Schematic of the Viking Lander Biological Experiment System
The two Viking landers each carried four types of biologicalexperiments to the surface of Mars in 1976. These were the first Mars landers to carry out experiments to look for biosignatures of microbial life on Mars.
The landers used a robotic arm to put soil samples into sealed test
containers on the craft. The two landers were identical, so the same
tests were carried out at two places on Mars' surface, Viking 1 near the equator and Viking 2 further north.
The experiments
Four
experiments are presented here in the order in which they were carried
out by the two Viking landers. The biology team leader for the Viking
program was Harold P. Klein (NASA Ames).
Gas chromatograph — mass spectrometer
A gas chromatograph — mass spectrometer (GCMS) is a device that separates vapor components chemically via a gas chromatograph and then feeds the result into a mass spectrometer, which measures the molecular weight of each chemical. As a result, it can separate, identify, and quantify a large number of different chemicals. The GCMS (PI: Klaus Biemann,
MIT) was used to analyze the components of untreated Martian soil, and
particularly those components that are released as the soil is heated to
different temperatures. It could measure molecules present at a level
of a few parts per billion.
The GCMS measured no significant amount of organic molecules in the Martian soil. In fact, Martian soils were found to contain less carbon than lifeless lunar soils returned by the Apollo program.
This result was difficult to explain if Martian bacterial metabolism
was responsible for the positive results seen by the Labeled Release
experiment (see below). A 2011 astrobiology
textbook notes that this was the decisive factor due to which "For most
of the Viking scientists, the final conclusion was that the Viking missions failed to detect life in the Martian soil."
Experiments conducted in 2008 by the Phoenix lander discovered the presence of perchlorate
in Martian soil. The 2011 astrobiology textbook discusses the
importance of this finding with respect to the results obtained by Viking
as "while perchlorate is too poor an oxidizer to reproduce the LR
results (under the conditions of that experiment perchlorate does not
oxidize organics), it does oxidize, and thus destroy, organics at the
higher temperatures used in the Viking GCMS experiment. NASA
astrobiologist Chris McKay has estimated, in fact, that if Phoenix-like
levels of perchlorates were present in the Viking samples, the organic
content of the Martian soil could have been as high as 0.1% and still
would have produced the (false) negative result that the GCMS returned.
Thus, while conventional wisdom regarding the Viking biology
experiments still points to "no evidence of life", recent years have
seen at least a small shift toward "inconclusive evidence"."
According to a 2010 NASA press release: "The only organic
chemicals identified when the Viking landers heated samples of Martian
soil were chloromethane and dichloromethane
-- chlorine compounds interpreted at the time as likely contaminants
from cleaning fluids." According to a paper authored by a team led by Rafael Navarro-González of the National Autonomous University of Mexico,
"those chemicals are exactly what [their] new study found when a little
perchlorate -- the surprise finding from Phoenix -- was added to desert
soil from Chile containing organics and analyzed in the manner of the
Viking tests." However, the 2010 NASA press release also noted that:
"One reason the chlorinated organics found by Viking were interpreted as
contaminants from Earth was that the ratio of two isotopes of chlorine
in them matched the three-to-one ratio for those isotopes on Earth. The
ratio for them on Mars has not been clearly determined yet. If it is
found to be much different than Earth's, that would support the 1970s
interpretation." Biemann has written a commentary critical of the Navarro-González and McKay paper, to which the latter have replied; the exchange was published in December 2011.
Gas exchange
The gas exchange (GEX) experiment (PI: Vance Oyama,
NASA Ames) looked for gases given off by an incubated soil sample by
first replacing the Martian atmosphere with the inert gas Helium.
It applied a liquid complex of organic and inorganic nutrients and
supplements to a soil sample, first with just nutrients added, then with
water added too. Periodically, the instrument sampled the atmosphere of the incubation chamber and used a gas chromatograph to measure the concentrations of several gases, including oxygen, CO2, nitrogen, hydrogen, and methane.
The scientists hypothesized that metabolizing organisms would either
consume or release at least one of the gases being measured. The result
was negative.
Labeled release
The labeled release (LR) experiment (PI: Gilbert Levin, Biospherics Inc.) gave the most promise for exobiologists.
In the LR experiment, a sample of Martian soil was inoculated with a
drop of very dilute aqueous nutrient solution. The nutrients (7
molecules that were Miller-Urey products) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive 14CO2 gas as evidence that microorganisms in the soil had metabolized
one or more of the nutrients. Such a result was to be followed with the
control part of the experiment as described for the PR below. The
result was quite a surprise, considering the negative results of the
first two tests, with a steady stream of radioactive gases being given
off by the soil immediately following the first injection. The
experiment was done by both Viking probes, the first using a sample from
the surface exposed to sunlight and the second probe taking the sample
from underneath a rock; both initial injections came back positive.
Subsequent injections a week later did not, however, elicit the same
reaction, and according to a 1976 paper by Levin and Patricia Ann Straat
the results were inconclusive. In 1997, Levin, Straat and Barry DiGregorio co-authored a book on the issue, titled Mars: The Living Planet.
A CNN
article from 2000 noted that "Though most of his peers concluded
otherwise, Levin still holds that the robot tests he coordinated on the
1976 Viking lander indicated the presence of living organisms on Mars." A 2006 astrobiology
textbook noted that "With unsterilized Terrestrial samples, though, the
addition of more nutrients after the initial incubation would then
produce still more radioactive gas as the dormant bacteria sprang into
action to consume the new dose of food. This was not true of the Martian
soil; on Mars, the second and third nutrient injections did not produce
any further release of labeled gas."
The 2011 edition of the same textbook noted that "Albet Yen of the Jet
Propulsion Laboratory has shown that, under extremely cold and dry
conditions and in a carbon dioxide atmosphere, ultraviolet light
(remember: Mars lacks an ozone layer, so the surface is bathed in
ultraviolet) can cause carbon dioxide to react with soils to produce
various oxidizers, including highly reactive superoxides (salts containing O2−)
When mixed with small organic molecules, superoxidizers readily oxidize
them to carbon dioxide, which may account for the LR result. Superoxide
chemistry can also account for the puzzling results seen when more
nutrients were added to the soil in the LR experiment; because life
multiplies, the amount of gas should have increased when a second or
third batch of nutrients was added, but if the effect was due to a
chemical being consumed in the first reaction, no new gas would be
expected. Lastly, many superoxides are relatively unstable and are
destroyed at elevated temperatures, also accounting for the
"sterilization" seen in the LR experiment."
In a 2002 paper published by Joseph Miller, he speculates that
recorded delays in the system's chemical reactions point to biological
activity similar to the circadian rhythm previously observed in terrestrial cyanobacteria.
On 12 April 2012, an international team including Levin and Straat published a peer reviewed paper suggesting the detection of "extant microbial life on Mars", based on mathematical speculation through cluster analysis of the Labeled Release experiments of the 1976 Viking Mission.
