PNAS
2025 Vol. 122 No. 2 e2404253121
https://doi.org/10.1073/pnas.2404253121
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PERSPECTIVE
Mars Sample Return: From collection to curation of samples
from a habitable world
Francis M. McCubbin
a,1
, Kenneth A. Farley
b
,c
, Andrea D. Harrington
a
, Aurore Hutzler
d
, and Caroline L. Smith
e,f
Edited by Harry McSween, The University of Tennessee Knoxville, Knoxville, TN; received April 1, 2024; accepted July 2, 2024
NASA’s Mars 2020 mission has initiated collection of samples
from Mars’ Jezero Crater, which has a wide range of ancient
rocks and rock types from lavas to lacustrine sedimentary
rocks. The Mars Sample Return (MSR) Campaign, a joint effort
between NASA and ESA, aims to bring the Perseverance
collection back to Earth for intense scientific investigation.
As the first return of samples from a habitable world, there
are important challenges to overcome for the successful
implementation of the MSR Campaign from the point of
sample collection on Mars to the long
-
term curation of the
samples on Earth. In particular, the successful execution
of planetary protection protocols adds well
-
warranted
complexity to every step of the process from the two
MSR Program flight elements to the ground element at
the sample receiving facility (SRF). In this contribution,
we describe the architecture of the MSR Campaign, with
a focus on infrastructure needs for the curation (i.e., the
clean storage, processing, and allocation) of pristine Martian
samples. Curation is a science
-
enabling and planetary
protection
-
enabling activity, and the curation practices
described in this contribution for the SRF and any long
-
term
curation facility will enable the sample safety assessment,
initial scientific investigations of the samples, and establish
the MSR collection as a scientific resource that will enable
generations of science and discovery through studies of
the returned Mars samples. The planetary protection and
curation processes established for MSR will provide critical
insights into potential future sample return missions from
other habitable worlds like Enceladus and Europa.
Jezero Crater | astromaterials | astrobiology | planetary protection |
Martian
The origin and history of the solar system, as well as the
origin and history of its distinct parent bodies, are chronicled
within the solar system rock record. That rock record could
hold clues about some of the most interesting phenomena
that humanity seeks to understand, such as the origin of life
and the process of abiogenesis, the processes that have led
to the diversity in parent bodies across the solar system, and
the sources of material that comprise our solar system. For
centuries, humans have used the Earth’s rock record to
understand Earth’s natural history and to understand life’s
origins. The latter remains one of the most important unan
-
swered questions of our time. Although we have excellent
access to Earth’s surface and near
-
surface rocks, Earth rep
-
resents only one of many planetary bodies that comprise the
solar system.
Extraterrestrial samples enable comparisons between Earth
and other worlds, allowing us to further develop an under
-
standing of natural processes beyond Earth. Furthermore, the
ubiquitous secondary overprint of Earth’s crustal rocks coupled
with active plate tectonics has further limited what can be
gleaned about early Earth from its own rock record. Our access
to other portions of the solar system rock record is limited to
the extraterrestrial samples we can collect on Earth such as
meteorites and cosmic dust, as well as material that we collect
on other parent bodies and return to Earth through robotic
and human space exploration. Analyses of these astromate
-
rials have led to unprecedented advances in our understand
-
ing of Earth and the solar system broadly, including the
development of models for the origin of the Earth–Moon sys
-
tem ( 1 – 4 ), the age of our solar system ( 5 ), the bulk composi
-
tions of planets and the solar system ( 6 , 7 ), and identification
of many of the fundamental processes that have governed
and shaped the solar system and its parent bodies to this point
( 8 – 14 ). However, much remains unknown, and each new extra
-
terrestrial sample or set of samples we acquire helps to further
elucidate the formation and evolution of our solar system and
its parent bodies, including Earth. One lesson learned through
decades of planetary sample science is that the scientific value
of any sample from the solar system is impactful beyond the
parent body from which it was derived and adds to our overall
knowledge of the solar system rock record.
Meteorites and cosmic dust represent our most bountiful
resource for understanding the solar system rock record.
These samples represent a valuable and renewable scientific
resource that continues to yield a wealth of information
about the formation and evolution of our solar system and
its parent bodies. These materials naturally rain down upon
the Earth, and a subset is collected, curated, and allocated
for scientific study. However, as a consequence of their
delivery mechanism, these materials have a high degree of
terrestrial contamination and lack a priori context about
Author affiliations:
a
Astromaterials Research and Exploration Science Division, NASA
Johnson Space Center, Houston, TX 77058;
b
Jet Propulsion Laboratory/California Institute
of Technology, Pasadena, CA 91109;
c
Division of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA 91125;
d
European Space Agency/
European Space Research & Technology Centre, Keplerlaan 1, Postbus 299, 2200 AG
Noordwijk, The Netherlands;
e
Science Group, Natural History Museum, London SW7 5BD,
United Kingdom; and
f
School of Geographical and Earth Sciences, University of Glasgow,
Glasgow G12 8QQ, United Kingdom
Author contributions: F.M.M., K.A.F., A.D.H., A.H., and C.L.S. designed research; performed
research; and wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Copyright © 2025 the Author(s). Published by PNAS. This open access article is distributed
under
Creative Commons Attribution
-
NonCommercial
-
NoDerivatives License 4.0
(CC BY
-
NC
-
ND)
.
1
To whom correspondence may be addressed. Email: francis.m.mccubbin@nasa.gov.
This article contains supporting information online at
https://www.pnas.org/lookup/
suppl/doi:10.1073/pnas.
2404253121/-
/DCSupplemental
.
