Eastern Elongation Research Articles

Amateur Observers Witness the Return of Venus’ Cloud Discontinuity

Firstly identified in images from JAXA’s orbiter Akatsuki, the cloud discontinuity of Venus is a planetary-scale phenomenon known to be recurrent since, at least, the 1980s. Interpreted as a new type of Kelvin wave, this disruption is associated to dramatic changes in the clouds’ opacity and distribution of aerosols, and it may constitute a critical piece for our understanding of the thermal balance and atmospheric circulation of Venus. Here, we report its reappearance on the dayside middle clouds four years after its last detection with Akatsuki/IR1, and for the first time, we characterize its main properties using exclusively near-infrared images from amateur observations. In agreement with previous reports, the discontinuity exhibited temporal variations in its zonal speed, orientation, length, and its effect over the clouds’ albedo during the 2019/2020 eastern elongation. Finally, a comparison with simultaneous observations by Akatsuki UVI and LIR confirmed that the discontinuity is not visible on the upper clouds’ albedo or thermal emission, while zonal speeds are slower than winds at the clouds’ top and faster than at the middle clouds, evidencing that this Kelvin wave might be transporting momentum up to upper clouds.

Detection of a hydrogen corona at Callisto

AbstractIn December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far‐ultraviolet spectral images of Jupiter's moon Callisto. The leading and trailing hemispheres were observed in the spectral range 1190 Å to 1720 Å when the moon was at eastern and western elongations, respectively. We analyzed the hydrogen Lyman α (1216 Å) signal in the two observations and found that faint atmospheric emissions extending up to several moon radii away are present in addition to the solar flux reflected off the surface. We show that the detected atmospheric Lyman α emissions are consistent with an escaping hydrogen corona with a vertical column density in the range of (6–12) ×1011 cm−2. The derived hydrogen abundance is about 2 times higher when the moon is at eastern elongation, possibly related to increased water sublimation when the visibly darker leading hemisphere is illuminated by the Sun. The detected hemispheric difference is larger than the measurement uncertainties providing first evidence for variations in Callisto's neutral atmosphere.

HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF EUROPA'S ATMOSPHERIC ULTRAVIOLET EMISSION AT EASTERN ELONGATION

We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e., Europa’s leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa’s atmospheric Oi 1304 A and Oi 1356 A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 ± 14 and 226 ± 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modeling the reflected light from Europa’s surface. The total emission also shows a clear dependence of Europa’s position with respect to Jupiter’s magnetospheric plasma sheet. We derive a lower limit for the O2 column density of 6 × 10 18 m −2 . Previous observations of Europa’s atmosphere with the Space Telescope Imaging Spectrograph in 1999 of Europa’s trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian versus sub-Jovian asymmetry with respect to the central meridian on the leading side and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 ◦ west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa’s UV emission can also be caused by inhomogeneous surface properties, an optically thick atmospheric contribution of atomic oxygen, and/or by Europa’s complex plasma interaction with Jupiter’s magnetosphere.

Seasonal photometric variability of Titan, 1972–2006

Measurements at Lowell Observatory of Titan in the b (472 nm) and y (551 nm) filters of the Strömgren photometric system at thirty four consecutive apparitions (282 nights) from 1971/72 to 2006 show a 10% sinusoidal variation that lags seasonal extremes by about 1/8 of a Titan year. The seasonal variations are asymmetric: the autumn lightcurve maxima of the northern and southern hemispheres differ significantly as do the spring lightcurve minima. Changes also occur from one Titan year to the next: Titan was ∼3% fainter in b and ∼1% fainter in y following the 2002 southern summer solstice than it was one Titan year earlier in 1973. These changes appear to be intrinsic to Titan's atmosphere and cannot be explained by instrumental effects and changing geometries. Orbital variations visible in recent Hubble Space Telescope images at 673 nm and Voyager orange images (590–640 nm) may have a small ( 0.002 ± 0.001 mag ) counterpart in the b, y photometric record (eastern elongation brighter, consistent with the Cassini near-infrared albedo map).

