NONTHERMAL ESCAPEOF THE ATMOSPHERES OF VENUS, EARTH, AND MARS BernieD. Shizgal• andGregory G. Arkos Department of Earth and Ocean Sciences, Universityof British Columbia, Vancouver, British Columbia, Canada Abstract. Atmospheric loss from planetary atmo- sphere. The translationalenergy required for escape spheresis an importantgeophysical problem with impli- could be thermal energyand proportionalto the ambi- cations for planetary evolution. This is a multidisci- ent temperatureor the result of some collisionalpro- plinaryresearch field that requiresan expertisein a wide cessesenergizing the speciesabove thermal energies. range of subjectsincluding statisticalmechanics, fluid These collisionalprocesses, which include charge ex- mechanics,plasma physics, collision theory, and surface change and dissociativerecombination between ener- science.This paper is a reviewof the current stateof our getic ions, neutrals, and electrons, are referred to as understandingof atmosphericloss from the terrestrial nonthermalescape processes. We highlightthe similar- planets.A detailed discussionis provided of the basic itiesand differencesin the importantescape mechanisms conceptsrequired to understandthe processesoccurring on the terrestrialplanets and commenton applicationof in the high-altitudeportion of a planetaryatmosphere thesemechanisms to evolutionarytheories of the terres- referred to as the exosphere.Light atomic specieswith trial atmospheres.The emphasisin this paper is directed sufficienttranslational energy can escapefrom an atmo- toward the need to considerthe exosphereas collisional. 1. INTRODUCTION processesthat are referred to as nonthermal escape mechanisms.An example of such collisionalprocesses One of the more challengingmultidisciplinary prob- are collisionsbetween hot protonsand coolerhydrogen lems in geophysicsand atmosphericscience is the study atoms resulting in energetic hydrogen atoms that can of the evolutionand escapeof planetary atmospheres. escape.There are many other nonthermal processes An atmosphereis boundincompletely to a planet by the includingphotodissociation, sputtering [Johnson, 1994], planetary gravitational field. At very high altitudes, and solar wind pickup. Hunten [1982] has provided a atomic speciesof low mass,such as hydrogenand he- comprehensivelist of the varioustypes of nonthermal lium, can attain speedsin excessof the escapespeed of processesthat occurin planetaryatmospheres. Since the the planet and escape,provided they suffer no further particledensities of the exosphereare small,an approx- collision.The escapeflux referred to as thermal escape imate model of the exosphere[Chamberlain, 1963] as- or Jeans[1925] escapedepends on the ambienttemper- sumesthat exosphericatoms move, without colliding, ature and occursover a range of altitudes where the under the influenceof the planetarygravitational field. atmosphereis approximatelycollisionless, which is in the The studyof exosphericprocesses and escapemech- vicinity of the exobase.The exobaseis the altitude for anismsfrom the planets is of considerableinterest to whichthe mean free path of atmosphericconstituents is planetary scientistsin their effort not only to better equal to the densityscale height and is defined in detail understandthe present state of planetary atmospheres in section2. The region abovethe exobaseis referred to but also to model atmosphericevolution. This is a mul- as the exosphere.For Earth the exobaseis at approxi- tidisciplinarystudy which involvescoupling of different mately 500 km, and the exosphere,in the vicinityof the atmosphericregions in addition to the interactionof the exobase,is predominantlyatomic oxygen with hydrogen exospherewith the solarwind plasmaand the planetary and helium as minor species.Heavier speciessuch as surface.The expertiseof scientistsin atmosphericchem- oxygen,nitrogen, and carboncan escapefrom the atmo- istry and dynamics,ionospheric plasma properties, geol- spheresof the terrestrialplanets as a resultof collisional ogy and planetary surfacemorphology, planetary mag- netic fields, the solar wind plasma, and other fields is required. •Also at Departmentof Chemistry,University of British There havebeen severalexcellent reviews which pro- Columbia, Vancouver, British Columbia, Canada. vide a good overviewof the basicconcepts involved in Copyright1996 by the American GeophysicalUnion. Reviewsof Geophysics,34, 4 / November 1996 pages 483-505 8755-1209/96/96 RG-02213515.00 Paper number 96RG02213 ß 483 ß 484 ß Shizgaland Arkos:NONTHERMAL ESCAPE 34, 4 / REVIEWSOF GEOPHYSICS the study of exospheres.The review by Chamberlain translationallyexcited. Some of the exothermicityof the [1963]provides a very detailedtheoretical description of reaction appearsas translationalenergy of the product