Solar Longitude Research Articles

Ertel potential vorticity versus Bernoulli streamfunction on Mars

Scatter plots of Ertel potential vorticity, Q, versus Bernoulli streamfunction, B, on potential temperature surfaces, θ, are investigated for Mars using the global Mars Analysis Correction Data Assimilation (MACDA) reanalysis, which spans Mars Year (MY) 24.39 to 27.24. In midlatitudes, Mars exhibits monotonic, function‐like Q(B) correlations on θ surfaces similar to those observed for Earth. We quantify this with linear regressions of Q versus B over the vertical range θ=400 to 900 K (∼30 to 60 km). In autumn, winter and spring, in both hemispheres, the non‐dimensionalized correlation generally lies between zero and unity and gradually decreases with height, whereas in northern summer, it swings negative. These characteristics match Earth's lower mesosphere (θ= 2000 to 3000 K; z≈ 48 to 62 km) during the same seasons. The exception is southern summer, when the correlation on Mars nearly vanishes. In time series, the transition into and out of northern summer is sinuous and centred just after solar longitude Ls = 90°, whereas in southern summer it is abrupt and spans ΔLs≈120°, which is one third of a Mars year. A striking feature seen on Mars but not on Earth is a large range of Q over the narrow domain of B poleward of each winter polar jet, particularly in the north, which is consistent with the known annular structure of the Martian polar vortex. Froude number calculations suggest the existence of a planetary‐scale hydraulic jump associated with the winter polar jet.

Orionids and Eta Aquariids in the IAU MDC database

The structure of the Orionids and the Eta Aquariids meteor streams was studied using the IAU MDC Photographic Orbits Database. Both the streams are associated with one of the most active comet known in the cometary population – 1P/Halley. The stream is therefore constantly supplemented by new particles, what creates conditions for a formation of a complex internal structure. We found that two frequency maxima of the photographic Orionids identified at the solar longitudes λ⊙=208.5° and λ⊙=210.5° coincide with the positions of the maximum activity of the visual meteors and bolides. One of five identified associations (O1) probably does not belong to the Orionids, i.e. meteor stream 008 ORI, but regards the September omicron Orionids - 479 SOO. The orbital parameters of the filament O4 correspond to the mean orbit of the Orionids resulting from the meteor radar observations in the period 2002–2012. There is no spatial relation between meteors observed in May (031 ETA) and October (008 ORI). While Eta Aquariids are in fact the core of the stream, the orbits of Orionids are much further from the cometary orbit.

Long‐term variability of dust optical depths on Mars during MY24–MY32 and their impact on subtropical lower ionosphere: Climatology, modeling, and observations

AbstractDust optical depths (τ) for nine Martian years (MY24–MY32) in the subtropical region (25–35°S) have been used to classify distinct dust scenarios. These data are based on observations at 9.3 µm from the Mars Global Surveyor and Mars Odyssey missions and encompass the regional dust storms which occur every year around solar longitude (Ls) ~ 220° and the two major dust storms of MY25 and MY28. Constrained by these observations and the Mars Climate Sounder observations of detached dust layers, we estimate altitude profiles of dust concentrations. We discuss the characteristics of dust aerosol particles of different size between 0.2 and 3.0 µm by assuming a modified gamma distribution. We then use a comprehensive ion‐dust model to calculate ion densities and conductivities in the lower ionosphere of Mars in the absence of dust storm at τ = 0.1 and Ls = 150° and for three dust storm periods viz., (1) major dust storm at τ = 1.7 and Ls = 210°, (2) major dust storm at τ = 1.2 and Ls = 280°, and (3) regional dust storm at τ = 0.5 and Ls = 220°. The model with 12 neutral species considers galactic cosmic rays as a source of ionization. Results show that the density of the dominant hydrated cluster ions and the electrical conductivity are reduced by an order of magnitude near the surface for a few months until the dust storm settles down to its normal condition.