Near-infrared study of Titan's resolved disk in spectro-imaging with CFHT/OASIS

We present observations of Titan taken on November 17, 2000, with the near-infrared spectro-imaging system OASIS, mounted downstream of the CFHT/PUEO adaptive optics system. We have spatially resolved Titan's disk at Greatest Eastern Elongation. Our spectra cover the 0.86– 1.02 μ m range with a spectral resolution of 1800. By studying Titan at these wavelengths, we have recovered several pieces of information on the vertical and latitudinal structure of the atmosphere and surface of the satellite. The observing conditions were sufficiently good (AO-corrected seeing of 0.34”) so as to allow us to separate the disk into 7 independent elements. From the flux contained in the 0.890 μ m methane band, we find that at higher altitudes on Titan, the North–South asymmetry is undergoing changes with respect to previous years when the South was much brighter than the North. This asymmetry still prevails in the troposphere, but at higher levels the well-known “Titan smile”—previously reported—disappears. We believe that we even have evidence for a reversal. The year 2000 may then represent the beginning of a seasonal change in Titan's haze distribution in the near-infrared, something which has been confirmed since but was not visible in the previous years. By comparing regions on Titan's disk with similar surface and stratospheric characteristics, we find an differences in the latitudinal distribution of the aerosol content in the intermediate altitude levels. Reflectivity measurements derived in the 0.94 μ m window (and hence pertaining to the surface conditions) show that the equatorial regions of the leading side are brighter than the surrounding areas, due to the presence of the large bright zone observed since 1994. Given our spatial resolution, we find this region to be 6% brighter than northern latitudes, 7% brighter than the South pole and in total we have a contrast of 9% between the darker and the brighter areas distinguishable on our images. The 0.94 μ m methane window yields a geometric albedo of about 0.26 for the bright center of Titan's disk. This region is affected by a strong H 2 O telluric absorption and therefore we could not derive any precise information on the surface composition from the original spectrum. We have, however, been able to correct for the telluric lines by using a stellar spectrum taken just before our Titan observations. We were then able to apply our radiative transfer code and after modeling surface albedo values of about 0.37 and 0.29 for the brightest and darkest areas respectively were found. We investigate possible surface components, compatible with our data, such as water ice, hydrocarbon liquid, tholin deposits or silicates.

The sodium tail of Mercury

Abstract— Mercury is difficult to observe because it is so close to the Sun. However, when the angle of the ecliptic is near maximum in the northern hemisphere, and Mercury is near its greatest eastern elongation, it can be seen against the western sky for about a half hour after sunset. During these times, we were able to map sodium D2 emission streaming from the planet, forming a long comet‐like tail. On 2001 May 26 (U.T.) we mapped the tail downstream to a distance of ˜40 000 km. Sodium velocities in the tail increased to ˜11 km s−1 at 40 000 km as the result of radiation pressure acceleration. On 2000 June 5 (U.T.) we mapped the cross‐sectional extent of the tail at a distance of ˜17 500 km downstream. At this distance, the half‐power full‐width of the emission was ˜20 000 km. We estimated the transverse velocity of sodium in the tail to range from 2 to 4 km s−1. The velocities we observed imply source velocities from the planet surface of the order of 5 km s−1, or 4 eV. Particle sputtering is a likely candidate for production of sodium atoms at these velocities. The total flux of sodium in the tail was ˜1 times 1023 atoms s−1, which corresponds to 1 to 10% of the estimated total production rate of sodium on the planet.

