As with so much of this mission, the goals began as two separate set of goals. ESA looked to extend the science that the Mars Science Laboratory (MSL) will begin next year -- in-depth analysis of samples on the surface of Mars. The ExoMars rover has a larger suite of instruments than MSL, and unlike MSL which will collect its samples near the surface, MSL would drill up to two meters below the surface. NASA's rover, by contrast, would focus on collecting and caching samples from within a few centimeters of the surface. If the samples were eventually returned (requiring two follow on missions), the full sophistication of instruments on Earth would be used to unveil the secrets of the Red Planet
Now that ESA and NASA have decided to pool their resources to fly a joint 2018 rover, a science working group has begun defining the goals for the combined mission. The group shared it's early thinking at a joint European and American Mars Exploratory Analysis Group (MEPAG) meeting last month.
A number of presentations discussed the goals and implementation of the merged rover, with the focus of the discussion on the sample collection goals. This may reflect the relative maturity of the previous rover concepts. While NASA's rover was in early definition, the ExoMars rover was ready to cut metal and proceed towards launch prior to the merger of the two programs. However, what unites these two missions (other than the need to pool financial resources) is their goal to, "Search for evidence of past and present life on Mars."
Proposed science and sampling goals. From On-Mars measurements and strategies, summary
Mars may or may not prove to have signs of life (at least at the spots we explore), but it is unique in at least one respect. It retains a record of conditions on a world with a significant atmosphere and liquid water from when it and the solar system was young. While the first and mandatory goal is to search for signs of life or habitability, five mission goals focus on understanding the early formation of Earth-like worlds and the processes acting on their surface, especially those involving water. (Two goals relate to evaluating Mars for potential human exploration.)
The science goals lead to a list of required samples to cache. Highest priority goes to samples that have been exposed to water in the earliest epochs of Mars to look for signs of life and to better understand the role liquid water played in that planet's history. The next goal is to sample igneous rocks to determine the process of early planet formation. Rounding out the requirements would be samples of regolith (the dust, sand, and gravel on the surface) and the atmosphere.
The atmosphere can be sampled from any location, and regolith will be present almost anywhere. Studies from orbiters have revealed many locations altered by water and many with volcanic rocks. An early assessment of potential landing sites is showing that finding ancient water-altered material and volcanic rocks at the same site may prove difficult within reasonable roving distances. ESA and NASA plan to follow the same open process used to select the Mars Science Laboratory's landing site, and the science community has five to six years to find one or more sites that possess the right surface materials.
Resources: You can read the complete set of presentations at the MEPAG website: http://mepag.jpl.nasa.gov/meeting/jun-11/index.html
Editorial Thoughts: I am a strong supporter of the 2018 merged rover mission. Even if samples are never returned to Earth, the mission will continue the work of sophisticated analysis of samples at Mars that will begin next year with the landing of the Mars Science Laboratory (MSL). Presumably, this rover will go to a second site, but if MSL finds strong signs of possible life, it may follow up MSL's discoveries at Gale Crater. The investment of $1-1.5B by each space agency just for this goal seems to me to be a worthwhile investment.
The merged rover also offers the opportunity for a far more robust sample caching mission. NASA's original plan had been to collect samples from near the surface, where multiple forces can destroy organic material. ESA's deep drill offers the opportunity to collect samples from depths where more pristine samples are likely. There will have to be some engineering work done to enable the transfer from ESA's drill to NASA's sample cache system, so this is not a done deal, but it is a great opportunity.
Beyond the immediate great science that can be done on the surface of Mars by the rover, there is the hope that with samples collected and waiting on Mars, that governments will pony up the further investments needed to return those samples to Earth.
This mission still may face significant budgetary challenges -- it is a Flagship-scale mission in an era where budgets are tight on both sides of the Atlantic. I have my fingers crossed.