the collisionlessexosphere. Several additional reviews atoms,and a large populationof energeticor hot atoms have been written by Hunten and Donahue [1976], Tin- is produced. sley[1978], Fahr and Shizgal[1983], Hunten [1982,1990], Hot oxygenatoms resulting from dissociativerecom- andMahajan and Kar [1990].This reviewis motivatedby bination were predicted to occur on both Mars and the need to emphasizethe importanceof nonthermal Venus by McElroy et al. [1982a] and Rodriguezet al. processesin planetary exospheresand to reconsider [1984]. The presenceof hot oxygenon Venus was con- currentexospheric models. This reviewis appropriateat firmed by the 130.4-nm measurementof the ultraviolet this time in view of upcomingmissions to Mars and the spectrometeron the Pioneer Venus Orbiter [Nagyet al., importantobservational constraints on theoreticalmod- 1981]. A hot corona of atomic oxygenhas also been els that the new data will provide.Our aim in this paper observedfor the terrestrialexosphere [Yee et al., 1980]. is to provide the geophysicscommunity with a back- Theoretical estimates of the nonthermal character and ground of the basic conceptsin exosphericphysics as extentof thesehot oxygencoronae in the exospheresof well as an understandingof the important outstanding the terrestrial planets have been provided by several researchproblems. While someof the topicscovered in researchers[Nagy and Cravens,1988; Nagy et al., 1995; the previousreviews are repeatedfor completeness,the Ip, 1988, 1990;Lammer and Bauer, 1991;Shematovich et main emphasisof this paper will be collisionalphenom- al., 1994]. Enhanced loss of hydrogenwith respect to ena and nonthermal processes.We provide an up-to- deuterium,as implied by the D/H ratio observedby the date review, and we cite primarily the most recent pa- Pioneer Venus Orbiter, if coupledwith an appropriate pers sinceabout 1980which impacton currentresearch lossof oxygen,may be indicativeof the lossof water. in this area. We refer the reader interested in the his- The observed Pioneer Venus Orbiter D/H ratio has led torical developmentof the subjectto the reviewsmen- to suggestionsthat an Earth equivalentocean of water tioned previouslyand the exhaustivereference lists con- existedon Venus in the distantpast and hasdissipated as tained in those works. a result of this loss of atomic hydrogen and oxygen. The presentunderstanding of planetaryexospheres is Other isotopicfractionations, such as the enrichmentof determinedlargely by satelliteand ground-basedobser- •SNover •4N on Mars,can be explainedon the basis of vationswhich are predominantlymeasurements of the similar nonthermalprocesses. emissionsof exosphericconstituents. These includeLy- Nonthermal processeshave also been employedin man a and Lyman • emissionsof atomic hydrogenat order to understanda discrepancyin the terrestrial he- 121.6nm and 102.6nm, respectively,emission of helium lium budget [Axford, 1968; Bates and McDowell, 1959; at 58.4 nm, and emissionof atomic oxygenat 130.4 nm. Lie-Svendsenet al., 1992].The productionof 4He is These observationsof the exospheretogether with in predominantlyfrom the radioactive decay of 238U,with situ massspectrometric measurements provide density anestimated flux of Fprod • (0.9-1.9)x 106 cm-2 s-• and temperatureprofiles of neutral and chargedconstit- [Torgerson,1989]. For helium the exosphericescape uents.For example,data from the Pioneer Venus large energy on Earth is approximately2.5 eV, and for a probe neutral mass spectrometerindicated an enrich- densityof 106 cm-3 andtemperature of 1000K at the ment of the deuteriumto hydrogen(D/H) ratio in the exobasethe thermal Jeans flux is Fj = 0.4 cm-2 s-• (see exosphereof Venus by a factor of 100 relative to the section2), so that Fprod/F J • (2.3-4.8)x 106.Clearly, terrestrialvalue [Donahueet al., 1982]. As discussedin the calculatedrate of outgassingof 4He is far greater section 3, this enrichment of deuterium relative to hy- than the loss due to thermal Jeans escape, and the drogenis believedto arisefrom the enhancedescape of atmospherichelium contentshould be far abovewhat is hydrogen due to nonthermal processes.Nonthermal observed.Furthermore, as helium is chemicallyinert, processesrefer to collisionsbetween exospheric species there are no reactive processesto accountfor the ob- and translationallyenergetic species (both ionsand elec- serveddiscrepancy. In order to reconcilethis, there must trons), generally of ionosphericorigin. This includes exist additional loss processeswhich remove helium processessuch as the collision of hot plasmaspheric from the atmosphere.Lie-Svendsen et al. [1992] and protonswith exospherichydrogen, H + + H -• H* + Lie-Svendsenand Rees[1996a, b] recentlystudied several H +, whicheffectively converts the energeticprotons