Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Lunar and Planetary Science XLVIII (2017) 1341.pdf EXOSPHERIC ESCAPE: A PARAMETRICAL STUDY. R. M. Killen1 and M. H. Burger2, 1NASA Goddard Space Flight Center ([email protected]), 2Space Telescope Science Institute ([email protected]). Method: We assume that in a collisionless exosphere a neutral species escapes if it achieves escape velocity defined as: vesc = ( 2GM 1/2 8π ) = ( G ρ R 2 )1/2 R 3 where G is the gravitational constant, R is the object radius, M is the object radius, and ρ is the object density. where vth is the thermal speed defined as kT/m, k is Boltzmann's constant, T is the temperature, and m is the mass of the neutral species. The temperature could refer to the surface temperature, the temperature of the vapor cloud produced by impact vaporization, or other process that results in a thermal distribution (e.g., photon-stimulated desorption ejects Na with a ~1200 K Maxwellian distribution [4] (). We also consider a sputtering distribution in the form: f (v) ∝ EU (E +U)3 where U is the binding energy. Results Figure 1 shows the temperature at which 50% of an ejected neutral species escapes from a body as a function of the object radius and species mass, assuming the body has a density of 3 g cm-3; i.e., it is the temperature at which half the Maxwellian distribution is above the escape velocity. A small body such as Phobos (radius=XX km) cannot hold any atmosphere at almost any temperature above 0 K. Figure 2 shows the fraction of sputtered gas that escapes from an object as a function of object radius and the species mass, assuming the surface binding energy U=2 eV, consistent with Na bound to a regolith [5]. 9 0. 5 0. 2 0. 5 1 0. 01 .9 0 0. 9 1.0 0.8 1000 0.6 0.4 5 0.2 0 5 .1 0.0 1 100 0.2 0.0 10 10 100 1000 Object Radius (km) Gravitational Escape Fraction Gravitational Escape Fraction, Test Mass = 18 AMU Phobos Ceres Moon Earth 10000 0. spheres, i.e., atmospheres that are collisionless down to an object's surface, are the most common type of atmosphere in the Solar System. They are known to occur at the Moon, Mercury, and several outer planet moons. The primary mechanisms for these exospheres vary for different objects and exospheric species, but include micrometeoroid impact vaporization, photon-stimulated desorption, ion sputtering, and thermal desorption. Several objects, such as Enceladus [1], Europa [2] and Ceres [3] show evidence of active venting of water. We have conducted a parametrical study of exospheres to distinguish between the bound and escaping components of exospheres (an escaping exosphere is commonly referred to as a corona). We describe a method for determining whether an exospheric species will escape from a body as a function of the object radius (mass) and surface temperature, source velocity distribution, and the species mass. This can be applied to extra-solar planets as well. f (v) ∝ v 3 exp(−v 2 / vth2 ) 0.9 Gravitational Escape Fraction: Surface-bounded exo- Neutral species are ejected from the surface with a Maxwellian flux distribution in the form: 0.99 stand the long-term loss of volatiles from planetary bodies due to interactions of planets, satellites, and small bodies with the interplanetary medium (solar wind, meteors, and dusk), solar radiation, and internal forces including diffusion and outgassing. Recent evidence for water and OH on the moon has spurred interest in processes involving chemistry and sequestration of volatile species at the poles and in voids. In recent years, NASA has sent spacecraft to asteroids including Vesta and Ceres, and ESA sent Rosetta to the asteroids Lutetia and Steins. OSIRIS-REX will return a sample from a primitive asteroid, Bennu, to Earth. It is possible that a Phobos-Deimos flyby will be a precursor to a manned mission to Mars. Exospheric particles are derived from the surface and thus reflect the composition of the body's regolith, although not in a one-to-one ratio. Observation of an escaping exosphere, termed a corona, is challenging. We have therefore embarked on a parametrical study of exospheres as a function of mass of the exospheric species, heliocentric distance, rotation rate of the primary, and composition of the body (asteroid type or icy body). These parameters will be useful for mission planning as well as quick look data to determine the size and location of bodies likely to retain their exospheres and observability of exospheric species. It is also of interest to be able to determine the extent of contamination of the pristine exosphere due to spacecraft sent to make measurements. Source Temperature (K) Introduction: The study of exospheres can help us under- 10000 Figure 1. Gravitational escape fraction for a Maxwellian distribution at source temperature vs. Object radius (km). Ceres, the Moon and Earth are marked by the black lines. Lunar and Planetary Science XLVIII (2017) 0.25 1.0 0.8 0.5 0.9 20 0.25 30 0.6 0.4 0.2 10 10 0.0 100 1000 Object Radius (km) Gravitational Escape Fraction Earth 0.5 0.9 0.99 40 Sputtering Flux Distribution, U = 2 eV Ceres Moon 0.99 Species Mass (amu) Phobos 50 1341.pdf 10000 Figure 2. Gravitational escape fraction is shown here for a sputtering velocity distribution with binding energy 2 eV. References: [1] (Porco C. C. et al. (2006) Science 311, 13931401. [2] Roth L. et al. (2014) Science, 343, 171-174. [3] Kueppers M. et al. (2014) Nature 505, 525-527. [4] Yakshinskiy B. V. and Madey T. E. (2004) Icarus 168, 53-59. [5] McGrath M. A., Johnson R. E. and Lanzerotti L. J. (1986) Nature 323, 694-696.