Pyrolytic release
The pyrolytic release (PR) experiment (PI: Norman Horowitz, Caltech) consisted of the use of light, water, and a carbon-containing atmosphere of carbon monoxide (CO) and carbon dioxide (CO2), simulating that on Mars. The carbon-bearing gases were made with carbon-14 (14C), a heavy, radioactiveisotope of carbon. If there were photosynthetic organisms present, it was believed that they would incorporate some of the carbon as biomass through the process of carbon fixation, just as plants and cyanobacteria
on earth do. After several days of incubation, the experiment removed
the gases, baked the remaining soil at 650 °C (1200 °F), and collected
the products in a device which counted radioactivity. If any of the 14C
had been converted to biomass, it would be vaporized during heating and
the radioactivity counter would detect it as evidence for life. Should
a positive response be obtained, a duplicate sample of the same soil
would be heated to "sterilize" it. It would then be tested as a control
and should it still show activity similar to the first response, that
was evidence that the activity was chemical in nature. However, a nil,
or greatly diminished response, was evidence for biology. This same
control was to be used for any of the three life detection experiments
that showed a positive initial result.
Scientific conclusions
Organic
compounds seem to be common, for example, on asteroids, meteorites,
comets and the icy bodies orbiting the Sun, so detecting no trace of any
organic compound on the surface of Mars came as a surprise. The GC-MS
was definitely working, because the controls were effective and it was
able to detect traces of the cleaning solvents that had been used to
sterilize it prior to launch. At the time, the total absence of organic material on the surface made the results of the biology experiments moot, since metabolism
involving organic compounds were what those experiments were designed
to detect. However, the general scientific community surmise that the
Viking's biological tests remain inconclusive.
Most researchers surmise that the results of the Viking biology
experiments can be explained by purely chemical processes that do not
require the presence of life, and the GC-MS results rule out life.
Despite the positive result from the Labeled Release experiment, a
general assessment is that the results seen in the four experiments are
best explained by oxidative chemical reactions with the Martian soil.
One of the current conclusions is that the Martian soil, being
continuously exposed to UV light from the Sun (Mars has no protective ozone layer), has built up a thin layer of a very strong oxidant.
A sufficiently strong oxidizing molecule would react with the added
water to produce oxygen and hydrogen, and with the nutrients to produce carbon dioxide (CO2).
On August 2008, the Phoenix lander detected perchlorate, a strong oxidizer when heated above 200 °C. This was initially thought to be the cause of a false positive LR result. However, results of experiments published in December 2010
propose that organic compounds "could have been present" in the soil
analyzed by both Viking 1 and 2, since NASA's Phoenix lander in 2008
detected perchlorate, which can break down organic compounds. The
study's authors found that perchlorate can destroy organics when heated
and produce chloromethane and dichloromethane
as byproduct, the identical chlorine compounds discovered by both
Viking landers when they performed the same tests on Mars. Because
perchlorate would have broken down any Martian organics, the question of
whether or not Viking found organic compounds is still wide open, as
alternative chemical and biological interpretations are possible.
In 2013, astrobiologist Richard Quinn at the Ames Center
conducted experiments in which perchlorates irradiated with gamma rays
seemed to reproduce the findings of the labeled-release experiment.
He concluded that neither hydrogen peroxide nor superoxide is required
to explain the results of the Viking biology experiments.
Controversy
Before the discovery of the oxidizer perchlorate
on Mars in 2008, some theories remained opposed to the general
scientific conclusion. An investigator suggested that the biological
explanation of the lack of detected organics by GC-MS could be that the
oxidizing inventory of the H2O2-H2O solvent well exceeded the reducing power of the organic compounds of the organisms.
It has also been argued that the Labeled Release (LR) experiment
detected so few metabolising organisms in the Martian soil, that it
would have been impossible for the gas chromatograph to detect them. This view has been put forward by the designer of the LR experiment, Gilbert Levin, who believes the positive LR results are diagnostic for life on Mars.
He and others have conducted ongoing experiments attempting to
reproduce the Viking data, either with biological or non-biological
materials on Earth. While no experiment has ever precisely duplicated
the Mars LR test and control results, experiments with hydrogen peroxide-saturated titanium dioxide have produced similar results.
While the majority of astrobiologists still conclude that the Viking biological experiments were inconclusive or negative, Gilbert Levin
is not alone in believing otherwise. The current claim for life on Mars
is grounded on old evidence reinterpreted in the light of recent
developments.
On 2006, scientist Rafael Navarro demonstrated that the Viking
biological experiments likely lacked sensitivity to detect trace
amounts of organic compounds. In a paper published in December 2010,
the scientists suggest that if organics were present, they would not
have been detected because when the soil is heated to check for
organics, perchlorate destroys them rapidly producing chloromethane and
dichloromethane, which is what the Viking landers found. This team also
notes that this is not a proof of life but it could make a difference in
how scientists look for organic biosignatures in the future. Results from the current Mars Science Laboratory mission and the under-development ExoMars program, may help settle this controversy.
On 2006, Mario Crocco went as far as proposing the creation of a new nomenclatural rank that classified some Viking results as 'metabolic' and therefore representative of a new form of life.
The taxonomy proposed by Crocco has not been accepted by the scientific
community, and the validity of Crocco's interpretation hinged entirely
on the absence of an oxidative agent in the Martian soil.
Critiques
James Lovelock
argued that the Viking mission would have done better to examine the
Martian atmosphere than look at the soil. He theorised that all life
tends to expel waste gases into the atmosphere, and as such it would be
possible to theorise the existence of life on a planet by detecting an
atmosphere that was not in chemical equilibrium.
He concluded that there was enough information about Mars' atmosphere
at that time to discount the possibility of life there. Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate. Although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb. later in 2013 and in 2014, measurements by Curiosity did detect methane, suggesting a time-variable source. The planned ExoMars Trace Gas Orbiter,
launched on March 2016, will implement this approach and will focus on
detection, characterization of spatial and temporal variation, and
localization of sources for a broad suite of atmospheric trace gases on
Mars and help determine if their formation is of biological or
geological origin. The Mars Orbiter Mission
is also attempting —since late 2014— to detect and map methane on Mars'
atmosphere. A press commentary argued that, if there was life at the
Viking lander sites, it may have been killed by the exhaust from the
landing rockets. That is not a problem for missions which land via an airbag-protected
capsule, slowed by parachutes and retrorockets, and dropped from a
height that allows rocket exhaust to avoid the surface. Mars Pathfinder's Sojourner rover and the Mars Exploration Rovers each used this landing technique successfully. The Phoenix Scout lander descended to the surface with retro-rockets, however, their fuel was hydrazine,
and the end products of the plume (water, nitrogen, and ammonia) were
not found to have affected the soils at the landing site.
Future missions
Urey design
The question of life on Mars
will probably not be resolved entirely until future missions to Mars
either conclusively demonstrate the presence of life on the planet,
identify the chemical(s) responsible for the Viking results, or both.
The Mars Science Laboratory mission landed the Curiosity rover on August 6, 2012, and its goals include investigation of the Martian climate, geology, and whether Mars could have ever supported life, including investigation of the role of water and planetary habitability. Astrobiology research on Mars will continue with the ExoMars Trace Gas Orbiter in 2016, ExoMars rover on 2018, and the Mars 2020 rover in 2020.
The Urey instrument was a funded study for sensitive organic compound detector, but has not been sent to Mars but was considered for ExoMars program of the 2000s
Proposed missions
The Biological Oxidant and Life Detection (BOLD) is a proposed Mars mission that would follow up the Viking soil tests by using several small impact landers. Another proposal is the Phoenix lander-based Icebreaker Life.