Images of Titan at 1.3 and 1.6 μm with Adaptive Optics at the CFHT

Titan was observed with the Adaptive Optics Bonette at the Canada–France–Hawaii Telescope during October 27th 1998 (UTC), when the satellite was at greatest eastern elongation (GEE) with respect to Saturn and its leading hemisphere was seen from the Earth. The seeing was excellent during these observations (with peaks at 0.3″ in the visible), and this allowed us to successfully image Titan for the first time in the J band, where the adaptive optics correction is highly dependent on the atmospheric conditions. Images were obtained in the center of the so-called “methane windows,” where the absorption is weak (at 1.29 –J1– and 1.6 μm –H1–) and also in the wings of the CH 4 bands (at 1.18 μm –J2– and 1.64 μm –H2–) with narrowband filters. The latter wavelengths yield information on Titan's stratosphere and allow us, by subtracting its contribution from the J1 and H1 images, to infer furthermore direct information on the surface properties. The resolution on the J and H diffraction-limited images is about 0.08–0.1″, translating into about 10 independent resolution elements or 20 pixels on the Titan disk. We have obtained reconstructed PSFs of very high signal-to-noise ratio, with associated Strehl ratios of about 35% in J and 50% in H. Hence, new and more efficient deconvolution methods (such as MISTRAL) were applied to the images, reducing ringing defects and restoring the initial photometry while enhancing the contrast on the observed features. Thus, the main features of the Titan atmosphere (bright south pole) and surface (large bright equatorial region and darker areas)—as previously observed with ADONIS/ESO, as well as with the HST and the Keck telescopes—are apparent, but with a much higher level of detail and contrast (about 30%) at these wavelengths. In addition, on the J1 and J2 images, along with the north–south asymmetry (which is probably due to seasona l effects), another bright feature is reported for the first time seen on Titan's morning limb (anti-Saturn during GEE). This feature may be diagnostic of diurnal effects (such as morning fog) at altitudes of 70–90 km (due to the cycle of the condensable species), but requires further investigation before its origin can be firmly identified. On the surface images (corrected for limb effects and atmospheric contribution), the large equatorial feature is found to be bright also at 1.3 μm (thus further constraining models of the surface composition). The high quality of the data allows us to resolve this area into three or more individual peaks, possibly towering over a mountainous plateau covered with ice (a plausible candidate being ethane ice). In any event, our images show this part of Titan's leading hemisphere to be more complex than previously suggested by models of Titan's surface. From our albedo maps, it appears that the darker areas have reflectances that are about three times lower than the bright equatorial region.

Model Calculations for Io's Atmosphere at Eastern and Western Elongations

Numerical calculations are performed for Io's sublimation atmosphere at eastern and western elongations using an improved version of the multispecies hydrodynamic model first developed by Wong and Johnson (1996). Subsolar SO 2 frost temperatures of 113 and 120 K are adopted to cover a range of plausible atmospheric abundances that are consistent with recent observations. In the model, the incoming plasma ions are allowed to reaccelerate to the unperturbed corotational speed after colliding with a neutral in the atmosphere above the ion gyropause where the local ion gyration frequency is higher than the neutral collision frequency. This reenergization of ions gives an energy deposition rate per ion that can be up to 2.5 times its upstream corotational energy rate, depending on the local atmospheric column. It is found that, in general, SO 2 is the dominant species in the dayside atmosphere while the noncondensibles, O 2 and SO, are the dominant species in the nightside atmosphere. The gas-phase reactions among these noncondensibles can produce a substantial amount of SO 2 in the nightside atmosphere. In all four cases considered here (high and low density; eastern and western elongations) we find that there exists a global exobase above all of the surface and that the ratio of dayside to nightside average SO 2 abundance is surprisingly limited to the range 17–95, even though surface vapor pressure on the nightside is many orders of magnitude lower. The dayside [SO]/[SO 2] mixing ratio is ∼3.2–7.1%, consistent with recent observations by Lellouch (1996) if SO is colocated with SO 2. We have also included simple NaX chemistry in the model and found that atomic sodium is far more abundant than molecular sodium because of fast reactions of NaX with O. Depending on the solar zenith angle and Io's orbital location, the exobase altitude ranges from 30 to 465 km while its temperature ranges from 220 to 2800 K. It is also found that at western elongation the dayside atmospheric flow is enhanced by the impinging plasma ions, resulting in an overall hotter and bigger atmosphere which has an average SO 2 column that is about twice as high as what would result from a hydrostatic atmosphere in vapor pressure equilibrium with the surface. At eastern elongation, the plasma energy is added to the nightside atmosphere, increasing its pressure and causing the formation of a standing oblique shock that occurs farther upstream in the day-to-night gas flow than in the case for western elongation. Consequently, the dayside atmospheric temperature is lower, and the day-to-night transport rate is reduced, which results in lower relative abundance for all other species except SO. The results from the high density case at western elongation give a line-of-sight tangential column density at the terminator for S, O, and Na, respectively, of 1.7×10 15, 2.5×10 15, and 4.3×10 13 cm −2. These profiles above the surface are reasonably consistent with those in Io's corona inferred from recent HST measurements for S and O (F. Roesler 1999, private communication) and from eclipse measurements for Na (Schneider et al. 1991a).

