Over the past decades, due to an increasing number of successful orbiter and lander space missions, interest in planet Mars experienced an enormous growth. To complement Mars missions, studies using samples from Earth as analogue for Mars contribute to the increasing knowledge about Mars. This study is the first to use basaltic rock samples from the shield volcano Mauna Loa (Hawaii) as Martian analogue for a hydrous alteration study.

First, samples from differently aged lava flows were collected during the EMMIHS-I simulated Moon mission at the HI-SEAS habitat. The HISEAS habitat is situated on the north-eastern flank of the shield volcano Mauna Loa (Big Island, Hawaii). In order to obtain the most representative samples, only the top 10 cm of horizontally emplaced pahoehoe lava flows were sampled in a radius of 6 km around the HISEAS habitat. From the samples, thin sections were analysed using petrographic microscopy, SEM-EDS and SWIR hyperspectral imagery. Combining the datasets from these three methods made it possible to estimate the chemical composition of the present hydrated minerals. Additionally, XRF was used to study the elemental changes along the depth profiles through the samples and calculate geochemical weathering indices. The Hawaiian rocks were then compared to datasets from Mars. The Hawaiian XRF dataset was compared with calibrated APXS data from the Mars Exploration Rovers (MER). Hawaiian mineral spectra obtained using SWIR hyperspectral imagery were compared with remote sensing data from Mars Orbital spacecrafts.

In the field, hydrous alteration can be recognized by the removal of metallic surface layers, presence of brown horizontal surface layers and horizons enriched in ferric iron. The main detected alteration products include Fe,Mg-smectites, hydrated silica, Al-silicates, Al-hydroxide and palagonite. These products formed due to the interaction between meteoric water and basaltic rocks. The calculated geochemical weathering indices indicate alteration and weathering processes acting on the top 2-3 cm of the rock samples from lava flows with ages >400 years. The values indicate the alteration products formed in an arid to semi-arid environment at cold to moderate temperatures.

Comparisons between the Hawaiian datasets and the Martian datasets mentioned in the Methods section [5,6,7,8], show that the alteration products on Mars are similar to the hydrous alteration products found on Mauna Loa. When calculating the already mentioned geochemical weathering indices for the MER datasets, the Martian values indicate a low degree of weathering and alteration processes, similar to the values of the Hawaiian rocks. This suggests that the Martian alteration products formed in a similar environment as the Hawaiian alteration products. These observations are consistent with the assumption of a present active hydrosphere on an early Mars.

This study shows analogies between rock samples from the Hawaiian shield volcano Mauna Loa and Martian datasets in terms of mineralogy and alteration environments. The study suggests the same type of clay minerals as identified on Mauna Loa have formed in an arid to semi-arid environment at cold to moderate temperatures on an early Mars. These findings add to the assumption that Mars once had a more Earth-like climate with an active hydrosphere and relatively thick atmosphere.@en