International Journal of
Astrobiology, 5, Issue 01,
January 2006, pp. 17-22 (Cambridge University Press).
THE SULPHUR DILEMMA:
Are there biosignatures on Europa's icy and patchy surface?
_____________________________________________________________
J. Chela-Flores
The Abdus Salam International Centre for Theoretical Physics,
Strada Costiera 11; 34014 Trieste, Italy and Instituto de Estudios
Avanzados, Apartado Postal 17606 Parque Central, Caracas 1015A,
R. B. Venezuela.
e-mail:chelaf@ictp.it, URL: http://users.ictp.it/~chelaf/index.html
Abstract: We discuss whether sulphur traces on Jupiter's moon Europa could be of biogenic origin. The compounds detected by the Galileo mission have been conjectured to be endogenic, most likely of cryovolcanic origin, due to their non-uniform distribution in patches. The Galileo space probe first detected the sulphur compounds, as well as revealing that this moon almost certainly has a volcanically heated and potentially habitable ocean hiding beneath a surface layer of ice. In planning future exploration of Europa there are options for sorting out the source of the surficial sulphur. For instance, one possibility is searching for the sulphur source in the context of the study of the "Europa Microprobe In Situ Explorer" (EMPIE), which has been framed within the Jovian Minisat Explorer Technology Reference Study (ESA). It is conceivable that sulphur may have come from the nearby moon Io, where sulphur and other volcanic elements are abundant. Secondly, volcanic eruptions in Europa's seafloor may have brought sulphur to the surface. Can waste products rising from bacterial colonies beneath the icy surface be a third alternative significant factor in the sulphur patches on the Europan surface? Provided that microorganisms on Europa have the same biochemical pathways as those on Earth, over geologic time it is possible that autochthonous microbes can add substantially to the sulphur deposits on the surface of Europa. We discuss possible interpretations of the non-water ice elements (especially the sulphur compound mercaptan) in the context of the studies for future missions. To achieve reliable biosignatures it seems essential to go back to Europa. Our work highlights the type of biogenic signatures that can be searched for when probing Europa's icy and patchy surface.
Key words: Jupiter's
moon Europa, biogenic signatures, sulphur patches on the Europan
surface, missions to Europa, biogeochemistry.
Introduction: the patchiness of the icy surface of Europa
The two Voyager spacecrafts crossed
the orbit of Jupiter in 1979. The images that were retrieved from
the icy surface of Europa were significant: they presented a young
terrain with very few craters. Io was a surprise due to its volcanic
activity. Sixteen years later the Galilean satellites revealed
more physical, chemical and geophysical data. The Galileo mission
showed evidence of internal liquid water oceans in Europa and
Callisto (Showman and Malhotra, 1999). Unlike the ocean on Europa,
Callisto has presented planetary science with an unusual internal
structure, since data from the Galileo mission suggests a lower
than expected moment of inertia. The internal structure consists
of a nucleus of ice and rock with the outermost 200 kilometres
of water ice, or liquid water (compatible with the presence of
an ocean) with an outer layer of dust accumulated throughout its
history by impacting bodies. In spite of this significant discovery,
in the present paper we focus on Europa's ocean, since its internal
structure suggests that this satellite provides an environment
more favourable to the presence of life. Indeed, its structure
is compatible with an outermost water layer of about 1 gm/cm3
density and from 80 to 200-kilometre thickness, an intermediate
silicate rock mantle, and perhaps a metallic core (Fe-FeS).
On the other hand, even before the Voyager and Galileo missions,
it was evident that the surface of Europa is dominated by water
ice (Johnson and McCord, 1971). It has also been equally clear
that there is much spectroscopic evidence for the presence of
non-ice substances on the surface (Delitsky and Lane, 1998). In
particular, Galileo Near-Infrared Mapping Spectrometer (NIMS)
evidence for the presence of sulphur compounds has been discussed
in detail (Carlson et al. 2002). It had been suggested
earlier that the sulphur contamination was due to the implantation
of sulphur from the Jovian magnetosphere (Lane et al. 1981).
