Phosphorus as a potential guide in the search for extinct life on Mars

Adv. S/me Rex Vol. 15. No. 3, pp. (3)185-(3)191, 1995 Copylisht Q1994 COSPAR Printed in Great Britain. All rights re.wved. 0273-1177/95 $7.00 + 0.00

PHOSPHORUS AS A POTENTIAL GUIDE IN THE SEARCH FOR EXTINCT LIFE ON MARS G. Weckwerth* and M. Schidlowski** * Deutsche Forschungsanstaltfr Luf- wad Raumfahrt, Hauptabteilung Systemanalyse Raumfahrt, Linder HOhe, D-5000 Kdn 90, Germany ** Max-Planck-Institutfr Chemie, Abteilung Biogeochemie, Saarstr. 23, D-6500 Mainz, Germany

ABSTRACT In contrast to the search for extant organisms, the quest for fossil remains of life on Mars need not be guided by the presence of water and organic compounds on the present surface. An appropriate tracer might be the element phosphorus which is a common constituent of living systems. Utilizing terrestrial analogues, it should preferentially exist in the form of sedimentary calcium phosphate (phosphorites), which would have readily resisted changing conditions on Mars. Moreover, higher ratios of P/Th in phosphorites in comparison to calcium phosphates from magmatic rocks give us the possibility to distinguish them from inorganically formed phosphorus deposits at or close to the Martian surface. Identification of anomalous phosphorus enrichments by remote sensing or in situ analysis could be promising approaches for selecting areas preferentially composed of rocks with remains of extinct life.

INTRODUCTION Initial expectations to 6nd extraterrestrial life on Mars were strongly damped by the first pictures from the Martian surface (Mariner 4,1964), and by the detection of its thin, almost pure COz-atmosphere. Low atmospheric pressure and stlrface temperatures indicated that liquid water (as an essential requirement of life) would, in all probability, not be available on the present Martian surface [l]. On the other hand, surface structures such as ancient river valley systems have been found all over the planet, implying the former existence of liquid water in high quantities, but at least two billion years ago. This would indicate that atmospheric conditions (pressure and temperatures) on Mars in those days could not have been too different &om conditions on the Archaean Earth where atmospheric compositions were likewise dominated by carbon dioxide and nitrogen.

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G. Weckwerth and M. Schidlowski

Therefore, it cannot be excluded that biological evolution had proceeded on Mars during the first billion years of the planet’s existence but, in contrast to the Earth, had been halted by decreasing temperatures. As a result, water and the bulk of atmospheric carbon dioxide were frozen, this bringing about a substantial reduction of the atmospheric pressure. There is little doubt that organic compounds that had possibly existed three billion years ago close to the Martian surface would not have resisted changing environmental conditions on the planet. Average surface temperatures more than 50°C lower than on Earth, an atmospheric pressure of only 1% of the terrestrial pressure, up to 100 times higher fluxes of high-energy radiation [2] and high temperature fluctuations along with wind erosion were bound to destroy all traces of former organic substances, and notably their volatile components [3]. Therefore, in constraet to a search for extant organisms (in the polar ice or in permafrost regions), the immediate surface of the planet as exposed today is probably not a promising hunting ground for organically preserved relics of extinct life.

THE SIGNIFICANCE

OF PHOSPHORUS

Apart from the water-laid sediments occurring in the conspicuous river valleys, a promising indicator of suitable places to search for extinct life could be the element phosphorus (P) which is a common constituent of living systems, specifically because of its key role If terrestrial analoguea can be utilized, it should in biological energy transductions. preferentially exist in the form of calcium phosphates, similar to phosphorites, which can be found in sediments all over the Earth [4], mainly on the ocean floor (Fig, 1). These phosphorites (primarily made up of carbonate fluorapatite or “francolite” with 3 ) EF s.a8Fz) were formed by processes the general formula Cars_X_rN~Mgy(PO~)s_~(CO involving biological mediation, thus being remains of former life. Phosphorite formations on Mars would have readily resisted changing conditions there over the last billion years. On the other hand, calcium phosphates apparently unrelated to life processes have also been found in meteorites and on the lunar surface. Such inorganically formed calcium phosphate in the form of apatite and whitlockite is also present on the Martian surface in quantities up to 1 or 270 according to the information provided by the SNC meteorites. Therefore, an important corollary of the possible use of phosphorus as a tracer of former life on Mars is that biogenic calcium phosphate can be distinguished from inorganically formed varieties of this mineral. Table 1 shows the main differences of these two mineral variants. In particular, the inorganically formed calcium phosphates associated with magmatic rocks take up a host of incompatible elements, with P-concentrations in such high-temperature environments consequently positively correlated with REE, Th, U and other incompatibles. Fig. 2 shows the excellent correlation of P-concentrations with the concentrations of the rare earth element (REE) neodymium (Nd) wh ic h is found in all primary magmatic rocks [5]. Thorium (Th) and uranium (U) al so belong to this group of elements which are incompatible with the minerals of the main silicate series, and hence their concentrations in magmatic rocks are similarly correlated with phosphorus. Since the absolute concentration of thorium is about 10 times lower than that of neodymium, common magmatic P/Thabundance ratios are between 100 and 1000.

