pass over a specific :w:t :it a !);irt iculnr t inw of t!:ly. If the orl)it is >I:irs sy~~chrot~~us (i.c., a pcriott of 24" :C"`) the vicwirq interval !vill be 011~ day. Intcr~n1.s of 1w1 or mow da\3 can be rcadil;. :~chic~\wl \Yit II orbits of soturm4r:It longer !wiot!s, !wrniltiir~g 1n0w ~~J:III one :ITC:I to bc rc\%ilcd frcqucntly uliclc>r simil:kr ligtit ing condi- t ions. The twhniqucs tlcscribcd so fnr--groulicl-1J:1~c(l ob- wrvnt.ions, flytjys, and orbiters-will neccw::~rily i1.e remote sewing methods. Thcv wilt ma132 vn!u:~blc cOIltriblltiOIl~ t0 Ollr ~lIldCYst:\Il~!~Ilg of the :LtIn~J.qhcrc and surface, and may also lwotlucc results that are sug- gestive of 311 indigenous hiota (e.g., the presence of atmospheric constitucrits in tioticquilil,riii~n amounts nnti i~~csplainable by abiological processes). However, it is gcncrully agreed that in order to dctect~ uncquivo- cnlty the pre.wice or absence of life, we must land on the surface ant! make direct measurements. This stage WC are now entering with the l'ikillg Project. The l'iking Project consists of two combination orbiter-lander missions in 1973. The orbiter's role has not been complctcly dcterminctl, but it will work with the lander as a team. It ml accomplish this in at least three different. ~I>-s: (1) t )y surveying possif)!c lander sites before the lander is detnchcd from the orbiter, (2) by observing the atmosphrre :111d surface in the neighbor- lwod of ttw I:III&~ in order to rclatc the 1:mtlcr's men- surements to its cnvironmcnt, and (3) by serving as a high capncit~. data relay link bet.necn the lander and earth. Once its primary n+sion is over, the orbiter ma). be uacd to estrapolnte the detailctl findings of the / team to other areas of the planet and to do site survey / The l:mdcr's lifr is :1 minimum of tliwc tl:iys \Vit!i :L Ko:tl of ninety tl:l~~. l'on~r \5ill Pomc from 1):jt Icricy lvit Ii eit,hcr solar cells or :\ r:~ttioisotope thrrmoclcrt ric g:(uII- erator (RTG) serving as t tic source for IWri(J(ls IrJ~igcI than the t hrcc-day minimum. Other Missmns SAS:\ l)l:uis to pl:tcr a sm:1ll q)ii~-stnbilizrcl >l):ic`c- craft, cnllcd a pluwt ary cs!~lorc~r, in orbit arollntl lI:krs in 1973. The object ivck of this niissioll will :~lniozt err- tainly bc investigationa of t Iic so!:lr wilitl i~itcr:~ction v;ith the pl:tnct and of tlic ionos!)hcre. Tt1115, it n-ill be of minor interest for csobiologic:~! studiw. Additional csl)!or:ltion of the surface by Iandw, including rovers, is clc:irly de.sirat)!c. IIowcver, it is premature to present a schedule and r:~pnbi!it ie.; for such . . mlssmns. borne Exobiological Speculstions In putting this issue together, I request4 .Joshu:\ Lederbcrg to offer ~ornc commcllts on the thcorcticn! aspects of exobiology. IIis cOIItribJ.itkJll is brief. but significant, since it gilrcs the flavor of the subject. Rather than include it as a separate paper, I Il;~\~c ndtlctl it to this Tntroductio~i. work for fu(urc Inriders. The orbiter payload n-ill nit, be determilwd until at least Dcccmbcr 1969. How-ever, it is expected that high priority measurements n-ill be visible imagery and ir observations to determine water vapor and surface tempcrnture. The most exciting part. of this n&ion is certainly the lander. :ifter separation from the orbiter, the lander capsule will enter the atmosphere, slow dowi by aero- dynamic bralting, deploy a parachute, and finally jet- tison the parachute and turn on retro-rockets for a soft landing. During tleaccnt, measurements of ntmo- spheric structure, including composition by a mass spectrometer, are envisioned. After a soft landing, a variety of investigations will be carried out. Final selection of the payload will not be made ultt il December 1969. Typical of n-hat could constitute the actual payload are: Facsimile c:~mcra: both high and low resolution pict,urcs. Surface x~~ml)lcr-p~ro!~zer-g3s chromnt.ograph mass spectrometer: organic analysis of the soil, analysis of t tic i\t nloSptlCrC. Direct biology mensurcmcnt : measure various life-related functions such as metabolism, growth, etc. A. Lederberg The editor of this feature had asked me to comment. on the theoretical espcctations for the character of life on >Iars, if any, and how this might be rc~enletl b>- the missions of 1969 and 1971 wllich emph:~.Gze optiwl methods of analysis. Such a comment vxs much easier to write a fete \.enrs ago'*? when the chance of espcrimcntal vcrifiwtion MX~ leas imminent,. It must also be :ttldct! that our go\\-ing knowletlge of the 3Iartian cnvironnncrlt since 19ti.5 (hlariner IV) has made it. difficult to paint :uly facile picture of >lnrtinn life. The most ttiflicult obstnclc is the apparent lack of water, :wcor(ling to s:cvcr:it