Titan’s Atmosphere and Surface from Infrared Spectroscopy and Imagery

AbstractWe have performed observations of Titan in the near-infrared since 1991, using both spectroscopy and imagery. A higher geometric albedo is observed near the Greatest Eastern Elongation than near the Greatest Western Elongation or Conjunctions. The surface is within reach for telescopes at 1.075, 1.28, 1.6 and 2.0 micron, so we simulated Titan’s geometric albedo in order to derive the surface albedo at all longitudes. Titan’s leading hemisphere appears brighter than the trailing one by 20-25% at 1.08; 1.28 and 1.6 μm, and by 35% at 2.0 μm, with spectra compatible with H2O everywhere. We have then simulated the presence of a mountain on Titan covering 50% of the leading hemisphere disk, varying its height and introducing a higher surface albedo when required. We find that a relief can not reproduce the whole geometric albedo differences observed between the two hemispheres. Some other additional component bright at 1.0 and 2.0 μm must then be invoked. Candidates are tholins or CH4, which can exist on the top of a mountain if the temperature is lower than 90 K.

Titan's Rotational Light-Curve

Recent observations demonstrate that near-infrared spectroscopy can probe Titan's surface through its haze. In a 5-week period during September and October 1993 we observed Titan's methane windows at 1.1, 1.3, 1.6, and 2 μm. At 1.1 and 1.3 μm observations were consistent with observations in 1992 at the same phases reported by Lemmon et al. (1993, Icarus 103, 329-332). Our new observations indicate that Titan was brighter near eastern elongation than near western elongation by 23 ± 2% at 1.6 μn and 32 ± 3% at 2 μn. With almost daily observations at 2 μn during one orbit, we observed Titan to be dark near western elongation, to brighten as it approached eastern elongation, and to darken as it returned to western elongation. We determine that the observed light-curve is due to surface albedo variations and Titan's rotational period is 15.950 ± 0.025 days. By considering the work of other observers we constrain the rotational period to be 15.949 ± 0.006 days; this constraint is consistent with synchronous rotation. Models of Titan's surface reflectivity are inconsistent with the presence of a strong 2-μm water ice absorption feature and do not require the presence of any surface absorption features; however, we cannot rule out models of the surface as dirty water ice or silicates that have been suggested by others.

B, V photometry of Thebe (JXIV)

Thebe (JXIV), a small inner moon of Jupiter, was discovered by Synnott in 1980 from Voyager images. It orbits in a severe environment of bombardment by high-energy charged particles trapped in Jupiter's magnetosphere and other contaminants originating on Io. Since Thebe is too small for its surface to be affected by endogenic processes, Thebe's surface is useful as a probe for the remote sensing of this environment. B and V photometry of Thebe was obtained relative to Amalthea (JV). A TI 800 × 800 CCD chip was used with the USNO 61-in. astrometric reflector at Flagstaff. The results show Thebe to be 0.5 mag brighter in V on the leading side, near the eastern elongation, than on the trailing side, near the western elongation, suggesting synchronous rotation. A mean opposition magnitude, V = 15.7, is obtained when combining these results with Millis' photometry of JV. The average albedo in V, 0.04, is similar to that of JV and agrees with Synnott's discovery observations. A ( B - V) of 1.3 makes Thebe one of the reddest moons in the Solar System and suggests that it too has been resurfaced by sulfur from Io. Contrary to expectations, Thebe is found to be 0.2 mag bluer than Amalthea, suggesting other contaminants. Thebe also shares several photometric patterns with the Galileans and, thus, links the inner satellite system with the Galilean system. This suggests that the magnetospheric processes which have modified the surfaces of the Galileans have also modified the surfaces of the inner satellites. An implication of this conclusion is that the leading faces of the innermost satellites, Adrastea (JXV) and Metis (JXVI), should be darkened and reddened.

