Why is 90 sols a standard mission duration for robotic spacecraft on the surface of Mars?

Question

Several robotic Mars surface missions have a planned mission duration of 90 sols.

I would like to understand why this specific mission lifetime is commonly used by mission planners.

Below is a non-exhaustive list of a few past/present/proposed missions where this has been the case, where the original planned mission duration can be found in the associated hyperlink:

• Mars Exploration Rovers (Sprit and Opportunity):
• Phoenix:
• Zhurong Rover:
• Mars Polar Lander:
• Icebreaker:

Interestingly, these missions do not have similar landing site locations or primary mission objectives.

Clearly missions that successfully land can often go on to have an operational lifetime far longer than this (notably the Mars Exploration Rovers, Spirit and Opportunity, which far outlived their original planned mission duration by several years).

The rationale for including a deadline to meet all mission objectives is obvious; mission operations are expensive. A mission which can complete all its primary objectives in a reasonable timeframe is likely to be far more appealing to the stakeholders funding the mission. Similarly, proposing a robotic mission which drags-on indefinitely when its primary objectives can be achieved in a short period of time is unlikely to get funded.

But why specifically 90 sols? Why do many missions consider the same primary lifetime? Why not 80 or 100 sols?

I believe the answer has something to do with the fact that all these missions listed above are solar powered. Curiosity and Perseverance are notable admissions from the list – both use RTGs which are likely needed to achieve their respective mission objectives that cannot be achieved in a “short” mission.

Dust accumulation might be the reason, but I cannot find a source that states this directly. Furthermore, the Mars Global Dust Storm Season (GDSS) is commonly cited as occurring between solar longitudes Ls180 and Ls360 (i.e. from the start of NH Autumn to the end of NH Winter). The time between two potential GDSSs is ~372 sols, far greater than 90 sols.

Expected solar irradiation through NH Spring and Summer might be another reason, but mission duration could then be augmented to match the energy budget by varying solar array size – so the question remains, why 90 sols?

Answer 1

I believe I might have found an answer to this question in this interesting paper, which reviews the origin of duration and distance requirements for planetary mobility systems.

It appears the 90 sol value could tie its origin to the early studies for the Mars Exploration Rovers - Spirit and Opportunity. According to the paper, at the time of MER's early formulation it was reportedly in competition for selection against alternative mission concepts for the 2003 launch opportunity:

Specifically, a decision as to whether an orbiter or a lander/ rover would be developed for the 2003 opportunity was pivotal, and the anticipated scientific capability (and thus engineering requirements to support them) had to be attractive enough to merit selection.

In order to remain competitive, MER's mission design team were encouraged to extend the mission from an originally proposed 30-sol duration to be at least equal to the 86-sol lifetime of the 1997 Sojourner rover (Mars Pathfinder). According to design studies at the time, the team believed to be able to maintain a positive power margin up to 91 sols. The team then accepted a 90-sol mission.

As the paper states, it is an instance where the capability became the requirement.

It may be the case that several missions after MER have simply used this as a bench mark requirement.