Planetocentric Latitude Research Articles

Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004

Many observers in the past gave detailed descriptions of the telescopic aspect of Venus during its extremely rare transits across the Solar disk. In particular, at the ingress and egress, the portion of the planet’s disk outside the Solar photosphere has been repeatedly perceived as outlined by a thin, bright arc (“aureole”). Those historical visual observations allowed inferring the existence of Venus’ atmosphere, the bright arc being correctly ascribed to the refraction of light by the outer layers of a dense atmosphere. On June 8th, 2004, fast photometry based on electronic imaging devices allowed the first quantitative analysis of the phenomenon. Several observers used a variety of acquisition systems to image the event – ranging from amateur-sized to professional telescopes and cameras – thus collecting for the first time a large amount of quantitative information on this atmospheric phenomenon. In this paper, after reviewing some elements brought by the historical records, we give a detailed report of the ground based observations of the 2004 transit. Besides confirming the historical descriptions, we perform the first photometric analysis of the aureole using various acquisition systems. The spatially resolved data provide measurements of the aureole flux as a function of the planetocentric latitude along the limb. A new differential refraction model of solar disk through the upper atmosphere allows us to relate the variable photometry to the latitudinal dependency of scale-height with temperature in the South polar region, as well as the latitudinal variation of the cloud-top layer altitude. We compare our measurements to recent analysis of the Venus Express VIRTIS-M, VMC and SPICAV/SOIR thermal field and aerosol distribution. Our results can be used a starting point for new, more optimized experiments during the 2012 transit event.

A giant thunderstorm on Saturn

Lightning discharges in Saturn's atmosphere emit radio waves with intensities about 10,000 times stronger than those of their terrestrial counterparts. These radio waves are the characteristic features of lightning from thunderstorms on Saturn, which last for days to months. Convective storms about 2,000 kilometres in size have been observed in recent years at planetocentric latitude 35° south (corresponding to a planetographic latitude of 41° south). Here we report observations of a giant thunderstorm at planetocentric latitude 35° north that reached a latitudinal extension of 10,000 kilometres-comparable in size to a 'Great White Spot'-about three weeks after it started in early December 2010. The visible plume consists of high-altitude clouds that overshoot the outermost ammonia cloud layer owing to strong vertical convection, as is typical for thunderstorms. The flash rates of this storm are about an order of magnitude higher than previous ones, and peak rates larger than ten per second were recorded. This main storm developed an elongated eastward tail with additional but weaker storm cells that wrapped around the whole planet by February 2011. Unlike storms on Earth, the total power of this storm is comparable to Saturn's total emitted power. The appearance of such storms in the northern hemisphere could be related to the change of seasons, given that Saturn experienced vernal equinox in August 2009.

THE 1998 NOVEMBER 14 OCCULTATION OF GSC 0622-00345 BY SATURN. I. TECHNIQUES FOR GROUND-BASED STELLAR OCCULTATIONS

On 1998 November 14, Saturn and its rings occulted the star GSC 0622-00345. We observed atmospheric immersion with NSFCAM at the National Aeronautics and Space Administration's Infrared Telescope Facility on Mauna Kea, Hawaii. Immersion occurred at 55.5\circ S planetocentric latitude. A 2.3 {\mu}m, methane-band filter suppressed reflected sunlight. Atmospheric emersion and ring data were not successfully obtained. We describe our observation, light-curve production, and timing techniques, including improvements in aperture positioning, removal of telluric scintillation effects, and timing. Many of these techniques are known within the occultation community, but have not been described in the reviewed literature. We present a light curve whose signal-to-noise ratio per scale height is 267, among the best ground-based signals yet achieved, despite a disadvantage of up to 8 mag in the stellar flux compared to prior work.

THE IMPACT OF A LARGE OBJECT ON JUPITER IN 2009 JULY

On 2009 July 19, we observed a single, large impact on Jupiter at a planetocentric latitude of 55^{\circ}S. This and the Shoemaker-Levy 9 (SL9) impacts on Jupiter in 1994 are the only planetary-scale impacts ever observed. The 2009 impact had an entry trajectory opposite and with a lower incidence angle than that of SL9. Comparison of the initial aerosol cloud debris properties, spanning 4,800 km east-west and 2,500 km north-south, with those produced by the SL9 fragments, and dynamical calculations of pre-impact orbit, indicate that the impactor was most probably an icy body with a size of 0.5-1 km. The collision rate of events of this magnitude may be five to ten times more frequent than previously thought. The search for unpredicted impacts, such as the current one, could be best performed in 890-nm and K (2.03-2.36 {\mu}m) filters in strong gaseous absorption, where the high-altitude aerosols are more reflective than Jupiter's primary cloud.