Published January 6, 2025.
OPEN ACCESS
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their point of origin. Both of these factors limit the scientific
utility of meteorites to answer scientific questions that are
dependent upon context and limited terrestrial contamina
-
tion (e.g., 15 ).
Sample return missions offer an alternative approach to
astromaterials acquisition where samples from another parent
body can be deliberately collected and returned to Earth in
a controlled and prescribed manner to a controlled environ
-
ment, designed with specific scientific questions in mind
( 16 – 18 ). The first example of such an endeavor is the Apollo
11 mission to the Moon, where astronauts landed on the
Moon, collected 21.55 kg of samples from the lunar surface,
and returned to Earth with those samples. These samples
were delivered to a curation facility that was designed to keep
the samples in a pristine “as returned” state in perpetuity.
One of the biggest benefits of sample return missions is the
ability to curate samples in an environment that can preserve
the scientific integrity of the samples over many years. The
careful curation and conservation of samples allows for their
availability when new scientific questions arise. Furthermore,
as technology advances and new generations of scientists
emerge, the samples can be used to answer questions that
were not possible to answer, or even conceived of, at the
time of sample return.
In total, humans have successfully executed 15 sample
return missions, 10 from the Moon, three from asteroids, one
from the coma of a comet, and one that collected solar wind
at Earth
-
Sun Lagrange point one ( 18 – 21 ). Other than the first
few Apollo missions, there was little concern about the poten
-
tial for extraterrestrial life to be within the returned samples
and/or for the returned samples to pose a threat to life on
Earth. This lack of concern was motivated by the fact that all
of the parent bodies or collection points thus far are not con
-
sidered to be habitable. However, NASA’s Mars 2020 mission,
through the Perseverance Rover, has initiated collection of
samples from the planet Mars ( 22 ) in Jezero Crater. A joint effort
between NASA and ESA referred to as the Mars Sample Return
(MSR) Campaign is designed to bring the Perseverance collec
-
tion back to Earth by 2040 ( 23 ). This endeavor would be human
-
ity’s first return of samples to Earth from another planet and
the first return of samples from a habitable world.
The MSR Campaign was described as the highest scientific
priority of NASA’s robotic exploration efforts for the last two
Planetary Science Decadal Surveys ( 24 , 25 ). The MSR Campaign
consists of the Mars 2020 mission, two MSR Program flight
elements, and one ground element ( Fig. 1 ). Each of these
portions of the MSR campaign architecture has immense,
but not insurmountable, challenges that must be overcome,
from sample collection to long
-
term curation. In this contri
-
bution, we highlight the functional steps in the MSR architec
-
ture, with a focus on aspects related to planetary protection
and the curation of the samples after they arrive on Earth.
Although we highlight curation activities that occur in an SRF
and in a long
-
term curation facility, we focus on the aspects
of curation that will best enable the sample safety assess
-
ment, initial scientific investigations of the samples, and
establish the MSR collection as a scientific resource that will
allow generations of science and discovery through studies
of the returned Mars samples.
Collection of Scientifically Compelling Samples
and Considerations for Their Curation
The Mars 2020 mission is tasked with collecting a scientifically
compelling set of samples that are worth delivering to Earth
for intense scientific examination ( 22 ). The processing, sub
-
division, and handling of these samples, prior to allocation
for scientific study or for life
-
detection studies associated
Fig. 1.
Notional MSR Campaign architecture (Image Credit: Adapted with permission from ref. 26). The cartoon is intended to demonstrate functional steps for
the MSR Campaign and, other than the Mars 2020 Mission, may not represent the final campaign mission architecture.
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2025 Vol. 122 No. 2 e2404253121
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with planetary protection requirements, will be done by cura
-
tion personnel. Planning for curation activities in the SRF and
in a long
-
term curation facility requires knowledge about the
types of samples that will be returned and the types of sci
-
entific investigations that will be prioritized. All of the sam
-
ples collected by the Perseverance rover are in sealed sample
tubes, but these tubes host a wide variety of sample types
that need to be considered when designing and planning for
curation infrastructure. As of Spring 2024, the Perseverance
Rover has collected and sealed a total of 21 rock cores (8
igneous and 13 sedimentary), two regolith samples, one sample
of Martian atmosphere, and three witness tubes designed to
document Earth
-
sourced contamination ( 27 ). The rover holds
17 of these samples, and 10 of them have been placed in the
Three Forks sample depot on the Martian surface to provide
flexibility to the plans to return the samples.
Based on the samples collected thus far, the sample pro
-
cessing infrastructure in any MSR curation facility (i.e., SRF or
long
-
term curation facility) will need the capability to process
rock cores, unconsolidated regolith (and partially broken or
disintegrated rock cores), and gas samples (including head
-
space gases in all the sample tubes). Furthermore, there is
substantial diversity in the rock types being returned that will
also have important implications for curation infrastructure
needs for handling and processing the returned samples.
Samples collected from the floor of Jezero Crater by the
Perseverance rover during the Crater Floor Campaign include
basaltic lava flows from the heavily cratered Máaz Formation
and an older igneous olivine cumulate lithology that underlies
the lava flows, referred to as the Séítah Formation ( 28 ). Both
of these lithologic units have experienced varying degrees of
aqueous alteration ( 29 – 31 ) that could prove challenging for
sample processing if the aqueously altered portions of the
samples are friable or result in partial or full disintegration of
the core samples before opening or during extraction and
subsequent processing. Understanding the state of the sam
-
ples prior to opening, through methods such as X
-
ray com
-
puted tomography (XCT), will be crucial for understanding the
state of each sample tube before it is opened and to develop
a sample processing plan for each sample tube ( 32 , 33 ).