ExoMars goals are to search for signs of past life on Mars,
investigate how the Martian water and geochemical environment varies,
investigate atmospheric trace gases and their sources and by doing so
demonstrate the technologies for a future Mars sample return mission. The mission will search for ancient biosignatures of Martian life, employing several spacecraft elements to be sent to Mars on two launches.
The ExoMars Trace Gas Orbiter (TGO) and a test stationary lander called Schiaparelli were launched on 14 March 2016. TGO entered Mars orbit on 19 October 2016 and will proceed to map the sources of methane (CH 4)
and other trace gases present in the Martian atmosphere that could be
evidence for possible biological or geological activity. The TGO
features four instruments and will also act as a communications relay
satellite. The Schiaparelli experimental lander separated from TGO on 16 October and was maneuvered to land in Meridiani Planum, but it crashed on the surface of Mars. The landing was designed to test new key technologies to safely deliver the 2020 rover mission.
In 2020, a Roscosmos-built lander (ExoMars 2020 surface platform) is to deliver the ESA-built ExoMars Rover to the Martian surface. The rover will also include some Roscosmos built instruments. The second mission operations and communications will be led by ALTEC's Rover Control Centre in Italy.
Since its inception, ExoMars has gone through several phases of
planning with various proposals for landers, orbiters, launch vehicles,
and international cooperation planning, such as the defunct 2009 Mars Exploration Joint Initiative (MEJI) with the United States.
Originally, the ExoMars concept consisted of a large robotic rover being part of ESA's Aurora Programme as a Flagship mission
and was approved by the European Space Agency ministers in December
2005. Originally conceived as a rover with a stationary ground station,
ExoMars was planned to launch in 2011 aboard a Russian SoyuzFregat rocket.
ExoMars begun in 2001 as part of the ESA Aurora program for the human exploration of Mars. That initial vision called for rover in 2009 and later a sample return mission. Another mission intended to support the Aurora program is a Phobos sample return mission. In December 2005, the different nations composing the ESA gave approval to the Aurora program and to ExoMars.
Aurora is an optional program and each state is allowed to decide which
part of the program they want to be involved in and to what extent
(e.g. how much funds they want to put into the program).
The Aurora program was initiated in 2002 with support of twelve
nations: Austria, Belgium, France, Germany, Italy, the Netherlands,
Portugal, Spain, Sweden, Switzerland, the United Kingdom and Canada.
In 2007, Canadian-based technology firm MacDonald Dettwiler and
Associates Ltd. (MDA) was selected for a one-million-euro contract with EADS Astrium
of Britain to design and build a prototype Mars rover chassis for the
European Space Agency. Astrium was also contracted to design the final
rover.
On July 2009 NASA and ESA signed the Mars Exploration Joint Initiative,
which proposed to utilise an Atlas rocket launcher instead of a Soyuz,
which significantly altered the technical and financial setting of the
ExoMars mission. On 19 June, when the rover was still planned to
piggyback the Mars Trace Gas Orbiter,
it was reported that a prospective agreement would require that ExoMars
lose enough weight to fit aboard the Atlas launch vehicle with a NASA
orbiter.
Then the mission was combined with other projects to a multi-spacecraft mission divided over two Atlas V-launches: the ExoMars Trace Gas Orbiter
(TGO) was merged into the project, piggybacking a stationary
meteorological lander slated for launch in January 2016. It was also
proposed to include a second rover, the MAX-C.
In August 2009 it was announced that the Russian Federal Space Agency (now Roscosmos) and ESA had signed a contract that included cooperation on two Mars exploration projects: Russia's Fobos-Grunt project and ESA's ExoMars. Specifically, ESA secured a Russian Proton rocket as a "backup launcher" for the ExoMars rover, which would include Russian-made parts.
On 17 December 2009, the ESA governments gave their final
approval to a two-part Mars exploration mission to be conducted with
NASA, confirming their commitment to spend €850 million ($1.23 billion)
on missions in 2016 and 2018.
In April 2011, because of a budgeting crisis, a proposal was announced to cancel the accompanying MAX-C rover, and fly only one rover in 2018 that would be larger than either of the vehicles in the paired concept.
One suggestion was that the new vehicle would be built in Europe and
carry a mix of European and U.S. instruments. NASA would provide the
rocket to deliver it to Mars and provide the sky crane landing system. Despite the proposed reorganisation, the goals of the 2018 mission opportunity would have stayed broadly the same.
Under the FY2013 Budget President Obama released on 13 February
2012, NASA terminated its participation in ExoMars due to budgetary cuts
in order to pay for the cost overruns of the James Webb Space Telescope. With NASA's funding for this project completely cancelled, most of these plans had to be restructured.
On 14 March 2013, representatives of the ESA and the Russian space agency (Roscosmos), signed a deal in which Russia became a full partner. Roscosmos will supply both missions with Proton launch vehicles with Briz-M upper stages and launch services, as well as an additional entry, descent and landing module for the rover mission in 2018. Under the agreement, Roscosmos was granted three asking conditions:
Roscosmos will contribute two Proton launch vehicles as payment for the partnership.
The Trace Gas Orbiter payload shall include two Russian instruments that were originally developed for Fobos-Grunt.
All scientific results must be intellectual property of the European Space Agency and the Russian Academy of Sciences (i.e. Roscosmos will have full access to research data).
ESA had originally cost-capped the ExoMars projects at €1 billion, (USD 1.3 billion) but the withdrawal of the U.S. space agency (NASA) and the consequent reorganisation of the ventures will probably add several hundred million euros to the sum so far raised. So on March 2012, member states instructed the agency's executive to look at how this shortfall could be made up. One possibility is that other science activities within ESA may have to step back to make ExoMars a priority.
On September 2012 it was announced that new ESA members, Poland and
Romania will be contributing up to €70 million to the ExoMars mission. ESA has not ruled out a possible partial return of NASA to the 2018 portion of ExoMars, albeit in a relatively minor role.
Russia's financing of ExoMars could be partially covered by
insurance payments of 1.2 billion rubles ($40.7 million USD) for the
loss of Fobos-Grunt, and reassigning funds for a possible coordination between the Mars-NET and ExoMars projects. On 25 January 2013, Roscosmos fully funded the development of the scientific instruments to be flown on the first launch, the Trace Gas Orbiter (TGO).
As of March 2014, the lead builder of the ExoMars rover, the British division of Airbus Defence and Space, had started procuring critical components, but the 2018 rover mission was still short by more than 100 million euros, or $138 million. The wheels and suspension system are paid by the Canadian Space Agency and are being manufactured by MDA Corporation in Canada.
Status
A prototype of the ExoMars Rover at the 2015 Cambridge Science Festival
In January 2016 it was announced that the financial situation of the 2018 mission 'might' require a 2-year delay. Italy is the largest contributor to ExoMars, and the UK is the mission's second-largest financial backer.
The rover was scheduled to launch in 2018 and land on Mars in early 2019,
but in May 2016 ESA announced that the launch would occur in 2020 due
to delays in European and Russian industrial activities and deliveries
of the scientific payload.