However, based on combined spectral reflectance data from the
Solid State Imaging (SSI) experiment, the NIMS and the Ultraviolet
Spectrometer (UVS), it has been argued that the non-water ice
materials are endogenous in three diverse, but significant terrains
(Fanale et al. 1999). Effusive cryovolcanism is clearly
one possible endogenous source of the non-water-ice constituents
of the surface materials (Fagents, 2003). The most striking feature
of the non-water surficial elements is their distribution in patches.
Indeed, implantation would be expected to produce a more uniform
surface distribution if the source were ions from the Jovian plasma.
We refer to this phenomenon as 'the patchiness of the icy surface
of Europa'. It may be argued that if the plasma from the magnetosphere
were responsible for the sulphur distribution, some geologic process
has to be invoked to allow for a non-uniform distribution. Such
possibilities have been discussed (Carlson et al. 1999).
Alternatively, the sulphurous material on the surface may be endogenous.
Some mechanisms for the contamination of the surficial water ice
come to mind, based on fluid-dynamic arguments (Thomson and Delaney,
2001): it is possible to interpret the non-water elements on the
icy surface as the product of eruptions on the seafloor that were
subsequently raised to the icy surface. This assumption is especially
reasonable in the chaos-type features, such as melt-through structures
that are formed by rotationally confined oceanic plumes that have
risen form heated regions on the seafloor. In other words, the
cryovolcanism on Europa would not be from its core, but rather
from the bottom of the global ocean. It might be more like the
"black smokers" that are found on the Earth seafloor.
The compounds produced at the bottom of the ocean would make their
way up to the surface. In the next section we use models of Europa
to support the view that there is sufficient sulphur to be raised
from the bottom of the ocean. These models suggest that chondrites
are capable of carrying a sufficient amount of sulphur (3.25%).
This property renders the chondrite as an appropriate model of
a planetesimal that contributed to the formation of Europa (Oro
et al. 1992).
Can waste products rising from bacterial colonies beneath the
icy surface be a significant factor in the sulphur patches on
the Europan surface? (Singer, 2003). The implications of biogenicity
on Europa have intrigued science for some time (Chela-Flores,
2003). In fact, the above-mentioned patchiness of the icy surface
of Europa presents us with the following dilemma: In forthcoming
missions we could test the endogenicity of the non-water ice contaminants.
The search would help us to decide whether sulphur contamination
of the water ice is due to either cryovolcanism, or alternatively
whether the water-ice contamination is due to endogenic factors,
including biogenicity. In the rest of the present paper we shall
explore some possibilities that could be made available in the
foreseeable future for solving this 'sulphur dilemma'.
Biogeochemistry of the Europa icy surface
Of all the biogenic elements, sulphur
has the most relevant isotopic fractionation for the detection
of traces of biogenic activity (Kaplan, 1975): Once the primordial
planetary mantle material (for example, on the Earth), or satellite
internal silicate nucleus (for example, on Europa) had entered
their corresponding geochemical cycles, their initial isotope
mixtures began to be redistributed. The Earth upper mantle and
crust are believed to reflect broadly the isotopic distribution
patterns of chondritic meteorites (Libby, 1971). In this context
we should stress that carbon, through its ¥ 13C [0/00, PDB]
parameter, can be used as a good biosignature. On the Earth biota,
for instance, there is ample evidence that photosynthetic bacteria,
eukaryotic algae and plants have typical significant deviations
that yield values of up to -30 and beyond, due to biological processes
(Schidlowski et al. 1983a). These results are analogous
to the deviations shown by fractionation due to bacterial sulfate
reduction. The point we make here is that for an extraterrestrial
test of biogenicity, as for instance, in lunar fines, where we
know that life is absent, significant negative deviations in ¥
13C do in fact occur, but are absent in the corresponding sulphur
parameter (cf., Figure 11 in Kaplan, 1975). Thus, without prior
knowledge whether we are in the presence of life in a given environment,
negative values of ¥13C do not arise exclusively from biogenic
sources. For this reason we have mentioned above that sulphur
is more relevant for studying possible biosignatures.
