Mapping shallow subsurface water (ice) on near-equatorial Mars is critical for future (manned) exploration, yet existing research shows poor correlation, leaving its presence debated.

This thesis assesses dual-spacecraft Bi-Static Radar (BSR) measurement feasibility at Ultra High Frequency, providing near-global coverage ideal for comparison with gamma/neutron spectrometry hydrogen maps. The Mars Express lander relay antenna transmits a continuous signal probing a few metres into the subsurface (shallow depths which cannot be mapped by conventional low-frequency radar). The ExoMars Trace Gas Orbiter receives the echo, whose amplitude directly reflects permittivity variations induced by compositional changes, e.g. water (ice) deposits.

Models were created to optimize measurement planning and simulate the received power spectrum against BSR data. The current match is limited, reflecting the method’s novelty, and surface composition is yet to show a strong signature. However, after calibration, resolution increase and improving direct signal, seasonal and polarization effects modelling, reliable detections appear possible.

Glacial isostatic adjustment (GIA) is the phenomenon where the solid Earth responds to ice shelves that grow or shrink. As the weight of an ice shelf on the Earth reduces, the Earth rebounds in that location and gravity increases. This rebound process has an instant component, elastic rebound, but also a delayed component as the mantle material slowly flows to a new equilibrium position with timescale determined by the mantle viscosity. GIA affects both the vertical land motion (VLM) and therefore relative sea level (RSL). When predicting sea-level change in the near-future ice mass and sea level changes need to be monitored. However, measurements of current ice mass change are obscured by uplift and gravity changes due to GIA. With a large portion of the world population and economic activity located in coastal areas it is important to monitor sealevel rise and therefore to understand the GIA contribution to sea level rise itself and measurements of the processes....

Venus presents a compelling case study of the greenhouse effect, resulting in the highest surface temperatures of any planet in our solar system. Despite its similarities to Earth in origin and size, Venus has a vastly different atmosphere, dominated by CO2 and thick sulfuric acid clouds undergoing retrograde super-rotation. To better understand Venus’ climate and characterise it, this thesis employs the technique of polarimetry to develop precise computational models of light scattering in its atmosphere. These models are compared with existing observations to investigate dynamic atmospheric phenomena, such as gravity waves and variations in cloud top altitude. Planet-wide gravity waves were observed by the Japanese Akatsuki spacecraft in thermal infrared wavelengths on Venus. These waves were attributed to the underlying mountainous topography and are generated when wind flows over mountains and propagate to themiddle and upper cloud layers. In chapter 2, we explore the possibility of whether orographic gravity waves of similar nature would be observable through polarimetry. As gravity waves propagate, they alter atmospheric density and aerosol distribution, making them observable through various imaging techniques. While direct and thermal imaging have primarily been used to detect these waves, those affecting higher altitudes cause much smaller changes in atmospheric density as compared to the cloud tops. Consequently, these high-altitude waves are challenging to observe with direct imaging and have been detected primarily through in-situ sensors or night-glow measurements. Unlike direct imaging, which captures both polarized and unpolarized light, polarimetry is more sensitive to density variations due to the high degree of polarization imparted by gas molecules, which also varies with solar and viewing geometry.

Since Galileo Galilei’s first discovery of natural satellites orbiting around other planets, observing and reconstructing their dynamics has been at the core of our efforts to understand and characterise these distant worlds. Far from following perfect, frozen in time Keplerian orbits, the dynamics of these satellites keep evolving, with tides as driving mechanism. The dissipation of energy in natural bodies due to their visco-elastic response to tidal forcing both heats up the moons’ interiors and causes their orbits to expand or shrink, as well as to become more circular or elliptical. Refining the moons’ ephemerides (i.e., tabulated solutions of their motion as a function of time) is thus key to studying not only their present-day dynamics, but also the long-termthermal-orbital evolution of planetary systems....