Infrared observations of the dark side of Iapetus

Broadband (J, H, K, L, and L′) and narrowband (2.8–3.8 μm) circularly variable filter photometric observations were made of the dark side of Iapetus (Eastern Elongation). The data show a 3- μm absorption feature which is too deep to be a residual water frost feature from the bright material around the poles. Laboratory simulations show that the dark material must have a strong 3- μm absorption and could be a hydrated silicate mixed with a small amount of water frost.

Observations of Saturn's inner satellites and the orbit of Janus in 1980

Systematic observations of faint satellites were conducted at Pic-du-Midi with a focal coronograph from 1980 March 20 to 24, during which 150 exposures covering 17 hr were obtained; in addition, the 1966 discovery plates of satellites S10 Janus were reexamined together with other 1966 observations. Janus had its greatest eastern elongation on 1966 December 15.720 (±0.003) + light time, at a distance of 2.53 R eq. It is recognized that some of the observations thought to be 1966 S2 were in fact reobservations of Janus a few days after its discovery. Among the 1980 observations, differences in magnitudes indicate that is satellite 1980 S1 which corresponds to Janus; its greatest eastern elongation was observed on March 23.876 (±0.002) + light time. Subjected to corrections for librations, the mean period over the past 14 years has most probably one of three values: P 1 = 0.69458 day, P 2 = 06.9448 day or P 3 = 0.69438 day. The fainter satellite, S11, which is also 1980 S3, gravitates in an orbit similar to that of Janus and was leading it by +190° in March 1980; this difference of longitude was +224° in December 1966. An object of magnitude 15–16 was seen not detached from the ring; it could be a condensation in the external part of the rings or an additional faint inner satellite.

Venus I. Carbon monoxide distribution and molecular-line searches

An observational program to study variations of the vertical distribution of CO in the Venus atmosphere is presented. Measurements of the J = 0 → 1 absorption line at 2.6 mm wavelength are reported for two phase angles in 1977, one near eastern elongation (Feb.) and the other near inferior conjunction (Apr.). The two spectra are significantly different, with the April absorption line being narrower and deeper. The results of numerical inversion calculations show that the CO mixing ratio increases a factor of ∼ 100 between 78 and 100 km and that the CO abundance above ∼ 100 km is greatest on the night-side hemisphere. These conclusions are in qualitative agreement with theoretical models. In addition to the CO observations, a search for other molecules was made to provide further information on the composition of the Venus middle atmosphere. The J = 0 → 1 transition of 13CO was detected and upper limits were derived for nine other molecules.

The stability of an SO2 atmosphere on Io

Active volcanism observed on Io could provide a continuous source of gases from Io's interior. Only a small fraction of what is coming out of Io's volcanoes is escaping; the bulk of it is being recycled. There must be an atmospheric reservoir which maintains the gases until they have had a chance to recycle. Such an atmosphere must be protected from Jupiter's corotating plasma in order to be stable. Only the dayside atmosphere is subject to escape, while the cold nightside and poles would serve as a sink for atmospheric gases. However, the scavenging of atmospheric gases is maximum at the eastern elongation and minimum at the western elongation. A stable SO2 atmosphere on Io shows promise if the volcanic activity is continuous. If the volcanoes stop erupting, the atmosphere will be depleted.