Models of Europa suggest that a type of chondrites carry sufficient
amount of water (13.35%), carbon compounds (2.46%) and sulphur
(3.25%) to stand as good models of the planetesimals that gave
rise to the proto-Europa (Oro et al. 1992). The meteorite
in question is petrographic type-2 carbonaceous chondrite of chemical
class CM, i.e., similar to the prototypical Mighei meteorite (Cronin
and Chang, 1993). This shows that in an ice-ocean model of Europa,
collisions with the proto-satellite planetesimals of this composition
would have carried with them sufficient amounts of water to account
for an ocean on Europa (up to 7 % of the mass of the satellite).
Other models have been discussed during the last decade independently
(Kargel et al. 1999). There would have been also sufficient
carbon input for eventually inducing a substantial biota. The
redistribution of the primordial isotopic mixtures can be followed
up in terms of the appropriate parameter, namely the parameter:
For simplicity this function will be
referred to as the delta-34 parameter, or simply as the delta
parameter. Its value is close to zero when the sample coincides
with the corresponding value of the Canyon Diablo meteorite (CDM),
a triolite (FeS) that was found in a crater north of Phoenix,
Arizona. This parameter allows a comparison of a sample (sa) with
the standard (st) CDM. The relevant terms are the dominant sulphur
isotope (32S) and the next in abundance (34S). In fact, (34S/32S)st
coincides with the average terrestrial fraction of the two most
abundant isotopes of sulphur. We obtain positive values of the
delta-parameter when by comparison we have a larger quantity of
the less abundant isotope 34S. Nevertheless, the advantage of
having defined such a parameter is that negative values will indicate
an abundance of the most abundant isotope 32S. Besides, we remark
that in non-terrestrial solar system materials (such as lunar
dust, or meteorites), the values of the delta parameter are close
to the CDM average. This, in turn, signifies that biological processes
will be more easily recognizable when sulphur, rather than the
other biogenic elements (hydrogen, carbon or nitrogen) will be
considered. There is an overwhelming amount of data supporting
the view that metabolic pathways of sulphur bacteria have enzymes
that preferentially select the isotope 32S over 34S. As pointed
out above, this will be reflected in habitats that are depleted
of 34S. In other words, in lakes, seas or oceans, where the sulphur
microbes are present, the value of the delta34S parameter will
have characteristic large negative values.
This suggests that focusing on sulphur might be more reliable
means for estimating biological effects (if any) on Europa. In
contrast, to the isotopes of hydrogen, carbon or nitrogen, sulphur
shows fractionation with a relatively narrow distribution range
in meteorites, as well as the Moon fines, breccias and fine-grained
basalts retrieved by the Apollo missions. In the case of meteorites
these values are about 2o/oo relative to the standard CDM average
(Kaplan, 1975, Farquhar and Wing, 2005). The measurements of isotopic
ratios of the biogenic elements were not considered during the
Galileo Mission. Fortunately, they are in principle measurable
in future missions to Europa.
The Galileo results and the interpretation of the NIMS data
Some arguments militate in favour of
focusing on spectrometry measurements of Europa in situ.
There are very clear signals associated with sulphate-reducing
bacteria living in reducing environments. Dissimilatory sulphate
reduction releases hydrogen sulphide with associated turnover
rates of sulphur unlike the significantly much smaller assimilation
processes. The consequence of this biochemical cycle is that the
dissimilatory sulphur reducers are responsible for the well-observed
large-scale interconversion of sulphur between oxidized and reduced
reservoirs in lacustrian, marine, or oceanic environments. For
instance, seawater sulphate has a delta34S parameter value of
+ 20 o/oo, in sharp contrast with, for instance, biogenic insoluble
sulphide in marine environments. (We find mostly biogenic pyrite,
since sulphate-reducing bacteria unite H with S atoms from dissolved
sulphate of seawater to form hydrogen sulphide; the H2S then combines
with Fe in sediments to form grains of pyrite). In these biogenic
cases the delta34S parameter can have values of even less than
- 40 o/oo (Schidlowski et al. 1983b).