In addition to the igneous samples collected from the floor
of Jezero Crater, the Perseverance rover has also collected
13 sedimentary samples and 2 regolith samples from the
Delta Front, Upper Fan, and Margin Campaigns ( 27 ). The
Delta Front samples include a variety of sandstones and
mudstones that record fluvial, deltaic, and lacustrine settings
in Jezero Crater ( 34 ). These sedimentary rocks are composed
of detrital igneous grains and products of aqueous alteration.
These aqueous alteration products include various clay min
-
erals, sulfates, oxides, chloride salts, hydrated silica, and
carbonates ( 27 ). Furthermore, many of these samples host
cements composed of aqueous alteration products. Special
consideration of the curation environment will be needed to
prevent alteration of the most sensitive phases that could
undergo phase changes over time as a function of relative
humidity, temperature, and pressure including some clay and
sulfate minerals. In fact, there will likely be some time
-
sensitive measurements that will need to occur in the SRF
shortly after opening select sample tubes to avoid loss of
science ( 35 ).
In addition to considering the physical forms of the sam
-
ples that will be returned within the sample tubes and the
various rock and mineral types they will host, the types of
scientific investigations that will be prioritized must also be
considered when designing the infrastructure and proce
-
dures associated with sample processing. Such considera
-
tions will ensure that Earth
-
based contamination is kept
below any threshold values that could compromise the pri
-
oritized scientific investigations. Based on the samples that
are present within the Perseverance rover and at the Three
Forks Depot, the scientific investigations of the samples will
span a broad range of topics about Mars and the solar sys
-
tem at large. For example, the igneous rocks collected during
the Crater Floor Campaign hold invaluable information about
the interior of Mars, and they will enable transformative
advances in our understanding of the physicochemical evo
-
lution of Mars and the nature of its mantle and crust.
Furthermore, these samples will provide key insights into the
chemical composition of the building blocks that formed
Mars, which will aid in understanding the nature and distri
-
bution of materials that formed the terrestrial planets at the
nascent stages of solar system evolution, including processes
such as the delivery of volatiles like water ( 36 – 43 ). Both the
igneous and sedimentary samples from Jezero Crater will be
targeted for detailed organic studies as both exhibit lumi
-
nescent properties that offer tantalizing evidence of potential
organics in the samples, although inorganic sources could
also be responsible for the observed luminescence features
( 31 , 44 ). Studies of the aqueous alteration products pre
-
served in the Jezero Crater samples will yield key insights into
Martian surface processes and elucidate further on the hab
-
itable conditions that once existed at the Martian surface
( 45 ). The presence of aqueous alteration phases, particularly
in fine
-
grained samples from subaqueous paleoenviron
-
ments recorded in the facies 2 rocks of the Shenandoah
Formation, has a high potential for biosignature preservation
( 34 ). These investigations represent only a minor fraction of
the types of studies that will be done on the returned sam
-
ples ( 46 ), but they highlight the need for stringent inorganic,
organic, and biological contamination control measures in
the curation facilities where the samples will be opened, pro
-
cessed, and allocated ( 32 , 47 ). In addition to contamination
control measures, contamination knowledge strategies will
be needed to facilitate quantification of contamination
throughout sample processing activities. These strategies will
include the deployment of witness materials within the cura
-
tion environment that will act as procedural blanks that are
processed alongside the Martian samples ( 48 ) and could also
include analysis of hardware coupons or processing tools
( 49 ). Additionally, they include active monitoring of the cura
-
tion environment for inorganic, organic, and biological con
-
tamination ( 16 – 18 ).
The Martian samples collected thus far by Perseverance
have an abundance of science value, regardless of whether it
is the samples on the Perseverance rover or the samples from
the Three Forks Depot that are ultimately returned. Returning
these samples for study in terrestrial laboratories will enable
hypothesis testing and independent validation of important
rover observations using laboratory
-
based instruments with
capabilities that cannot be achieved with flight instruments.
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With the successful execution of stringent contamination con
-
trol and contamination knowledge measures during the MSR
Campaign, including in the curation facilities, that covers inor
-
ganic, organic, and biological contamination, returning either
set of samples collected thus far would transform our under
-
standing of the solar system rock record broadly. The Perse-
verance rover continues to explore the Jezero Crater margin,
and it will eventually arrive at the crater rim where additional
samples of high scientific value could be collected that will only
add to the scientific value of the collection of samples on the
Perseverance rover. With a scientifically compelling set of sam
-
ples collected, the first major challenge to a successful MSR
Campaign has already been surpassed.
Sample Retrieval and Return to Earth
The two flight elements for the MSR Program are still in devel
-
opment, but the top
-
level functional needs for these flight
elements have been identified. The first flight element is the
Sample Retrieval Lander that will retrieve up to
~
30 sealed
sample tubes and place them within transport hardware called
the Orbiting Sample (OS) for return to Earth. The retrieval of
the samples will either occur directly from the Perseverance
rover, or they will be collected from the Three Forks Sample
Depot. Having both options mitigates the risk of one of the
two options not being tenable at the time of retrieval. The
Sample Retrieval Lander will also have a launch capability
referred to as the Mars Launch System that will launch the OS
into Mars orbit. The second flight element consists of an Earth
Return Orbiter that is responsible for capturing the OS from
Mars orbit and placing it within a secondary containment ves
-
sel that will be capable of safely delivering the sample to Earth
through an Earth Entry System (EES). A cartoon depicting the
functional execution of the MSR Campaign, including the Mars
2020 mission, the two MSR Program flight elements, and the
ground element of MSR is depicted in Fig. 1 .
