centration and detection by various means. Common probes are antibodies, FAb fragments, DNA/RNA aptamers, and molecular imprinted polymers or capture resins. Detection of the target molecules can occur via optical means (fluorescence or colorimetry), mass spectrometry, or surface plasmon resonance.
Sample-handling technology. A major hurdle for in situ investigations of biomarkers is the availability of robust and flexible sample-handling systems. Most measurements for biosignatures require that the sample be pretreated in some fashion. The extraction of biomolecules and their introduction into instruments without cross-contamination or biasing the concentrations of molecules of interest, allowing the concentration of the sample to increase the likelihood of detection, and ensuring that no poisoning compounds or ions come into contact with the sample, is critical.
There are currently more than three-dozen examples of the so-called SNC (shergottite, nakhlite, and chassignite) meteorites believed to have originated on Mars.8 All of these meteorites are igneous rocks whose source localities on Mars are unknown. Most of the known martian meteorites have crystallization ages younger than ~1.3 billion years.9 Only one, the orthopyroxenite ALH 84001 with its crystallization age of ~4.5 billion years, is likely to represent a sample of the ancient crust of Mars that is most likely to have had environments that could have hosted life.10,11
Given the lack of geological context for the martian meteorites, their origin from a limited number of mostly young volcanic terrains on Mars, alteration by shock and cosmic irradiation during their ejection from Mars and delivery to Earth, and subsequent weathering and contamination by organisms in a terrestrial environment, the martian meteorites are not ideal for astrobiological investigations. Nevertheless, some of them contain clear evidence for low-temperature alteration by aqueous fluids, and investigations of these samples have provided significant constraints on the near-surface environment and the history of water on Mars.12,13 Ancient ALH 84001 in particular has been the focus of numerous (often controversial) studies that may have implications for past biogenic activity on Mars.14–17 Therefore, although not optimal, the martian meteorites still provide a relatively inexpensive means of investigating the potential for past and/or present life on Mars.
In recent years, many new members of this clan have been recovered from hot and cold desert regions of the world. Although the terrestrial residence time of martian meteorites collected in hot and cold deserts is about the same, the former are much more subject to terrestrial weathering and biological contamination than the latter.18,19 Because those meteorites collected in Antarctica have spent most of their time on Earth fully encased in ice, they do not, in general, display the veins and cracks filled with alteration minerals commonly seen in meteorites found in hot deserts. Consequently, a concerted effort for the collection of martian meteorites in Antarctica should continue, given the potential scientific return from the recovery of new samples, particularly if they could provide a record of ancient and/or water-rich environments on Mars.
In the development of a shorter-term strategy for the investigation of Mars from an astrobiological perspective, sample return must also be accorded a higher priority than indicated by current NASA planning.20 Returned samples, which can be analyzed in Earth-based laboratories, offer significant advantages (Box 5.1) over remote analyses of samples by landers and rovers.21 For the foreseeable future, the capabilities for remote analyses cannot hope to match those available in Earth-based laboratories in terms of sensitivity, accuracy, and precision. Moreover, given their severe mass- and power-limitations, spacecraft can only be equipped with a limited suite of analytical instruments, whereas the entire range of analytical capabilities available in Earth-based laboratories can be brought to bear on returned samples. Many analytical techniques require extensive sample preparation, which cannot be duplicated in remote spacecraft protocols. Analysis of chemical gradients in rocks requires spatial (depth) resolution that is difficult to achieve on Mars but would be relatively easy if done on Earth.
If properly stored and isolated from the terrestrial environment, returned samples can be examined by more sensitive analytical techniques and methodologies that are likely to be developed in the future. Returned samples