Sklute E C, Jensen H B, Rogers A D, Reeder R J
Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, USA.
Now at Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts, USA.
J Geophys Res Planets. 2015 Apr;120(4):809-830. doi: 10.1002/2014JE004784. Epub 2015 Mar 25.
Current or past brine hydrologic activity on Mars may provide suitable conditions for the formation of amorphous ferric sulfates. Once formed, these phases would likely be stable under current Martian conditions, particularly at low- to mid-latitudes. Therefore, we consider amorphous iron sulfates (AIS) as possible components of Martian surface materials. Laboratory AIS were created through multiple synthesis routes and characterized with total X-ray scattering, thermogravimetric analysis, scanning electron microscopy, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III)(SO) · ~ 6-8HO) from sulfate-saturated fluids via vacuum dehydration or exposure to low relative humidity (<11%). Amorphous ferrous sulfate (Fe(II)SO · ~1HO) was synthesized via vacuum dehydration of melanterite. All AIS lack structural order beyond 11 Å. The short-range (<5 Å) structural characteristics of amorphous ferric sulfates resemble all crystalline reference compounds; structural characteristics for the amorphous ferrous sulfate are similar to but distinct from both rozenite and szomolnokite. VNIR and TIR spectral data for all AIS display broad, muted features consistent with structural disorder and are spectrally distinct from all crystalline sulfates considered for comparison. Mössbauer spectra are also distinct from crystalline phase spectra available for comparison. AIS should be distinguishable from crystalline sulfates based on the position of their Fe-related absorptions in the visible range and their spectral characteristics in the TIR. In the NIR, bands associated with hydration at ~1.4 and 1.9 μm are significantly broadened, which greatly reduces their detectability in soil mixtures. AIS may contribute to the amorphous fraction of soils measured by the Curiosity rover.
火星当前或过去的卤水水文活动可能为无定形硫酸铁的形成提供适宜条件。一旦形成，这些物相在当前火星条件下可能是稳定的，尤其是在低至中纬度地区。因此，我们认为无定形硫酸铁（AIS）可能是火星表面物质的组成成分。通过多种合成路线制备了实验室AIS，并利用全X射线散射、热重分析、扫描电子显微镜、可见/近红外（VNIR）、热红外（TIR）和穆斯堡尔技术对其进行了表征。我们通过真空脱水或暴露于低相对湿度（<11%）的环境中，从硫酸盐饱和流体中合成了无定形硫酸铁（Fe(III)(SO)·6 - 8H₂O）。通过水绿矾的真空脱水合成了无定形硫酸亚铁（Fe(II)SO₄·1H₂O）。所有AIS在超过11 Å的尺度上缺乏结构有序性。无定形硫酸铁的短程（<5 Å）结构特征与所有晶体参考化合物相似；无定形硫酸亚铁的结构特征与纤铁矾和水纤铁矾相似但又不同。所有AIS的VNIR和TIR光谱数据显示出与结构无序一致的宽而柔和的特征，并且在光谱上与所有用于比较的晶体硫酸盐不同。穆斯堡尔光谱也与可用于比较的晶相光谱不同。基于其在可见光范围内与铁相关吸收的位置及其在TIR中的光谱特征，AIS应该能够与晶体硫酸盐区分开来。在近红外区域，与~1.4和1.9 μm处水合作用相关的谱带显著变宽，这大大降低了它们在土壤混合物中的可探测性。AIS可能对好奇号火星车测量的土壤无定形部分有贡献。
