In the short BBC video Spain's otherworldly red river, Ricardo Amils, a researcher in Spain's Astrobiology Center says the following (amateur transcription):

Río Tinto is a peculiar place in our planet that has many similarities with Mars. The most important one is iron. Mars is red because of its iron, and Río Tinto is the red river.

The river’s singular red colour comes from ferric ion, iron oxidised due to the river’s acidity...

Considering the similar geochemistry and minerology, this place is a good analogue to Mars. Life on Mars would probably look like life here.

During his trips to South America, Charles Darwin found life in extreme saline conditions. He then predicted that we would find life in the subsoil. It took us 200 years to prove him right. If there is life on Mars, as some of us believe, it has to be in the subsoil.

What definitely equialised Río Tinto and Mars was the discovery of jarosite in Mars. This iron and potassium sulphate is produced in great quantities in Río Tinto.

Upon drilling the river’s subsoil, scientists found a great variety of bacteria and microorganisms. These could thrive in extreme conditions, and need no oxygen.

Wikipedia's Jarosite: Mars exploration says:

Ferric sulfate and jarosite have been detected by the two martian rovers Spirit and Opportunity. These substances are indicative of strongly oxidizing conditions prevailing at the surface of Mars. In May 2009, the Spirit rover became stuck when it drove over a patch of soft ferric sulfate that had been hidden under a veneer of normal-looking soil. Because iron sulfate has very little cohesion, the rover's wheels could not gain sufficient traction to pull the body of the rover out of the iron sulfate patch. Multiple techniques were attempted to extricate the rover, but the wheels eventually sank so deeply into the iron sulfate that the body of the rover came to rest on the martian surface, preventing the wheels from exerting any force on the material below them. As the JPL team failed to recover the mobility of Spirit, it signified the end of the journey for the rover.

Question: How did Spirit and Opportunity identify ferric sulfate and jarosite on Mars?

The BBC News videos don't always remain viewable indefinitely, so here are some screen shots for background and context. click for full size

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Reference [1] describes the finding of jarosite in the Mawrth Vallis region of Mars. The NASA Mars rovers include IR spectroscopy in their instrumentation, and that was the method used. The finding of potassium jarosite, $\text{KFe}_3\text{(SO}_4\text{)}_2\text{(OH}\text{)}_6$, is rendered graphically in this figure taken from the reference. The jarosite peaks lie between 2.2 and 2.7 micrometers wavelength. The upper plot shown an additional peak at 1.92 micrometers which is attributed to a hydrated oxide phase, $\text{Fe}_{10}\text{O}_{14}\text{(OH}\text{)}_2$. Not shown in the figure, but mentioned in the article, is goethite, $\text{FeOOH}$. In all these compounds the iron works out to an oxidation state of +3, thus making these two identifications consistent in terms of chemistry.

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1. William H. Farrand, Timothy D. Glotch, James W. Rice Jr., Joel A. Hurowitz, Gregg A. Swayze, "Discovery of jarosite within the Mawrth Vallis region of Mars: Implications for the geologic history of the region",Icarus, Volume 204, Issue 2 (December 2009), Pages 478-488.

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