The early stages of future missions may be initially tested on
Earth, in environments that are similar to Europa, namely the
dry valley lakes of southern Victoria Land of Antarctica (Doran
et al. 1994; Parker et al. 1982; Priscu et al.
1999). One large lake lies underneath the Vostok Station, the
Russian Antarctic base about 1,000 km from the South Pole. A lake,
the size of Lake Michigan, was discovered beneath this Station
in 1996 (Ellis-Evans and Wynn-Williams, 1996), after having drilled
in that area since 1974. The lake lies under some 4 kilometres
of ice. Lake Vostok, as it is known, may harbour a unique micro
flora. The retrieval of biota from Lake Vostok will serve as a
test for handling a larger aquatic medium, such as the proposed
Europan submerged ocean that may be teeming with life. At the
time of writing the lake itself has not been sampled, prevented
by the bioethical principles of planetary protection. On the other
hand, in the dry valley lakes there is already a well-studied
biota that consists of abundant microorganisms living underneath
their iced surface. The estimated annual sulphur removal is over
one hundred kilograms in the case of the Lake Chad in the dry
valleys (Parker et al. 1982). Thus, endogenic sulphur and
other chemical elements will be, at any time, found on the icy
surface of the dry valley lakes. These environments will help
us to decide on the experiments that should be performed with
the help of the forthcoming Europa missions.
Sulphur is a non-water ice constituent on the surface of the Galilean satellites
Right from the beginning of the Galileo Mission the icy surface of Europa, and other icy Galilean satellites, were studied by spectroscopic means (Noll et al., 1995). Subsequent measurements with NIMS (Fanale et al. 1999; McCord et al. 1998) have provided some evidence for the presence of various chemical elements on their surfaces (cf., Table 1).
Although the NIMS data allows various
interpretations (a situation that ought to improve during future
missions), we should discuss at present the implications of some
of these possibilities, in preparation for the planning of what
type of biogenic signatures should be searched for, when probing
the Europa icy surface for signs of life. We should recall in
this context that on Earth there are chemical compounds that are
associated with metabolism, or microbial decomposition. Mercaptan,
for example, is one of the most intriguing interpretations of
the data that is available (Bhattacherjee and Chela-Flores, 2004).
We should discuss the possible interpretation, and the equipment
that could test biogenicity of such sulphur compounds.
Mercaptans can be the product of the decay of animal, or vegetal
matter. (Consequently, it is also found in petroleum.) The term
'mercaptan' applies specifically to ethyl mercaptan C2H5SH. This
is a biogenic, volatile compound of sulphur that is found in bacteria
that is eventually obtained from reduced cellular sulphate. Nevertheless,
in this context it should also be pointed out that the six-atom
interstellar molecule CH3SH could be present in the Jovian system
from the time of its formation, since the molecule has been identified
in interstellar dust (Ehrenfreund and Charnley, 2001). Consequently,
if a signal at 3.88 micrometers is present on the Europan surface,
before it can be attributed to a biogenic constituent, it is necessary
to test its source with the appropriate instrumentation. In other
words, if the presence of mercaptan is due to a relic of the interstellar
medium preserved during the satellite formation, then the biogenic
hypothesis could be excluded by appropriate use of the delta-34
S parameter. This enquiry is not beyond the reach of present technology,
as we shall illustrate in the following section.
Planned missions can contribute to solve the sulphur dilemma
New missions for Europa are possible
according to preliminary studies. One example is the "Europa
Microprobe In Situ Explorer" (EMPIE), which has been framed
within the Jovian Minisat Explorer Technology Reference Study
of the European Space Agency (Renard et al. 2005). In
this study a set of four microprobes are intended to land on the
icy surface with a mass constraint of 1.7 kg. (Each lander is
constrained to some 350 gm.) Their penetration in the ice could
be up to just over 70 cm (Velasco et al. 2005). These studies
could be sufficient to allow adequate instrumentation that is
capable of constraining the possibilities of a biota lying beneath
the surface.