Photometry of Dione, Tethys, and Enceladus on the UBV system

UBV measurements of Dione, Tethys, and Enceladus were made with an area-scanning photometer on several nights during the 1972/1973 and 1973/1974 apparitions of Saturn. The observed brightness variations have been separated into two components—one a function of orbital position, the other a function of solar phase angle. Dione and Tethys are brightest near greatest eastern elongation and faintest near greatest western elongation. The reverse is true of Enceladus. Opposition surges are observed for Dione and Tethys.

SPECTROSCOPIC OBSERVATIONS OF VENUS FOR ROTATION MADE AT MOUNT WILSON IN 1956

Spectroscopic measures on the rotation of Venus were made by V. M. Slipher in 1903 at the Lowell Observatory,1 and by St. John and Nicholson at the Mount Wilson Observatory about 1923. 2 These early observations were confined to the blue and violet spectral regions, using an iron arc comparison spectrum to correct for the inclination of the spectrum lines. It seemed desirable to repeat the work with the fast emulsions now available in the red, using telluric lines in the A 6300 region for comparison, so that solar absorption lines might be measured against atmospheric absorption lines. The eastern elongation of Venus in the spring of 1956 offered a favorable opportunity to launch such a program. It was decided to take the spectrograms with the Snow telescope on Mount Wilson, which, although designed for solar work, had the advantage of being available on any evening. Additional spectra were photographed in October 1956, when Venus was west of the sun, at the coude focus of the 100-inch telescope.

ASPECT OF THE HEAVENS FOR JANUARY AND FEBRUARY, 1955

Mercury is at greatest eastern elongation, 18° from the sun, on January 28. During the last week of January it should be easily visible in the evening twilight just above the southwestern horizon. It reaches inferior conjunction on February 12. Venus, magnitude 4.0, reaches greatest western elongation, 47° from the sun, on January 25. At this time it is rising in the southeast about three hours before the sun.

ASPECT OF THE HEAVENS FOR JANUARY AND FEBRUARY, 1954

Mercury is in superior conjunction on January 14 and at greatest eastern elongation, 18° from the sun, on February 13. During the second week of February the planet should be visible just above the western horizon in the evening twilight. Venus appears too close to the sun to be easily observed now, being in superior conjunction on January 29. Mars is increasing in brightness as its distance from the earth decreases, its magnitude changing from +1-6 on January 1 to +0.8 on February 28. It is moving through the constellations of Libra and Scorpius, passing 1 ° south of Saturn on January 2 and 5° north of Antares on February 24. At that time it is rising about one hour after midnight. Jupiter, magnitude 2. 1, is in Taurus, ending its retrograde motion on February 10. At that time it is high in the southern sky during the early evening and sets about three hours after midnight. Table I lists the transits of Jupiter's satellites and their shadows, which can be observed at convenient times on the Pacific Coast.

The Night Sky in May

FULL moon occurs May 12d. 12h. 51m., U.T., and new moon or May 27d. 22h. 24m. The following conjunctions with the moon take place : May 6d. 20h., Samuni 3° S. ; May 17d. 14h., Jupiter 5° N. ; May 26d 16h., Mars 2° S. ; May 27d. 12h., Mercury 0 8° S. Mercury is an evening star, setting at 21h. 21h. 43m. and 20h. 16m., at the beginning, Middle and end of the month, respectively ; but in the latter case it is too close to the sun to be observed. The planet attains its greatest eastern elongation on May 10. Venus is an evening star and can be seen for a short time in the western sky after the middle of the month, setting 35 min. and nearly an hour after the sun on May 15 and 31, respectively. Mars is a morning star but rises too soon before the sun for favourable observation. Jupiter rises at Ih. 30m., Oh. 35m. and 23h. 33m., on May 1, 15 and 31, respectively, and is visible for a few hours before sunrise. The planet is stationary on May 20. Saturn is visible until after midnight, setting at 2h. 40m., Ih. 45m. and Oh. 45m., at the beginning, middle and end of the month, respectively, and is stationary on May 1. Only one occultation of stars brighter than magnitude 6 occurs, λ Cane, disappearing at 21h. 30·6m. on May 4.