The device used by Nasa rovers to study minerals on Mars could return confusing results, complicating the search for signs of water, an expert has said.
It is claimed the rovers' Moessbauer spectrometers will not adequately tell between minerals that form in the presence of water and ones that do not.
But a senior Nasa scientist has rejected the claims and says the device is the best instrument for the job.
Details are available in the journal Earth and Planetary Science Letters.
The Moessbauer spectrometer carried by Nasa's twin Mars rovers Spirit and Opportunity is designed to determine the mineral composition of rocks on the surface of the Red Planet.
"
It won't allow us to distinguish between minerals that have hydrogen in their structure and minerals that don't, which of course is the million dollar question
"
M Darby Dyar, Mount Holyoke College
But one expert on Moessbauer spectroscopy told BBC News Online the technique has limitations which could make mineral data from Mars difficult to interpret.
Million dollar question
Mission scientists will pay close attention to Martian minerals containing iron, because it interacts strongly with liquid water.
But a lack of variety in the way iron bonds with different atoms in molecules could make Moessbauer measurements difficult to interpret, said M Darby Dyar, associate professor of astronomy and geology at Mount Holyoke College, US.
"It won't allow us to distinguish between minerals that have hydrogen in their structure and minerals that don't, which of course is the million dollar question," she said.
The presence of iron hydroxides, for example, would strongly indicate the past presence of water on Mars.
But Dr Goestar Klingelhoefer, head of Nasa's Mars rover Moessbauer team, countered: "[This paper] ignores lots of details that we have already published."
"For instance, goethite (a hydrogen-bearing mineral) in the Moessbauer spectrum has really a unique pattern."
Dr Dyar said: "It's particularly bad if it's Fe3+, the oxidised form of iron - which is the one we expect to find on Mars. The range of Moessbauer parameters for this form of iron is very small, which means that almost everything with Fe3+ in it looks the same."
"
This will enable us to identify hydrogen-bearing minerals
"
Goestar Klingelhoefer, Nasa
But Dr Klingelhoefer suggested Dr Dyar was referring in large part to very rare minerals that do not occur frequently in nature.
Temperature change
The reddish soil of Mars is down to the presence of iron oxides; it has essentially rusted.
This could be due to ongoing interaction between the oxidising atmosphere and surface rocks rather than the action of water in the past.
In addition, the Moessbauer spectra of minerals look different depending on the temperature of that mineral, she claims.
Martian temperatures are known to vary between about -133 Celsius (140 Kelvin) and -3 Celsius (270 Kelvin).
But Dr Klingelhoefer dismissed this claim saying that the team was collecting data on how changes in temperature affected Moessbauer readings on Mars and that this was informing their analysis.
"It's a fingerprint method," he said. "We can measure between daytime and night-time, the whole temperature range, in [increments] of plus or minus five [Celsius].
"This will enable us to identify hydrogen-bearing minerals," he said.
Moessbauer spectrometers work by spraying gamma rays at a target mineral and then measuring the fraction of atoms which do not recoil.
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