One of the biggest challenges to overcome during the two
flight elements of the MSR Program architecture is the process
referred to as “breaking the chain” and represents the step
for which biocontainment of the OS is achieved prior to being
sealed into the secondary vessel that is integrated within the
EES. Although a specific implementation for breaking the
chain has not been selected, the functional requirement for
this step will involve sterilization of the outer portion of the
OS so that it is not a risk to Earth’s biosphere after it lands.
This sterilization step is part of the overall planetary protec
-
tion strategy for the MSR Campaign. Notably, Jezero Crater
was chosen, in part, as the sampling target for MSR because
it is a place of past aqueous activity on Mars and has the
potential to preserve an ancient record of aqueous processes
and potential past life. In addition, Jezero Crater is not in a
“special region” that is thought to potentially host extant
Martian life ( 50 ), and life
-
detection is not one of the scientific
objectives of MSR ( 22 , 51 ). However, life
-
detection investiga
-
tions of the MSR samples will be part of the sample safety
assessment to meet planetary protection requirements ( 52 ).
The United Nations’ (UN) Outer Space Treaty of 1967 ( 53 )
outlines the basic framework of planetary protection in its
article IX, which declares “...States Parties to the Treaty shall
pursue studies of outer space, including the moon and other
celestial bodies, and conduct exploration of them so as to
avoid their harmful contamination and also adverse changes
in the environment of the Earth resulting from the introduction
of extraterrestrial matter and, where necessary, shall adopt
appropriate measures for this purpose...” The United States
and ESA’s member states are signatories of the Outer Space
Treaty, and this treaty motivated NASA Procedural Requirement
(NPR) 8715.24 entitled: Planetary Protection Provisions for
Robotic Extraterrestrial Missions. This NPR defines planetary
protection designations for the return phases of sample return
missions as being either Class V Unrestricted or Class V
Restricted Earth
-
return. For samples that are returned from a
habitable world, like Mars, the return phase of the flight mis
-
sion is designated as Class V Restricted Earth
-
return. Although
the likelihood of returned Martian samples from Jezero Crater
having a deleterious impact on the terrestrial biosphere is
exceptionally low ( 22 , 54 ), these planetary protection policies
represent important safeguards to establish procedures for
mitigating the risks associated with returning samples that
could harbor life. These policies are motivated by the idea that
“preventing harmful biological contamination of Earth’s bio
-
sphere is the highest priority.” The term “break the chain”
comes from NPR 8715.24, and it requires a strategy to be
implemented that will isolate and robustly contain the return
samples prior to delivery to the Earth–Moon system. Several
break the chain implementation strategies are under evalua
-
tion and include methods such as redundant layering, chem
-
ical, heat, or ultraviolet (UV) sterilization ( 55 ).
Once the chain is broken, the sample capsule will be per
-
mitted to enter the Earth–Moon system. Once the EES is “Go”
for Earth Entry, it will enter the Earth’s atmosphere and land.
At this time, the EES is being designed such that it can with
-
stand a terminal velocity landing without the aid of a para
-
chute given the risks associated with parachute reliance and
the challenges associated with mass and volume that come
with including a parachute. Although the engineering imple
-
mentation for the two flight elements of the MSR Program will
offer many challenges, there is nothing in the current archi
-
tecture that is considered to be impossible to overcome.
As the EES is heading through the Earth’s atmosphere,
recovery operations on the ground will already be underway
to 1) assess whether it is a nominal landing, 2) have a team
ready to recover the capsule from the landing site, and 3)
have a team ready to transport the EES to a temporary stag
-
ing area, possibly for the first steps of hardware deintegra
-
tion, before it is transported to the SRF. The ground element
of MSR is referred to as the Sample Receiving Project (SRP),
and this step begins once the samples land on Earth and are
transported to a facility where curation activities, early sci
-
ence activities, and sample safety assessment activities will
be conducted ( Fig. 1 ). Other than the additional biocontain
-
ment requirements that come with a Restricted Earth
-
return
mission, much of the recovery operations can take advantage
of lessons learned from recent sample recovery operations
for unrestricted sample return missions like Genesis, Stardust,
Hayabusa, Hayabusa2, and OSIRIS
-
REx.
The most significant departure from an unrestricted
return is to develop a baseline biocontainment plan that, at
least in part, considers the possibility that the EES partially
breaches upon landing because it may not be possible to
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confirm whether the EES remained intact until a detailed
engineering inspection occurs within the SRF. The additional
risk mitigation steps include but may not be limited to: moving
the EES into a biocontainment box at the landing site, sterilizing
the outside of the biocontainment box before it is trans
-
ported to the temporary staging area, and remediating the
landing site utilizing best practices dictated by the Center for
Disease Control and/or Department of Agriculture.
Even the biocontainment portions of the initial ground
operations can borrow heritage from mobile biocontainment
labs that have been used in field settings by the medical
profession for over a decade ( 56 ). These mobile biocontain
-
ment labs can be designed to meet necessary contamination
control requirements ( 57 ). The availability of a “clean” bio
-
containment laboratory would enable some initial flight hard
-
ware disassembly to occur at the temporary staging area to
limit the time the Sample Containment Vessel (SCV) is outside
of an inert environment and in proximity to the highly con
-
taminating Earth entry vehicle. This clean biocontainment
lab could also be utilized as a robust sample triage area in
the event of a major breach of the EES.