Thermal properties of Rhea's poles: Evidence for a meter-deep unconsolidated subsurface layer

Cassini's Composite Infrared Spectrometer (CIRS) observed both of Rhea's polar regions during a close (2000km) flyby on 9th March 2013 during orbit 183. Rhea's southern pole was again observed during a more distant (51,000km) flyby on 10th February 2015 during orbit 212. The results show Rhea's southern winter pole is one of the coldest places directly observed in our Solar System: surface temperatures of 25.4±7.4K and 24.7±6.8K are inferred from orbit 183 and 212 data, respectively. The surface temperature of the northern summer pole inferred from orbit 183 data is warmer: 66.6±0.6K. Assuming the surface thermophysical properties of the two polar regions are comparable then these temperatures can be considered a summer and winter seasonal temperature constraint for the polar region. Orbit 183 will provide solar longitude (LS) coverage at 133° and 313° for the summer and winter poles respectively, while orbit 212 provides an additional winter temperature constraint at LS 337°. Seasonal models with bolometric albedo values between 0.70 and 0.74 and thermal inertia values between 1 and 46Jm−2K−1s−1/2 (otherwise known as MKS units) can provide adequate fits to these temperature constraints (assuming the winter temperature is an upper limit). Both these albedo and thermal inertia values agree within the uncertainties with those previously observed on both Rhea's leading and trailing hemispheres. Investigating the seasonal temperature change of Rhea's surface is particularly important, as the seasonal wave is sensitive to deeper surface temperatures (∼tens of centimeters to meter depths) than the more commonly reported diurnal wave (typically less than a centimeter), the exact depth difference dependent upon the assumed surface properties. For example, if a surface porosity of 0.5 and thermal inertia of 25MKS is assumed then the depth of the seasonal thermal wave is 76cm, which is much deeper than the ∼0.5cm probed by diurnal studies of Rhea (Howett et al., 2010). The low thermal inertia derived here implies that Rhea's polar surfaces are highly porous even at great depths. Analysis of a CIRS focal plane 1 (10–600cm−1) stare observation, taken during the orbit 183 encounter between 16:22:33 and 16:23:26 UT centered on 71.7°W, 58.7°S provides the first analysis of a thermal emissivity spectrum on Rhea. The results show a flat emissivity spectrum with negligible emissivity features. A few possible explanations exist for this flat emissivity spectrum, but the most likely for Rhea is that the surface is both highly porous and composed of small particles (<∼50µm).

A strong seasonal dependence in the Martian hydrogen exosphere

Hubble Space Telescope (HST) and Mars Express (MEX) observed unexpected rapid changes in the martian hydrogen exosphere involving a decrease in scattered Lyman α intensity in Fall 2007(solar longitude, Ls = 331°-345°). These changes detected were speculated to be a combination of seasonal variation and/or dust storms and lower atmospheric dynamics. Here we present HST observations of Mars in 2014 over a broad range of heliocentric distances and seasons (Ls = 138°-232°) which indicate a factor of ~3.5 change in martian Lyman α brightness associated with a factor ~5.4 variation of hydrogen escape flux in the absence of global dust storms and significant solar variability. We thus conclude that seasonal effects have a strong influence on the hydrogen exosphere, which in turn has major implications for the processes that control water supply to the martian upper atmosphere and the history of water escape from Mars.

Dust storm and electron density in the equatorial D region ionosphere of Mars: Comparison with Earth's ionosphere from rocket measurements in Brazil

AbstractWe report the first model result for the dust densities and electron densities in the D region ionosphere of Mars for aerosol particles of different sizes during a major dust storm that occurred in Martian Year (MY) 25 at low latitude. These calculations are made at latitude 10°S and solar longitudes (Ls) = 200°, 220°, 250°, and 280° for high, medium, low, and absence of dust storms, respectively. Four corresponding dust layers were found at 50 km, 50 km, 38 km, and 25 km during these events. During high dust storm period, the optical depth and dust density increased by a factor of ~20 from its normal condition. The electron densities estimated for the D region ionosphere of Mars for submicron sized dust particles are largest as compared to that estimated for larger particles. The electron density reduced by ~2 orders of magnitude during high dust storm. The estimated electron density in the clear atmosphere of Mars is compared with measurements of Earth's ionosphere at nearly the same geophysical condition.

