Minami Yasui

Young mare volcanism in the Orientale region contemporary with the Procellarum KREEP Terrane (PKT) volcanism peak period ∼2 billion years ago

[1]xa0The crater retention ages of the mare deposits within the Orientale multi-ring impact basin are investigated using 10-m resolution images obtained by the SELENE (Kaguya) spacecraft, in order to constrain the volcanic history of the Moon around the nearside-farside boundary. Precise crater-counting analyses reveal that mare deposits in the Orientale region are much younger than previously estimated: ∼2.9xa0Ga mare basalt in the eastern part of Mare Orientale and ∼1.8–2.2xa0Ga mare deposits in Lacus Veris and Lacus Autumni, maria along the northeastern rings of the basin. The latter age estimates indicate that the Orientale region experienced volcanic activities ∼2 billion years after the basin-formation impact. The dominance of a uniform surface age across the mare deposits in the peripheral regions strongly suggests that these volcanic eruptions are contemporary with the elevated volcanic activity episode proposed for the Procellarum KREEP Terrane on the lunar nearside at ∼2xa0Ga and that this activity peak is much more widespread than previously estimated. The longevity of mare volcanism in the Orientale region further suggests high initial temperatures and/or high content of heat-producing elements in the underlying mantle of this region.

Experimental studies on mechanical properties and ductile‐to‐brittle transition of ice‐silica mixtures: Young's modulus, compressive strength, and fracture toughness

We measured Youngs modulus, fracture toughness, and compressive strength of ice and of ice-0.25-μm hard silica bead mixtures in controlled systematic experiments to determine the effect of silica on the ductile-to-brittle transition. Unconfined compressive strength was measured in a cold room at -10o C under a constant strain rate ranging from 10-5 to 6 x 10-1 s-1 of mixtures with silica volume fraction f of 0, 0.06, and 0.18. In the brittle regime, the compressive strength σpeakwas a maximum at the transitional strain rate and then decreased with increasing strain rate. In the ductile regime, the σpeak increased exponentially with increasing strain rate ease=B⋅σpeakn. The stress exponent n for f=0.06 and 0.18 was ~6, twice as large as the value of pure ice, n~3. The transitional strain rate increased with increasing silica volume fraction; 10-3-10-2 s-1 for pure ice, 10-2-10-1 s-1 for f=0.06 and > 6 x 10-1 s-1 for f=0.18. Fracture toughness and Youngs modulus were measured over the range 0 ≤ f ≤ 0.34. Fracture toughness scaled as f0.5 while Youngs modulus increased linearly with f. Finally, a theoretical model of the transitional strain rate proposed by Schulson [1990] and Renshaw and Schulson [2001] was compared to the measured transitional strain rates. Model predictions were in accord with measured transitional strain rates for pure ice but somewhat higher than observed for ice-silica mixtures. Large model uncertainty was due to high sensitivity of the transitional strain rate to the stress exponent n.