Space debris and micrometeoroids are abundant around our home planet and offer both an opportunity of research for scientists and a threat to human’s endeavours in space. Impacts with even the smallest particles can happen at such high velocities that they can damage or destroy an operational spacecraft, creating even more debris and possibly endangering its crew, if manned. Detecting and cataloguing the debris and micrometeoroid population has thus been a priority for many space agencies around the world to ensure safe space access. Unfortunately, small debris in high orbits cannot be detected by ground based observatories that can detect objects with a minimum diameter of 10 cm in LEO and 1m in GEO. This is why space based detectors are needed. Current detectors are small in size and can often detect only high velocity impacts due to relying on ionisation of particles upon impact. This MSc thesis investigated the possibility of using the whole spacecraft as a detector, by employing vibration sensors on its structure to detect impact-induced oscillations. These vibrations could contain information on the impactor’s mass, speed, material class and other features. Impacts of pieces of debris were simulated in a laboratory environment and the vibrations generated in a spacecraft-like structure were measured and processed to study the effectiveness of such a detection method. Three types of projectiles were used, beads of glass (3mm diameter, 0.04g average mass), teflon (3.2 mm average diameter and 0.033g average mass) and steel (3 mm average diameter and 0.11 g average mass). As target, to represent a spacecraft, a 3mm thick aluminium panel, a 5 mm thick aluminium- skin aluminium-honeycomb sandwich panel and a flight spare model of a solar array were used. An impact-location estimation algorithm was developed that can successfully determine the point of impact based on the signal of at least 4 sensors with centimetre accuracy. Analysis of the waveforms acquired by the sensors showed that deformation phenomena upon impact affect the frequency content of the signal, showing that to preserve as much as possible the high frequency content of the signal stiffer parts of a satellite are more indicated for this type of technology. Lastly, it was observed that the maximum voltage measured grows linearly with the momentum of the particles, making it possible to estimate the impact momentum solely based on the signal acquired. The dependence of voltage from the impact momentum together with the experimentally calculated coefficient of vibration dissipation of -0.05 cm^(−1) was used to determine the maximum distance of sensors from the point of impact as a function of particle speed and mass for successful detection of the impact. Particles as light as 0.03 g (the lightest used in the experiments) travelling at speeds as low as 15 m/s are expected to be detectable from sensors as far as 50cm, while for objects of 0.2 g travelling at speeds exceeding 200 m/s it is expected that a sensor can be as far as 1 meter away from the point of impact. The method, for its simplicity and low-cost provides an easy way to greatly increase the amount of data on small pieces of debris and micrometeoroid around our planet, enhancing space exploration safety and supporting future missions.

As a result of climate change, sea level is changing all over the world at unprecedented rates. Sea-level change can have significant impacts on coastal communities, infrastructure and global economy, as most of the major cities are located near to or at the coast. Rising sea levels can lead to, for instance, more severe and more frequent flooding, increasing coastal erosion and salt water intrusion. In addition, sea-level change can also influence coastal ecosystems, by altering the habitats of many plant and animals species. Therefore, it is crucial that we understand what is causing sea-level change and at what rate sea levels are changing.

Global mean sea level has been rising at a rate of about 3.4 millimetres per year over the last 30 years. Regionally, however, sea level can be changing at a much higher or lower rate. That is because local processes, such as ocean dynamics and gravitational effects associated with continental ice mass changes, cause regional deviations from the global average. But what is causing sea level to change at a specific location? Is sea level changing because the oceans are warming, and thus expanding? Or because the ice from glaciers and ice sheets are melting? The attribution of sea-level change to these and other drivers can be done using a sea-level budget approach. Sea-level budget studies can be used to constrain missing or poorly known contributions and to validate climate models. While the global mean sea-level budget is considered closed within uncertainties, closing the budget on a regional to local scale is still challenging.

In this thesis, I focused on the question: Can we close the regional sea-level budget in the satellite altimetry era on a sub-basin scale consistently for the entire world? For this, we need not only high quality observations of sea-level change and each component, but also of the uncertainties within each process. Therefore, in Chapter 2 and 3, I explored the main drivers of regional sea-level change, focusing on the uncertainty characterization of each component. I then looked at which spatial scale is optimal for analysing the regional sea-level budget, and compared the sum of the drivers with the total observed change in these regions in Chapter 4.