Gas chromatography-mass spectrometry (GC-MS) is a possible appropriate
instrumentation for the detection of such sulphur-related compounds.
Indeed, there is a wide variety of miniaturized instruments available,
the development of which has been required by other missions of
planetary exploration, especially the Mercury-bound Bepi Colombo
mission that is due to be launched next decade (Sheridan et
al. 2003). Originally the mission intended to have a GC-MS
on a lander, but this mission has now been restricted to an orbital
probe.
Endogenic, non-living sources (cryovolcanism) can, in principle,
be tested with the technology of landers and probes, such as the
combined microprobe concept studied by EMPIE (with its intrinsic
constrain on the four small payloads that would reach the icy,
patchy surface). A more advanced lander concept has been put forward
envisaging landers on the icy and patchy surface, such as the
JPL study (Gershman et al. 2003).
We have argued above that measuring deviations of the delta-34
S parameter from its mean CDM value can test our biogenic hypothesis.
Sufficiently large negative values of the delta-34 S parameter
would militate in favour of biogenicity.
Understanding the exogenous contribution of Europa's surface patchiness
Besides the early Earth, the most likely
scenarios for early life are Mars, Europa, and even Titan (Fortes,
2000, Chela-Flores, 2001). A whole fleet of missions are likely
to be planned in the foreseeable future in the search life in
planetary, or satellite environments. On the other hand, different
sets of missions that focus on solar exploration are adding substantially
to the knowledge of our nearest star. Ulysses, a solar probe,
is one of them. It has made significant measurements of the Sun
from a polar orbit, but in spite of not being planned for the
exploration of the Jovian system it has also unexpectedly discovered
the presence of abundant streams of chemical elements originating
form Jupiter and its moon Io. Such a surprising discovery is relevant
to the accumulation of exogenous sulphur on the surface of Europa.
From the point of view of identifying the source of the patchiness
of Europa's surface, there is a valid reason for persevering with
solar missions of the Ulysses-type (Messerotti and Chela-Flores,
2006). Solar probes such as Ulysses could contribute to testing
to what extent the patchy surface of Europa is being influenced
by Io's volcanic activity, but clearly further solar missions
alone could not settle the question of reliable biosignatures.
In other words, we need to be more certain of the mechanisms that
the rotating magnetosphere of Jupiter uses for distributing the
chemical elements that it receives from the volcanic activity
of Io that is merely some 6 Jovian radii away from the giant planet.
The Jovian system is emitting streams of volcanic particles at
passing spacecraft. The discovery of this phenomenon dates back
to 1992 when Ulysses was hit by a stream of volcano dust while
approaching within 1 AU from Jupiter (Grun et al. 1993).
These particle streams were detected not only by Ulysses, a solar
mission, but also subsequently by two of the most successful planetary
exploration missions: Galileo and Cassini. It is now agreed that
Io's volcanoes are the dominant source of the Jovian dust streams
(Graps et al. 2000). In September 2004 the impact rate
of the stream of dust particles was recorded once again by the
instrumentation on Ulysses. However, Cassini's dust detector was
more capable than the instrumentation on Ulysses when faced with
a similar event (Sarma et al. 2000). In addition to mass,
speed, charge and trajectory, Cassini measured elemental composition,
finding sulphur and other elements of volcanic origin. Further
measurements of the interplanetary distribution of sulphur that
is spread by the dust streams of Jupiter's magnetosphere would
help us to understand to what extent the exogenic and non-biogenic
sulphur accumulates on the patchy surface of Europa (cf.,
Discussion).
