EVA 3

For EVA 3, we decided to continue on from where we had ended EVA 1 two days ago. We thought this was a good idea as we were doing a base-line study again. The aim was to get much further down Candor Chasma without the constraints of the space suits in hope of finding a unit of much differing character so as to define a unit older than the typical rocks of the canyon.

We traversed approximately 600m west to east along the valley without finding a bed that could represent a new unit.

The start of EVA 3 is stratigraphically lower (i.e. older) than the rocks at the start of EVA 1 and the conglomerate frequently seen at the start of EVA 1 was much less frequent. We saw more of the interbedded, dark brown mudstones and white-greenish fine sandstones at our onset. What really caught our attention was the presence of a large, nodular bed of a white-grained rock. The nodules were on the order of 50 cm - 1 m in diameter and the white grains look to be loosely-consolidated white crystals. Both above and below the nodular bed were sub-horizontal veins of a similar white mineral, however, in the veins this mineral appears more fibrous. The sub-horizontal beds ranged in thickness from 2 mm - 2 cm. Through some of the mudstone beds, this mineral occurred in very fine veins forming a plant-root-like network.

We tested the mineral with HCl (hydrochloric acid) and found it didn't fizz - this proved that our initial diagnosis of calcite was incorrect. We then noted that the mineral was easily scratched by a fingernail and so had a hardness of less than 2.5, with the one definite cleavage plane we determined that the mineral was in fact gypsum. The mineral in the nodular beds also did not fizz and we believe it too is gypsum. It shows how the use of such simple tools as a finger nail and HCl can really be useful in diagnosis in the field as our first diagnosis was made whilst we were wearing space suits.

As we moved east down section, the dark brown mudstone/ white-green sandstone alternating beds became more uniform and these formed almost vertical cliff faces with scree slopes at the base. A number of veins and nodular gypsum beds were still visible in these cliff faces.

On the south side of the valley, the rocks had a lot of accumulated dust, whereas the north cliffs had none, this appeared to be due to prevailing winds. We came to this conclusion after seeing a dust devil skirting the top of the southern cliff. A similar dust/sediment was seen on the canyon floor and appears to be predominantly quartz-rich sand and varying lithics.