W. Schmidt

The loss of ions from Venus through the plasma wake

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.

Pressure observations by the Curiosity rover: Initial results

[1]xa0REMS-P, the pressure measurement subsystem of the Mars Science Laboratory (MSL) Rover Environmental Measurement Station (REMS), is performing accurate observations of the Martian atmospheric surface pressure. It has demonstrated high data quality and good temporal coverage, carrying out the first in situ pressure observations in the Martian equatorial regions. We describe the REMS-P initial results by MSL mission sol 100 including the instrument performance and data quality and illustrate some initial interpretations of the observed features. The observations show both expected and new phenomena at various spatial and temporal scales, e.g., the gradually increasing pressure due to the advancing Martian season signals from the diurnal tides as well as various local atmospheric phenomena and thermal vortices. Among the unexpected new phenomena discovered in the pressure data are a small regular pressure drop at every sol and pressure oscillations occurring in the early evening. We look forward to continued high-quality observations by REMS-P, extending the data set to reveal characteristics of seasonal variations and improved insights into regional and local phenomena.

LYMAN-ALPHA OBSERVATIONS OF COMET HYAKUTAKE WITH SWAN ON SOHO

Abstract The SWAN instrument on board SOHO is a Lyman-α (Lα) photometer able to map the sky intensity with a resolution of 1°, and a capability of microstepping (0.1°). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background, but was in addition extensively used to map the Lα emission of several comets since its launch in December 1995. Here we report observations of comet C/1996 B2 (Hyakutake). Its Lα emission cloud extended over more than 60° while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distance to the Sun, pre-perihelion. The derived H2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H2CO, CH3OH and CH3CN. Most important, the D H ratio in comet Hyakutake is now found at 3 × 10−4, as in comet Halley, while a previous estimate based on a wrong H2O number had indicated a value twice lower, with important cosmogonic consequences. The time evolution showed a fast surge on 21 March, coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the detailed comparison of H column densities (observed vs model) as a function of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 March, and then a progressive filling-in of the H envelope. After the surge, there was a plateau for 16 days around 1.8 × 1029 H2O mol s−1, and then an increase following approximately a R−2 law. This behavior is interpreted as the surge and plateau corresponding to the fragmentation and total disruption/evaporation of a fragment of the nucleus, of approximately 200 m. Finally, it is argued that the first detection of ethane C2H6 in this comet (IR observations) might have been the result of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich comets as argued by other authors.

Mars Science Laboratory relative humidity observations: Initial results

The Mars Science Laboratory (MSL) made a successful landing at Gale crater early August 2012. MSL has an environmental instrument package called the Rover Environmental Monitoring Station (REMS) as a part of its scientific payload. REMS comprises instrumentation for the observation of atmospheric pressure, temperature of the air, ground temperature, wind speed and direction, relative humidity (REMS-H), and UV measurements. We concentrate on describing the REMS-H measurement performance and initial observations during the first 100 MSL sols as well as constraining the REMS-H results by comparing them with earlier observations and modeling results. The REMS-H device is based on polymeric capacitive humidity sensors developed by Vaisala Inc., and it makes use of transducer electronics section placed in the vicinity of the three humidity sensor heads. The humidity device is mounted on the REMS boom providing ventilation with the ambient atmosphere through a filter protecting the device from airborne dust. The final relative humidity results appear to be convincing and are aligned with earlier indirect observations of the total atmospheric precipitable water content. The water mixing ratio in the atmospheric surface layer appears to vary between 30 and 75 ppm. When assuming uniform mixing, the precipitable water content of the atmosphere is ranging from a few to six precipitable micrometers. Key Points Atmospheric water mixing ratio at Gale crater varies from 30 to 140 ppm MSL relative humidity observation provides good data Highest detected relative humidity reading during first MSL 100 sols is RH75%

Convective vortices and dust devils at the MSL landing site: Annual variability

Two hundred fifty-two transient drops in atmospheric pressure, likely caused by passing convective vortices, were detected by the Rover Environmental Monitoring Station instrument during the first Martian year of the Mars Science Laboratory (MSL) landed mission. These events resembled the vortex signatures detected by the previous Mars landers Pathfinder and Phoenix; however, the MSL observations contained fewer pressure drops greater than 1.5u2009Pa and none greater than 3.0u2009Pa. Apparently, these vortices were generally not lifting dust as only one probable dust devil has been observed visually by MSL. The obvious explanation for this is the smaller number of strong vortices with large central pressure drops since according to Arvidson et al. [2014] ample dust seems to be present on the surface. The annual variation in the number of detected convective vortices followed approximately the variation in Dust Devil Activity (DDA) predicted by the MarsWRF numerical climate model. This result does not prove, however, that the amount of dust lifted by dust devils would depend linearly on DDA, as is assumed in several numerical models of the Martian atmosphere, since dust devils are only the most intense fraction of all convective vortices on Mars, and the amount of dust that can be lifted by a dust devil depends on its central pressure drop. Sol-to-sol variations in the number of vortices were usually small. However, on 1u2009Martian solar day a sudden increase in vortex activity, related to a dust storm front, was detected.

Preliminary retrieval of solar wind latitude distribution from Solar Wind Anisotropies/SOHO observations

The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar-induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.

Comets in full sky

The SWAN instrument onboard the SOHO spacecraft is a Lyman α scanning photometer cabable of mapping the whole sky with

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maps of the SWAN instrument - I. Survey from 1996 to 1998

resolution. Since January 1996 the instrument has produced on average three full sky maps a week with the principal scientific objective of observing the distribution of heliospheric neutral hydrogen. In addition, these systematic observations are a valuable source for studying comets brighter than a visual magnitude of 7-11, the observing limit depending on the abundance ratios of produced radicals and the location of the comet relative to the galactic plane. When the data before the temporary loss of control of SOHO at the end of June 1998 were processed, altogether 18 comets were positively identified, of which one is a new discovery and another 5 can be detected on SWAN images before their actual discovery date. This demonstrates the feasibility of SWAN as an instrument for cometary surveys. The observations are used to estimate the water production rates of the detected comets near their perihelion passages.

LYMAN-ALPHA OBSERVATIONS OF COMET 46 P/WIRTANEN WITH SWAN ON SOHO: H2O PRODUCTION RATE NEAR 1997 PERIHELION

Abstract The SWAN instrument on board SOHO is a Lyman-α photometer able to map the sky intensity with a resolution of 1°, primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background. In addition SWAN was extensively used to map the Lyman α emission of several comets since launch in December 1995. Here we report observations of Comet 46 P/Wirtanen near perihelion. From the recorded Lyman α intensity the H 2 O production rate was derived for 45 observations from 21 December 1996–17 May 1997, with a peak of 1.6±0.4×10 28 mol/s just before perihelion. This should help to constrain the physical models of 46 P/Wirtanen for Rosetta mission planning purposes.