Discussion
As mentioned in the above section on
Biogeochemistry, large negative values of the delta-34 S parameter
(i.e., in the range from - 20 to beyond - 40 o/oo ) would be a
strong and clear signal of biogenic activity. Studies of extraterrestrial
materials (both lunar dust and meteorites) suggest that in the
solar system no natural process, other than biological activity,
yields such large corresponding depletion of the less abundant
isotope 34S, compared to the more abundant isotope 32S. In fact,
the small deviations from the average CDM value that are known
could be due to various physical processes, as in the case of
Moon dust. For example, hydrogen stripping due to solar wind proton
bombardment of Moon dust can lead to minor deviations from the
average CDM value < - 15 o/oo. (For the relevant literature
the reader is advised to refer to Kaplan, 1975.)
In this work we have addressed several questions: Why should
the search for biosignatures focus mainly on the sulphur isotopes?
Would a combination of sulphur and carbon isotope anomalies give
the best biosignature that would show that biology is involved?
We have argued that sulphur is unique amongst the main biogenic
elements in the sense that sulphur, unlike carbon, shows a very
narrow range of values about zero per mil in isotopic fractionation
in extraterrestrial material (lunar fines and meteorites).
Similarly, an additional question raised by our proposed selection
of sulphur isotopic fractionation, rather than the corresponding
analysis in term of carbon, is: Can you accept some contribution
of sulphur from Io, and still find the biogenic fraction in those
sulphur deposits? In this case since we are assuming from the
data that only biogenic processes alter the null values of the
isotopic sulphur fractionation, the contribution from the exogenous
(Io) sulphur would not give the telltale signals for life that
would otherwise be produced by the endogenous sulphur, if it were
the product of bacterial metabolism.
Thus, indirect tests of sulphur deposition involving the solar
missions do not have the same profound significance than the direct
biogeochemical results that we have suggested. Clearly, it would
not be possible to rule out any exogenous sources. Indeed,
such an attempt would be unreasonable and might be asking too
much, since we know that the sulphur distribution is patchy on
Europa's icy surface, and hence an exclusive exogenous source
of sulphur is not to be expected in the first place. It could
even be argued that when Ulysses and Cassini went past the neighbourhood
of Jupiter and Io, special episodes of sulphur distribution were
occurring that were not representative of the long-term sulphur
distribution processes generated by the Jupiter magnetosphere.
But in a future sequence of flybys the situation could possibly
be more representative of the effect of the Jovian magnetosphere.
For these reasons we have suggested that additional solar missions
should be supported, but in this paper we have instead focused
sharply on the question of sulphur isotope fractionation, arguing
that further solar missions could nevertheless still add a small
contribution to our understanding of the exogenous source of the
non-water ice chemical elements on Europa's surface.
Surely, the proposed tests for biosignatures can tolerate some
exogenous sulphur, and still identify the biogenic sulphur as
we have suggested above by searching along vents, or cracks, where
sulphur would be concentrated. This approach suggests significant
strategies for identifying those places where future landers could
search for the biosignatures. The most likely sites would be where
the salt deposits, or organics, are concentrated, as suggested
by the NIMS data. For instance, the search for biosignatures could
focus on the area north of the equatorial region, between 0 and
30 N and between the longitudes 240 and 270 (cf., McCord et
al. 1998, Fig. 2A). But a more intriguing and smaller patch
would be the narrow band with high-concentration of non-ice elements
that lies east of the Conamara Chaos, between the Belus and Asterius
lineae, namely, between 18 - 20 N, and longitudes 198 - 202 (cf.,
McCord et al. 1998, Fig. 2D).
Definite answers can be searched in situ on the icy surface
with GC-MS instrumentation for the corresponding measurements
of the delta-34 S parameter. But even before the biogeochemical
research we have briefly sketched above can be performed by four
miniprobes (EMPIE studies) or by landers (JPL studies), valuable
additional information about the distribution of sulphur throughout
the solar system (and especially in the neighbourhood of Europa
itself) could make a modest contribution to the overall question
of settling one of the most significant problems in astrobiology,
namely the sulphur dilemma.
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