Journal of Molecular EvOlution by Springer-Verlag. 1979

Size: px

Start display at page:

Download "Journal of Molecular EvOlution by Springer-Verlag. 1979"

Eustacia Kelley
5 years ago
Views:

1 ~ J. Ml. Evl. 14, (1979) Jurnal f Mlecular EvOlutin by Springer-Verlag Slar Radiatin Incident n the Martian Surface W.R. Kuhn and S.K. Atreya Department f Atmspheric and Oceanic Science, The University f Michigan, Ann Arbr, Michigan 48109, U.S.A. Summary. Calculatins indicate that the maximum daily slar radiatin reaching the Martian surface is abut 325 calcm 2 during suthern hemisphere summer at latitude f abut 40 S. In the ultravilet regin f the spectrum, the radiatin reaching the surface at wavelengths greater than 2800 A is within 10% f the radiatin incident n the atmsphere. There is significant extinctin f radiatin in the spectral regin near 2500 A in mid and high latitudes due t absrptin f radiatin by zne; radiatin reaching the surface may be reduced t ne ne-thusandth f that incident n the atmsphere during winter. Virtually n radiatin f wavelengths less than 1900 A reaches the surface because f absrptin by the large clumn abundance f carbn dixide. Daily and latitudinal distributins f radiatin are presented fr wavelengths f 3000, 2500 and 2000 A. Key wrds: Mars -- Slar radiatin Intrductin The slar radiatin reaching the surface f a planet fr a given latitude and time depends upn the energy incident n the atmsphere, the amunt f thse gases and dust within the atmsphere that absrb the radiatin, and the scattering f radiatin by mlecules, dust, and cluds. In additin the eccentricity f the planetary rbit, the inclinatin f the rtatin axis t the ecliptic, and the revlutin and rtatin rate influence the spatial and tempral distributin f the radiatin. Mars is half again as far frm the sun as the earth, s that the slar radiatin incident n the Martian atmsphere is 43% f that incident n the earth's atmsphere. Thus the "slar cnstant" fr Mars, i.e. the energy incident n ne square centimeter at the mean distance f Mars frm the sun is 0.83 calriescm2min. On earth, as slar radiatin traverses the atmsphere, abut 19% is absrbed by water vapr and t a lesser extent by dust, zne, and cluds. 20% is reflected back t space by cluds, while air mlecules scatter 6%. Thus apprximately 55% f the ttal slar radiatin reaches

2 58 W.R. Kuhn and S.K. Atreya the surface. The situatin is much different n Mars. The clumn abundance 1 f the water vapr in the atmsphere is nly 10-3 f that fund in the terrestrial atmsphere; since water vapr is ne f the primary absrbers f slar radiatin (X > 0.7am), mst will penetrate the Martian atmsphere and reach the surface fr wavelengths lnger than 3000 A. Althugh there is thirty-five times as much carbn dixide in the Martian atmsphere as n earth, carbn dixide des nt absrb slar radiatin in the visible prtin f the spectrum but nly at wavelengths greater than abut 1.5am where the amunt f energy is small. Thus, mlecular absrptin f visible radiatin in the clear atmsphere f Mars is nt nearly s imprtant as it is n earth. As previusly mentined, air mlecules scatter abut 6% f the terrestrial radiatin back t space. This scattering, knwn as Rayleigh scattering, depends n the atmspheric clumn density which is prprtinal t the air pressure at the surface f the planet. On Mars this is less than 10-2 f that n earth. Thus Rayleigh scattering f visible radiatin n Mars is small, being less than 1% fr wavelengths greater than 3500 A. Rayleigh scattering des becme mre imprtant at the shrter wavelengths since the scattering varies inversely as the furth pwer f the wavelength. Dust in a planetary atmsphere attenuates the slar radiatin by bth absrptin and scattering. Dust amunts in bth the Martian and terrestrial atmspheres are highly variable in bth space and time. The attenuatin is a functin f wavelength and depends upn the aersl number densities, size distributins and indices f refractin. In the earth's atmsphere, the extinctin f visible slar radiatin by aersls is generally less than 20% (see e.g., Paltridge and Platt, 1976). Dust in the Martian atmsphere is highly variable. During the great dust strm f 1971, the ptical depth 2 was 2 (Masursky, et al., 1972) crrespnding t a transmissin f radiatin f nly 15% Fr clear sky cnditins, the ptical depth is a few tenths. Cluds in the terrestrial atmsphere bscure apprximately 50% f the earth's surface and thus reflect much f the visible radiatin back t space. Cluds als ccur in the Martian atmsphere althugh they are nt as extensive. They appear in the fall and winter seasns abve latitudes f abut 45 and are highly variable in bth space and time. They smetimes appear as wave cluds but als as clud layers which may be thick r quite diffuse (see e.g., Barth and Dick, 1974). The ptical prperties f these cluds are nt knwn, and we have nt attempted t include them in ur calculatins f the radiatin reaching the surface. Thus ne shuld keep in mind that ur estimates f radiatin at the Martian surface are smewhat large fr wintertime, high latitude cnditins. 1 The clumn abundance f a gas in an atmsphere is the amunt in a clumn ne square centimeter in crss sectin extending frm the surface t the tp f the atmsphere. Units may be grams f the gas (gmscm 2) r the height f the clumn f that gas if reduced t standard temperature mad pressure (e.g., ~m-atm, r cm-atm). 2 Optical depth (T) is a measure f the extinctin f radiatin in an atmsphere. It is the prduct f the absrptin r scattering crss sectin times the clumn abundance f the absrbing r scattering cnstituent. The transmissin f radiatin is exp(-t).

