxidation (redox) reactions at an adequate rate to sustain life (e.g. Vance et al., 2016). Frictional strength of europan plate boundaries is one of the most important factors in whether or not constant subsumption of icy plates occurs. In this study, we performed friction experiments on ice and salt, and numerical calculations to estimate the strength of the plate boundary on Europa.
The friction experiments with constant slip velocity were conducted using a biaxial testing machine at Hiroshima University, at low temperature conditions (3 mm/s, T = –41 ~ –7 ℃, σn = 2.5–5.0 MPa). The mixture samples of H2O (Ice-Ih) and MgCl2･6H2O were prepared with various mixing ratios (the volume fractions of MgCl2･6H2O were 0, 25, 50, and 100 vol.%), with grain sizes of 45–75 mm. The samples were sandwiched by the stainless blocks in the double direct assembly. The temperature was monitored by 4 or 2 thermocouples inserted into holes drilled in the stainless blocks 2 mm from samples. The friction coefficient was calculated from the measured shear stress and normal stress. Our numerical calculations were conducted to simulate the evolution of the temperature structure of Europa's ice shell along with the plate boundary assuming various subsumption rates (0–4.8 cm/yr) (e.g. Howell and Pappalardo, 2019). In this simulation, a two-dimensional convection-diffusion equation was calculated.
Our experimental results for H2O (Ice-Ih) samples are comparable to that of Zoet et al. (2013) and McCarthy et al. (2017), and the friction coefficient was dependent on the square root of homologous temperature linearly at higher than T/Tm > 0.85. The friction coefficient of the mixture samples was smaller than those of H2O (Ice-Ih) samples and independent of the volume fraction of MgCl2･6H2O in the range of 25–50 vol.%. The temperature-friction models were constructed from our experimental results of the ice and the mixture samples to evaluate the frictional strength of the plate boundary on Europa.
The frictional strength of Europa's plate boundaries was estimated from the friction coefficient models and the numerically calculated temperature structure of the plate boundary. In this study, the frictional strength was converted into a friction coefficient. As a result, the friction coefficient of the plate boundary based on the results of H2O (Ice-Ih) was estimated to be 0.66-0.82. On the other hand, the friction coefficient calculated from the results of the ice-salt mixture was 0.57–0.79. This indicates that the strength of the plate boundary is decreased due to the existence of salts on the surface of Europa. However, the calculated friction coefficient of the plate boundary on Europa was too high to drive the constant subsumption of the ice plate, suggesting that greater driving forces are necessary to affect constant subduction (Howell and Pappalardo, 2019). It may be necessary to consider other causes which may decrease the frictional strength of Europan plate boundaries, such as other salts (that would further decrease the melting point), or intermittent plate subsumption with a stick-slip.
The friction experiments with constant slip velocity were conducted using a biaxial testing machine at Hiroshima University, at low temperature conditions (3 mm/s, T = –41 ~ –7 ℃, σn = 2.5–5.0 MPa). The mixture samples of H2O (Ice-Ih) and MgCl2･6H2O were prepared with various mixing ratios (the volume fractions of MgCl2･6H2O were 0, 25, 50, and 100 vol.%), with grain sizes of 45–75 mm. The samples were sandwiched by the stainless blocks in the double direct assembly. The temperature was monitored by 4 or 2 thermocouples inserted into holes drilled in the stainless blocks 2 mm from samples. The friction coefficient was calculated from the measured shear stress and normal stress. Our numerical calculations were conducted to simulate the evolution of the temperature structure of Europa's ice shell along with the plate boundary assuming various subsumption rates (0–4.8 cm/yr) (e.g. Howell and Pappalardo, 2019). In this simulation, a two-dimensional convection-diffusion equation was calculated.
Our experimental results for H2O (Ice-Ih) samples are comparable to that of Zoet et al. (2013) and McCarthy et al. (2017), and the friction coefficient was dependent on the square root of homologous temperature linearly at higher than T/Tm > 0.85. The friction coefficient of the mixture samples was smaller than those of H2O (Ice-Ih) samples and independent of the volume fraction of MgCl2･6H2O in the range of 25–50 vol.%. The temperature-friction models were constructed from our experimental results of the ice and the mixture samples to evaluate the frictional strength of the plate boundary on Europa.
The frictional strength of Europa's plate boundaries was estimated from the friction coefficient models and the numerically calculated temperature structure of the plate boundary. In this study, the frictional strength was converted into a friction coefficient. As a result, the friction coefficient of the plate boundary based on the results of H2O (Ice-Ih) was estimated to be 0.66-0.82. On the other hand, the friction coefficient calculated from the results of the ice-salt mixture was 0.57–0.79. This indicates that the strength of the plate boundary is decreased due to the existence of salts on the surface of Europa. However, the calculated friction coefficient of the plate boundary on Europa was too high to drive the constant subsumption of the ice plate, suggesting that greater driving forces are necessary to affect constant subduction (Howell and Pappalardo, 2019). It may be necessary to consider other causes which may decrease the frictional strength of Europan plate boundaries, such as other salts (that would further decrease the melting point), or intermittent plate subsumption with a stick-slip.