Question

Three models are proposed to explain the formation of hematite ($\text{Fe}_2\text{O}_3$) spherules, commonly called "blueberries," discovered in the Meridiani Planum region of Mars.

*Model 1*
Spherules formed in situ within porous basaltic volcanic rock. Upwelling volcanic fluids heated to temperatures between $150^\circ\text{C}$ and $250^\circ\text{C}$ circulated through underground aquifers. These fluids, neutral in pH and rich in dissolved iron, encountered sudden pressure drops, causing hematite to precipitate symmetrically in all directions within spherical pore spaces (vesicles). Because the vesicles were free of mineral grains, the resulting spherules consist of pure, crystalline hematite with no internal sedimentary inclusions.

*Model 2*
Spherules formed as chemical concretions in a shallow, highly acidic ($pH < 3.0$), hypersaline surface lake. Liquid water containing dissolved $\text{Fe}^{3+}$ ions seeped downward through porous quartz sandstone. The acidic water reacted with localized, alkaline carbonate minerals within the sandstone, raising the pH and causing hematite to precipitate outward from nucleation centers. Consequently, these spherules grew around and enveloped surrounding quartz sand grains, resulting in a concentric internal structure containing micro-grains of quartz.

*Model 3*
Spherules are impact spherules created during a hypervelocity meteorite impact on the Martian surface. The impact vaporized iron-rich basaltic target rocks and the iron-nickel meteorite itself, ejecting a plume of vapor and molten droplets into the upper atmosphere. As the droplets fell back toward the surface, they cooled and solidified into spherical shapes. The spherules accumulated as a distinct, widespread air-fall layer on top of preexisting rock units, rather than growing within them.

Match each description of a spherule's formation mechanism or physical constraint to the specific model that proposes it.