Investigating Hydrated Silica in Syrtis Major: Implications for the Intensity of Water–Rock Interaction

We use the crystallinity of hydrated silica, represented by the 1.4 μm absorption position in orbiter spectroscopic data, as a proxy for the longevity of water–rock interaction in the Syrtis Major region. Geological maps and crater size-frequency distribution analyses are employed to contextualize mineral detections and estimate surface ages. Hydrated silica is detected within two distinct geological units: a younger “volcanic terrain” (vt) unit (~2.4 Ga); and an older “highland terrain” (ht) unit (3.5–3.7 Ga). Hydrated silica in the vt unit typically has a band position <1.41 µm, consistent with amorphous opal-A, suggesting these younger terrains have experienced limited interaction with water. In contrast, hydrated silica in the older highlands typically has a position >1.41 µm, indicating opal-CT, suggesting that these deposits have had more time to interact with water over which to mature opal-A to opal-CT, while also producing accessory minerals such as kaolinite and Fe/Mg phyllosilicates.