The transportation of Restricted Earth
-
return samples to
the SRF also requires major considerations since the trans
-
portation container, simultaneously, needs to minimize the
risk to the samples and minimize the risk to the terrestrial
biosphere. Traditionally, unrestricted samples are packaged
to mitigate contamination risks to the samples and are trans
-
ported by a United States Military aircraft to the receiving/
curation facility. It is anticipated that if the EES lands within
the United States, this paradigm would also be deemed the
safest route for MSR samples.
Role of a Sample Receiving Facility
The SRF is a specialized facility in which the samples will be
initially curated and assessed for their safety. As with a tra
-
ditional curation facility, the SRF must have capabilities to
accept and disassemble the flight hardware, preserve the
scientific integrity of the Martian samples, perform initial
characterization of the samples to inform the collection cat
-
alog, and allocate samples to scientists within and outside
(for sterilized samples or samples deemed safe for release)
the SRF. While there are any number of potential implemen
-
tation strategies that depend on the desired facility scope,
there are two fundamental differences between a traditional
curation facility and an SRF. The first, and most apparent, is
the capability of an SRF to maintain the samples under high
-
containment. The second is the enhanced analytical capability
that is necessary to complete a sample safety assessment,
conduct sample sterilization, and, in some cases, perform
select time
-
or sterilization
-
sensitive analyses.
The National Institute of Health defines a high
-
containment
biological laboratory at Biosafety Level (BSL)
-
3 and
-
4. To
meet these requirements, the facility must meet both infra
-
structural and procedural parameters necessary to ensure
the personnel working within the laboratory and public are
protected ( 58 , 59 ). Although the probability is extremely low
( 22 , 50 , 54 ), it is not known if the returned Mars samples
contain a pathogen transmissible by air for which there is no
known treatment. Therefore, due to the nonzero potential
for an unknown, NASA’s Planetary Protection Office recom
-
mends the utilization of containment at “the highest level
based on current technology” to safeguard the terrestrial
biosphere. This stance is in line with Article IX of the UN Outer
Space Treaty ( 53 ).
While UN and NASA Planetary Protection recommenda
-
tions give clear guidance that an SRF for MSR should provide
containment equivalent to a BSL
-
4 facility, there is currently
no requirement to formally commission a high
-
containment
facility that does not house a select agent or known infectious
pathogen. This nuance may be especially important since
formal classification as a BSL
-
4 facility may levy certain
requirements on the facility that are not conducive to long
-
term sample integrity. One of these requirements could be
to sterilize all samples removed from the facility, even if the
sample safety assessment deems the samples safe for release
(i.e., the samples are devoid of Martian life). The “unneces
-
sary” sterilization of all extraterrestrial samples could be cat
-
astrophic to the scientific value of the MSR collection,
especially if standard high
-
containment sterilization proce
-
dures were required, such as wet heat in the form of an auto
-
clave. Therefore, the current assumption is that the SRF
should be structurally equivalent to a BSL
-
4 facility and main
-
tain a laboratory that is negative pressure relative to the sur
-
rounding area, but not be formally classified as BSL
-
4.
There are two ways in which to achieve the infrastructural
requirements necessary to meet BSL
-
4 equivalence. The
most common method is to operate a negative pressure lab
-
oratory with full body, air
-
supplied, positive pressure suits,
known as a BSL
-
4 suit laboratory. Traditionally these facilities
are constructed with outer walls of thick single
-
poured con
-
crete. However, new stainless
-
steel technology is coming on
the market ( 57 ) and will be evaluated for its potential use in
the SRF. The second method for meeting BSL
-
4 equivalence
is to utilize negative pressure glove boxes called Class III
Biosafety cabinets (BSC
-
III), known as a cabinet laboratory.
In the United States, there is a strong preference for operat
-
ing a suit laboratory due to some ergonomic limitations of
working in the BSC
-
III, as well as the greater flexibility of the
working space. Given the wide range of processes to occur
within the SRF, it is likely that both implementation strategies
would be utilized. The implementation of either option will
depend on an array of factors, including sample integrity,
SRF footprint, and schedule. For example, for operations
required to be performed in ultraclean, inert environments,
the utilization of a custom BSC
-
III may enable both high
-
containment and contamination control compliance.
In addition to containing the samples and enabling the
sample safety assessment, a major priority of the SRF is to
maintain sample integrity by protecting the samples from
terrestrial contamination. It is anticipated that, given the sci
-
entific objectives associated with MSR, the contamination
control requirements levied on the SRF will be stringent for
select operations, specifically for activities involving pristine
sample handling. Unlike previous sample return missions
that emphasized inorganic and organic contamination control
measures, protecting the samples from biological contami
-
nation will be vital for not only achieving science goals but
also for minimizing the potential risk of false positives during
the sample safety assessment. A false positive during the
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sample safety assessment would not only delay the unster
-
ilized release of the remaining MSR collection from the SRF,
but it would also likely require more samples to be utilized/
consumed for the sample safety assessment ( 52 ).
There are four fundamental considerations when developing
contamination
-
controlled environments within curation infra
-
structure: 1) materials restrictions, 2) needed rates of air
-
exchanges to meet particle counts and flush organic outgassing,
3) sample sensitivity to the terrestrial atmosphere or atmos
-
phere of the curated environment, and 4) the cleaning pro
-
cesses necessary to achieve cleanliness with respect to organic,
inorganic, and microbial contamination. Each of these consid
-
erations is customized for each collection and for specific oper
-
ations within the curatorial process. While the implementation
of technology that provides an inert environment utilizing strin
-
gent material restrictions is standard practice within curation,
the integration of negative pressure environments with the
positive pressure barrier(s) necessary to minimize terrestrial
contamination is novel. However, this concept has been under
consideration for over two decades ( 60 ) and ESA has made sig
-
nificant investments into developing the necessary technology,
in the form of the Double Walled Isolator (DWI) ( 61 ), to achieve
clean handling of pristine samples in containment. Fig. 2
updates the Draft Test Protocol pressure paradigm ( 60 ) and
represents potential implementation strategies for integrating
positive and negative pressure environments.
