Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p53f..08k&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P53F-08
Other
[5460] Planetary Sciences: Solid Surface Planets / Physical Properties Of Materials, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [8404] Volcanology / Volcanoclastic Deposits
Scientific paper
Thermal conductivity of granular materials is an important parameter to characterize the nature of planetary surface. Interpretation of thermal inertia obtained by planetary explorations requires physical understanding of the thermal properties. Because of this extensive laboratory investigations have been conducted to characterize them( Preseley and Christensen 1997,Huetter et al 2008 for examples). The basic principal factor in controlling thermal conductivity of granular aggregates at martian surface pressure is widely considered as grain size, but there are also other critical factors to be considered. Interpretation of thermal inertia value in terms of only grain size sometimes would overlook essential nature. Here we present how the thermal conductivity of heterogeneous granular aggregates is modeled by grain size, grain porosity, packing porosity and existence of paste-phase based on measurements of thermal conductivity of vesiculated materials under vacuum and atmospheric pressure. We should note different role of packing porosity and grain porosity. The samples are collections of vesiculated pyroclasts of pumice/scoria having grain porosity up to 0.7 and vesiculated glass beads. Monodispersed/polydispersed samples are prepared by arranging shivered fractions. The average grain size is 40 microns to 6mm. For monodispered aggregate prepared by normal tapping thermal conductivity seems to follow a unique function of the bulk porosity under atmospheric pressure. The conductivity changes from 0.25 to 0.05 W/mK in the bulk porosity of 0.4 to 0.96, respectively. The packing porosity is about 0.4 for all the monodispersed samples under normal tapping preparation. This corresponds to random close packing state, where similar number of grain contact mostly controls thermal conduction between grains. Polydispersed samples are prepared by mixing two grain sizes:50 micron and 4000 micron with variable fractions. Change of the conductivity in terms of bulk porosity is more evident for polydispersed samples than monodispersed ones, which indicates the conductivity is controlled by increasing number of contact points between grains. Under vacuum the conductivity decreases depending on the grain size, which completely follows the model (Piqueux and Christensen 2009 ) . An interesting crossover occurs: the sample with higher grain porosity and larger grain size has a similar value as the sample with lower grain porosity and smaller grain size around 1000Pa. Similar crossover also occurs in polydispersed samples. On the surface of Mars ice phase can condensate/sublimate in the surface granular layer. This process is proposed to critically control the thermal conductivity (Presley et al 2009,Piqueux and Christensen 2009) because it fills the space around grain contacts and it increases/decreases contact area. To see this effect we add soft agar to glass beads sample as an analog of ice phase. Agar is easily deformable material which glues contact points well. Small addition is found to increase the conductivity largely, which confirms this effect. We consider this should play a significant role in controlling thermal inertia value on the surface of Mars, thermal inertia feedback.
Baratoux David
Iwasaki Atsushi
Kurita Kazuyoshi
Toyota Takenobu
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