3 Slar Radiatin Incident n the Martian Surface 59 Distributin f Ttal Slar Radiatin Incident n the Martian Surface We shw in Fig. 1 the ttal slar radiatin reaching the Martian surface fr clear sky cnditins (ptical depth f 0.35 which crrespnds t a transmissin f 70%). The methdlgy was given in a previus paper and will nt be repeated here (Levine et al., 1977). The verall pattern f the radiatin field n Mars is similar t that fr earth since the angles f inclinatin f the axes f rtatin are nearly the same. One shuld nte hwever the asymmetry between the crrespnding seasns; during suthern hemisphere summer, the radiatin is sme 50% larger than that reaching the surface during nrthern hemisphere summer. This ccurs because the rbital eccentricity f Mars is quite large, 0.093, and during suthern hemisphere summer, Mars is clser t the sun than during nrthern hemisphere summer. During summer there is little variatin in the incident radiatin with latitude, the maximum ccurring near 40 latitude. During winter the radiatin decreases unifrmly with increasing latitude. It is interesting t nte that during nrthern hemisphere summer and winter, Mars receives abut 55% as much radiatin as des the earth at mid-latitudes, while fr the suthern hemispheres the percentages are abut 84% and 53% fr summer and winter, respectively. The larger terrestrial clud cver cmpensates t sme extent the smaller radiatin incident n Mars because it is farther frm the sun AO 20 0 t~ 20 d Z~O 60 Star radiatin incident at the martian surface [cal cm 2 (martian day) q] fr 2"= Slar tngitude ( Seasn ) Fig. 1. Seasnal and latitudinal distributin f ttal slar radiatin reaching the Martian surface. The ptical depth r = 0.35 crrespnds t an average transmissin f 70%. The dashed line is the slar declinatin. Slar lngitudes f, 90, 180, and 270 crrespnd t nrthern hemisphere spring, summer, fall, and winter respectively. The Viking landers are at latitudes f 22.5 and 47.9 N