Detection of visible lightning on Saturn

Until now, evidence for lightning on Saturn has been indirect – through radio emissions and cloud morphology. Here we report the first visible detection of lightning, on the night side on August 17, 2009 at −36.4° ± 0.1° planetocentric latitude and 10.6° ± 0.9° west longitude. No other locations produced lightning detectable by either imaging or radio. The lightning images are consistent with a single cloud flashing once per minute. The visible energy of a single flash is comparable to that on Earth and Jupiter, and ranges up to 1.7 × 109 Joules. The diameter of the lightning flashes is ∼200 km, which suggests the lightning is 125–250 km below cloud tops. This depth is above the base of the liquid H2O‐NH3 cloud and may be either in the NH4SH cloud or in the H2O ice cloud. Saturn's lower internal heat transport and likely 5–10 fold enrichment of water largely explain the lower occurrence rate of moist convection on Saturn relative to Jupiter.

Seeing double at Neptune’s south pole

Keck near-infrared images of Neptune from UT 26 July 2007 show that the cloud feature typically observed within a few degrees of Neptune’s south pole had split into a pair of bright spots. A careful determination of disk center places the cloud centers at −89.07 ± 0.06° and −87.84 ± 0.06° planetocentric latitude. If modeled as optically thick, perfectly reflecting layers, we find the pair of features to be constrained to the troposphere, at pressures greater than 0.4 bar. By UT 28 July 2007, images with comparable resolution reveal only a single feature near the south pole. The changing morphology of these circumpolar clouds suggests they may form in a region of strong convection surrounding a neptunian south polar vortex.

Evolution of the cloud field and wind structure of Jupiter's highest speed jet during a huge disturbance

Aims. Despite the banded visual aspect of cloud patterns in Jupiter, high resolution images indicate that these regions are markedly turbulent. One region of particular interest is the north temperate belt (NTB) at 21 ◦ N planetocentric latitude, where the most intense Jovian jet resides with eastward peak speeds of 160−180 m s −1 . Almost every 15 years, the NTB is known to experience an eruption or disturbance that dramatically changes its appearance, a phenomenon known as NTB disturbance (NTBD). In this work, we characterize the morphology of the disturbed cloud field in the wake of the plumes that caused the perturbation, and check for changes in the velocity or shape of the jet. Methods. The 2007 disturbance was witnessed with unprecedented resolution by the Hubble Space Telescope and by a long-term survey based on the “International Outer Planet Watch” (IOPW) network. Our analysis is based on the brightness spectral distribution to characterize both the typical spatial frequency of the perturbation and its turbulent and wavy nature. We also compare our characterization with non-linear dynamical simulations of the disturbance using the EPIC dynamical model. Finally, we obtain a renewed wind profile for the region of interest by cloud tracking. Results. We detect a change in the power spectral slope of the cloud brightness following the disturbance that is related to a change in the typical size of the observed structures. We model the initial disturbance as a Rossby wave. A comparison of the jet profile in the NTB just after the disturbance ended (June 2007) with one observed a year later (July 2008), illustrates a net change occurred in the westward jet at 16 ◦ N with a speed change of 25 m s −1 . As implied by the power spectra analysis, the disturbance and its related Rossby wave dissipate. We propose that this dissipation produced a momentum transfer to the anticyclonic side of the NTB jet increasing the speed of the westward jet at 16 ◦ N as also supported by numerical simulations.

Lightning storms on Saturn observed by Cassini ISS and RPWS during 2004–2006

We report on Cassini Imaging Science Subsystem (ISS) data correlated with Radio and Plasma Wave Science (RPWS) observations, which indicate lightning on Saturn. A rare bright cloud erupt at ∼35° South planetocentric latitude when radio emissions (Saturn Electrostatic Discharges, or SEDs) occur. The cloud consisting of few consecutive eruptions typically lasts for several weeks, and then both the cloud and the SEDs disappear. They may reappear again after several months or may stay inactive for a year. Possibly, all the clouds are produced by the same atmospheric disturbance which drifts West at 0.45 °/day. As of March 2007, four such correlated visible and radio storms have been observed since Cassini Saturn Orbit Insertion (July 2004). In all four cases the SEDs are periodic with roughly Saturn's rotation rate ( 10 h 39 m ), and show correlated phase relative to the times when the clouds are seen on the spacecraft-facing side of the planet, as had been shown for the 2004 storms in [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243–1247]. The 2000-km-scale storm clouds erupt to unusually high altitudes and then slowly fade at high altitudes and spread at low altitudes. The onset time of individual eruptions is less than a day during which time the SEDs reach their maximum rates. This suggests vigorous atmospheric updrafts accompanied by strong precipitation and lightning. Unlike lightning on Earth and Jupiter, where considerable lightning activity is known to exist, only one latitude on Saturn has produced lightning strong enough to be detected during the two and a half years of Cassini observations. This may partly be a detection issue.