Since the first radio occultation measurements were performed to study planetary atmospheres, the calculations returning refractivity profiles always adopted the so-called "spherical symmetry assumption". This hypothesis imposes a null horizontal gradient of the atmospheric parameters within the region sampled during the occultation: a scenario that can be quite unrealistic for regions in proximity of the terminator line. This study suggests that another way of calculating refractivity profiles is doable, dropping the spherical symmetry assumption and assuming a straight line propagation of the radio signal through the atmospheric medium (which, according to the laws of refraction, should travel in a curved line, corresponding to the fastest trajectory according to Fermat's law). The study proved that the theoretical difference between the two methods is negligible (within 1%) and that numerical methods can be developed to account for the asymmetric regions within the atmosphere.

Jupiter is the most visited outer solar system planet, but the exact variation in atmospheric properties along its disk remains largely a mystery. This is where polarimetry fits into the picture. Its added value to spectrometry by additionally measuring the polarisation degree and direction of light makes it a suitable remote sensing tool for the characterisation of planetary atmospheres. It can potentially be used to detect and characterise exoplanets as starlight is originally unpolarised [Kemp et al., 1971] while light reflecting from an object is not. The degree of polarisation is sensitive to the atmospheric properties and its coupling with the wavelength, phase angle and absorption are used to derive the approximate upper atmospheric structure of Jupiter. For this purpose, polarimetric observations of the Torino Polarimeter are compared to the results of a numerical model coded in Fortran. This numerical model uses a doubling-adding radiative transfer algorithm to simulate the polarisation properties of the designated atmospheric profile. The atmospheric profile consists of gas and aerosols, the latter modelled by spherical particles using Mie scattering theory. The numerical model results are processed and compared to the observations using a Matlab script. The particle properties are constrained by the observations using the wavelength filters, the implemented methane absorption and by using a variable cloud pressure and haze optical thickness. The numerical model results best matching the observations show higher altitude clouds in the higher polarisation degree regions known as the zones, and lower altitude clouds in the belts. The optical thickness of the haze layer turns out to be low or zero in the zones and higher in the belts. To better characterise Jupiter's atmospheric structure, several aspects relating to the observations and the numerical model have to be investigated in more detail in order to improve the matching of the two.

This thesis' work aims to evaluate the potential added benefit to spacecraft orbit determination procedures upon using non-conventional measurements for orbit reconstruction. As a spacecraft orbits a celestial body, its ground tracks will naturally cross previous ground tracks at many points. These locations, known as crossover points, yield valuable information about the orbited body and the spacecraft trajectory using the spacecraft altitude measured during both passages at each crossover location. To evaluate the impact of altimetry crossover measurements on orbit determination, the mission scenario of the planetary mission Jupiter Icy moons Explorer (JUICE) by the European Space Agency (ESA) is used as case study. As the mission's measurements will only be available several years from now, the resulting analysis is done with synthetic measurements obtained through numerical simulations. Herein, the necessary mathematical expressions for the inclusion of crossover measurements into orbit determination algorithms are presented, verified and evaluated. In doing so, it is shown that a first-order approximation of these expressions, as used in previous efforts, is insufficient and a more detailed expression is developed. Furthermore, the used crossover determination algorithm is presented in detail as well as the crossover selection filters in accordance to mission requirements. Finally, the sensitivities and intricacies of crossover measurements are discussed and their added value to orbit determination schemes is shown.