Fundamentally the DWI would be a custom, material
-
compliant BSC
-
III with positive pressure interstitial regions that
create an inert, positive pressure barrier between the labora
-
tory and the samples. It is conceivable that DWI technology
could be implemented within the SRF to not only meet high
-
containment requirements but also to ensure that the samples
remain pristine during handling and processing. The core
functionality of the DWI is based on the use of remote manip
-
ulation instead of gloves, to minimize human
-
related contam
-
ination, glove
-
related contamination, and to add a more solid
biobarrier between the pristine samples and the environment.
However, while remote manipulation is preferred to meet con
-
tamination control requirements, it is not required for plane
-
tary protection, and the utilization of gloves could be considered
for certain processes such as flight hardware disassembly.
In addition to curatorial isolation technology, the SRF must
also accommodate an array of analytical instruments neces
-
sary for initial characterization of the samples [i.e., pre
-
basic
characterization, basic characterization, preliminary examina
-
tion] ( 32 ) and the sample safety assessment. Depending on
the type of analysis, it may be feasible to integrate these instru
-
ments with DWIs or custom isolation cabinets. However, initial
reports from MSR science working groups have recommended
an array of instruments where integration with or within a DWI
would be challenging or necessitate costly custom develop
-
ment (e.g., mass spectrometers) ( 23 , 32 , 62 ). For all instruments
not able to be integrated with isolators, either within them or
measurements made through windows or viewports, suit lab
-
oratories would be required. Additional suited laboratory space
may also be needed for sample preparation, tool and cabinet
cleaning, cold storage, and/or sample sterilization.
The cleaning and sterilization steps implemented during
hardware disassembly are another major consideration for
the SRF design. The degree to which cleaning and sterilization
steps are integrated will depend on the final mission archi
-
tecture and acceptable risks. For example, if the hardware is
hermetically sealed, then more aggressive measures such as
vapor phase hydrogen peroxide could be utilized for sterili
-
zation of the outside container. However, the risk of false
positives should be weighed against the potential degrada
-
tion of the samples if there is an undetected breach in the
outer container. Therefore, it may be advisable to simply
Ultraclean
Glov
ebox
/
Isolator
(Samples
)
ISO Class
Cleanroom
(Personne
l)
BSCIII
Glov
ebox
/
Isolator
(Samples
)
High-Contain
me
nt
Laboratory
(Personne
l)
A
B
High-Contain
ment
Spac
e
(Plenum,
Personnel,
or
Select
Instrumentaon
)
Ultraclean
BSCIII
Double-Wall
ed
Isolato
r
(Samples
)
High
Purity
Inert Ga
s
C
D
Ultraclean
BSCIII
Double-Wall
ed
Isolato
r
(Samples
)
High
Purity
Inert Ga
s
Prisne
Cleanroo
m
within
High-Contain
me
nt
(Personn
el
&
Sele
ct Instrumentaon
)
E
ISO
6-8
Clean Plenum
High-Containmen
t
Primary
High-Containmen
t
High-Containmen
t
Cleanroom
High-Containmen
t
Prisne
Isolator
ISO
3-5
Cleanroom
within
High-Containment
(Personnel
& Select
Instrumentaon
)
Fig. 2.
Airflow direction in various clean (positive pressure) and biosafety
(negative pressure) laboratory settings. The legend refers to a plenum, which
is a part of a building that facilitates air circulation through various pathways for
airflow. Panels (
A
and
B
) include traditional airflow in cleanrooms and biosafety
labs, respectively. Panels (
C
–
E
) illustrate various strategies for achieving clean
high
-
containment environments that could be utilized for MSR in the SRF.
The implementation choice for meeting clean and contained environmental
requirements will be dictated by the level of high containment, the cleanliness
requirements, and the necessary operations. (
A
) Typical cleanroom positive
-
pressure technology; (
B
) typical high
-
containment negative
-
pressure technology;
(
C
) cleanroom within high
-
containment lab, operated by personnel in positive
pressure suits (for activities with less stringent contamination control
requirements and larger equipment); (
D
) specialized ultraclean isolator (for
activities with more stringent contamination control requirements and for
activities with inert gas requirement); (
E
) specialized isolator installed within a
cleanroom in an overall contained space operated by personnel in cleanroom
attire for nominal operations (for isolators, the negative versus positive pressure
configuration is reversed to ensure planetary protection requirements are met).
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wipe the hardware down and trace contamination by pre
-
serving those wipes as contamination knowledge samples
for early hardware disassembly. Another trade
-
off when
planning hardware disassembly is the number of isolators
utilized for each process. Balancing the desire to isolate each
of the disassembly steps into individual isolators and provid
-
ing isolation chambers for cleaning and/or sterilization steps
will have to be weighed against other priorities within the
facility. Therefore, combining processes within a given isolator
must be strategic.
Sample and Science Management in the SRF
A close working relationship between the project science and
curation teams is integral to the success of any sample return
mission. The relationship becomes even more critical and
intertwined for Restricted Earth
-
return missions due to the
additional planetary protection considerations and the result
-
ing complexity of designing, constructing, and operating an
SRF. For MSR, the necessity to develop a strong working rela
-
tionship is further mandated by the mission’s design (e.g., the
flight hardware and the return of 10 to 30 individual sample
tubes), the additional sample safety assessment considera
-
tions, and the diversity of the sample collection ranging from
igneous to sedimentary rock cores, samples of atmosphere,
unconsolidated regolith, and witness tube assemblies.