The first detection of propane on Saturn

We report the first detection of propane, C 3H 8, in Saturn's stratosphere. Observations taken on September 8, 2002 UT at NASA's IRTF using TEXES, show multiple emission lines due to the 748 cm −1 ν 21 band of C 3H 8. Using a line-by-line radiative transfer code, we are able to fit the data by scaling the propane vertical mixing ratio profile from the photochemical model of Moses et al. [2000. Icarus 143, 244–298]. Multiplicative factors of 0.7 and 0.65 are required to fit the −20° and −80° planetocentric latitude spectra. The resultant profiles are characterized by a 5 mbar mixing ratio of 2.7 ± 0.8 × 10 −8 at −20° and 2.5 −0.8 +1.7 × 10 −8 at −80° latitude. These results suggest that the time scale for meridional circulation lies between the net photochemical lifetimes of C 2H 2 and C 3H 8, ≈ 30 – 600 years.

Meridional variations of temperature, C 2H 2 and C 2H 6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C 2H 2 and C 2H 6 abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C 2H 6 in Saturn's middle and lower stratosphere ( ∼ 40 – 10 , 000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C 2H 2 ( ∼ 1 – 7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13–14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a L S ≈ 270 ° , corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm −1 with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈ 10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C 2H 2 at − 1 ° and − 83 ° planetocentric latitude to be 9.2 −3.8 +6.4 × 10 −7 and 2.5 −1.0 +1.8 × 10 −7 , respectively. The C 2H 2 mixing ratio at 0.12 mbar is found to be 1.0 −0.3 +0.5 × 10 −5 at − 1 ° planetocentric latitude and 2.6 −0.9 +1.3 × 10 −6 at − 83 ° planetocentric latitude. The 2.3 mbar mixing ratio of C 2H 6 inferred from the data is 7.5 −1.7 +2.3 × 10 −6 and 1.0 −0.2 +0.3 × 10 −5 at − 1 ° and − 83 ° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C 2H 2 and C 2H 6 are affected by seasonal forcing and/or stratospheric circulation.

Jupiter's Polar Regions in the Ultraviolet as Imaged by HST/WFPC2: Auroral-Aligned Features and Zonal Motions

Jupiter's polar regions are examined using ultraviolet (UV, 120–320 nm) images obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 over the period of 1994–1997. These images probe the stratospheric aerosols at pressures of a few tens of millibars. We discovered a UV-darkened segment in the polar shading just equatorward of the north polar hood that remained aligned with the southern-most portion of the north auroral oval. The darkened segment generally extended over 100°–260° System III longitude, and 39°–53°N planetocentric latitude in the F160W, F218W, and F255W images. No single type of feature in the F218W and F255W images appeared consistently aligned within the outline of the north auroral oval, but several transient features were observed. It is not certain whether these transient features were associated with auroral processes, or just coincidentally aligned. A “south dark patch” was consistently observed within the outline of the south auroral oval and was temporally variable in size and shape. By tracking selected features, we obtained the first direct measurements of the zonal motions in Jupiter's high-latitude stratosphere. Primarily retrograde motions were observed from 41°N to 60°N. Both prograde and retrograde motions appeared from 48°S to 71°S. We propose that the asymmetric appearance of the polar hoods (R. A. West 1979, Icarus 38, 12–33; R. Wagener and J. Caldwell 1988, Icarus 74, 141–152) is influenced, in part, by meridional mixing in the stratosphere. Enhanced meridional mixing would tend to be more effective at dispersing the polar aerosols to lower latitudes in the north than in the south.