lines of eviclcncc. On ttic other hand , since thr avcr:q sub- surface tcmpcraturc of >lars is :tbout --cO"C, the hlartinn crust might contain any amount of \wtcr (:I' ice) in the range of mil!igr:un:: to liil0tOIl~ !w iclWIx~ mcter.3 The ez;tcnt of the solid phnsc is, of tour,r, im- material to the equilibrium with the otwrv;li)le atmo- July 1969 / Vol. 8. No. 7 / APPLIED OPTICS 1269 sphere and consistent with any observation so far. It is, tiowevcr, quite crucial t.0 any plausible models of bio- logical systems on the planet. An important challenge to forthcoming reconnaissance missions is the evaluation of subsurface ice (permafrost). It is, ho\ve\.er, not easl to predict just how such a feature will be rcvcaled, if at all, to remote observation. At subliilometcr resolutions, it is barely possible that volcanic formations and asso- ciated clouds might, be delineated photographically and confirmed by ir mapping. 1Iore comprehensive land form ob.-;ervations or analysis of diurnal and seasonal changes might also be contributory. In the face of t.hese discouragements, most biologists (including Inyself) have hccn quite conservative in our stated expectations about Alnrtian life (ul~avs with the qualification, "if my"). Published speculations center on "primitive vegetation." 111 fact, this view is no bet- ter justified than any ot.her. The contemporary life system is surely coupled to solar energy, but it might have originated "in the beginning" from cosmic rather than atmospheric processes.' The aridity and thin at- mosphere of Mars do not, then, necessarily bear on the liketihood of the origination of life or on its initial com- plexity. If solar uv flus is high at the surface, or- ganisms will need shielding, but this is en.zily available with a fern micro& thickness of iron oxides. Hen-ever, the conveyance of solar energy from the surface with moisture a meter below will require an elaborate struc- ture (like the root system of a leafy plant) or as an nlter- native, a food chain of mobile microorganisms. Once this is graut.etl, howver, there are no theoretical limits to the evolution of herbivores and their pkdators, roughly nnnlogous to animal life 011 earth. Sor is there any rigorous argument against intelligent life which, indeed, might have sequcxtcrcd all the nvnilnhlc mois- ture in more congenial, subsurface habitats. To assert these possibilities is not to esprcss my dcep- seated conviction for or ag:linst, them. It speaks rather to the need to keep nn open mind and an open eye for the unespected that will be the main harvest from the mis- sions now initiated. Recent, snc~ly`scs of the ;\I:lrs at.mosphcre are trying to offer SOIW more concrctc possibi1itic.s for the metabolic system (which has n limited bearing on the over-all complexity of my organisms). C:lrbon may be present 011 A[ars in many different forms, but only CO and CO2 are 110~ rcvc:kxl by direct measurement ,5 \Vith both coIl~pourlds present, however, it is hard to imagillc that their mctnbolic interconversion is cscluded from Alar- T. tinn life. (That CO is n general cell poison on earth is immaterial; note that tcrrcstrinl bacteria are known that also oxidize CO.) DC In fact, the exergonic reaction: X0 + E-I20 -t CO, t CII?O + 22 cal might allow for the svnthcsis of carbo- hydrat.e without. photosynthesis within the organism, being driven ultimately by the inorganic photolysis of coz. PI It is not obvious how the actual occurrence of such reactions could he verified short of a lauder mission. At this stage, such proposals are. hardly more than idle speculations. However, they illustrate the way in which more detailed, rigorous information about the 3Iartinn environment can guide our efforts to frame the most. ren~onnble designs for detecting that elusive 1Iartian life, if ari~r.~ References 1. J. Lcderberg, Science 132, 303 (1DGO). 2. J. Lederbcrg, Satwe 207. 9 (I%G). 3. J. bderherg and I). B. Cowie, Science 127, I-liB (l!),jg). 4. C. Sap, and J. Lederberg, Proc. Satl. Acad. Sci. is, 1473 (1962). 5. I,. Iiapl:m, Jet Proprdsiou Laboratory; private commnnica- tion. ct. For general accounts see: S.4S.l SP-3030, IIancl6ook o{ (he Physical Properties of Ae Planel .lJars (Govcrnmellt Printing Of&e, MVwhington, D.C., 19K); SM.4 S-179, The I!ook of .lJars, by Samuel Glwstone (Government Printing Office, Washington, D.C., 1968); Biology and tile Erploralion of dlars C. S. Pittendrigh, W. Vishniac, and J. I'. T. Pearman, Eds. (Nntiowd Acndamy of Science-Satiollal Rwxrch Council, Wzwhington, D.C., 1966). Int (1 rod clue grc: of F ccrl. iltte cow itivt InElI; this rock for11 surf: S'ent nnd inti;, irltci Il~OSi rrga; mntc cart 1, Wt I-(! gr0p' ar1d I or of Ciplit :l!ld ti nlltl g pwcc