2016 first spacecraft launch
The spacecraft containing ExoMars Trace Gas Orbiter (TGO) and Schiaparelli launched on 14 March 2016 (Livestream began at 08:30 GMT [03:30 AM EDT]).
Four rocket burns occurred in the following 10 hours before the descent
module and orbiter were released. Signal from the Orbiter was
successfully received at 21:29 GMT of the same day, which confirmed that
the launch was fully successful and the spacecraft is on its way to
Mars. Shortly after separation from the probes, the Briz-M upper booster stage possibly exploded a few kilometers away, however apparently without damaging the orbiter or lander. The spacecraft, which housed the Trace Gas Orbiter and the Schiaparelli
lander, took its nominal orbit towards Mars and was seemingly in
working order. Over the next two weeks, controllers continued to check
and commission its systems, including the power, communications,
startrackers, and guidance and navigation system.
Mission objectives
The scientific objectives, in order of priority, are:
to access the subsurface with a drill able to collect samples down to a depth of 2 metres (6.6 ft)
to develop surface exploration capability using a rover.
Mission profile
ExoMars is a joint programme of the European Space Agency (ESA) and the Russian space agency Roscosmos.
According to current plans, the ExoMars project will comprise four
spacecraft: two stationary landers, one orbiter and one rover. All
mission elements will be sent in two launches using two heavy-lift Proton rockets.
Russian-built landing system and surface science platform will deliver the rover to the surface. Russia will provide various scientific instruments for the lander and rover.
The two landing modules and the rover will be sterilised
in order not to contaminate the planet with Earth life forms. Cleaning
will require a combination of sterilising methods, including ionising radiation, UV radiation, and chemicals such as ethyl and isopropyl alcohol.
First launch (2016)
Trace Gas Orbiter
The Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and
atmospheric gas analyzer mission that was launched on 14 March 2016.
The spacecraft arrived in the Martian orbit in October 2016. It
delivered the ExoMars Schiaparelli EDM lander and then proceed to map
the sources of methane on Mars and other gases, and in doing so, help select the landing site for the ExoMars rover
to be launched in 2020. The presence of methane in Mars' atmosphere is
intriguing because its likely origin is either present-day life or
geological activity. Upon the arrival of the rover in 2021, the orbiter
would be transferred into a lower orbit where it would be able to
perform analytical science activities as well as provide the
Schiaparelli EDM lander and ExoMars rover with telecommunication relay.
NASA provided an Electra telecommunications relay and navigation
instrument to ensure communications between probes and rovers on the
surface of Mars and controllers on Earth. The TGO would continue serving as a telecommunication relay satellite for future landed missions until 2022.
Schiaparelli EDM lander
Model of the ExoMars Schiaparelli EDL Demonstrator Module (EDM). During its descent it returned 600 MB of data, but it did not achieve a soft landing.
The Entry, Descent and Landing Demonstrator Module (EDM) called Schiaparelli, was intended to provide the European Space Agency (ESA) and Russia's Roscosmos with the technology for landing on the surface of Mars. It was launched together with the ExoMars Trace Gas Orbiter
(TGO) on 14 March 2016 and was scheduled to land softly on 19 October
2016. No signal indicating a successful landing was received, and on 21 October 2016 NASA released a Mars Reconnaissance Orbiterimage showing what appears to be the lander crash site. The lander was equipped with a non-rechargeable electric battery with enough power for four sols. The soft landing should have taken place on Meridiani Planum
during the dust storm season, which would have provided a unique chance
to characterise a dust-loaded atmosphere during entry and descent, and
to conduct surface measurements associated with a dust-rich environment.
Once on the surface, it was to measure the wind speed and
direction, humidity, pressure and surface temperature, and determine the
transparency of the atmosphere. It carried a surface payload, based on the proposed meteorological DREAMS
(Dust Characterisation, Risk Assessment, and Environment Analyser on
the Martian Surface) package, consists of a suite of sensors to measure
the wind speed and direction (MetWind), humidity (MetHumi), pressure
(MetBaro), surface temperature (MarsTem), the transparency of the
atmosphere (Optical Depth Sensor; ODS), and atmospheric electrification
(Atmospheric Radiation and Electricity Sensor; MicroARES).
The DREAMS payload was to function for 2 or 3 days as an environmental
station for the duration of the EDM surface mission after landing.
Second launch (2020)
Russian landing system
The
second mission, scheduled for launch in July 2020, will have an 1800 kg
Russian-built landing platform system derived from the 2016
Schiaparelli EDM lander, to place the ExoMars rover on the surface of Mars. This lander platform will be built 80% by the Russian company Lavochkin, and 20% by ESA.
Lavochkin will produce most of the landing system's hardware, while ESA
will handle elements such as the guidance, radar and navigation
systems.
Lavochkin's current landing strategy is to use two parachutes; one will
open while the module is still moving at supersonic speed, and another
will deploy once the probe has been slowed down to subsonic velocity.
The heat shield will eventually fall away from the entry capsule to
allow the ExoMars rover, riding its retro-rocket-equipped lander, to
come for a soft landing on legs or struts. The surface platform lander
will then deploy ramps for the rover to drive down.
Critics have stated that while Russian expertise may be
sufficient to provide a launch vehicle, it does not currently extend to
the critical requirement of a landing system for Mars.
Surface platform
After landing on Mars in 2021, the rover will descend from the
platform via a ramp. The platform is expected to image the landing site,
monitor the climate, investigate the atmosphere, analyse the radiation
environment, study the distribution of any subsurface water at the
landing site, and perform geophysical investigations of the internal
structure of Mars. Following a March 2015 request for the contribution of scientific instruments for the landing system, there will be four instruments; the two European-led instruments selected are:
the Lander Radioscience experiment (LaRa) will
study the internal structure of Mars, and will make precise measurements
of the rotation and orientation of the planet by monitoring two-way
Doppler frequency shifts between the surface platform and Earth. It will
also detect variations in angular momentum due to the redistribution of
masses, such as the migration of ice from the polar caps to the
atmosphere.
the HABIT (HabitAbility: Brine, Irradiation and Temperature)
package will investigate the amount of water vapour in the atmosphere,
daily and seasonal variations in ground and air temperatures, and the UV
radiation environment.
two Russian-led instruments will monitor pressure and humidity, UV
radiation and dust, the local magnetic field and plasma environment.
The platform is expected to operate for at least one Earth year, and its instruments might be powered by a radioisotope thermoelectric generator to provide long-term power.
The ExoMars rover will land in 2021 and is designed to navigate autonomously across the Martian surface.
Instrumentation will consist of the exobiology laboratory suite, known as "Pasteur analytical laboratory" to look for signs of biomolecules and biosignatures from past life. Among other instruments, the rover will also carry a 2-metre (6.6 ft) sub-surface core drill to pull up samples for its on-board laboratory. The rover will have a mass of about 207 kg (456 lb).
Oxia Planum, near the equator, is the selected landing site for its potential to preserve biosignatures and smooth surface
A primary goal when selecting the rover's landing site is to identify
a particular geologic environment, or set of environments, that would
support —now or in the past— microbial life. The scientists prefer a
landing site with both morphologic and mineralogical evidence for past
water. Furthermore, a site with spectra indicating multiple hydrated minerals such as clay minerals is preferred, but it will come down to a balance between engineering constraints and scientific goals.
