NASA's proposed action seems likely to fail legal review, since a BSL-4 facility can't comply with the 2012 European Space Foundation study's limit (Ammann et al, 2012:14ff):

"The release of a single unsterilized particle larger than 0.05 µm is not acceptable under any circumstances".

Their rationale: viable starvation limited ultramicrobacteria can pass through a 0.1 µm filter (Miteva et al, 2005).

This limit is easier to achieve in water under high pressure. One study achieved 100% removal of 0.03 µm polioviruses using carbon nanotubes loaded with silver. (Kim et al, 2016) (Singh et al, 2020:6.3).

However aerosol filters are less effective. Even ULPA level 17 filters remove only 99.999995%. Also those filters are only tested to 0.12 µm (BS, 2009:4). At the ESF's 0.05 µm, an experimental 6-layer charged nanofiber filter for coronaviruses filtered out 88% of ambient aerosol particles (Leung et al, 2020), far from 100% containment.

The ESF also said the chance of release of even a single unsterilized particle at 0.01 µm must be less than 1 in a million, to stop gene transfer agents which readily transfer novel capabilities to unrelated species of archaea overnight in sea water (Maxmen, 2010).

The ESF said both requirements need regular review, as later research might reduce size limits further.

A review board could consider research since 2012 into small synthetic minimal cells (Lachance, 2019), and protocells (Joyce et al, 2018). Also, ideas for simpler "RNA world" cells without ribosomes or proteins (Benner et al, 2010: 37) could be revisited using new research on ribocells (Kun, A., 2021). Panel 4 for the 1999 "Size limits" workshop calculated that such a primitive free living lifeform could be as small as 0.014 µm in diameter and 0.12 µm in length, if there is an efficient mechanism for packing its RNA (Board et al, 1999: 117). 

Biologists have searched for a shadow biosphere of nanobes (Cleland, 2019, pp 213-214) which could co-exist with modern life. They didn't find these nanobes, but they are biologically credible, because such small cells have an advantage in an environment with low nutrient concentrations, as they have a larger surface to volume ratio, and so take up nutrients more efficiently. They would also avoid protozoan grazing (Ghuneim et al, 2018).

If Mars has early life nanobes, even with less sophisticated biology, they might be able to compete in a shadow biosphere on Earth. In a worst case scenario, mirror numbers with the right enzymes (isomerases) would convert normal organics in an ecosystem into mirror organics that only mirror life can use, or rare terrestrial microbes with the ability to metabolize mirror organics.

This size limit review, and the following legal process, may change requirements. They are best completed before we launch the Earth return orbiter, Earth Entry Vehicle, and Mars Ascent Vehicle or build the return facility.

The legal process can also conclude that the required technology doesn't yet exist.

Uhran et al estimate a minimum of 6-7 years to complete the legal process starting from the Environmental Impact Statement date, so that's 2028 at earliest. This can be significantly extended if challenged in the courts. International bodies like the WHO and FAO likely get involved and international treaties triggered (Uhran et al, 2019).

Also, NASA is required to provide preliminary design and engineering details for the Sample Return Facility before they start a build, and with a life-cycle cost over $250 million must also commit to Congress on cost and schedule (NASA, Science Engineering Handbook: section 3.5).

Uhran et al estimate 9 years to build or repurpose the facility. It needs 2 years to train scientists because of many lapses in Apollo sample handling. 

So, if the build starts in 2028, the earliest the facility can be ready is 2039.

I propose two solutions.

1. sterilize samples first, e.g. during the return journey with low energy nanoscale X-ray emitters. Any present day life would be recognizable after sterilization,
OR
2. return unsterilized samples to a safe orbit where astrobiologists study them remotely using miniature instruments such as those designed for life detection on Mars. Return sterilized sub-samples to Earth immediately;

My paper recommends adding dust capture (Jakovsky et al, 2021) to the ESF Fetch rover to search duststorms for viable spores, and capability to take a scoop of dirt to follow up Viking results, for higher chance to return life in 2.

2. needs care. A return to the ISS doesn't break the chain of contact with Mars, and COSAPAR say the Moon must be kept free of contamination for future astronauts and tourists (COSPAR, 2011).

My paper solves both issues with a return to the Laplace plane above GEO. This is where ring particles orbit in a ring system, and a stable orbit for any satellite debris.

For details and cites, see my preprint at: https://osf.io/rk2gd (in progress). DOI 10.31219/osf.io/rk2gd