Imaging Jupiter's Aurora at Visible Wavelengths

On November 9, 1996 and again on April 2, 1997, the Galileo spacecraft's Solid State Imaging (SSI) camera targeted the northern auroral region of Jupiter. These observations represent (i) the first spatially resolved images of the jovian auroral oval either at visible wavelengths or on the nightside of the planet, (ii) the first image at visible wavelengths of an auroral footprint of the Io Flux Tube (IFT), (iii) the first unambiguous detection at visible wavelengths of auroral emission on the jovian limb, and (iv) the first images of the aurora with spatial resolution below 100 km per pixel (46 and 35 km, respectively). Relative to many prior expectations, the visible aurora is (i) lower in altitude, (ii) associated with magnetic field lines that cross the equator closer to the planet, and (iii) more variable in time and space. The 1996 images used a clear (broadband) filter, while the 1997 images used both the clear filter and five narrower filters over wavelengths ranging from violet to 968 nm. The filtered images imply that the visible auroral emission contains atomic hydrogen lines, although there is also a continuum component. We were able to position the aurora in three-dimensional space and found the limb emission to be ∼240 km above the surface of a standard (P≈ 1 bar) reference ellipsoid. Our most accurate analysis of the equatormost part of the oval placed it at 54.5° planetocentric latitude and 168° west longitude. Combined with the latest magnetic field models, our results imply that the particles that cause the aurora originate in Jupiter's equatorial plane ∼13RJfrom the center of the planet. The oval was brighter and wider in the 1996 images than in the 1997 images. The broadband radiance of a typical place on the oval as seen directly overhead varied from ∼80 kR in 1997 to ∼300 kR in 1996. Our estimates of the full width of the oval varied from under 500 km to over 8000 km, partly depending on the signal-to-noise ratio of the image. The radiated power per unit length along the oval ranged from ∼60 to ∼700 W/m, with the associated radiated power from the entire oval varying from ∼109to ∼9 × 1010W. Appreciable auroral emission also occurred both north and south of the main oval. One image contains the northern footprint of the IFT, which appears as a central ellipse with a tail of emission that lies downstream with respect to the plasma flow past Io. The central ellipse is ∼1200 km downstream by ∼500 km cross stream. The IFT is comparable in brightness to the nearby auroral oval (∼250 kR) and has a total radiated power of ∼3 × 108W.

Stellar Occultation Observations of Saturn's North-Polar Temperature Structure

We have observed a stellar occultation of GSC5249-01240 by Saturn's north polar region on November 20, 1995 from NASA's Infrared Telescope Facility (IRTF). This is the first recorded occultation by the polar region of a giant planet. The occulted region extends 88 km in vertical height and 660 km in horizontal length, over a region from 82.5° to 85° in planetocentric latitude and from 20° to 30° in planetocentric longitude. Based on isothermal model fits to the light curve, we find an equivalent isothermal temperature of 130 ± 10 K at a pressure level of 1.6 ± 0.1 μbar, which corresponds to a half-light latitude of 83.2 ± 0.2° and longitude of 24.1 ± 0.5°. Using numerical inversion procedures, we have retrieved the temperature profile of the occulted region, which suggests an increase in temperature (with radius) of 14.5 K between 6 and 10 μbar. We also find temperature fluctuations of 1 to 5 K along the path probed by the occultation; if the observed temperature gradients of these fluctuations apply to the vertical direction only, then this region is super-adiabatic. More likely, these thermal gradients are due to a combination of diffractive scintillations and horizontal temperature variations. Given that isothermal model fits and numerical inversions cannot separate individual contributions to observed temperature gradients, such as from vertical variations, horizontal variations, and scintillations, this occultation requires further study.

Clouds and Circulation on Neptune: Implications of 1991 HST Observations

Images of Neptune recorded by the Hubble Space Telescope (HST) Wide Fields/Planetary Camera in late 1991 provide a basis for the first significant comparison with the circulation results obtained from Voyager 2 observations 2 years earlier. The HST images using red (F675W) and methane band (F889N) filters contain isolated bright cloud features suitable for tracking atmospheric motions near planetocentric latitudes of 67°S, 57.5°S, 50°S, 5°N, and 27°N. Wind speeds inferred from the feature rotational periods are consistent with the latitudinal wind profile established by the 1989 Voyager observations. The latitudinal distribution of clouds also matches the Voyager distribution reasonably well, except for a notable lack of clouds between the equator and 40°S, a region where the Great Dark Spot (GDS) and its companion cloud were observed in 1989. The 1991 HST images contain no cloud feature at any latitude with an integrated brightness comparable to the GDS companion, suggesting that the GDS itself may not have survived its projected November 1990 arrival at Neptune's equator.

The solar radiation incident at the top of the atmospheres of Uranus and Neptune

The latitudinal and seasonal variation of the direct solar radiation incident at the top of the atmosphere of Uranus and Neptune has been recalculated by use of updated values for the period of axial rotation and the oblateness. Values for the solar radiation are given in Watt per square meter instead of the unit used in earlier papers (calories per square centimeter per planetary day). The solar radiation averaged over a season and a year as a function of planetocentric latitude has also been reviewed. In addition, attention is made to the ratio of the solar radiation incident on an oblate planet to that incident on a spherical planet.