THE 2014 KCG METEOR OUTBURST: CLUES TO A PARENT BODY

The Kappa Cygnid (KCG) meteor shower exhibited unusually high activity in 2014, producing ten times the typical number of meteors. The shower was detected in both radar and optical systems and meteoroids associated with the outburst spanned at least five decades in mass. In total, the Canadian Meteor Orbit Radar, European Network, and NASA All Sky and Southern Ontario Meteor Network produced thousands of KCG meteor trajectories. Using these data, we have undertaken a new and improved characterization of the dynamics of this little-studied, variable meteor shower. The Cygnids have a di use radiant and a significant spread in orbital characteristics, with multiple resonances appearing to play a role in the shower dynamics. We conducted a new search for parent bodies and found that several known asteroids are orbitally similar to the KCGs. N-body simulations show that the two best parent body candidates readily transfer meteoroids to the Earth in recent centuries, but neither produces an exact match to the KCG radiant, velocity, and solar longitude. We nevertheless identify asteroid 2001 MG1 as a promising parent body candidate.

HOW COMMON ARE HOT MAGNETIC FLUX ROPES IN THE LOW SOLAR CORONA? A STATISTICAL STUDY OF EUV OBSERVATIONS

We use data at 131, 171, and 304 Å from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to search for hot flux ropes in 141 M-class and X-class solar flares that occurred at solar longitudes equal to or larger than 50°. Half of the flares were associated with coronal mass ejections. The goal of our survey is to assess the frequency of hot flux ropes in large flares irrespective of their formation time relative to the onset of eruptions. The flux ropes were identified in 131 Å images using morphological criteria and their high temperatures were confirmed by their absence in the cooler 171 and 304 Å passbands. We found hot flux ropes in 45 of our events (32% of the flares); 11 of them were associated with confined flares while the remaining 34 were associated with eruptive flares. Therefore almost half (49%) of the eruptive events involved a hot flux rope configuration. The use of supplementary Hinode X-Ray Telescope data indicates that these percentages should be considered as lower limits of the actual rates of occurrence of hot flux ropes in large flares.

Mars Orbiter Camera climatology of textured dust storms

We report the climatology of “textured dust storms”, those dust storms that have visible structure on their cloud tops that are indicative of active dust lifting, as observed in Mars Daily Global Maps produced from Mars Orbiter Camera wide-angle images. Textured dust storms predominantly occur in the equinox seasons while both solstice periods experience a planet-wide “pause” in textured dust storm activity. These pauses correspond to concurrent decreases in global atmospheric dust opacity. Textured dust storms most frequently occur in Acidalia Planitia, Chryse Planitia, Arcadia Planitia, and Hellas basin. To examine the nature of the link between textured dust storms and atmospheric dust opacity, we compare the textured dust storm climatology with a record of atmospheric dust opacity and find a peak global correlation coefficient of approximately 0.5 with a lag of 20–40° in solar longitude in the opacity compared to the solar climatology. This implies that textured dust storms observed at 1400 local time by MOC are responsible for a large fraction of atmospheric dust opacity and that other mechanisms (e.g., dust devil lifting or storm-scale lifting not observed in this study) may supply a comparable amount of dust.

Reevaluated martian atmospheric mixing ratios from the mass spectrometer on the Curiosity rover

The Sample Analysis at Mars (SAM) instrument suite of the Mars Science Laboratory (MSL) Curiosity rover is a miniature geochemical laboratory designed to analyze martian atmospheric gases as well as volatiles released by pyrolysis of solid surface materials (Mahaffy et al., 2012). SAM began sampling the martian atmosphere to measure its chemical and isotopic composition shortly after Curiosity landed in Mars׳ Gale Crater in August 2012 (Mahaffy et al., 2013). Analytical methods and constants required for atmospheric measurements with SAM׳s quadrupole mass spectrometer (QMS) were provided in a previous contribution (Franz et al., 2014). Review of results obtained through application of these constants to repeated analyses over a full martian year and supporting studies with laboratory instruments offer new insights into QMS performance that allow refinement of the calibration constants and critical reassessment of their estimated uncertainties. This report describes the findings of these studies, provides updated calibration constants for atmospheric analyses with the SAM QMS, and compares volume mixing ratios for the martian atmosphere retrieved with the revised constants to those initially reported (Mahaffy et al., 2013). Sufficient confidence is enabled by the extended data set to support calculation of precise abundances for CO rather than an upper limit. Reanalysis of data acquired on mission sols 45 and 77 (at solar longitudes of 175° and 193°, respectively) with the revised constants leads to the following average volume mixing ratios: CO2 0.957(±0.016), N2 0.0203(±0.0003), Ar 0.0207(±0.0002), O2 1.73(±0.06)×10−3, CO 7.49(±0.026)×10−4.

