Summary: Regarding Saturn and Jupiter, a number of key scientific questions on their general circulation need to be addressed: what drives the observed cloud-level jets? What causes the equatorial superrotation? How deep is the observed cloud level wind? Notably, the origin, structure and evolution of the very intense prograde equatorial jet of Saturn is one of the most controversial aspects of the atmospheric dynamic on giant planets.
The Cassini spacecraft obtained high-resolution observations of Jupiter in 2000 [Porco2003]. Several full zonal wind profiles at different cloud altitudes of Saturn's atmosphere have been also successfully obtained by Cassini [Porco2005; GarcíaMelendo2011] and by the Hubble Space Telescope [SánchezLavega2003, SánchezLavega2012].
The scientific objective of this PhD proposal is to help constrain the atmospheric dynamics of the Giant Planets (Saturn and Jupiter), detect and study atmospheric gravity waves, storms, measure wind velocities and their spatial and temporal variability.
The first two years of this PhD theme proposal are planned to take place in the University of Lisbon, while the third year is planned to take place in the University of Porto.
State of the art: The gas giants (Jupiter and Saturn) are fluid planets with atmospheres primarily made of hydrogen and helium. The part of their atmospheres accessible to remote sensing occupies only a small fraction of their radii (0.05%). Clouds and hazes form around the 1-bar altitude pressure level and extend vertically, according to the thermochemical models, in a layer with a thickness of approximately 200 – 500 km where temperature increases with depth (usually known as the weather layer). Clouds made of NH3, NH4SH, H2O (in Jupiter and Saturn), cover the planet in stratified layers that are mixed with unknown chromophore agents. Dynamical phenomena in the weather layer shape different cloud patterns that define the visible appearance of these planets.
In the thermal part of the spectrum clouds act as opacity sources providing brightness contrasts. The ensemble of cloud morphologies in terms of shapes, sizes and albedos allows their use as tracers of the atmospheric motions in the weather layer. This is, till now, the main tool employed so far to study the winds on these planets. However, recently the supervisor of this project, and collaborators, had developed and fine-tuned tools to retrieve winds at cloud top region using Doppler velocimetry and high-resolution spectra. This method proofed the near instantaneous capability of the Doppler velocimetry techniques to better constrain Venus’ atmospheric dynamics. This Doppler velocimetry technique also allowed to inter-compare for the first time two different techniques using ground-based (CFHT/ESPaDOnS) Doppler velocimetry in the visible range, and coordinated/simultaneous space-based (Vex/VIRTIS) tracking of cloud features at Venus’ cloud-top, based on images acquired in orbit from the Venus Express spacecraft (Machado et al. 2014, 2017). The good consistency between the two approaches to obtain averaged wind velocities contributed to a cross-validation of both methods. Combined results from both techniques offer interesting synergies for constraining different aspects of planetary atmospheres´ dynamics.
The framework of this PhD theme proposal relies in the adaptation of our Doppler velocimetry method to the case of the Solar System´ Giant Planets. Recently, the adaptation of the Doppler velocimetry technique to our Saturn VLT/UVES datasets produced promising preliminary results.
As a by-product of the adaptation of the Doppler velocimetry techniques to Jupiter and Saturn high-resolution spectra, we propose to explore the high frequency capabilities of VLT-ESPRESSO in order to perform seismology of Jupiter and Saturn. This ESO detector will have its first light very soon (in 2017) and is being built with the participation of Portugal, giving us privileged access to its use. This use of ESPRESSO is a unique opportunity to apply our Doppler method to sunlight reflected at cloud level. As a first step we will test our method using a re-analysis of VLT-UVES observations. The objective of this task is to investigate the low frequency variations induced by global acoustic and/or gravity waves at cloud top (~0.7 bar).