Temperature and aerosol structure of the nightside Uranian stratosphere from Voyager 2 photopolarimeter stellar occultation measurements

The Voyager 2 photopolarimeter experiment measured the ultraviolet flux from the star γ Pegasi as the star emerged from behind the Uranus limb at planetocentric latitude 68.9°N on the nightside. We have compared these measurements to model atmospheres with, and without, an aerosol component. If the atmosphere is free of aerosols, the temperature is 85 ± 2.3 K at a pressure of 2.7 mbar, 90 ± 6 K at 1 mbar, and 96 ± 13 K at 0.37 mbar, and the radius of the 1‐mbar level is 25,219 ± 6.3 km. The radius value is close to the inferred radius (25,225 km) of the 1‐mbar level at latitude 69° in the southern hemisphere (G. Lindal, private communication, 1987). This agreement argues for hemispheric symmetry in the shape of the 1‐mbar geopotential and, therefore, in the latitude profile of the zonal wind velocity which was not measured in the northern hemisphere. Our measurement, when coupled with a 1‐mbar equatorial radius of 25,734 km (G. Lindal, private communication, 1986), leads to an effective planetary oblateness of ϵ= 0.0230 ± 0.0004, slightly smaller than the value (0.024) determined from earlier work. We place upper limits to the amount of aerosol that can form a discrete layer or haze‐top boundary; the aerosol extinction coefficient, kext ≤ ∼10−4 km−1, if the layer is at an altitude equal to or higher than the 1‐mbar altitude. Our ability to sense haze boundaries diminishes rapidly below 3 mbar. A well‐mixed haze may be present. Candidates for this haze include dust from meteor and ring particle infall, photochemically formed diacetylene condensing at temperatures below 103 K, and other photochemical species which condense at temperatures higher than about 80 K. We show that dust from the rings and meteors contributes no more than a few tenths of a percent to the extinction profile. Current predictions for the diacetylene abundance reveal that diacetylene aerosols may contribute as much as a few tens of percent to the extinction profile and can be accommodated by our observations. More detailed photochemical‐transport‐aerosol models are needed to explore these questions further.

A comparison of red spots in the atmosphere of Jupiter

Comparative broadband relative photometry has been obtained for the Great Red Spot, a well-developed Little Red Spot at 19.°2 planetographic or 16.°9 planetocentric latitude, and selected belts and zones. These results reveal the presence of a UV absorber in both the GRS and the LRS that is not present in the belts. Mapping of the latitudinal dimensions obtained from measurements of ground-based photographs onto the latitudinal dependence of zonal winds derived from the Voyager data indicates that the position of the LRS and GRS relative to the zonal winds is similar and suggest that the LRS is an anticyclonic system. Questions raised by this analysis are presented for further investigation.

Investigations of the optical properties of Saturn's atmosphere carried out at the main astronomical observatory of the Ukrainian Academy of Sciences

The results of polarimetric, photometric, and spectroscopic investigations of Saturn are given. Polarimetric and photometric investigations show that in the long wavelength spectral range ( λ > 0.5 μm) spherical particles play the dominant role in the reflected radiation. But for λ < 0.5 μm, oriented nonspherical particles are revealed in the equatorial region within planetocentric latitudes ±30°. It is suggested that Saturn's rings may be the source of these particles. According to photometric measurements, the brightness coefficient depends on the phase angle and marked temporal variations of the brightness coefficient were absent during the period from 1977 to 1981. A number of weak methane absorption bands were measured for the first time, and the ammonia absorption band at λ = 0.648 μm is present in our low-resolution photoelectric spectrophotometry. The reduction of high-resolution spectrograms has confirmed the presence of 20 ammonia lines belonging to the band at 0.648 μm. The observations can be fit to spherical aerosols with 1.35 ≤ n r ≤ 1.42, r 0 = 1 μm, σ 2 = 0.12; or n r = 1.93, r 0 ≅ 0.4 μm, σ 2 = 0.10; volume concentration 20 cm −3; and ammonia abundance (relative to methane) ≅ 0.01; and the optical thickness of the overlying gas layer is τ s = 0.2 to 0.4 for λ = 0.316 μm.

Dynamical features in the northern hemisphere of Saturn from Voyager 1 images

Results of the analysis of Voyager measurements of divergence and inferred vertical velocity of convective cloud features in the northern hemisphere of Saturn are presented here. The winds in this region have been measured by tracking the cloud elements observed in sequences of images. The derived zonal winds imply that these convective features reside at the maximum of strong easterly jets centred at 41° N planetocentric latitude and that the jet is barotropically unstable. The divergences and local dynamics of the Saturn features are found to differ from those of the jovian equatorial plumes.