Engineering constraints call for a flat landing site in a
latitude band straddling the equator that is only 30° latitude from top
to bottom because the rover is solar-powered and will need best sunlight
exposure. The landing module carrying the rover will have a landing ellipse that measures about 105 km by 15 km.
Scientific requirements include landing in an area with 3.6 billion years old sedimentary rocks that are a record of the past wet habitable environment. The year before launch, the European Space Agency will make the final decision. By March 2014, the long list was:
Following additional review by an ESA-appointed panel, four sites,
all of which are located relatively near the equator, were formally
recommended in October 2014 for further detailed analysis:
On 21 October 2015, Oxia Planum was reported to be the preferred landing site for the ExoMars rover.
The delay of the rover mission to 2020 from 2018 meant that Oxia
Planum was no longer the only favourable landing site due to changes in
the possible landing ellipse.
Both Mawrth Vallis and Aram Dorsum, surviving candidates from the
previous selection, could be reconsidered. ESA convened further
workshops to re-evaluate the three remaining options and in March 2017
selected two sites to study in detail;
A Viking lander being prepared for dry heat sterilization – this remains the "Gold standard" of present-day planetary protection.
Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and the Earth
in the case of sample-return missions. Planetary protection reflects
both the unknown nature of the space environment and the desire of the
scientific community to preserve the pristine nature of celestial bodies
until they can be studied in detail.
The potential problem of lunar and planetary contamination was first raised at the International Astronautical Federation VIIth Congress in Rome in 1956.
In 1958 the U.S. National Academy of Sciences
(NAS) passed a resolution stating, “The National Academy of Sciences of
the United States of America urges that scientists plan lunar and
planetary studies with great care and deep concern so that initial
operations do not compromise and make impossible forever after critical
scientific experiments.” This led to creation of the ad hoc Committee on
Contamination by Extraterrestrial Exploration (CETEX), which met for a
year and recommended that interplanetary spacecraft be sterilized,
and stated, “The need for sterilization is only temporary. Mars and
possibly Venus need to remain uncontaminated only until study by manned
ships becomes possible”.
In 1959, planetary protection was transferred to the newly formed Committee on Space Research (COSPAR). COSPAR in 1964 issued Resolution 26 affirming that:
the search for extraterrestrial
life is an important objective of space research, that the planet of
Mars may offer the only feasible opportunity to conduct this search
during the foreseeable future, that contamination of this planet would
make such a search far more difficult and possibly even prevent for all
time an unequivocal result, that all practical steps should be taken to
ensure that Mars be not biologically contaminated until such time as
this search has been satisfactorily carried out, and that cooperation in
proper scheduling of experiments and use of adequate spacecraft
sterilization techniques is required on the part of all deep space probe
launching authorities to avoid such contamination.
Signatories
of the Outer Space Treaty - includes all current and aspiring space
faring nation states. By signing the treaty, these nation states have
all committed themselves to planetary protection.
In 1967, the US, USSR, and UK ratified the United Nations Outer Space Treaty. The legal basis for planetary protection lies in Article IX of this treaty:
"Article IX: ... 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...
This treaty has since been signed and ratified by 104 nation states.
Another 24 have signed but not ratified. All the current space-faring
nation states have both signed and ratified it. Amongst nations with
space faring aspirations, some have not yet ratified: the United Arab
Emirates, Syria and North Korea have signed but not yet ratified.
The Outer Space Treaty has consistent and widespread
international support, and as a result of this, together with the fact
that it is based on the 1963 declaration which was adopted by consensus
in the UN National Assembly, it has taken on the status of customary
international law. The provisions of the Outer Space Treaty are
therefore binding on all states, even those who have neither signed nor
ratified it.
For forward contamination, the phrase to be interpreted is
"harmful contamination". Two legal reviews came to differing
interpretations of this clause (both reviews were unofficial). However
the currently accepted interpretation is that “any contamination which
would result in harm to a state’s experiments or programs is to be
avoided”. NASA policy states explicitly that “the conduct of scientific
investigations of possible extraterrestrial life forms, precursors, and
remnants must not be jeopardized”.
COSPAR recommendations and categories
The Committee on Space Research (COSPAR) meets every two years, in a gathering of 2000 to 3000 scientists,
and one of its tasks is to develop recommendations for avoiding
interplanetary contamination. Its legal basis is Article IX of the Outer
Space Treaty.
Its recommendations depend on the type of space mission and the celestial body explored. COSPAR categorizes the missions into 5 groups:
Category I: Any mission to locations not of direct interest for chemical evolution or the origin of life, such as the Sun or Mercury. No planetary protection requirements.
Category II: Any mission to locations of significant interest
for chemical evolution and the origin of life, but only a remote chance
that spacecraft-borne contamination could compromise investigations.
Examples include the Moon, Venus, and comets.
Requires simple documentation only, primarily to outline intended or
potential impact targets, and an end of mission report of any
inadvertent impact site if such occurred.
Category III: Flyby and orbiter missions to locations of
significant interest for chemical evolution or the origin of life, and
with a significant chance that contamination could compromise
investigations e.g., Mars, Europa, Enceladus.
Requires more involved documentation than Category II. Other
requirements, depending on the mission, may include trajectory biasing,
clean room assembly, bioburden reduction, and if impact is a
possibility, inventory of organics.
Category IV: Lander or probe missions to the same locations
as Category III. Measures to be applied depend on the target body and
the planned operations. "Sterilization of the entire spacecraft may be
required for landers and rovers with life-detection experiments, and for
those landing in or moving to a region where terrestrial microorganisms
may survive and grow, or where indigenous life may be present. For
other landers and rovers, the requirements would be for decontamination
and partial sterilization of the landed hardware."
Missions to Mars in category IV are subclassified further:
Category IVa. Landers that do not search for Martian life
- uses the Viking lander pre-sterilization requirements, a maximum of
300,000 spores per spacecraft and 300 spores per square meter.
Category IVb. Landers that search for Martian life. Adds stringent extra requirements to prevent contamination of samples.
Category IVc. Any component that accesses a Martian special region
(see below) must be sterilized to at least to the Viking
post-sterilization biological burden levels of 30 spores total per
spacecraft.
Category V: This is further divided into unrestricted and restricted sample return.
Unrestricted Category V: samples from locations judged
by scientific opinion to have no indigenous lifeforms. No special
requirements.
Restricted Category V: (where scientific opinion is unsure) the
requirements include: absolute prohibition of destructive impact upon
return, containment of all returned hardware which directly contacted
the target body, and containment of any unsterilized sample returned to
Earth.
For Category IV missions, a certain level of biological burden
is allowed for the mission. In general this is expressed as a
'probability of contamination', required to be less than one chance in
10,000
of forward contamination per mission, but in the case of Mars Category
IV missions (above) the requirement has been translated into a count of Bacillus spores per surface area, as an easy to use assay method.
More extensive documentation is also required for Category IV.
Other procedures required, depending on the mission, may include
trajectory biasing, the use of clean rooms during spacecraft assembly
and testing, bioload reduction, partial sterilization of the hardware
having direct contact with the target body, a bioshield for that
hardware, and, in rare cases, complete sterilization of the entire
spacecraft.