Objectives and Work plan:
The main goal of this PhD project is to use high resolution spectroscopy with UVES (UV-Visual Echelle Spectrograph) at ESO Very Large Telescope (VLT) to better understand the nature of the processes governing the overall dynamics in the atmosphere of Saturn and Jupiter following the techniques successfully developed by our Team to retrieve Doppler winds on Venus [Machado2017]. Winds on Giant Planets are known from cloud tracking and by the thermal wind equation but direct measurements were only taken by the Galileo spacecraft in a single point of Jupiter.
Our Doppler velocimetry method allows direct measurement of planetary winds based on high resolution spectroscopy. We plan to measure latitudinal profiles of zonal winds on Jupiter and Saturn and to search for wave motions at cloud level (~ 0,7 bar). Doppler winds will be compared with selected archive data retrieved from cloud tracking on Voyager, Galileo, Cassini, HST, relevant for the goals of this project. An improved cloud tracking method, based in phase correlation between images, will be also used in order to retrieve complementary results. Those measurements will complement space-based data such as Cassini mission, whose "grand finale" is planned for September 2017.
The work plan comprises: to improve our current data analysis algorithms, to correct for limb darkening and for the fast planetary rotation rate. We also plan to apply our method for the first time on planetary absorption lines that forms bands centred at 619 nm, 727 nm and 890 nm for methane and 645,3 nm for ammonia. This will allow the applicant to obtain a first approximation of a three-dimensional global view of the atmospheric circulation. The interpretation of observations requires the knowledge of the atmospheric region probed at different wavelengths, accordingly we will use the NEMESIS radiative transfer model [Irwin2008].
Observations: Our team already possesses data from UVES/VLT (Saturn), MUSE/VLT (Jupiter and Saturn). Those data will be used to retrieve mean zonal and meridional wind profiles at 0.7 bar level. New proposals will be submitted. However the success of those proposals is not critical to accomplish the proposed goals.
First task: High-resolution visible and infrared spectroscopic capabilities applied to the Solar System planets opens a new window by accessing atmospheric composition, mixing ratios and isotopic ratios. In particular, the measurement of spectral lines' Doppler shifts allow the retrieval of wind velocities and this technique has a strong potential to be used to constrain planetary circulation dynamics also on exoplanets.
The goal of this PhD project's second task is two-fold. First, to adapt our method to the infrared wavelength range, to sound deeper layers of the atmosphere of Jupiter and Saturn and planets radiation emission's contribution. Second, to test our Doppler technique using, for the first time, measurements from ESPRESSO/VLT [Pepe2013] (the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), in order to detect and characterize infrasound waves on Gas Giants. This will be a starting point for the development of versatile atmosphere characterization tools, provided that with the future E-ELT we are likely to be able to detect the reflected light spectrum on Neptunes and Super-Earths [Martins2015], and in selected cases to constrain atmosphere composition and to derive wind velocities [Kawahara2012].
Second task: The internal structure of giants planets is difficult to constrain without an observation of normal modes of vibration. A recent study allowed to observe part of these modes in the ring structure of Saturn. However, Jupiter is a better candidate for such a detection, because the cloud layer offer a strong reflection of sunlight [Gaulme and Mosser, 2005]. Previous attempt to detect oscillations of Jupiter's clouds were based on the Doppler effect of a single line [Mosser et al., 2000]. Despite a long development and design of specific sensors [Schmider et al;, 2007], only the spacing in between consecutive oscillation modes were constrained [Gaulme et al., 2011].
In this study we propose to use the Doppler velocimetry technique on the full range of reflected solar radiation emissions by using ESPRESSO/VLT in order to observe Jupiter and Saturn. In a first step, the student will apply our Doppler velocimetry method to UVES/VLT instrument, in order to develop and fine-tune the adaptation of our technique to these purposes. Despite a lower signal to noise ratio at spectral line level, the use of many spectral lines allows to improve the signal to noise ratio drastically. Previously used in the Venus case, this method allowed a precise determination of zonal and meridional winds at cloud level [Machado et al., 2012].