Europa is one of the most interesting celestial world that has been ever observed. The habitability condition, met for the liquid water layer covering the moon, yields to astonishing speculations concerning what might exist in the interior of the tiniest moon of Jupiter. The Voyager and Galileo programs detected a frozen and brittle layer, deeply battered by lineament features. The observed crevasses on the moon's icy surface can be considered as results of a strong and variating stress field applied to the brittle icy shell that eventually reaches critical deformation conditions locally, and favors the process of crevasse propagation. The superimposition of secular widening to diurnal components is the source of stress that continuously deforms the brittle surface of Europa and induces the ice to crack, similarly to the processes observed with crevasses in large terrestrial ice sheets. The research's aim is to improve the existing models of fracture propagation for the Europa ice shell, dealing with analogs observed in Earth's crevasses on large ice shelves, by the implementation and the usage of linear elastic fracture mechanics (LEFM). Two different LEFM approaches are included in the document, one dealing with the estimation of global areas on the moon that are more favorable to host propagation and one dealing with the estimation of fractures' lengths for specific observed features. Results describe the existence of critical and non-critical areas centered in the equatorial zone which are respectively prone or not to host vertical propagation. Maximum critical depths for surface crevasses reach values of 120 meters, while critical heights for bottom crevasses show values up to 1.5 kilometers. Beside the outcomes of the vertical simulation, a mathematical manipulation of the LEFM analysis allowed the determination of horizontal cracks’ growth. Knowing the aspect of an observed lineament, the current model could calculate fracturing events’ intensity. These reach propagation rates of kilometers per second, namely almost instantaneous episodes. The outcomes of the current research are particularly interesting when seen in relation with the future exploration missions to Europa: ESA's JUICE, NASA Europa Clipper and its potential lander. Specific areas that are more prone to host propagation and the determination of growth rates are helpful elements in the preliminary description of target landing areas and the fracturing events’ detection possibility. The built model yields to a further and more accurate understanding of the dynamics for the interior of one of the most promising celestial object, in term of searching for a biosphere, hence extraterrestrial life.

In the last decade, the gravity field of the Earth has been observed with increased coverage due to dedicated satellite missions, which resulted in higher resolution and more accurate global gravity field models than were previously available. These models make it possible to study large scale processes such as solid Earth deformation after large loading events such as retreat of ice sheets or to study lateral density variation in the lithospheric part of the upper mantle. However, to use the gravity data successfully, unwanted signal needs to be removed in order to extract the information of interest. For example, with lithosphere studies the gravity signal coming from the crust and the deep mantle needs to be removed. This is commonly done by filtering out long-wavelength signals from the solution to remove deep mantle effects, and by removing the crustal signal by forward modelling seismic-derived crustal models. With improved models of crustal structure and more accurate gravity data, new information about the upper mantle and lithosphere can be obtained. Adopting the increased resolution and accuracy of the global gravity field models, I have developed new approaches that focus on spectral analysis of the gravity field, which result in new insights of the upper mantle.

The forward gravity field modelling method that I improve upon in this dissertation is mostly used for topographic/isostatic mass reduction of gravity data. The methodology is able to transform density-models into gravitational potential fields using a spherical harmonic representation. I show that this methodology in the existing form is not suited to be used for density layers in lower crustal and upper mantle regions. The binomial series inherent to this methodology do not converge when applied to deep mass structures, and therefore it is not possible to truncate the series at a low degree to approximate the mass. This approximation is crucial for the computational efficiency of the methodology. I propose a correction that mitigates this erroneous behaviour, which enables this methodology to efficiently compute the potential field of deep situated masses. I benchmark the improved methodology with a tesseroid-based gravity-field modelling software, and I show that my software is accurate within ±4 mGal, when modelling the Moho density interface (with a range in signal of ±500 mGal. The improved methodology is used in the studies described in this thesis.

With an efficient and accurate forward modelling methodology, I am able to use global gravity field data in studies of the solid Earth. In the central part of Fennoscandia the crust is currently uplifting, because of the delayed response of the viscous mantle to melting of the regional Late Pleistocene ice sheet. This process, called glacial isostatic adjustment (GIA), causes a negative anomaly in the present-day static gravity field as isostatic equilibrium has not been reached yet. Several studies have used this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures had not been properly taken into account. In revisiting this problem, I show that the GIA gravity signal overlaps with mantle convection signals, such that a simple spherical harmonic truncation is not sufficient to separate these two phenomena. Furthermore, I find that, in contrast to the other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of the relative large uncertainties in the crustal density models. Therefore, I propose to correct the observed gravity field for GIA with numerical modelling results when constructing geophysical models that assume isostatic equilibrium. I show that correcting for GIA results in a significant vertical readjustment of the geometry of structural layers in the modelled crust of 5-10 percent. Correcting the gravity field for GIA prior to assuming isostatic equilibrium might be relevant in other areas with ongoing post-glacial rebound such as North America and the polar regions.

