Atmospheric acoustics of Titan, Mars, Venus, and Earth

Planetary atmospheres are complex dynamical systems whose structure, composition, and dynamics intimately affect the propagation of sound. Thus, acoustic waves, being coupled directly to the medium, can effectively probe planetary environments. Here we show how the acoustic absorption and speed of sound in the atmospheres of Venus, Mars, Titan, and Earth (as predicted by a recent molecular acoustics model) mirror the different environments. Starting at the surface, where the sound speed ranges from ∼200 m/s for Titan to ∼410 m/s for Venus, the vertical sound speed profiles reveal differences in the atmospheres' thermal layering and composition. The absorption profiles are relatively smooth for Mars, Titan, and Earth while Venus stands out with a noticeable attenuation dip occurring between 40 and 100 km. We also simulate a descent module sampling the sound field produced by a low-frequency “event” near the surface noting the occurrence of acoustic quiet zones.     

Water vapor variations in the Venus mesosphere from microwave spectra

Mm-wave spectra of HDO in the Venus mesosphere (65-100 km) were obtained over the period March 1998 to June 2004. Each spectrum is a measurement of the hemispheric-average H₂O vapor mixing ratio in the Venus mesosphere. Observations were conducted for wide ranges of Venus solar elongations (46° W to 47° E), and fractional disk illuminations (f=0% to 99%), yielding water vapor abundances on 17 dates and over a full range of local solar time (LST) at the sub-Earth point on Venus. Our mesopheric H₂O values are more numerous and far more precise than the earliest mm-derived H₂O measurements [Encrenaz, Th., Lellouch, E., Paubert, G., Gulkis, S., 1991. First detection of HDO in the atmosphere of Venus at radio wavelengths: An estimate of the H₂O vertical distribution. Astron. Astrophys. 246, L63-L66; Encrenaz, Th., Lellouch, E., Cernicharo, J., Paubert, G., Gulkis, S., Spilker, T., 1995. The thermal profile and water abundance in the Venus mesosphere from H₂O and HDO millimeter observations. Icarus 117, 162-172], allowing an analysis of variability that was previously impossible. Measured 65-100 km H₂O ranged from 0.0±0.06 to 3.5±0.3 ppmv, with significantly different variability than found in previous infrared (lower altitude, cloudtop) studies. Strong global variability on a 1-2 month timescale is clear and unambiguous. A limited number of excellent s/n measurements tentatively indicate the 1-2 month variability manifests most rapidly as change in the lower mesosphere, and more slowly as change in the upper mesosphere. Neither long term (1998-2004) nor diurnal variability in 65-100 km H₂O is evident. While six-year and/or diurnal variabilities are not ruled out, they are weaker than the 1-2 month timescale variation. These conclusions are supported by initial (2004) sub-mm measurements.     

Observational definition of the Venus mesopause: vertical structure, diurnal variation, and temporal instability

Submillimeter line observations of CO in the Venus middle atmosphere (mesosphere) were observed with the James Clerk Maxwell Telescope (JCMT, Mauna Kea) about the May 2000, February 2002 superior and July 1999, March 2001 inferior conjunctions of Venus. Combined ¹²CO and ¹³CO isotope spectral line measurements at 345 and 330 gHz frequencies, respectively, provided enhanced sensitivity and vertical coverage for simultaneous retrievals of atmospheric temperatures and CO mixing ratios over the altitude region 75-105 km with vertical resolution 4-5 km. Supporting millimeter ¹²CO spectral line observations with the Kitt Peak 12-m telescope (Steward Observatories) provide enhanced temporal coverage and CO mixing sensitivity. Implementation of CO/temperature profile retrievals for the 2000, 2002 dayside (superior conjunction) and 1999, 2001 nightside (inferior conjunction) periods yields a first-time definition of the vertical structure and diurnal variation of a low-to-mid-latitude mesopause within the Venus atmosphere. At the times of these 1999-2002 observations, the Venus mesopause was located at a slightly lower level in the nightside (0.5 mbar, ∼87 km) versus the dayside (0.2 mbar, ∼91 km) atmosphere. Average diurnal variation of Venus mesospheric temperatures appears to be ≤ 5 K at and below the mesopause. Diurnal variation of Venus thermospheric temperatures increases abruptly just above the mesopause, reaching 50 K by the 0.01-mbar pressure level (∼102 km). Atmospheric temperatures above and below the Venus mesopause exhibited global-scale (≥4000 km horizontal) variations of large amplitude (7-15 K) on surprisingly short timescales (daily to monthly) during the 2001 nightside and 2002 dayside observing periods. Venus dayside mesospheric temperatures observed during the 2002 superior conjunction were also 10-15 K warmer than observed during the 2000 superior conjunction. A characteristic timescale for these global temperature variations is not defined, but their magnitude is comparable to previous determinations of secular variability in nightside mesospheric temperatures (Clancy and Muhleman, 1991).     

9. Life On Earth... And Elsewhere?

This concluding chapter is divided into two main parts. The first part is a summary of the main facts and events which constitute the present body of knowledge of the chronology of life in the solar system, in the form of “highlights” in astronomy, geology, chemistry and biology. The second part raises the interrogation “Is life universal?”, and tries to provide answers based on these facts and events. These answers turn out to differ widely among the various disciplines, depending on how far they feel able to extrapolate their current knowledge.     

