The loss of ions from Venus through the plasma wake

Abstract

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth's also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Why an intrinsic magnetic field does not protect a planet against atmospheric escape

“…Scaling in this way introduces an uncertainty. However, the ionospheric electron densities and the composition of the escaping plasma are similar at the three planets -the H + to O + ratio of the escaping plasma is about 1 for Mars (Lundin et al 2009), 2 for Venus (Barabash et al 2007), and 0.25 for Earth (Table A.1) -and therefore the scaling procedure provides a reasonable approximation of the value and position of the peak at cusp outflow saturation. Modelling the escape rate according to Eq.…”

“…The results are applied to the terrestrial planets, computing escape rates for hypothetical Venus-like, Earth-like, and Marslike planets that have the atmospheric properties these planets have today. Venus, Earth, and Mars are all rocky planets with atmospheres for which satellite-based measurements of atmospheric escape are available (e.g., Barabash et al 2007;Strangeway et al 2005;Lundin et al 2004). For present-day conditions, the escape rates we arrive at in this work are about 0.5 kg s −1 for Venus, 1.4 kg s −1 for Earth, and between 0.7 kg s −1 and 2.1 kg s −1 for Mars (Fig.…”

Oxygen ion escape from Venus in a global hybrid simulation: role of the ionospheric O<sup>+</sup> ions

“…23 in Brace and Kliore, 1991). Estimates of the total solar wind induced oxygen ion escape from Venus range from the order of 10 24 s −1 to 10 26 s −1 including observational and simulation studies (e.g., Brace et al, 1987;Moore and McComas, 1992;Terada et al, 2004;Lammer et al, 2006;Kallio et al, 2006b;Barabash et al, 2007a;Martinecz et al, 2009). We select to call the value 10 25 s −1 as the nominal O + escape rate.…”

“…VEX studies the Venusian plasma environment by its particle instrument ASPERA-4 (Barabash et al, 2007b) and the MAG magnetometer (Zhang et al, 2006). Barabash et al (2007a) reported observations of the O + escape in the near Venus' tail. The statistically presented measurements show the O + , H + and He + flux away from the planet and energization in the direction of the convection electric field typical to the E × B pickup.…”

Hungarian national report on IAGA 2007–2010

“…The ASPERA-4 data contributed to that by measuring major loss processes. Barabash et al (2007b) reported Venus Express measurements showing that the dominant escaping ions are O + , He + and H + . The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere.…”