4 60 W.R. Kuhn and S.K. Atreya Ultravilet Radiatin Incid. ent n the Martian Surface The ultravilet radiatin reaching the surface f Mars is very much greater than that reaching the earth's surface and it has been speculated that this radiatin may react with the Martian sils in as yet unknwn ways. Ultravilet radiatin f wavelengths less than 3000 A des nt penetrate t the earth's surface because it is absrbed by mlecular zne and xygen. Fr example, at 3000 A nly 4% f the radiatin reaches the surface f earth while virtually nne reaches the surface at 2000 A. The zne clumn abundance n Mars is highly variable but it can be as large as 60~m-atm during high latitude winter cnditins (Barth, 1974) which is still nly 2% f that in the terrestrial atmsphere. The methdlgy fr calculatin f the ultravilet radiatin reaching the surface is similar t that used by Levine et al. (1977). The radiatin incident n the planet per unit area, per Martian day, per unit wavelength can be fund by numerically integrating the fllwing expressin: h E=2Sa2cr 2 ] (cshcs6csqs+sin6sin~b) x do exp (-'r(cs h cs 6 cs ~b + sin ~ sin ~b))dh (1) where S is the ultravilet radiatin (energya s) incident n the atmsphere when Mars is at its mean distance (a) f A.U. frm the sun, r is the instantaneus distance f Mars frm the sun, c is the angular speed f rtatin f the planet, 6 is the slar declinatin, ~b is latitude, and h is the hur angle; h is the hur angle f sunrise. The ptical depth r includes Rayleigh scattering and absrptin by carbn dixide and zne. We calculated the Rayleigh crss sectin t be 9.03 x 10-12X 4 where X is the wavelength in Angstrms. The zne crss sectins and slar flux incident n the atmsphere are frm Ackerman (1971) and carbn dixide crss sectins are frm Banks and Kckarts (1973). We shw in Fig. 2 the calculated ultravilet radiatin reaching Mars surface at 50 N latitude during spring and fall. Als included is the radiatin incident n the atmsphere. The minimum in the radiatin reaching the surface at 2550 A is due t absrptin by the Hartley band f zne. On earth nly f this incident radiatin reaches the surface, while n Mars the reductin is abut These calculatins were made fr a Martian clumn abundance f 35gin-arm; fr the maximum bserved zne amunt (60#m-atm), the reductin in slar radiatin wuld be abut During the summer the clumn abundance is less than 3#m-arm in the plar regins (attenuatin f 10%), while in equatrial regins n zne has been measured during any seasn. The secnd feature in Fig. 2 that ne shuld nte is that fr wavelengths f 2000 A and less, virtually n radiatin reaches the Martian surface. This is due t absrptin by carbn dixide which cmprises abut 95% f the Martian atmsphere. Althugh the absrptin crss sectin is nly 5 x cm 2 at 1950 A, the large amunt f carbn dixide (clumn abundance f 2.3 x 1023 cm -2) yields an ptical depth f 11.5 s that nly abut 10-5 f the incident radiatin reaches the surface. The absrptin crss sectin increases t nearly cm 2 in the spectral regin f 1300 t 1500 A (Banks and Kckarts, 1973). Belw 1200 A carbn dixide will dissciate.

5 Slar Radiatin Incident n the Martian Surface < lo 16 u "~c~ 1015 el_ x ~_ 101~ i,\, i "\ i \ ',,j i'""! 50 N Spring N Winter lo Wavetength ( ~, ) Fig. 2. A cmparisn f the radiatin incident n the Martian atmsphere and at the surface fr 50 N Spring and 50 N Winter. The uppermst curve fr each seasn crrespnds t the radiatin incident n the atmsphere The inizatin edge ccurs at 986 A. Crss sectins are abut cm 2. Thus carbn dixide will shield the surface frm any f this shrt wavelength radiatin (Hudsn, 1971). Minr cnstituents in the Martian atmsphere will als absrb this shrt wavelength radiatin but their cntributin is negligible in cmparisn t the absrptin by the large amunt f carbn dixide present. Water vapr absrptin will ccur beginning at wavelength abut 1860 A and reaching a maximum at 1450 A with crss sectin f 5 x cm 2. Mlecular xygen begins t absrb radiatin at 2400 A with crss sectin f cm 2 but its clumn abundance is nly abut 1020 cm -2 s that the extinctin f radiatin at A is small. Althugh scattering f radiatin by the Martian air mlecules (Rayleigh scattering) des cntribute t the ttal extinctin f ultravilet radiatin, the absrptin by carbn dixide dminates belw abut 2000 A. The crss sectins fr bth carbn dixide absrptin and Rayleigh scattering are equal (4 x cm 2) at abut 2200 A. The extinctin f ultravilet radiatin at 2500 A fr Rayleigh scattering is abut 7% and is less than 1% at 3000 A. The variatins f ultravilet slar radiatin with latitude reaching the surface at wavelengths 3000, 2500, and 2000 A are shwn in Figs. 3a, b, and c. At 3000 A there is little atmspheric absrptin and scattering s that the latitudinal distributin is similar t that f the slar radiatin incident n the atmsphere. The phtn flux