t?)187

Phosphorus as a Guide

fossil phosphorites o recent phosphorites l undated phosphoxites

l

Fig.

1. Distribution

their relationship

of recent

higher.

to the

Table

above

P/Th

between

1.1-2

100 000, far higher

related to the extreme formations,

insolubility

the analytical

this trend.

in water.

The

in the planet’s distinct

from

surface.

Therefore,

elements

becomes

of phosphorites

corresponding

uranium

is moderately

of both samples

sedimentary

cover,

their

trace

formed analytical

imperative.

methods

The elements

they

On the other hand, measurements

Mars

(admittedly

Observer

faint)

chance

is

are very suitable analytical

does not quite follow

due to its slightly higher are much lower than 1,

life on Mars

element

calcium

radioactivity.

US

enriched

of extinct

here because

The

of P/Th

is apparently

from an aqueous solution.

investigated

natural

ANALYTICAL

with Th-

rocks.

indicative

any inorganically

or X-ray

ratio

difference

in Table 2 show that uranium

the design of suitable

activation-

are much

from India,

abundance This

been precipitated

chosen

complicated

phosphorites

[7], showing that the Indian phosphate

potentially

a compelling

from

ANALOGUE

of Th in seawater

The Th/U-ratios

external

glaringly

and

sample.

data presented

formations

sea floor,

in any magmatic

which is also very unusual for other terrestrial If phosphorite

those

ratios,

have certainly

In most phosphorites,

solubility

AS A TERRESTRIAL

analyses

ppm [S].

like all phosphorites,

Moreover,

abundance

than

on the present

[4].

PHOSPHORITES

2 shows two representative

concentrations around

and fossil phosphorites

to areas of upwelling

SEDIMENTARY In contrast

--- areas of upwelling water

abundances

phosphates

should

on the

for determining

thorium

for an analysis

were preserved

the respective

and uranium

on Mars

have been

by virtue

of P-concentrations

be

Martian

of their

require more

methods.

METHODS mission,

launched

for a possible

in September

identification

1992,

of areas that

may

already

offer

are characterized

an by

G. Weckwerth and M. Schidlowski

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TABLE 1 Comparison Biogenic (Sedimentary)

Source

Crystallized

from:

of Inorganically Formed Phosphate Formations

(Magmatic)

and

Magmatic Rocks (apatite)

Sediments (“phosphorites”)

Residual

PO:--saturated pore waters Biological mediation

silicate

liquids

Cause of P-enrichment:

Incompatibility with main rock forming minerals

REE,

Correlated with P and other incompatibles

Markedly depleted due to different solubilities in water

ratios P/Th: Estimated Martian

N 100-1000

> 50 000

ratios

ff 10 000

> lo6

U and Th contents:

Terrestrial

abundance

P/Th:

TABLE 2 Instrumental Neutron Activation Analysis (INAA) for Selected Elements of Two Proterozoic Phosphorites from India (Samples by Courteousy D.M. Banerjee, 1985)

Phosphorite (2) (0.892 mg)

Phosphorite (1) (1.127 mg) Ca

34.1

Fe P Na K Mn

2.29 NlO 1940 2050 230

co Ga

20 2.4

As

141

Sr Ba La Ce

960 3160 120 115 24

Sm Eu DY Th U P/Th Th/U

5.8 34.8 <2 45.3

% % %

wm wm wm mm PPm PPm PPm PPm PPm PPm PPm PPm PPm PPm PPm

31.4 0.5 -10 270 2680 31.5 20 2 9.1 172 73 11.6 21 1.7 1.77 0.95 1.1 2.3

% % %

wm wm p?m wm PPm ppm

PPm PPm PPm PPm PPm PPm Pmm PPm PPm

Phosphorus as a Guide

L

LpPqnl -

basalts n komatiites + spine1 lherzolites A others

l

103 r

lo-’ Fig.