Ten years of Martian nitric oxide nightglow observations

AbstractWe present 10 years of Martian NO nightglow Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars observations in limb and stellar occultation modes. The NO nightglow is used as a tracer of the summer‐to‐winter hemispherical circulation in the upper atmosphere of Mars. Its distribution roughly follows the curve latitude = −80 sin(solar longitude), with deviations. We find that the peak brightness is 5 ± 4.5 kR, situated at 72 ± 10.4 km. It ranges from 0.23 to 18.51 kR and from 42 to 97 km. These values are consistent with previous studies. We also present maps of the brightness of the NO emission peak and its variability, an important factor that can reach up to 50% of the emission and is not reproduced by average brightness model maps. The characteristics and factors that may control the emission are investigated. In particular, we show that the solar activity exerts a positive influence on the number of detections. It does not influence, on the contrary, the brightness or altitude of the peak of the NO nightglow emission. Results presented in this study lead to future comparisons with global Martian atmospheric models and observational targets for the Imaging Ultraviolet Sprectrograph Mars Atmosphere and Volatile Evolution.

Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars

The large amount of multi-temporal high-resolution images acquired in the last few years offers the opportunity to identify morphological changes associated with recent geologic activity on the surface of Mars. In this study we focus on a single gully in Sisyphi Cavi, located in the south polar region at 1.44°E and 68.54°S. The gully incises the gullied equator-facing slope of an isolated polar pit within an infilled impact crater. It is important to notice that the following investigations describe the activity and modifications of an existing gully and not the formation of the gully itself. High-resolution image data analyses show new deposits at the terminus of the gully channel and on the gully apron within spring (after solar longitudes of 236°) of martian years (MY) 29 and 31. Our morphological investigations show that the identified new deposits were formed by dark flows through the entire gully deposited on top of the apron between solar longitudes (LS)∼218° and ∼226°. Thermal data show a temperature increase between LS ∼218° and ∼226°. Near-infrared spectral data show relatively constant band strengths of CO2 ice and H2O ice in this time range. After the formation of the dark flows (after LS∼226°), temperatures increase rapidly from ∼180K to >∼270K at LS∼250°. At this time, spectral data indicate that all volatiles on the surface sublimated. However, an earlier beginning of sublimation when the dark flows were observed (between LS∼218° and ∼226°) is likely, due to the fact that the instruments can only show the last phase of sublimation (decrease of volatile band strengths). Spectral modeling shows that from winter to mid-spring, the surface of the studied area is covered by translucent CO2 slab-ice contaminated by minor amounts of H2O ice and dust. Furthermore, our spectral modeling indicates that the dark material most likely flows on top of the CO2 slab-ice cover. Three different scenarios were proposed to explain the identified dark flows, including (1) flows supported by liquid H2O, (2) flows supported by CO2 gas, and (3) dry flows. On the basis of our study we find that scenario (1) is unlikely because of the very low temperatures. While scenario (2) is consistent with the observed beginning of CO2 ice sublimation in the study area, it is unlikely because of the limitation of the activity to only one gully compared to surrounding gullies that share the same morphologies, slope angles, and volatile contents. Also with scenario (3), dry flows, the activity of only one gully is difficult to explain. Thus, we propose a mixture of scenario (2 and 3), dry flows supported by the ongoing sublimation of CO2 ice within the gully, to be the most plausible scenario, when the observed active gully comprises different source material than the surrounding gullies, i.e., a higher content of probably sand-sized material from outcrops located in the alcove.

Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission

Dust aerosol plays a fundamental role in the behavior and evolution of the martian atmosphere. The first five Mars years of Mars Exploration Rover data provide an unprecedented record of the dust load at two sites. This record is useful for characterization of the atmosphere at the sites and as ground truth for orbital observations. Atmospheric extinction optical depths have been derived from solar images after calibration and correction for time-varying dust that has accumulated on the camera windows. The record includes local, regional, and globally extensive dust storms. Comparison with contemporaneous thermal infrared data suggests significant variation in the size of the dust aerosols, with a 1μm effective radius during northern summer and a 2μm effective radius at the onset of a dust lifting event. The solar longitude (LS) 20–136° period is also characterized by the presence of cirriform clouds at the Opportunity site, especially near LS=50° and 115°. In addition to water ice clouds, a water ice haze may also be present, and carbon dioxide clouds may be present early in the season. Variations in dust opacity are important to the energy balance of each site, and work with seasonal variations in insolation to control dust devil frequency at the Spirit site.

New observations of martian southern mid-latitude recurring slope lineae (RSL) imply formation by freshwater subsurface flows

Southern mid-latitude (SML) recurring slope lineae (RSL) are narrow (0.5–5m) dark albedo features that emanate from bedrock and incrementally lengthen down steep slopes that preferentially face the equator. We observe that SML RSL begin lengthening prior to southern summer at a solar longitude (Ls) of 245°±11° when Mars Global Surveyor Thermal Emission Spectrometer (TES)-derived near-maximum surface temperatures are 296±5K and Mars Reconnaissance Orbiter Mars Climate Sounder (MCS) – and Mars Odyssey Thermal Emission Imaging System (THEMIS)-derived mid-afternoon surface temperatures are >273K. SML RSL continue to lengthen for 104±38 sols with an average near-maximum surface temperature of 298±5K. The SML RSL then stop lengthening at Ls=314°±12° when mid-afternoon surface temperatures drop below 273K. They remain dark for another 116±41 sols (until Ls=16°±14°) as surface temperatures continue to fall. Although the RSL recharge mechanism remains unknown, our observation that the vast majority of RSL lengthen only when mid-afternoon surface temperatures are >273K supports the hypothesis that they are formed by shallow subsurface liquid water flows without significant freezing-point depression. The number and length of RSL at multiple sites increased dramatically following the Mars Year 28 globe-encircling dust storm. We interpret this increase to be due to warmer subsurface temperatures created by a dust-laden greenhouse effect that may be unique to the southern mid-latitudes near Ls=270°. Therefore SML RSL flow is quite sensitive to ground temperature and may only occur under favorable orbital parameters when mean insolation during the RSL lengthening season is above that of the current southern mid-latitude mean insolation value. This value is currently at a peak that has not been attained for the last ∼100ka. Meanwhile, the RSL-poor northern mid-latitude mean insolation is near a minimum and has a value 27% lower than the southern value. If SML RSL are indeed formed by shallow subsurface freshwater flows, then they may be some of the best locations on Mars to explore for extant life.

Atmospheric thermal structure and cloud features in the southern hemisphere of Venus as retrieved from VIRTIS/VEX radiation measurements

Thermal structure and cloud features in the atmosphere of Venus are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) aboard ESA’s Venus Express mission in the moderate resolution infrared mapping channel (M-IR, 1–5μm). New methodical approaches and retrieval results for the northern hemisphere have been recently described by Haus et al. (Haus, R., Kappel, D., Arnold, G. [2013]. Planet. Space Sci. 89, 77–101. http://dx.doi.org/10.1016/j.pss.2013.09.020). Now, southern hemisphere maps of mesospheric temperature and cloud parameter fields are presented that cover variations with altitude, latitude, local time, and mission time. Measurements from the entire usable data archive are utilized comprising radiation spectra recorded during eight Venus solar days between April 2006 and October 2008.Zonal averages of retrieved temperature altitude profiles in both hemispheres are very similar and give evidence of global N–S axial symmetry of atmospheric temperature structure. Cold collar and warmer polar vortex regions exhibit the strongest temperature variability with standard deviations up to 8.5K at 75°S and 63km altitude compared with about 1.0K at low and mid latitudes above 75km. The mesospheric temperature field strongly depends on local time. At altitudes above about 75km, the atmosphere is warmer in the second half of night, while the dawn side at lower altitudes is usually colder than the dusk side by about 8K. Local minimum temperature of 220K occurs at 03:00h local time at 65km and 60°S. Temperature standard deviation at polar latitudes is particularly large near midnight. Temperature variability with solar longitude is forced by solar thermal tides with a dominating diurnal component. The influence of observed cloud parameter changes on retrieved mesospheric zonal average temperature structure is moderate and does not exceed 2–3K at altitudes between 60 and 75km. The mesospheric thermal structure was essentially stable with Julian date between 2006 and 2008.Global N–S axial symmetry is also observed in cloud structures. Cloud top altitude at 1μm slowly decreases from 71km at the equator to 70km at 45–50° and rapidly drops poleward of 50°. It reaches 61km over both poles. Average particle size in the vertical cloud column increases from mid latitudes toward the poles and also toward the equator resulting in minimum and maximum zonal average cloud opacities of about 32 and 42 and a planetary average of 36.5 at 1μm. Zonal averages of cloud features are similar at different solar days, but variations with local time are very complex and inseparably associated with the superrotation of the clouds.

