¹Cyril Sturtz,¹Angela Limare,¹Stephen Tait,¹Édouard Kaminski
Journal of Geophysical research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2021JE007000]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France
Published by arrangement with John Wiley & Sons

This paper is the first of two companion papers presenting a theoretical and experimental study of the evolution of crystallizing magma oceans in planetesimals. We aim to understand the behavior of crystals formed in a convective magma ocean, and the implications of crystal segregation for the thermal and structural evolution of the convective system. In particular, we wish to constrain the possibility to form and preserve cumulates and/or flotation crusts by sedimentation or flotation of crystals respectively. We use lab-scale analog experiments to study the stability and the erosion of a floating lid composed of plastics beads lying over a convective viscous fluid volumetrically heated by microwave absorption. We propose a law for erosion and re-entrainment that depends only on two dimensionless numbers that govern these phenomena: (i) the Rayleigh-Roberts number, characterizing the vigor of convection and (ii) the Shields number, that encompasses the physics of the flow-particle interaction. We further consider the formation of a cumulate at the base of the convective layer by sedimentation of beads that are denser than the fluid. We find that particle deposition occurs at a velocity that scales with the Stokes velocity, a result consistent with previous experimental studies. We build up a model that describes the transient evolution of the convective system’s thermal state and the fraction of particles that segregated from the flow or that remain in suspension.

¹Aditi Pandey,²Elizabeth B. Rampe,²Douglas W. Ming,¹Youjun Deng,^2,3Candice C. Bedford,¹Paul Schwab
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115362]
¹Department of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
³Lunar and Planetary Institute, Universities Space Research Association, Houston, TX, USA
Copyright Elsevier

Data collected by orbiters and landers have consistently shown abundant amorphous materials on Mars. Developing analytical techniques that target amorphous materials in terrestrial analogs can help determine the environmental conditions under which the amorphous assemblages on Mars were formed. However, the inherent short-range order, chemical heterogeneity, and nanoscale phase interactions create challenges in characterizing these phases. This study is aimed to overcome these challenges by combining chemical dissolution rate analysis with spectroscopy-based mass balance calculations (MBC) to characterize different pools of amorphous materials in terrestrial analogs. The differences in dissolution rates between rapidly dissolving Si, Al, and Fe amorphous materials and the slowly dissolving crystalline minerals were modeled to predict amorphous composition in five palagonitic samples from Hawaii (Mauna Kea) and four hyaloclastite tuffs from the subglacial volcanoes in southwest Iceland. These samples were selected as potential analogs because of their spectral and compositional resemblance to Mars’s surface materials.

The amorphous Si, Al, and Fe compositions from both sites varied based on the degree of aqueous alteration, grain size, and location. The amorphous fractions of the unconsolidated palagonitic analogs from Hawaii are composed of alteration products such as opal CT and ferrihydrite with minor amounts of unaltered basaltic glass. In contrast, the amorphous fractions of the cemented Icelandic samples were composed mainly of unaltered glass and mixed Fe(II, III) iron phases. Amorphous compositions of the loose Hawaiian and the consolidated Icelandic palagonites are comparable with the Martian modern and ancient aeolian materials respectively. The amorphous compositions in the Hawaiian sample, HWMK101, closely resemble the amorphous materials in Rocknest, a modern aeolian material from the sand shadow, and the Icelandic samples are comparable to the ancient aeolian materials from the Stimson formation. Our analysis indicates the presence of hydrated secondary alteration products such as ferrihydrite and allophane in Rocknest, and a mixture of reduced iron phases and silicate glass outlined with finer altered opaline silicates in Greenhorn. The chemical extraction analysis combined with MBC can be used to characterize amorphous phases in terrestrial analogs to better constrain the formation and characterization of the abundant amorphous materials on Mars.