I think this sort of answers your question. Weight is one of the key factors as well as cost for any instrument for any planetary spacecraft.

The MECA Wet Chemistry Lab - Precursor of RCAL The Microscopy, Electrochemistry, & Conductivity Analyzer (MECA)

1-4] is a set of instruments that was designed to provide answers to many of the above questions. The MECA instrument package, funded by NASA's Human Exploration and Development of Space (HEDS) Enterprise, was developed at the Jet

Propulsion Laboratory in collaboration with ThermoOrion (Beverly, MA), Starsys Research (Boulder, CO), and the Kounaves Group at Tufts University. MECA was flight qualified for what would have been the Mars 2001 Odyssey Lander, but cancelled after the crash of the Mars 1999 Polar Lander. In May 2002 it was proposed as part of Phoenix, which in August of 2003 was selected by NASA to fly as the 2007 Mars Scout Mission. MECA (Figures 1 & 2) included a Microscopy Station [3] containing optical and atomic force microscopes, an electrical and thermal conductivity analyzer,a set of Adhesion/Abrasion Plates, and a Wet Chemistry Laboratory (WCL) [1,2,4]. The MECA-WCL contains an array of sensors which allow for determination of a wide variety of inorganic ions and electrochemical parameters, including, Ag+, Br-, Ca2+, Cd2+, Cl-, ClO4-, Cu2+, HCO3- , Hg2+, I-, K+, Li+, Mg2+, Na+, NH4+, NO3-, SO42-, Pb2+, pH, oxidants, reductants, redox potential, conductivity, dissolved O2 and CO2 .

RCAL & the 2009 Mars Science Laboratory

The RCAL is being proposed as a primary instrument for the 2009 Mars Science Laboratory (MSL). It expands on the MECA concept by providing twenty individual sealed sample chambers mounted on a rotating carousel. (Figure 3) The soil, after delivery by an external mechanism such as a robotic arm or sub-surface drill, is loaded into a dual soil hopper. Multiple small samples can then be taken from the hopper and delivered to the test chambers. After the chamber is punctured, one of a set of four electrodes mounted over the carousel can be inserted into the selected chamber. The RCAL will enable a variety of bench-top wet chemistry analyses of the Martian regolith, assessing its interaction with water and various reagents, and ultimately providing unique scientific information about the geochemical and biological history of Mars.Â

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