GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 6, PAGES 861-864, MAY 1990 INDIRECT EVIDENCES FOR A GAS/DUST TORUSALONG THE PHOBOS ORBIT Dubthin E.M., R. Lundin,N.F.Pissarenko, S.V.Barabash, A.V.Zakharov, H.Koskinen, K.Schwingenshuh ,Ye.G.Yeroshenko bsA t•r•. Observations from the PHOBOS-2 space- craft of plasma and magnetic field effects inthe solar wind v,•.ar Mars suggest thata neutral gas (dust?)torus/ring r•ides theorbit of the Martian satellite Phobos. along , Magnetic 'cavities (strong decreases of the magnetic field st-reng•h) coincident with strong plasma density increases (up to afactor of ten) are observed during the first el- !/pt•c transition orbits when thespacecraft approached the Phobos orbits. The characteristic transverse dimen- sion of the structures along the spacecraft orbitis in the range I00-!000 kin. 'Torus effects' also have character- is.6cs similar to the formation of a bow shock with in- creases of plasma density and ion temperature, and a c•acteristic deflection of the ion flow. This suggests a rather strong interaction between the solar wind plasma and plasma near Phobos orbit. The interaction appears q•t.e similar to that ofthe solar wind witha comet. The mtga•ssi. ngof matter from Phobos (and Deim0s ) is also suggested byplasma observations in thewake/tail of the Mart•ian satellites. Altogether, our observations imply thata neutral gas cloud -- possibly also associated with a faint dust ring-- exists along the Phobos orbit. Introduction The question of a dustring along the Phobos orbit can be tr.aced backto Sorer (197!). The dust ejectacan be generated by meteorite impactson the Phobos surface. Ana/ysing the motionof ejecta Sorer(1971) obtained a Pictm•e of toroidal distribution of dust particles around Phobos orbit. More recently this issuewas discussed by Banas•kiewicz and Ip (1990). According to their results d•sW/bution of ejecta is cor•qned in a disk configuraration The other important rational for the hypothesis of a dus• pa•ic!e population is that Phobos lies near the Ro- cl•e-li•t. Thereby the satellite is subject to strong tidal forces from Mars, expected to resultin collisions and •he jetrison of small velocity fragments into circumplanetary •rbits. Attempts to trace the Phobos ring by optical means (e.g. Duxbury and Ocampo, 1988) haveso far been unsuccessful. As for the gas toms, micrometeorite impacts on the Phobos surface, outgassing of Phobos and accretional in- ,•e'•ti.on with the hot oxygen corona of Mars may con- **fibute to it (ip,1988,Ip and Banaszkiewicz, 199'0). Ip and Banaszkiewicz (1990) have -calculated •;he radial dis- •butionof thenumber density of oxygen. Theenhance- ment in the number density from corona -- dustring •memction canbe important onlyfor an optical depth of ,¾ 10 -a. A number density in the center of the hot cmv•;en torus exceeding 10 acm -a is expected in the case of•l•e strong intrinsic outga•ing of Phobos. Copyright 1'990 by the •erican Geophysical Union. ?apa:r number 90GL00869 0!094• 8 •276/90/90GL- 00869 $ 03.O0 The first observation indicatingpossible outgassing from the Martian moons originates from a report by Bog- danov (198!), who observed an unusual solar wind dis- turbance in the wake behind the outer Martian satellite Deimos. In this report we demonstrate that the Martian inner moonPhobos indeed shows the signatures of an interac- tion with the solarwind that oneexpects to occur from a body that outgasses. Furthermore, observations of strong solar wind disturbances each time the P!tOBOS-2 space- craft crossed the Phobos orbit are also evidence for a Phobos gas/dust ring along the Phobos orbit. Instrumentation The PHOBOS-2 spacecraft carrieda set of plasma in- strumcutsfor measuring the plasmainteraction between the solar wind and the Martian environment. In this report we comparedata from the fluxgate magnetome- ter MAGMA (seee.g. Riedler et.al., 1989 for a brief description of the instrument) with datafromthe instru- ment ASPERA (Automatic Space PlasmaExperiment with a Rotating Analyser) that measures the composi- tion,energy, and angular distributionof ions with en- ergies 0.SeV/e + 24keV/e and electrons with energies 1eV + 50keV (Lundin et.al., 1989).The plasma analyser has a 360x field of view with one sector pointing the so- lar direction. This provides a coverage of the unit sphere either by own mechanicalscanner or by the spacecraft rotation. At times, when the spacecraft was three-axis stabilized and the scanner was switchedoff, ASPERA measured in a planenominally lying along the eclipticX- axis. The data to be presented here are mainly moments of the distribution function (density, flow velocity e.t.c.) determinedin the measurement plane of ASPERA. The time resolution of such moments are up to one setof mo- ments (e.g. N, T, V•, and V_t. ) every approximately 13s. Observations The data for studying the solar wind interaction with Phobos andthe plasma tail behind Phobos come fromthe first four transition orbits of PHOBOS-2, when the space- craftwas in a highly eccentric orbitnearthe equatorial plane.Fromthese spacecraft orbits we have identified 4 crossings of the Phobos/Deimos orbits in the solar wind and4 crossings of the Phobos/Deimos orbits in the Mar- tian tail (see Figure !). Fur•hemore, wehave identified a number of Phobosand Deimostail cross•gs. Except for two solar wind • Deimos tail crossings, all tail cross- ingsoccurred in the Martian plasma tail. Thus, some ambiguities existin terms of distinguishing the plasma characteristics of the Phobos/Deimos plasma tails with processes taking place in the Martianplasma tail. Figures la and lb give the spacecraft trajectory and the Phobos /Deimos orbits projected onto the ecliptic plane,with Figurela illustrating the spacecraft closest approach of the Martian satelhtes andFigure lb illustrat- ing •;h e spacecraft crossings of the Phobos andDeimos 861