For restricted Category V missions, the current recommendation
is that no uncontained samples should be returned unless sterilized.
Since sterilization of the returned samples would destroy much of their
science value, current proposals involve containment and quarantine
procedures. For details, see Containment and quarantine
below. Category V missions also have to fulfill the requirements of
Category IV to protect the target body from forward contamination.
Mars special regions
A special region
is a region classified by COSPAR where terrestrial organisms could
readily propagate, or thought to have a high potential for existence of
Martian life forms. This is understood to apply to any region on Mars
where liquid water occurs, or can occasionally occur, based on the
current understanding of requirements for life.
If a hard landing risks biological contamination of a special region, then the whole lander system must be sterilized to COSPAR category IVc.
Target categories
Some
targets are easily categorized. Others are assigned provisional
categories by COSPAR, pending future discoveries and research.
The 2009 COSPAR Workshop on Planetary Protection for Outer Planet
Satellites and Small Solar System Bodies covered this in some detail.
Most of these assessments are from that report, with some future
refinements. This workshop also gave more precise definitions for some
of the categories:
Category I
“not of direct interest for understanding the process of chemical evolution or the origin of life.”
…
where there is only a remote chance that contamination carried by a
spacecraft could jeopardize future exploration”. In this case we define
“remote chance” as “the absence of niches (places where terrestrial
microorganisms could proliferate) and/or a very low likelihood of
transfer to those places.”
Callisto, comets, asteroids of category P, D, and C, Venus, Kuiper belt objects (KBO) < 1/2 size of Pluto.
Provisional Category II
Ganymede, Titan, Triton, the Pluto-Charon system, and other large KBOs (> 1/2 size of Pluto), Ceres,
Provisionally, they assigned these objects to Category II. However,
they state that more research is needed, because there is a remote
possibility that the tidal interactions of Pluto and Charon could
maintain some water reservoir below the surface. Similar considerations
apply to the other larger KBOs.
Triton
is insufficiently well understood at present to say it is definitely
devoid of liquid water. The only close up observations to date are those
of Voyager 2.
In a detailed discussion of Titan, scientists concluded that
there was no danger of contamination of its surface, except short term
adding of negligible amounts of organics, but Titan could have a below
surface water reservoir that communicates with the surface, and if so,
this could be contaminated.
In the case of Ganymede, the question is, given that its surface
shows pervasive signs of resurfacing, is there any communication with
its subsurface ocean? They found no known mechanism by which this could
happen, and the Galileo spacecraft found no evidence of cryovolcanism.
Initially, they assigned it as Priority B minus, meaning that precursor
missions are needed to assess its category before any surface missions.
However, after further discussion they provisionally assigned it to
Category II, so no precursor missions are required, depending on future
research.
If there is cryovolcanism on Ganymede or Titan, the undersurface
reservoir is thought to be 50 – 150 km below the surface. They were
unable to find a process that could transfer the surface melted water
back down through 50 km of ice to the under surface sea.
This is why both Ganymede and Titan were assigned a reasonably firm
provisional Category II, but pending results of future research.
Icy bodies that show signs of recent resurfacing need further
discussion and might need to be assigned to a new category depending on
future research. This approach has been applied, for instance, to
missions to Ceres. The planetary protection Category is subject for review during the mission of the Ceres orbiter (Dawn) depending on the results found.
Category III / IV
“…where
there is a significant chance that contamination carried by a
spacecraft could jeopardize future exploration.” We define “significant
chance” as “the presence of niches (places where terrestrial
microorganisms could proliferate) and the likelihood of transfer to
those places.”
Unrestricted Category V: “Earth-return missions from bodies deemed by scientific opinion to have no indigenous life forms.”
Restricted Category V:
"Earth-return missions from bodies deemed by scientific opinion to be of
significant interest to the process of chemical evolution or the origin
of life."
In the category V for sample return the conclusions so far are:
If
there has been no activity for 3 billion years, it will not be possible
to destroy the surface by terrestrial contamination, so can be treated
as Category I. Otherwise, the category may need to be reassessed.
The Coleman-Sagan equation
The
aim of the current regulations is to keep the number of microorganisms
low enough so that the probability of contamination of Mars (and other
targets) is acceptable. It is not an objective to make the probability
of contamination zero.
The aim is to keep the probability of contamination of 1 chance in 10,000 of contamination per mission flown.
This figure is obtained typically by multiplying together the number of
microorganisms on the spacecraft, the probability of growth on the
target body, and a series of bioload reduction factors.
In detail the method used is the Coleman-Sagan equation.
.
where
= the number of microorganisms on the spacecraft initially
= Reduction due to conditions on spacecraft before and after launch
= Probability that microorganisms on the spacecraft reach the surface of the planet
= Probability that spacecraft will hit the planet - this is 1 for a lander
= Probability of microorganism to be released in the environment when on the ground, usually set to 1 for crashlanding.
= Probability of growth. For targets with liquid water this is set to 1 for sake of the calculation.
Then the requirement is
The
is a number chosen by Sagan et al., somewhat arbitrarily. Sagan and
Coleman assumed that about 60 missions to the Mars surface would occur
before the exobiology of Mars is thoroughly understood, 54 of those
successful, and 30 flybys or orbiters, and the number was chosen to
endure a probability to keep the planet free from contamination of at
least 99.9% over the duration of the exploration period.
Critiques
The
Coleman Sagan equation has been criticised because the individual
parameters are often not known to better than a magnitude or so. For
example, the thickness of the surface ice of Europa is unknown, and may
be thin in places, which can give rise to a high level of uncertainty in
the equation.
It has also been criticised because of the inherent assumption made of
an end to the protection period and future human exploration. In the
case of Europa, this would only protect it with reasonable probability
for the duration of the period of exploration.
Greenberg has suggested an alternative, to use the natural
contamination standard - that our missions to Europa should not have a
higher chance of contaminating it than the chance of contamination by
meteorites from Earth.
As long as the probability of
people infecting other planets with terrestrial microbes is
substantially smaller than the probability that such contamination
happens naturally, exploration activities would, in our view, be doing
no harm. We call this concept the natural contamination standard.
Another approach for Europa is the use of binary decision trees which is favoured by the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System under the auspices of the Space Studies Board. This goes through a series of seven steps, leading to a final decision on whether to go ahead with the mission or not.
Recommendation: Approaches to
achieving planetary protection should not rely on the multiplication of
bioload estimates and probabilities to calculate the likelihood of
contaminating Solar System bodies with terrestrial organisms unless
scientific data unequivocally define the values, statistical variation,
and mutual independence of every factor used in the equation.
Recommendation: Approaches to achieving planetary protection for
missions to icy Solar System bodies should employ a series of binary
decisions that consider one factor at a time to determine the
appropriate level of planetary protection procedures to use.
Containment and quarantine for restricted Category V sample return
In the case of restricted Category V missions, Earth would be
protected through quarantine of sample and astronauts in a yet to be
built Biosafety level 4 facility.
In the case of a Mars sample return, missions would be designed so that
no part of the capsule that encounters the Mars surface is exposed to
the Earth environment. One way to do that is to enclose the sample
container within a larger outer container from Earth, in the vacuum of
space. The integrity of any seals is essential and the system must also
be monitored to check for the possibility of micro-meteorite damage
during return to Earth.