Real-Time Flux Density Measurements of the 2011 Draconid Meteor Outburst

During the 2011 outburst of the Draconid meteor shower, members of the Video Meteor Network of the International Meteor Organization (IMO) provided, for the first time, fully automated flux density measurements in the optical domain. The data set revealed a primary maximum at 20:09 UT $\pm$5 min on 8 October 2011 (195.036 deg solar longitude) with an equivalent meteoroid flux density of (118 $\pm$10) x 10^-3 km^-2 h^-1 at a meteor limiting magnitude of +6.5, which is thought to be caused by the 1900 dust trail of comet 21P/Giacobini-Zinner. We also find that the outburst had a full width at half maximum (FWHM) of 80 minutes, a mean radiant position of alpha=262.2 deg, delta=+56.2 deg ($\pm$1.3 deg) and geocentric velocity of v_geo=17.4 km/s ($\pm$0.5 km/s). Finally, our data set appears to be consistent with a small sub-maximum at 19:34 UT $\pm$7 min (195.036 deg solar longitude) which has earlier been reported by radio observations and may be attributed to the 1907 dust trail.

Material ejection by the cold jets and temperature evolution of the south seasonal polar cap of Mars from THEMIS/CRISM observations and implications for surface properties

As the seasonal CO2 ice polar caps of Mars retreat during spring, dark spots appear on the ice in some specific regions. These features are thought to result from basal sublimation of the transparent CO2 ice followed by ejection of regolith‐type material, which then covers the ice. We have used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reflectance data, Thermal Emission Imaging System (THEMIS) visible images, and THEMIS‐derived temperature retrievals along with a thermal numerical model to constrain the physical and compositional characteristics of the seasonal cap for several areas exhibiting dark spots at both high spatial and temporal resolutions. Data analysis suggests an active period of material ejection (before solar longitude (Ls) 200), accumulation around the ejection points, and spreading of part of the ejected material over the whole area, followed by a period where no significant amount of material is ejected, followed by complete defrosting (≈ Ls 245). Dark material thickness on top of the CO2 ice is estimated to range from a few hundreds of microns to a few millimeters in the warmest spots, based on numerical modeling combined with the observed temperature evolution. The nature of the venting process and the amount of material that is moved lead to the conclusion that it could have an important impact on the surface physical properties.

NeMars: An empirical model of the martian dayside ionosphere based on Mars Express MARSIS data

Several models of the martian ionosphere have been developed in the last years. In this paper, a new empirical model for the dayside electron density of the martian ionosphere (primary and secondary layer), called NeMars, is described. The model is mainly based on MARSIS AIS data (Active Ionospheric Sounding from the Mars Advanced Radar and Ionospheric Sounding experiment aboard Mars Express mission) and to a lesser extent on radio occultation data from the Mars Global Surveyor mission. The model is able to reproduce to a reasonable degree the main characteristics of the electron density profiles obtained with the two techniques by considering solar zenith angle, solar flux F10.7 as a proxy of the solar activity, and heliocentric distance. The model partially assumes the Chapman theory, the scale height for the main layer of the model varies with the solar zenith angle and the altitude, and the Chapman-like photochemical processes dominate diffusion up to about 200km altitude. In addition, we assess the level of importance that parameters like heliocentric distance, solar longitude, solar activity or solar zenith angle have on the formation of the ionosphere. Eventually, we compare the Total Electron Content (TEC) derived from NeMars model with the values obtained from the MARSIS radar operating in subsurface mode.