In addition to the gain of S/N ratio expected from the Doppler shifts measured on a large amount of spectral lines, the spatial extent of UVES slit allows to perform stacking methods using all the pixels acquired on the slit. Moreover, the coverage of the space dimension also opens the way to perform specific stacking methods for vibration mode detection, and to perform correlation between pixels in order to recover the green function from one pixel to the other in a manner similar to what is performed for the seismology of the Sun [Duvall et al., 1997].
The expected frequencies of Jupiter and Saturn oscillations cover a frequency range starting at 0.1 mHz. In order to reach a precision in the frequency domain. The maximum frequency that can be reached, with our technique, is about 10 mHz. This frequency range is precisely the one in which we expect a transition from oscillations dominated by gravity waves to oscillations governed by acoustic waves. Knowing the signal to noise of these observations in this frequency range will allow to conclude firmly on the capability of UVES instrument, and further one with ESPRESSO, to perform atmospheric seismology of the Gas Giant's planets.
Methodology: The stepping stone to fulfil the objectives will be the infrared spectra retrieved using CARMENES spectrograph, observations were made in 13 June 2017 (the PhD's supervisor is co-I of the proposal). Those observations will allow to study the vertical wind shear of Saturn's equatorial jet, by applying the Doppler velocimetry technique to methane absorption bands (~1 bar). We also plan to perform this study for the case of Jupiter. Finally, we propose to use our Doppler velocimetry technique on the full range of reflected solar radiation using ESPRESSO, to observe Jupiter and Saturn with the aim of performing a planetary seismology study.
In order to increase the signal to noise ratio drastically, at spectral line level, we will use many spectral lines. Then, a time series of Doppler wind will be estimated to investigate the low frequency variations induced by acoustic and gravity waves at cloud top (0.7 bar). We will search for acoustic and gravity modes of Jupiter and Saturn using local helioseismology methods [Duvall1997]. The mean Doppler shift observed at each pixel will be interpreted as zonal wind, thus providing a very precise profile of zonal wind at cloud top.
The objective of this work is to obtain for each pixel of the high-resolution spectrograph instrument (assuming the slit position is fixed relative to the planet) a time series of Doppler wind estimates. These time series will be long enough to investigate the low frequency variations induced by acoustic and gravity waves at cloud top. These measurements will be performed as close as possible to the half phase angle meridian in order to minimize the effect of variations of zonal wind component (see Machado et al., 2017, for an example on Venus).
Once the winds at cloud top are obtained from the Doppler velocimetry technique, these data will be analysed by computing power spectral densities covering the 0.1mHz to 3 mHz frequency range. This range is particularly interesting because it covers the expected frequencies for low degrees Jupiter and Saturn oscillations and the transition from gravity waves, that will generate variations at frequencies below the Brunt-Vaissala frequency (~2mHz), to acoustic waves, which cover the frequency range above the acoustic cut-off frequency (~2.5mHz). The power spectral densities obtained will be compared to the ones obtained by previous observations relying on the Doppler of a single line [Mosser et al., 2000]. This comparison will allow an estimate of the power spectral density of the observation noise.
A second method will consist in the search of acoustic and gravity modes of Jupiter and Saturn. To do so, the applicant will stack the power spectral densities obtained at each pixel in order to increase the signal to noise ratio, and we will also perform a search of the interspacing of mode frequencies by computing power density spectra of spectral estimates [Gaulme et al., 2011]. Other stacking methods exploiting the spatial extent of the used instruments will also be implemented.
Finally, the increase of power below the Brunt-Vaissala frequency will be interpreted as background long wavelength gravity wave activity in the planet's clouds. This signal is providing both an insight into the dynamics of Jupiter's and Saturn's atmosphere at cloud level and an estimate of the main noise source for the detection of vibration modes.
High-resolution visible and infrared (in the CO2 transparency windows) spectroscopic capabilities to the Solar System planets opens a new window by accessing atmospheric composition, mixing ratios and isotopic ratios, in particular, the measurement of the spectral lines' Doppler shifts allows the retrieval of wind velocities and thus contribute to constrain planetary circulation dynamics.
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