The recommendation of the ESF report is that
“No uncontained Mars materials,
including space craft surfaces that have been exposed to the Mars
environment should be returned to Earth unless sterilised"
..."For
unsterilised samples returned to Earth, a programme of life detection
and biohazard testing, or a proven sterilisation process, shall be
undertaken as an absolute precondition for the controlled distribution
of any portion of the sample.”
No restricted category V returns have been carried out. During the
Apollo program, the sample-returns were regulated through the Extra-Terrestrial Exposure Law.
This was rescinded in 1991, so new legislation would need to be
enacted. The Apollo era quarantine procedures are of interest as the
only attempt to date of a return to Earth of a sample that, at the time,
was thought to have a remote possibility of including extraterrestrial
life.
Samples and astronauts were quarantined in the Lunar Receiving Laboratory. The methods used would be considered inadequate for containment by modern standards.
Also the lunar receiving laboratory would be judged a failure by its
own design criteria as the sample return didn't contain the lunar
material, with two failure points during the Apollo 11 return mission,
at the splashdown and at the facility itself.
However the Lunar Receiving Laboratory was built quickly with
only two years from start to finish, a time period now considered
inadequate. Lessons learned from it can help with design of any Mars
sample return receiving facility.
Design criteria for a proposed Mars Sample Return Facility, and
for the return mission, have been developed by the American National
Research Council, and the European Space Foundation.
They concluded that it could be based on biohazard 4 containment but
with more stringent requirements to contain unknown microorganisms
possibly as small as or smaller than the smallest Earth microorganisms
known, the ultramicrobacteria. The ESF study also recommended that it should be designed to contain the smaller gene transfer agents
if possible, as these could potentially transfer DNA from martian
microorganisms to terrestrial microorganisms if they have a shared
evolutionary ancestry. It also needs to double as a clean room facility
to protect the samples from terrestrial contamination that could confuse
the sensitive life detection tests that would be used on the samples.
Before a sample return, new quarantine laws would be required.
Environmental assessment would also be required, and various other
domestic and international laws not present during the Apollo era would
need to be negotiated.
Decontamination procedures
For all spacecraft missions requiring decontamination, the starting point is clean room assembly in US federal standard class 100 cleanrooms. These are rooms with fewer than 100 particles of size 0.5 µm or larger per cubic foot. Engineers wear cleanroom suits
with only their eyes exposed. Components are sterilized individually
before assembly, as far as possible, and they clean surfaces frequently
with alcohol wipes during assembly. Spores of Bacillus subtilis
was chosen for not only its ability to readily generate spores, but its
well-established use as a model species. It is a useful tracker of UV
irradiation effects because of its high resilience to a variety of
extreme conditions. As such it is an important indicator species for
forward contamination in the context of planetary protection.
For Category IVa missions (Mars landers that do not search for
Martian life), the aim is to reduce the bioburden to 300,000 bacterial
spores on any surface from which the spores could get into the Martian
environment. Any heat tolerant components are heat sterilized to 114 °C.
Sensitive electronics such as the core box of the rover including the
computer, are sealed and vented through high-efficiency filters to keep
any microbes inside.
For more sensitive missions such as Category IVc (to Mars special regions),
a far higher level of sterilization is required. These need to be
similar to levels implemented on the Viking landers, which were
sterilized for a surface which, at the time, was thought to be
potentially hospitable to life similar to special regions on Mars today.
In microbiology, it is usually impossible to prove that there are
no microorganisms left viable, since many microorganisms are either not
yet studied, or not cultivable. Instead, sterilization is done using a
series of tenfold reductions of the numbers of microorganisms present.
After a sufficient number of tenfold reductions, the chance that there
any microorganisms left will be extremely low.
The two Viking Mars landers
were sterilized using dry heat sterilization. After preliminary
cleaning to reduce the bioburden to levels similar to present day
Category IVa spacecraft, the Viking spacecraft were heat-treated for 30
hours at 112 °C, nominal 125 °C (five hours at 112 °C was considered
enough to reduce the population tenfold even for enclosed parts of the
spacecraft, so this was enough for a million-fold reduction of the
originally low population).
Modern materials however are often not designed to handle such
temperatures, especially since modern spacecraft often use "commercial
off the shelf" components. Problems encountered include nanoscale
features only a few atoms thick, plastic packaging, and conductive epoxy
attachment methods. Also many instrument sensors cannot be exposed to
high temperature, and high temperature can interfere with critical
alignments of instruments.
As a result, new methods are needed to sterilize a modern
spacecraft to the higher categories such as Category IVc for Mars,
similar to Viking. Methods under evaluation, or already approved, include:
Vapour phase hydrogen peroxide
- effective, but can affect finishes, lubricants and materials that use
aromatic rings and sulfur bonds. This has been established, reviewed,
and a NASA/ESA specification for use of VHP has been approved by the
Planetary Protection Officer, but it has not yet been formally
published.
Ethylene oxide
- this is widely used in the medical industry, and can be used for
materials not compatible with hydrogen peroxide. It is under
consideration for missions such as ExoMars.
Gamma radiation and electron beams
have been suggested as a method of sterilization, as they are used
extensively in the medical industry. They need to be tested for
compatibility with spacecraft materials and hardware geometries, and are
not yet ready for review.
Some other methods are of interest as they can sterilize the spacecraft after arrival on the planet.
Supercritical carbon dioxide
snow (Mars) - is most effective against traces of organic compounds
rather than whole microorganisms. Has the advantage though that it
eliminates the organic traces - while other methods kill the
microorganisms, they leave organic traces that can confuse life
detection instruments. Is under study by JPL and ESA.
Passive sterilization through UV radiation (Mars). Highly effective against many microorganisms, but not all, as a Bacillus
strain found in spacecraft assembly facilities is particularly
resistant to UV radiation. Is also complicated by possible shadowing by
dust and spacecraft hardware.
Passive sterilization through particle fluxes (Europa). Plans for missions to Europa take credit for reductions due to this.
Bioburden detection and assessment
The
spore count is used as an indirect measure of the number of
microorganisms present. Typically 99% of microorganisms by species will
be non-spore forming and able to survive in dormant states,
and so the actual number of viable dormant microorganisms remaining on
the sterilized spacecraft is expected to be many times the number of
spore-forming microorganisms.
One new spore method approved is the "Rapid Spore Assay". This is
based on commercial rapid assay systems, detects spores directly and
not just viable microorganisms and gives results in 5 hours instead of
72 hours.
Challenges
It is also long been recognized that spacecraft cleaning rooms harbour polyextremophiles as the only microbes able to survive in them. For example, in a recent study, microbes from swabs of the Curiosity
rover were subjected to desiccation, UV exposure, cold and pH extremes.
Nearly 11% of the 377 strains survived more than one of these severe
conditions.
This does not mean that these microbes have contaminated Mars.
This is just the first stage of the process of bioburden reduction. To
contaminate Mars they also have to survive the low temperature, vacuum,
UV and ionizing radiation during the months long journey to Mars, and
then have to encounter a habitat on Mars and start reproducing there.