The evolution of co‐orbiting material in the orbit of 2201 Oljato from 1980 to 2012 as deduced from Pioneer Venus Orbiter and Venus Express magnetic records

AbstractAsteroid 2201 Oljato passed through perihelion inside the orbit of Venus near the time of its conjunction with Venus in 1980, 1983, and 1986. During those three years, many interplanetary field enhancements (IFEs) were observed by the Pioneer Venus Orbiter (PVO) in the longitude sector where the orbit of Oljato lies inside Venus' orbit. We attribute IFEs to clouds of fine‐scale, possibly highly charged dust picked up by the solar wind after an interplanetary collision between objects in the diameter range of 10–1000 m. We interpret the increase rate in IFEs at PVO in these years as due to material in Oljato's orbit colliding with material in, or near to, Venus' orbital plane and producing a dust‐anchored structure in the interplanetary magnetic field. In March 2012, almost 30 yr later, with Venus Express (VEX) now in orbit, the Oljato‐Venus geometry is similar to the one in 1980. Here, we compare IFEs detected by VEX and PVO using the same IFE identification criteria. We find an evolution with time of the IFE rate. In contrast to the results in the 1980s, the recent VEX observations reveal that at solar longitudes in which the Oljato orbit is inside that of Venus, the IFE rate is reduced to the level even below the rate seen at solar longitudes where Oljato's orbit is outside that of Venus. This observation implies that Oljato not only lost its co‐orbiting material but also disrupted the “target material,” with which the co‐orbiting material was colliding, near Venus.

Seasonal variations of temperature, acetylene and ethane in Saturn’s atmosphere from 2005 to 2010, as observed by Cassini-CIRS

Acetylene (C2H2) and ethane (C2H6) are by-products of complex photochemistry in the stratosphere of Saturn. Both hydrocarbons are important to the thermal balance of Saturn’s stratosphere and serve as tracers of vertical motion in the lower stratosphere. Earlier studies of Saturn’s hydrocarbons using Cassini-CIRS observations have provided only a snapshot of their behaviour. Following the vernal equinox in August 2009, Saturn’s northern and southern hemispheres have entered spring and autumn, respectively, however the response of Saturn’s hydrocarbons to this seasonal shift remains to be determined. In this paper, we investigate how the thermal structure and concentrations of acetylene and ethane have evolved with the changing season on Saturn. We retrieve the vertical temperature profiles and acetylene and ethane volume mixing ratios from Δν̃=15.5cm-1 Cassini-CIRS observations. In comparing 2005 (solar longitude, Ls∼308°), 2009 (Ls∼3°) and 2010 (Ls∼15°) results, we observe the disappearance of Saturn’s warm southern polar hood with cooling of up to 17.1K±0.8K at 1.1mbar at high-southern latitudes. Comparison of the derived temperature trend in this region with a radiative climate model (Section 4 of Fletcher et al., 2010 and Greathouse et al. (2013, in preparation)) indicates that this cooling is radiative although dynamical changes in this region cannot be ruled out. We observe a21±12% enrichment of acetylene and a 29±11% enrichment of ethane at 25°N from 2005 to 2009, suggesting downwelling at this latitude. At 15°S, both acetylene and ethane exhibit a decrease in concentration of 6±11% and 17±9% from 2005 to 2010, respectively, which suggests upwelling at this latitude (though a statistically significant change is only exhibited by ethane). These implied vertical motions at 15°S and 25°N are consistent with a recently-developed global circulation model of Saturn’s tropopause and stratosphere(Friedson and Moses, 2012), which predicts this pattern of upwelling and downwelling as a result of a seasonally-reversing Hadley circulation. Ethane exhibits a general enrichment at mid-northern latitudes from 2005 to 2009. As the northern hemisphere approaches summer solstice in 2017, this feature might indicate an onset of a meridional enrichment of ethane, as has been observed in the southern hemisphere during/after southern summer solstice.