Traditionally, curation takes a minimalistic approach to ini
-
tial characterization ( 18 ). For example, the OSIRIS
-
REx collec
-
tion predominantly relied on high
-
resolution imaging
supplemented with some XCT and structured light scanning
to develop the initial sample catalog and to inform sample
requests. Due to timing, the initial sample catalog for MSR will
likely be predominantly composed of the Perseverance sam
-
ple dossiers, bulk magnetic properties, XCT measurements,
and high
-
resolution photographs of the samples and any
subsamples once extracted from their tubes. However, pro
-
ject science and sample safety assessment teams may request
the inclusion of additional measurements during any of the
three phases of initial characterization. For example, given
the prioritization of the sample safety assessment and life
detection (i.e., origins, extinct, extant) within the SRF, it is con
-
ceivable that systematic high
-
resolution, high detection sen
-
sitivity organic mapping of sample cores may help identify
high
-
priority regions of interest within the samples. This infor
-
mation could focus the sample requests from the sample
safety assessment and project science teams and optimize
the resulting sample processing and allocation by curation.
It is important to note that while curation staff may be
collecting the enhanced scans recommended by project sci
-
ence and sample safety assessment teams, given the restric
-
tive nature of a high
-
containment facility, these data are likely
to be scientifically compelling and suitable for inclusion in
peer
-
reviewed publications. From the curation perspective,
all information collected initially to characterize the samples
should be incorporated into the sample catalog. However,
publication of the data from pre
-
basic characterization, basic
characterization, and preliminary examination would be at
the discretion of the project science and sample safety
assessment teams, similar to the ground element of other
sample return missions like OSIRIS
-
REx.
The concept of operations for the SRF (
SI Appendix
and
Fig. 3 ) relies on the idea that project science and sample
safety assessment teams work with the curation team to
prioritize sample processing order and activities. While the
development of the sample catalog and the completion of
the sample safety assessment will take precedence within
the SRF, considerations may be made for select investigations
on samples that could degrade rapidly once the sample tubes
are punctured for gas extraction (e.g., hydrated clays, sul
-
fates, and salts). Those investigations are referred to as time
-
sensitive sample science investigations, and information
from the Perseverance Sample Dossiers and other data col
-
lected during initial characterization will help to identify sam
-
ples for which time
-
sensitive studies are necessary ( 35 ). To
further identify potential time
-
sensitive samples, we can also
rely on results from Hayabusa2 and OSIRIS
-
REx, which
returned aqueous alteration products similar to those col
-
lected in samples from Jezero Crater ( 19 ). The OSIRIS
-
REx
samples are curated in gaseous N
2
, which is also the baseline
plan for MSR samples ( 32 ). The OSIRIS
-
REx samples will ena
-
ble a critical assessment of the use of N
2
with volatile
-
rich
phases within the curation environment to better determine
what phases are most susceptible to change. In general, sig
-
nificant advanced planning for each tube, sample type, and
potential physical sample state (e.g., solid core, semifractured
core, highly fragmented core, regolith) is necessary to limit
sample degradation and mitigate work stoppages due to
unforeseen issues.
Practically, this means that the project science and sample
safety assessment teams should study the Perseverance
Sample Dossiers extensively before the samples arrive to
prioritize the order in which the samples will be processed.
Once the pre
-
basic characterization is initiated, the curation
team, in collaboration with the project science and sample
safety assessment teams, should then examine the data to
determine how best to execute sample extraction and pro
-
cessing, as well as identify areas of high interest for potential
additional XCT or organic mapping scans during preliminary
examination. If additional XCT or organic mapping scans are
requested by the project science or sample safety assess
-
ment teams, it is anticipated that these teams will need to
evaluate the data before further subsampling can occur.
Therefore, it may be prudent to store the sample after map
-
ping and begin processing and/or scanning the next sample
instead of waiting for the decision of the project science and
sample safety assessment teams. Given that not all poten
-
tialities can be identified in advance, the curation, project
science, and sample safety assessment teams will have to
be flexible to respond to unknowns. Communication and
understanding that issues will occur is vital to make forward
progress.
Despite the many challenges and the development of
novel concepts to implement the ground element of MSR,
the concept of operations at the landing site and within the
SRF is mature and many options are available for the suc
-
cessful execution of all the activities required to occur within
the SRF (
SI Appendix
). The SRF is not intended to be a long
-
term curation facility for MSR. Although Mars is a habitable
planet, Jezero Crater and the surrounding area are unlikely
to harbor extant life ( 22 , 50 , 54 ), and the baseline plan is that
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samples will be deemed safe for release at the conclusion of
the sample safety assessment. How the SRF will be utilized
after MSR is uncertain but, even if it is not used for other
Restricted Earth
-
return missions, the lessons learned during
MSR will be critical for the potential return of more complex
or high
-
risk samples from other habitable worlds in the outer
solar system like Enceladus and Europa.