Whether this has happened or not is a matter of probability. The aim of
planetary protection is to make this probability as low as possible. The
currently accepted target probability of contamination per mission is
to reduce it to less than 0.01%, though in the special case of Mars,
scientists also rely on the hostile conditions on Mars to take the place
of the final stage of heat treatment decimal reduction used for Viking.
But with current technology scientists cannot reduce probabilities to
zero.
New methods
Two recent molecular methods have been approved for assessment of microbial contamination on
spacecraft surfaces.
Adenosine triphosphate
(ATP) detection - this is a key element in cellular metabolism. This
method is able to detect non cultivable organisms. It can also be
triggered by non viable biological material so can give a "false
positive".
Limulus Amebocyte Lysate assay - detects lipopolysaccharides (LPS).
This compound is only present in Gram-negative bacteria. The standard
assay analyses spores from microbes that are primarily Gram-positive, making it difficult to relate the two methods.
Impact prevention
This
particularly applies to orbital missions, Category III, as they are
sterilized to a lower standard than missions to the surface. It is also
relevant to landers, as an impact gives more opportunity for forward
contamination, and impact could be on an unplanned target, such as a
special region on Mars.
The requirement for an orbital mission is that it needs to remain
in orbit for at least 20 years after arrival at Mars with probability
of at least 99% and for 50 years with probability at least 95%. This
requirement can be dropped if the mission is sterilized to Viking
sterilization standard.
In the Viking era (1970s), the requirement was given as a single
figure, that any orbital mission should have a probability of less than
0.003% probability of impact during the current exploratory phase of
exploration of Mars.
For both landers and orbiters, the technique of trajectory
biasing is used during approach to the target. The spacecraft trajectory
is designed so that if communications are lost, it will miss the
target.
Issues with impact prevention
Despite these measures there has been one notable failure of impact prevention. The Mars Climate Orbiter
which was sterilized only to Category III, crashed on Mars in 1999 due
to a mix-up of imperial and metric units. The office of planetary
protection stated that it is likely that it burnt up in the atmosphere,
but if it survived to the ground, then it could cause forward
contamination.
Mars Observer
is another Category III mission with potential planetary contamination.
Communications were lost three days before its orbital insertion
maneuver in 1993. It seems most likely it did not succeed in entering
into orbit around Mars and simply continued past on a heliocentric
orbit. If it did succeed in following its automatic programming, and
attempted the manoeuvre, however, there is a chance it crashed on Mars.
Three landers have had hard landings on Mars. These are Schiaparelli EDM lander, the Mars Polar Lander, and Deep Space 2. These were all sterilized for surface missions but not for special regions (Viking pre-sterilization only). Mars Polar Lander, and Deep Space 2 crashed into the polar regions which are now treated as special regions because of the possibility of forming liquid brines.
Controversies
Meteorite argument
Alberto G. Fairén and Dirk Schulze-Makuch published an article in Nature
recommending that planetary protection measures need to be scaled down.
They gave as their main reason for this, that exchange of meteorites
between Earth and Mars means that any life on Earth that could survive
on Mars has already got there and vice versa.
Robert Zubrin used similar arguments in favour of his view that the back contamination risk has no scientific validity.
Rebuttal by NRC
The meteorite argument was examined by the NRC in the context of back contamination. It is thought that all the Martian meteorites
originate in relatively few impacts every few million years on Mars.
The impactors would be kilometers in diameter and the craters they form
on Mars tens of kilometers in diameter. Models of impacts on Mars are
consistent with these findings.
Earth receives a steady stream of meteorites from Mars, but they
come from relatively few original impactors, and transfer was more
likely in the early Solar System. Also some life forms viable on both
Mars and on Earth might be unable to survive transfer on a meteorite,
and there is so far no direct evidence of any transfer of life from Mars
to Earth in this way.
The NRC concluded that though transfer is possible, the evidence
from meteorite exchange does not eliminate the need for back
contamination protection methods.
Impacts on Earth able to send microorganisms to Mars are also
infrequent. Impactors of 10 km across or larger can send debris to Mars
through the Earth's atmosphere but these occur rarely, and were more
common in the early Solar System.
Proposal to end planetary protection for Mars
In their 2013 paper "The Over Protection of Mars", Alberto Fairén and Dirk Schulze-Makuch suggested that we no longer need to protect Mars, essentially using Zubrin's meteorite transfer argument. This was rebutted in a follow up article "Appropriate Protection of Mars", in Nature by the current and previous planetary protection officers Catherine Conley and John Rummel.
Critique of Category V containment measures
The
scientific consensus is that the potential for large-scale effects,
either through pathogenesis or ecological disruption, is extremely
small.
Nevertheless, returned samples from Mars will be treated as potentially
biohazardous until scientists can determine that the returned samples
are safe. The goal is to reduce the probability of release of a Mars
particle to less than one in a million.
Proposal for extension of protection to non-biological considerations
A COSPAR workshop in 2010, looked at issues to do with protecting areas from non biological contamination. They recommended that COSPAR expand its remit to include such issues.
Recommendations of the workshop include:
Recommendation 3 COSPAR should add a separate and parallel policy to
provide guidance on requirements/best practices for protection of
non-living/nonlife-related aspects of Outer Space and celestial bodies
Ideas for doing this suggested since the workshop include protected special regions, or "Planetary Parks" to keep regions of the Solar System pristine for future scientific investigation, and also for ethical reasons.
Proposal to extend planetary protection
Astrobiologist Christopher McKay has argued that until we have better understanding of Mars, our explorations should be biologically reversible.
For instance if all the microorganisms introduced to Mars so far remain
dormant within the spacecraft, they could in principle be removed in
the future, leaving Mars completely free of contamination from modern
Earth lifeforms.
In the 2010 workshop one of the recommendations for future
consideration was to extend the period for contamination prevention to
the maximum viable lifetime of dormant microorganisms introduced to the
planet.
"'Recommendation 4.' COSPAR
should consider that the appropriate protection of potential indigenous
extraterrestrial life shall include avoiding the harmful contamination
of any habitable environment —whether extant or foreseeable— within the
maximum potential time of viability of any terrestrial organisms
(including microbial spores) that may be introduced into that
environment by human or robotic activity."
In the case of Europa,
a similar idea has been suggested, that it is not enough to keep it
free from contamination during our current exploration period. It might
be that Europa is of sufficient scientific interest that the human race
has a duty to keep it pristine for future generations to study as well.
This was the majority view of the 2000 task force examining Europa,
though there was a minority view of the same task force that such strong
protection measures are not required.
"One consequence of this view is
that Europa must be protected from contamination for an open-ended
period, until it can be demonstrated that no ocean exists or that no
organisms are present. Thus, we need to be concerned that over a time
scale on the order of 10 million to 100 million years (an approximate
age for the surface of Europa), any contaminating material is likely to
be carried into the deep ice crust or into the underlying ocean."
Exoplanets
The proposal by the German physicist Claudius Gros, that the technology of the Breakthrough Starshot project may be utilized to establish a biosphere of unicellular organisms on otherwise only transiently habitable exoplanets, has sparked a discussion, to what extent planetary protection should be extended to exoplanets.
The discussion involves in particular the long-term perspective of
complex vs. bacterial life and the diversity of life in the cosmos.