6 62 W.R. Kuhn and S.K. Atreya x A S---um m e r e - ~""-~ Equinx slstice n~ 0.8 I f ~x %'\ cl B, swiln~ere ' \ ',,i (nrth) Latitude (suth) ~, 1.6 ~ 1.4 x ~ 1.2 "~1.0,-r 0.8 u m 0.6 ~= O.Z~. ~ 0.2 slstice ~ ~ - ' ", Equinx slstice,~ '~ \ i \! 90,- r I ~ (nrth) Latitude (suth) 0.6r x 2000 A Winter slstice "~O.A - ~ Summe " F ~_ _ (nrth) Latitude (suth) Fig. 3(a). Latitudinal distributin f daily slar radiatin at 3000 A reaching the Martian surface. The arrws refer t the edge f the plar hd; (b). Latitudinal distributin f daily slar radiatin at 2500 A reaching the Martian surface. The arrws refer t the edge f the plar hd; (c). Latitudinal distributin f daily slar radiatin at 2000 A reaching the Martian surface. The arrws refer t the edge f the plar hd

7 Slar Radiatin Incident n the Martian Surface 63 is larger during suthern hemisphere summer than during nrthern hemisphere summer, since Mars is clser t the sun during suthern hemisphere summer. One shuld nte that we have nt cnsidered reflectin f radiatin by the plar hd, the ice clud that veils the plar cap, s that in plar latitudes the radiatin shwn is smewhat t high. At 2500 A (Fig. 3b) the phtn flux is abut ne-tenth that at 3000 A. The latitudinal distributin is smewhat different; there is enugh absrptin at 2500 A s that nw during the summer seasns the maximum in the radiatin is beginning t shift tward lwer latitudes. This is clearly seen in Fig. 3c where the absrptin is s large that the maximum cincides with the slar declinatin. The discntinuity which appears at the equatr results frm ur assumptin that the surface pressure in the suthern hemisphere is less than in the nrthern hemisphere. The mean surface elevatin f the suthern hemisphere is 4 km higher (Wiceshyn, 1974) than the elevatin in the nrthern hemisphere and thus the pressure and carbn dixide amunt in the suthern hemisphere are less, and mre radiatin reaches the surface. There is, f curse, nt an abrupt change in elevatin acrss the equatr and the radiatin reaching the surface wuld nt change as rapidly as shwn in Figs. 3c and 4. Finally we shw the yearly phtn flux fr fur wavelengths. At 3000 and 3500,& there is abut three times as much radiatin reaching the surface near the equatr than at the ples. The slight asymmetry is due t Mars being clser t the sun during C3 x x ~-~ ~1016 r" ~ ~ u 0 0 = 10~5 10 lz~ ( nrth ) Latitude (suth) Fig. 4. Latitudinal distributin f yearly slar radiatin incident n the Martian surface

8 64 W.R. Kuhn and S.K. Atreya suthern hemisphere than during nrthern hemisphere summer. At 2500 A, zne absrbs the radiatin at mid and high latitudes s that there is little latitudinal variatin beynd abut 45 latitude. At 2000 A, absrptin by carbn dixide dminates and there is mre than an rder f magnitude difference in the received radiatin between equatr and ple. Fr shrter wavelengths the phtn flux wuld be negligibly small. Thus we have shwn that the ttal radiatin incident n Mars surface, relative t the earth, is smewhat larger (10-20%) than ne wuld expect by simply cnsidering their relative distances frm the sun. In the ultravilet regin f the spectrum frm 2000 t 3000 A, Mars surface receives much mre radiatin than des the earth. Belw abut 1900 A, n radiatin reaches the surface. Acknwledgments. This wrk was supprted by NASA grants NSG 7308 and NSG References Ackerman, M. (1971). Messpheric Mdels and Related Experiments, G. Ficc, editr. Drdrecht, Hlland: D. Reidel Banks, P.M., Kckarts, G. (1973). Aernmy (Part A). New Yrk: Academic Press Barth, C.A. (1974). Ann Rev. Earth and Planetary Sci. 2, Barth, C.A., Dick, M.L. (1974). Icarus. 22, Hudsn, R.D. (1971). Rev. Gephys. Space Physics. 9, Levine, J.S., Kraemer, D.R., Kuhn, W.R. (1977). Icarus. 31, Masursky, H. et al. (1972). Science 175, Paltridge, G.W., Platt, C.M.R. (1976). Radiative Prcesses in Meterlgy and Climatlgy. New Yrk: Elsevier Wiceshyn, Peter M. (1974). Icarus. 22, Received December 22, 1978; Revised Marcb 19, 1979

NAME TEMPERATURE AND HUMIDITY. I. Introduction

NAME TEMPERATURE AND HUMIDITY. I. Introduction NAME TEMPERATURE AND HUMIDITY I. Intrductin Temperature is the single mst imprtant factr in determining atmspheric cnditins because it greatly influences: 1. The amunt f water vapr in the air 2. The pssibility