Which "exotic salt" can lower water's freezing point by 70 °C?
I recently got a chance to attend a talk by someone who was working on developing analytical instrumentation on Mars. The interesting story is that the initial results by ion-selective electrode was that Mars soil is full of nitrates. Nobody knew on Earth that the nitrate ion selective electrode is far more responsive to perchlorate than nitrate. After learning this, it was an eye opener for analytical chemists! Now they wish to use chromatography rather than electrochemistry. So this was a good lesson for us on Earth.

The perchlorate ion was discovered in 2008 by the nitrate selective electrode. No specific electrode was attached to detect perchlorate, it was rather an accidental discovery. The Science Report makes a footnote "Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site" (paper: Science 2009, 325 (5936), 64–67)

A Hofmeister anion ISE was intended to monitor nitrate from a $\ce{LiNO3}$ reference electrolyte that was part of the leaching solution, but was ultimately used for perchlorate detection


  [Footnote] The relative sensitivity of the Hofmeister series ISE to perchlorate
  over nitrate is 1000:1, and substantial quantities of perchlorate will
  overwhelm any other signal. If, as was observed, >1 mM perchlorate
  accounts for the observed signal, it would require >1000 mM nitrate to
  produce the same response. This would correspond to more than the mass
  of the entire sample.


Now that they know it is a perchlorate ion, people did some studies on supercooled brines. See this paper: Toner, J.; Catling, D.; Light, B. The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars. Icarus 2014, 233, 36–47 (also available here). They clearly show that if calcium or magnesium perchlorates are slowly cooled, one can get supercooled brines up to -120 Celcius. This is a rather amazing finding. They call it a glassy state.