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The loss of ions from Venus through the plasma wake

An Author Correction to this article was published on 09 May 2022

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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 System1. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry2, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape3 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.

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Figure 1: Venus Express orbit, and plasma and magnetic field measurements on 2006 December 12.
Figure 2: Composition and energy distribution of the escaping plasma in three spatial regions.
Figure 3: Spatial distribution of the escaping plasma.

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We thank the European Space Agency for providing the Venus Express opportunity, and national space agencies and organizations for supporting the investigators who contributed to the success of the Venus Express plasma package.

Author Contributions S.B. is the principal investigator of the Venus Express plasma package, Analyser of Space Plasmas and Energetic Atoms (ASPERA)-4. J.A.S. is the co-principal investigator. A.F. is the leading co-investigator of the ion mass analyser of ASPERA-4. A.C. is the leading co-investigator of the electron spectrometer of ASPERA-4. T.L.Z. is the principal investigator of the magnetometer. The remaining authors are co-investigators on either of the plasma analyser or the magnetometer investigations.

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Barabash, S., Fedorov, A., Sauvaud, J. et al. The loss of ions from Venus through the plasma wake. Nature 450, 650–653 (2007).

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