The essential energy, water, and nutrient requirements to support and sustain life on Mars are currently present, and the Martian geologic record offers tantalizing clue of many ancient habitable environments.
If life emerged and evolved on early Mars then it is possible, and indeed likely, that physical or chemical biosignatures are preserved in the exposed rock record.
These discoveries and inferences make a compelling case for a rover mission designed to explore for evidence of past Martian life.

For over a decade, the Mars Exploration Program has pursued a strategy of "follow the water".
While this strategy has been highly successful in the Mars missions of 1996-2007, it is increasingly appreciated that assessing the full astrobiological potential of Martian environments requires going beyond the identification of locations where liquid water was present.
Thus, in order to seek signs of past or present life on Mars, it is necessary to characterize more comprehensively the macroscopic and microscopic fabric of sedimentary materials.
This type of information would be critical to select and cache of relevant samples for addressing the life question in samples intended for study in sophisticated laboratories on Earth.

The possible strategy of using rovers to collect and cache geological samples for possible subsequent return to Earth has been discussed as far back as at least the mid-1990s.
In 2007 it was recommended sample caching on all surface missions that follow the Mars Science Laboratory Curiosity rover, in a way that would prepare for a relatively early return of samples to Earth.
In mid-2007, NASA directed that a very simple cache be added to the Curiosity rover and, although they endorsed the potential value of sample caching, experts raised serious concerns regarding sample quality for this specific implementation.
In November 2008, the cache was descoped to make room for tools to clean the rover's sample acquisition equipment, which were added due to sample handling problems encountered by the Phoenix lander.

A mid-range rover concept was originally included in the planning work of the Mars Architecture Tiger Team (MATT).
By the time of the MATT-3 report in 2009, the potential mission was referred to with several different working names, including both 'Mid-Range Rover' and 'Mars Prospector Rover', and the mission concept was generically envisioned as including a single Mars Exploration Rover or Mars Science Laboratory-class rover with precision landing and sampling/caching capability.
To provide a name that fit the mission concept better, it was changed in August 2009 from the generic Mid-Range Rover (MRR) to Mars Astrobiology Explorer-Cacher (MAX-C).

In April 2011, because of a budgeting crisis, a proposal was announced to fly only one rover in 2018 that would be larger than either of the vehicles in the paired concept, ExoMars (ESA) and MAX-C (NASA).
One suggestion was that the new vehicle be built in Europe and take on a combination of European and USA instruments.
NASA proposed to provide a launch rocket and the "Sky Crane" landing system.
In February 2012, NASA terminated its participation in ExoMars due to budgetary cuts, and when Congress reinstated funding for Mars exploration after a loud outcry from planetary scientists and enthusiasts, NASA announced in December 2012 the Mars 2020 rover and, later, that it would indeed prepare a sample cache.
The essential energy, water, and nutrient requirements to support and sustain life on Mars are currently present, and the Martian geologic record offers tantalizing clue of many ancient habitable environments. If life emerged and evolved on early Mars then it is possible, and indeed likely, that physical or chemical biosignatures are preserved in the exposed rock record. These discoveries and inferences make a compelling case for a rover mission designed to explore for evidence of past Martian life.
For over a decade, the Mars Exploration Program has pursued a strategy of "follow the water". While this strategy has been highly successful in the Mars missions of 1996-2007, it is increasingly appreciated that assessing the full astrobiological potential of Martian environments requires going beyond the identification of locations where liquid water was present. Thus, in order to seek signs of past or present life on Mars, it is necessary to characterize more comprehensively the macroscopic and microscopic fabric of sedimentary materials. This type of information would be critical to select and cache of relevant samples for addressing the life question in samples intended for study in sophisticated laboratories on Earth.
The possible strategy of using rovers to collect and cache geological samples for possible subsequent return to Earth has been discussed as far back as at least the mid-1990s. In 2007 it was recommended sample caching on all surface missions that follow the Mars Science Laboratory Curiosity rover, in a way that would prepare for a relatively early return of samples to Earth. In mid-2007, NASA directed that a very simple cache be added to the Curiosity rover and, although they endorsed the potential value of sample caching, experts raised serious concerns regarding sample quality for this specific implementation. In November 2008, the cache was descoped to make room for tools to clean the rover's sample acquisition equipment, which were added due to sample handling problems encountered by the Phoenix lander.
A mid-range rover concept was originally included in the planning work of the Mars Architecture Tiger Team (MATT). By the time of the MATT-3 report in 2009, the potential mission was referred to with several different working names, including both 'Mid-Range Rover' and 'Mars Prospector Rover', and the mission concept was generically envisioned as including a single Mars Exploration Rover or Mars Science Laboratory-class rover with precision landing and sampling/caching capability. To provide a name that fit the mission concept better, it was changed in August 2009 from the generic Mid-Range Rover (MRR) to Mars Astrobiology Explorer-Cacher (MAX-C).
In April 2011, because of a budgeting crisis, a proposal was announced to fly only one rover in 2018 that would be larger than either of the vehicles in the paired concept, ExoMars (ESA) and MAX-C (NASA). One suggestion was that the new vehicle be built in Europe and take on a combination of European and USA instruments. NASA proposed to provide a launch rocket and the "Sky Crane" landing system. In February 2012, NASA terminated its participation in ExoMars due to budgetary cuts, and when Congress reinstated funding for Mars exploration after a loud outcry from planetary scientists and enthusiasts, NASA announced in December 2012 the Mars 2020 rover and, later, that it would indeed prepare a sample cache.
Mars Astrobiology Explorer-Cacher