In your News story 'Phoenix fades away' (Nature 456, 8–9; 2008), you suggest that the polar soil analysed by NASA's Phoenix Lander “was like nothing else tested so far on Mars”. But the soils sampled may not be unusual at all. An alternative explanation may lie with the instruments used for the analysis, given that these had never been used before.
The Martian soils were found to be alkaline and to contain trace amounts of perchlorates and carbonates. But they may still be similar to the soils analysed by previous landers. The Phoenix measurement of soil pH (pH 8.3) was carried out by dissolving small amounts of soil in liquid water. Geochemical modelling indicates that adding water to the ionic composition of the most acidic environment described so far on Mars, Meridiani Planum, should give a pH of 9.2, according to my own calculations. The identification by the Mars Exploration Rover (MER) Opportunity of a set of minerals that could not precipitate in an alkaline solution, including sulphate evaporites, provides information about a post-settlement acidification at Meridiani. This was driven by gaseous carbon dioxide, sulphur dioxide and iron ions, neutralizing the pH-raising capacity of the basaltic minerals in the host rock and allowing the synthesis of acidic minerals.
Also, the discovery of perchlorate as a result of diluting Martian dust in water and searching for the soluble salts it contains is not necessarily a unique characteristic of the Phoenix soils. The inferred presence of chlorides at all Martian landing sites may not be the sole explanation after the identification of chlorine. It has been known since the Viking missions in the seventies that conditions at the Martian surface are strongly oxidizing. This could be due, for example, to chlorine salts such as chlorites, chlorates and perchlorates that originated photochemically in the atmosphere.
Moreover, the signal for calcium carbonate encountered by Phoenix is in fact the same footprint found by Mars Global Surveyor in soils all around the planet in 2003, and by the MER Spirit in Gusev crater in 2004. The only difference is that these previous identifications uncovered magnesium carbonate.
It is, therefore, possible that if the previous landers had incorporated the same analysis capabilities as Phoenix, the results might have been very similar.