2. Covariance

as an example specifically anomalously carries the

of phosphorus

for the positive

of mantle-derived

high phosphorus

a r-ray

of cosmic

loo

detector

rays from

Martian

surface

Using

phosphorus

y-ray

only exeptionally Certainly,

detector

at the planetary

surface.

identify phosphorus Likewise,

the natural

improved

terrains

we will probably on a balloon.

on the Martian

radioactivity

chances

sensing

of

to conduct

a

surface.

for this method.

To get a signal would

more 181, while SNC meteorites to its orbital

are unlikely

for a potential

have of the

and hence

to detection. to be exposed

have to wait until such an instrument Nevertheless,

any preliminary

attempt

surface would meet with paramount

of Th and U could be measured of similar

altitude,

would lend themselves

of such an areal extension

Thus,

3), but with the encounter

Markedly

for remote

of up to 0.5% only. Even worse, the area1 resolution

possibly

anomalies

rocks [5]

concentrations

the back-scattering

it is planned

on the Martian

is just 300 km, equivalent

formations

flies closer to the surface,

3), mainly

can be utilized

and probably

large phosphate-bearing

phosphorite

(Table

instrument,

is not the ideal element

yielded concentrations

Mars Orbiter

which this

of the main elements

abundance

magmatic

PZOS and REE

At the end of its long arm, the observer

the r-radiation surface,

Unfortunately,

(Table

concentrations.

composition.

require at least 1% absolute

in terrestrial

between

lo2

Nd [ppml

[ll].

the Martian

mapping

hitherto

and neodymium

correlation

melts

to measure

reconnaissance

10’

resolution

by this r-ray

to

interest. detector

problems.

identification

of phosphorites

are offered by

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G. Weckwerth and M. Schidlowski

TABLE Z$Analytical Method8 for Remote Sensing and In-Situ Analysis of Phosphorus, Thorium and Uranium Measurements

from Orbit

Gammaspectrometer on board of Mars Observer - detection limit P: l-10% - detection limit Th: N lppm - detection limit U: N lppm - areal resolution: N 300 km (- flight altitude) Measurements

at the Surface

Detection limits: - P N 0.1% [X-ray-fluorescence, INAA (108n*8-‘*cm-2)] - Th N 0.1 ppm (natural radioactivity) - U N 0.05 ppm (natural radioactivity)

analytical approaches carried out from lander8 directly at the planetary surface (Table 3). X-ray fluorescence measurements or activation analyses with a neutron source should be possible with reasonable detection limits [5] but, unfortunately, only from a limited number of miniscule sampling spots.

CONCLUSION Attempt8 to a88e88 the biogenicity of potentially detectable phosphorus enrichment8 on Mars have to await a substantial improvement of current analytical approaches and/or remote sensing techniques, although the basic analytical tool (y-ray spectroscopy) is already at our disposal. Moreover, relevant chemical surveys need to be supported by complementary investigations involving, for instance, lsC/12C work on rock carbon, or the search for biosedimentary structures and microfossils (9,lO). In all likelihood, these various approaches will have to rely on robotic sample return missions, if not manned missions. In any case, evidence pertaining to positive P-anomalies and appropriate concentration ratio8 Th/U could offer guidance for selecting promising sampling sites for future Mars missions.

REFERENCES 1. J.A. Wood, The Solar System, Prentice-Hall, Inc., Englewood Cliffs, N.J. (1979) 2. Guidance on Radiation Received in Space Activities, NCRP Report No. 98 (1989) 3. F. Miles, Aufbruch zum Mars, Franckh’sche Verlagsbuchhandlung,

Stuttgart,

1988

4. Y. Kolodny, Phosphorites, in: The Sea, ed. C. Emiliani, Wiley, New York 1980, Vol. 7, p. 981-1023

Phosphorus s a Guide

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5. G. Weckwerth, Anwendung der instrumentellen /3-Spektrometrie im Bereich der Kosmochemie, insbesondere zur Messung von Phosphorgehalten, Diploma Thesis, University of Mains (1983) 6. B. Spettel and D.M. Banerjee, private communication (1985) 7. A. Kaufman, The “‘Th concentration of surface ocean water, Geochim. Cosmochim. Acta 33, 717-724 (1969) 8. J. Briickner, private communication (1989) 9. M. Schidlowski, Stable carbon isotopes: Space Res. 12, No. 4, 101-110 (1992)

Possible clues to early life on Mars, A&.

10. L.J. Rothschild and D.J. Des Marais, Stable carbon isotope fractionation in the search for life on early Mars, Adv. Space Res. 9, No. 6, 159-165 (1989) 11. A.E. Beswick and I.S.E. Carmichael, Constraints on mantle source compositions imposed by phosphorus and rare-earth elements, Contrib. Minerd. Petrol. 67, 317330 (1978)