Radar sounding using the Cassini altimeter waveform modeling and Monte Carlo approach for data inversion observations of Titan's seas

Recently, the Cassini RADAR has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on Saturn's largest moon, Titan. Altimetry waveforms from observations over the seas are generally composed of two main reflections: the first from the surface of the liquid and the second from the seafloor. The time interval between these two peaks is a measure of sea depth, and the attenuation from the propagation through the liquid is a measure of the dielectric properties, which is a sensitive property of liquid composition. Radar measurements are affected by uncertainties that can include saturation effects, possible receiver distortion, and processing artifacts, in addition to thermal noise and speckle. To rigorously treat these problems, we simulate the Ku-band altimetry echo received from Titan's seas using a two-layer model, where the surface is represented by a specular reflection and the seafloor is modeled using a facet-based synthetic surface. The simulation accounts for the thermal noise, speckle, analog-to-digital conversion, and block adaptive quantization and allows for possible receiver saturation. We use a Monte Carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth, surface/subsurface intensity ratio, and subsurface roughness for the individual double-peaked waveform of Ligeia Mare acquired by the Cassini spacecraft in May 2013. This new analysis provides an update to the Ku-band attenuation and results in a new estimate for its loss tangent and composition. We also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over Ontario Lacus in December 2008

Liquid filled canyons on Titan

In May 2013 the Cassini RADAR altimeter observed channels in Vid Flumina, a drainage network connected to Titan’s second largest hydrocarbon sea, Ligeia Mare. Analysis of these altimeter echoes shows that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system

Possible explosion crater origin of small lake basins with raised rims on Titan

The Cassini mission discovered lakes and seas comprising mostly methane in the polar regions of Titan. Lakes of liquid nitrogen may have existed during the epochs of Titan’s past in which methane was photochemically depleted, leaving a nearly pure molecular nitrogen atmosphere and, thus, far colder temperatures. The modern-day small lake basins with sharp edges have been suggested to originate from dissolution processes, due to their morphological similarity to terrestrial karstic lakes. Here we analyse the morphology of the small lake basins that feature raised rims to elucidate their origin, using delay-Doppler processed altimetric and bathymetric data acquired during the last close flyby of Titan by the Cassini spacecraft. We find that the morphology of the raised-rim basins is analogous to that of explosion craters from magma–water interaction on Earth and therefore propose that these basins are from near-surface vapour explosions, rather than karstic. We calculate that the phase transition of liquid nitrogen in the near subsurface during a warming event can generate explosions sufficient to form the basins. Hence, we suggest that raised-rim basins are evidence for one or more warming events terminating a nitrogen-dominated cold episode on Titan

Synergy of Cassini SAR and altimeter acquisitions for the retrieval of dune field characteristics on Titan

This work focuses on the retrieval of Titan’s dune field characteristics addressing different radar modes. The main purpose of the proposed work is to exploit a possible synergy between SAR and altimeter acquisitions modes to provide information about dune field. Cassini has performed 86 Titan flybys in which several observations of dune fields have been collected in altimetry mode. There are several cases in which SAR and altimeter have been acquired over same areas covered by dune fields, such as during T28 (SAR) and T30 (altimeter) flybys. Altimetry together with SAR data have been used to derive the rms slopes of dunes (large scale) over Fensal area, this information has been employed to calculate SAR incidence angle with respect to dunes. We extracted backscattering coefficients of bright and dark areas detected in the analyzed SAR image in order to evaluate the angular response of scattering. Through the Geometric Optics model we retrieve roughness values (small scale rms slope) for both dune bright and dark areas

The SSDC Role in the LICIACube Mission: Data Management and the MATISSE Tool

Light Italian Cubesat for Imaging of Asteroids (LICIACube) is an Italian mission managed by the Italian Space Agency (ASI) and part of the NASA Double Asteroid Redirection Test (DART) planetary defense mission. Its main goals are to document the effects of the DART impact on Dimorphos, the secondary member of the (65803) Didymos binary asteroid system, characterizing the shape of the target body and performing dedicated scientific investigations on it. Within this framework, the mission Science Operations Center will be managed by the Space Science Data Center (ASI-SSDC), which will have the responsibility of processing, archiving, and disseminating the data acquired by the two LICIACube onboard cameras. In order to better accomplish this task, SSDC also plans to use and modify its scientific webtool Multi-purpose Advanced Tool for Instruments for the solar system Exploration (MATISSE), making it the primary tool for the LICIACube data analysis, thanks to its advanced capabilities for searching and visualizing data, particularly useful for the irregular shapes common to several small bodies