Progressive Build Up Of Co 2 In The AtmosphereOf Venus Through Multiple Volcanic Resurfacing Events

In absence of other mechanisms, the main input of CO₂into the Venusian atmosphere is via volcanic out gassing. Since Venus can be regarded as a planet-wide large igneous province, we can expect large quantities of CO₂ being transferred into its atmosphere via volcanic out gassing. We have quantified the maximum possible amount of CO₂ that can be out gassed via a single massive episode of resurfacing of the planet. This figure (5.6 × 10¹⁹ kg of CO₂) is about 8 times smaller than the total CO₂ present in the Venusian atmosphere (4.55 × 10²⁰ kg CO₂). The lack of planet-wide, efficient mechanisms for the recycling of CO₂ on Venus indicates that CO₂ has progressively accumulated in the atmosphere. Based on these considerations we suggest that the “equivalent” to eight global resurfacing episodes would be required to account for the present values of CO_{2 atmosphere}. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stream Interactions and Interplanetary Coronal Mass Ejections at 0.72 AU

We present comprehensive surveys of 203 stream interaction regions (SIRs) and 124 interplanetary CMEs (ICMEs) during 1979 – 1988 using Pioneer Venus Orbiter (PVO) in situ solar-wind observations at 0.72 AU and examine the solar-cycle variations of the occurrence rate, shock association rate, duration, width, maximum total perpendicular pressure (P _t), maximum dynamic pressure, maximum magnetic field intensity, and maximum velocity change of these two large-scale solar-wind structures. The medians, averages, and histogram distributions of these parameters are also reported. Furthermore, we sort ICMEs into three groups based on the temporal profiles of P _t, and we investigate the variations of the fractional occurrence rate of three groups of ICMEs with solar activity. We find that the fractional occurrence rate of magnetic-cloud-like ICMEs declined with solar activity, consistent with our former 1-AU results. This study at 0.72 AU provides a point of comparison in the inner heliosphere for examining the radial evolution of SIRs and ICMEs. The width of SIRs and ICMEs increases by 0.04 and 0.1 AU, respectively, and the maximum P _t decreases to about 1/3 from Venus to Earth orbit. In addition, our work establishes the statistical properties of the solar-wind conditions at 0.72 AU that control the solar-wind interaction with Venus and its atmosphere loss by related processes. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Structural evolution of Lavinia Planitia, Venus: Implications for the tectonics of the lowland plains

This work shows the results of a detailed structural analysis of the deformation belts of Lavinia Planitia. Ridge belts and graben and groove belts can be observed at the studied area, while wrinkle ridges and large individual grooves predominate in the smooth plains. Transcurrent components of displacement are commonly observed, and transpression and transtension zones are the rule rather than the exception at most of the studied belts. Along-strike azimuth changes of deformation belts are accommodated by internal variations in the predominance of contractional, transcurrent or extensional structures. The material of the surrounding plains embays most of these deformation belts. The kinematic analysis of this complex network of tectonic structures suggests a broadly synchronous activity of contractional, transcurrent and extensional structures. The maximum horizontal shortening axis determined in this work describes a steady, semi-circular pattern centered at Alpha Regio. This deformation continued, although with subdued activity, after embayment of the deformation belts by the material of the plains. Future study of the tectonic evolution of the lowland plains should take into account the importance of the coeval history of neighboring uplands and lowlands.     

Generation and emplacement of fine-grained ejecta in planetary impacts

We report here on a survey of distal fine-grained ejecta deposits on the Moon, Mars, and Venus. On all three planets, fine-grained ejecta form circular haloes that extend beyond the continuous ejecta and other types of distal deposits such as run-out lobes or ramparts. Using Earth-based radar images, we find that lunar fine-grained ejecta haloes represent meters-thick deposits with abrupt margins, and are depleted in rocks ?1 cm in diameter. Martian haloes show low nighttime thermal IR temperatures and thermal inertia, indicating the presence of fine particles estimated to range from ∼10 μm to 10 mm. Using the large sample sizes afforded by global datasets for Venus and Mars, and a complete nearside radar map for the Moon, we establish statistically robust scaling relationships between crater radius R and fine-grained ejecta run-out r^* for all three planets. On the Moon, r^* ∼ R^−0.18 for craters 5-640 km in diameter. For Venus, radar-dark haloes are larger than those on the Moon, but scale as r^* ∼ R^−0.49, consistent with ejecta entrainment in Venus’ dense atmosphere. On Mars, fine-ejecta haloes are larger than lunar haloes for a given crater size, indicating entrainment of ejecta by the atmosphere or vaporized subsurface volatiles, but scale as R^−0.13, similar to the ballistic lunar scaling. Ejecta suspension in vortices generated by passage of the ejecta curtain is predicted to result in ejecta run-out that scales with crater size as R^1/2, and the wind speeds so generated may be insufficient to transport particles at the larger end of the calculated range. The observed scaling and morphology of the low-temperature haloes leads us rather to favor winds generated by early-stage vapor plume expansion as the emplacement mechanism for low-temperature halo materials.     

Spatially-resolved high-resolution spectroscopy of Venus 2. Variations of HDO, OCS, and SO2 at the cloud tops

While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO₂ are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H₂O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H₂O with D/H ≈ 200, our observations at 2722 cm⁻¹ in the Venus afternoon show a H₂O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H₂O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H₂O mixing ratios refer to 74 km. The observed increase in H₂O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm⁻¹ confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO₂ at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO₂ lines look irregular in the observed spectra at 2476 cm⁻¹. The SO₂ abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO₂ mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO₂ above the clouds since the period of 1980-1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO₂ may be similar, and the SO₂/OCS ratio is ∼500 and may be rather stable at 65-70 km under varying conditions on Venus.     

Measurements of the helium 584 Å airglow during the Cassini flyby of Venus

The helium resonance line at 584 Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320 R, reaching ∼700 R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584 Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584 Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×10⁶ cm⁻³ at the level of where the CO₂ is 2×10¹⁰ cm⁻³.