Long
-
Term Curation
The curation of the MSR samples begins in the SRF. The infra
-
structural requirements and contamination control measures
implemented in the SRF to maintain the scientific integrity of
the samples are the same requirements and measures that
will be needed in any long
-
term curation facility for the MSR
collection. Keeping the sample processing and sample cura
-
tion infrastructure between the SRF and long
-
term curation
facilities the same will minimize the types of materials that
come into contact with the pristine samples over the lifetime
of the collection, which will help to maintain the long
-
term
scientific integrity of the MSR sample collection. Unlike the
SRF, a long
-
term or “uncontained” curation facility does not
have to provide high containment or support the extensive
array of analytical equipment that is needed for the sample
safety assessment. However, as with the SRF, an uncontained
facility must provide a contamination
-
controlled environment
to preserve sample integrity and must support sample pro
-
cessing and allocation of samples to scientists worldwide.
It is anticipated that a long
-
term curation facility will need
to be operational when the samples arrive on Earth, since a
portion of the collection may be sterilized soon after return
to enable research outside of the SRF before the sample
safety assessment is complete ( 23 , 63 , 64 ). Sterilized sam
-
ples will be housed in a long
-
term curation facility, and once
the samples are deemed safe for release, one or more long
-
term curation facilities will become the only MSR curation
facilities for the collection. A long
-
term curation facility will
operate at relative positive pressures, will provide an area
for pristine sample handling and storage, as well as space
to store and process returned samples and perform “dirty”
sample preparation (e.g., making thin sections). The pristine
sample handling and storage area is notionally planned to
Fig.
3.
The anticipated high
-
level concept of operations within the SRF is demonstrated in Stages 1 to 4. Infrastructural requirements within the high
-
containment
portion of the SRF will be determined by the operational functions and the cleanliness requirements. The center of the circle represents the environment with
the most stringent contamination control requirements, where the pristine samples are processed (Stage 3). Less stringent contamination control requirements
are anticipated during hardware disassembly (Stage 1 and 2) and within analytical suites (Stage 4), therefore, this work is positioned closer to the border of the
circle. The stages and activities highlighted represent notional examples and are not an exhaustive list of the potential activities or types of spaces that may be
implemented within the SRF. Previously undefined acronyms in the figure include: STIC, sample tube isolation container; SOCC, secondary outer containment
caps; SCV, secondary containment vessel; RSTA, returnable sample tube assemblies; Pre
-
BC, Pre
-
basic characterization; EEV, Earth entry vehicle.
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meet equivalent contamination control requirements as the
SRF and adopt the same thresholds for inorganic, organic,
and biological contamination. It will also implement similar
contamination knowledge strategies such as the deployment
of witness materials and contamination monitoring proce
-
dures that verify the levels of inorganic, organic, and biological
contamination within the curation environment. The equip
-
ment and laboratory materials will be restricted to the same
degree as in the SRF to minimize organic outgassing and par
-
ticle shedding, specifically operating ISO 3 equivalent isolation
chambers within an ISO 5 cleanroom. The isolation cabinets
may be operated with remote sample manipulation systems,
even though it would be for contamination control purposes
only. Similar to the SRF, the cabinets should be developed
with windows to allow for high
-
resolution imaging and geo
-
chemical sample mapping through the cabinet window, which
has been successfully implemented for Apollo samples,
Hayabusa2 samples, and OSIRIS
-
REx samples (e.g., 19 , 65 ).
Traditional gloveboxes could also be utilized in a long
-
term
curation facility, but their implementation should adhere to
the same materials restrictions that are implemented in the
SRF for processing and handling pristine MSR samples.
Planning for the long
-
term curation of the samples is crit
-
ical, and development of a curation strategy that maximizes
the science value of the samples from the time they are
received in the SRF, through their transfer to a long
-
term cura
-
tion facility, and throughout the decades that follow is the key
to successful curation of the samples. The development of
these facilities is a means to ensure a return on investment
for decades by enabling analyses of pristine Mars samples by
scientists that are not yet born, to answer questions that are
not yet conceived, using instrumentation that has not yet
been invented. Humanity has over 50 y of experience with
the long
-
term curation of returned astromaterials outside of
containment, and the lessons learned from past endeavors
have fed forward into the long
-
term curation strategy of MSR
samples. Although MSR will mark the first sample return from
a habitable world, the overarching strategies for maintaining
the scientific integrity of the collection over decadal times
-
cales is largely the same as those developed through earlier
sample
-
return missions from the Moon and asteroids.
Summary
There are some important challenges to overcome for the
successful implementation of the MSR Campaign, but paths
to overcome all those challenges are tenable. Furthermore,
the samples that have already been collected by Perseverance
are scientifically compelling and represent key gaps in our
solar system rock record. The scientific value of the MSR sam
-
ple collection is immeasurable and will contribute to ques
-
tions ranging from the origin of our solar system to the origin
of life. The scientific potential of these samples to answer
fundamental questions will not only draw planetary scientists
with interests that span across the solar system, but it will
bring in scientists outside of planetary science from the fields
of terrestrial geology, physics, chemistry, biology, and med
-
icine to take part in the discoveries that lie within the returned
Martian samples. As the first samples returned from a hab
-
itable world, the processes established will provide critical
insights into sample return missions from other habitable
worlds like Enceladus and Europa. Through the careful cura
-
tion and conservation of the samples, MSR will enable gen
-
erations of scientific discoveries about Mars and about the
solar system at large for decades to come.
Data, Materials, and Software Availability.
There are no data underlying
this work.
ACKNOWLEDGMENTS.
We are grateful to the Editor and two anonymous
reviewers who have improved the quality of this manuscript. F.M.M. and A.D.H.
acknowledge support from the Astromaterials Acquisition and Curation Office
at NASA JSC during this study. A.D.H. also acknowledges support from the Mars
Sample Return Project. The research described in this paper was partially carried
out at the Jet Propulsion Laboratory, California Institute of Technology, under a
contract with the NASA under Grant Award No. 80NM0018D0004.
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