Possible explosion crater origin of small lake basins with raised rims on Titan

The Cassini mission discovered lakes and seas comprising mostly methane in the polar regions of Titan. Lakes of liquid nitrogen may have existed during the epochs of Titan’s past in which methane was photochemically depleted, leaving a nearly pure molecular nitrogen atmosphere and, thus, far colder temperatures. The modern-day small lake basins with sharp edges have been suggested to originate from dissolution processes, due to their morphological similarity to terrestrial karstic lakes. Here we analyse the morphology of the small lake basins that feature raised rims to elucidate their origin, using delay-Doppler processed altimetric and bathymetric data acquired during the last close flyby of Titan by the Cassini spacecraft. We find that the morphology of the raised-rim basins is analogous to that of explosion craters from magma–water interaction on Earth and therefore propose that these basins are from near-surface vapour explosions, rather than karstic. We calculate that the phase transition of liquid nitrogen in the near subsurface during a warming event can generate explosions sufficient to form the basins. Hence, we suggest that raised-rim basins are evidence for one or more warming events terminating a nitrogen-dominated cold episode on Titan

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Observations of Titan liquid bodies by means of the Cassini RADAR altimeter

Twenty years after its great departure from Cape Canaveral the Cassini mission is going to have the “Grand Finale” it deserves. This spacecraft definitely marked a milestone in astronautical history as one of the best examples of collaboration among different space agencies (NASA-ESA-ASI) and a great model for any other future mission for planetary exploration. A discovery machine that unveiled the Titan’s lively world and that doesn’t stop giving surprises at every new observation. I’m Valerio Poggiali, phD candidate, and with my colleagues and professors of La Sapienza University of Rome I’m involved in the Cassini RADAR altimeter data processing, a task bequeathed to us by our Prof. Emeritus Giovanni Picardi who started believing in this project already at the beginning of nineties when he published a paper entitled “The Radar System for the Exploration of Titan”. It is 1992, and he states: “the knowledge of the ocean depth is particularly important and its determination is one of the main requirements for the radar instrument”. Actually he announced twenty-two years in advance our work on “The Bathymetry of a Titan Sea” in which we reported about an incredible observation made by the Cassini radar altimeter on May 23st, 2013. We were waiting for that fly-by by years as it was representing the only opportunity to test the instrument capability to plumb the bottom of a Titan sea and infer about its dielectric properties (besides proving once for all the liquid nature of those dark features). After Kraken and before Punga, the Ligeia Mare is the second greatest sea of Titan (roughly 260 x 217 miles). We have been able to track a deepest point of 180 yards and estimate its liquid to be composed by an extremely pure mixture of liquid hydrocarbons (laboratory experiments at JPL eventually confirmed components to be methane, ethane and nitrogen). The following period has been very busy for us, but also extremely rewarding. After the publication of Ligeia Mare bathymetry, the Cassini radar altimeter observed the Kraken Mare on August 21st, 2014 and the Punga Mare on January 11st, 2015. The measurements we had to perform were very challenging. Main difficulties came from the necessity to track the subsurface also in the shallowest parts of seas where we have to push the radar capabilities to the limits of its vertical resolution (nominally 33 yards but with some super-resolution algorithms improvable of a factor of about two). Another great test that we faced was in the production of the bathymetry of the largest lake (43 x 124 miles) of the southern polar area of Titan, the Ontario Lacus. The severe saturation of the receiver united to the shallowness of this liquid body (preliminary results suggested a 50 yards maximum depth along the altimeter ground track, that of course could be not the deepest point) imposed the necessity of developing a different method for determine its bathymetry and dielectric properties. Also if the Cassini radar will not observe Titan seas anymore (we will have just a last look to northern polar small lakes during Cassini’s last flyby of Titan on April 2017) we have still many analyses to perform on the already collected data, these will engage us for years to come. The more Cassini mission will come closer to its end on September 15, 2017, the more we will have to stay tuned to not miss anyone of the discoveries that will certainly follow as the spacecraft will finally approach Saturn, receiving, as if it was an engagement gift, the unique opportunity of a closer observation of its precious rings. In this PhD dissertation I would like to resume the main steps we followed for analyzing the Cassini radar altimeter data from which we obtained the first bathymetries of extraterrestrial liquid bodies, starting from a brief introduction to the RADAR instrument characteristics and its capabilities. Later I will focus on the backscattering models we adopted in order to effectively interpret the radar altimeter nadiral power returns from the surface of Titan. A detailed description will follow of the simulator we developed in order to reconstruct as accurately as possible how the Cassini radar receiving chain processes on board the incoming signal. Then, I will describe the results we obtained on Ligeia Mare and Ontario Lacus, giving some anticipations on the work ongoing for Punga and Kraken Mare. The last part will be finally dedicated to our discovery on Titan of liquid-filled canyons directly connected to seas. I will describe how the radar altimeter was able to measure their depth and infer about their nature of sea flooded valleys by means of a comparison of the levels of their liquids. Far from being just a library-based theoretical dissertation, this document will intentionally focus on the work we carried out and on the consequent results we obtained in the framework of the Cassini mission. The techniques described herein and, in particular, the simulator and the Bayesian method for the estimation of unknown parameters we adopted for this work are the result of the fruitful collaboration between our Dipartimento di Ingegneria Elettronica e Telecomunicazioni (DIET) of “La Sapienza” University of Rome and the Astronomy department of the Cornell University in Ithaca (NY)