Uncertainty in lithospheric density models is still the limiting factors in solid Earth studies and needs to be improved. Lithospheric density anomalies can, among other methods, be estimated from seismic tomography, gravity studies, or joint studies using both datasets. I compare different gravity-based density models of the lithosphere to a tomographic-derived solution and characterise the sources that introduce large uncertainties in the density models of the lithosphere. To study the uncertainty between global and regional crustal models, I select a region where the crust is explored in great measure with seismic profiles, namely the British Isles and surrounding areas, where I use three crustal models to quantify the crustal uncertainty: CRUST1.0, EUCrust-07, and a high-resolution regional P-wave velocity model of the region. The crustal models contribute to the uncertainty of the density of the lithosphere with ±110 kg/m³. Furthermore, I study various P-wave velocity-to-density conversions to quantify the uncertainty introduced by these conversion methods (±10 kg/m³. All different crustal density models are forward modelled into gravity anomalies using the improved methodology of Chapter 2 and these gravity anomalies are subsequently removed from the gravity observations. The unmodelled long-wavelength signal in the gravity field representing mass anomalies in the deep mantle are removed from the observation by spherical harmonic truncation, introducing an uncertainty of ±5 kg/m³. Also, the choice of density background model (±20 kg/m³) and lithosphere-asthenosphere boundary uncertainty (±30 kg/m³) have a small but significant effect on the estimated lithosphere densities. However, the inhomogeneous spatial distribution of profiles of controlled-source seismic exploration of the crustal thickness and density distribution proves to be the largest source of uncertainty (±110 kg/m³). The gravity-based lithospheric density solutions with a variation of ±100 kg/m³ are completely different in magnitude and spatial signature to the densities (±35 kg/m³) derived from a shear wave velocity model. This demonstrates that the tomographic model has a limited resolution, which can be related to regularisation that is used in the construction of global tomographic models. To account for this spectral imbalance, I spatially filter the gravity-based density models, resulting in similarities in spatial correlation and magnitude between that of the gravity-based and the tomographic-derived density. With the filtered gravity-based density I am able to estimate lateral varying conversion values between shear wave velocity and density for the lithosphere, which shows a correlation with major tectonic regions. This correlation shows that the independent gravity-based solutions, despite being filtered, can help in identifying different compositional domains in the lithosphere.

Satellite observations also provide global data on the temporal variations of the gravity field. In the last study, I show that global gravity-change observations from the GRACE satellite mission can be used to study GIA in the Barents Sea Region. The Barents Sea is subject to ongoing postglacial uplift since the melting of the Weichselian ice sheet that covered this region. The deglaciation history is not well known because there is only data from locations close to the boundary of the former ice sheet, in Franz Joseph Land, Svalbard, and Novaya Zemlya. At these locations the magnitude of the GIA uplift is limited, reducing the signal-to-noise of the data. The GRACE mission measures the gravity-change due to GIA at the center of the Barents Sea, where the maximum uplift and ongoing gravity-change is situated. I show that the linear trend in the gravity-change derived from a decade of observations from the GRACE satellite mission can constrain the volume of the ice sheet after correcting for current ice-melt, hydrology and far-field gravitational effects. Regional ice loading models based on new geologically-inferred ice margin chronologies show a significantly better fit to the GRACE data than the global ice models ICE-5G and ICE-6G_C. The regional ice models in this study contain less ice mass during LGM in the Barents Sea than ICE-5G (5-6.3 m equivalent sea level vs. 8.5 m). Also, I show that the GRACE gravity-change is sensitive to the upper mantle viscosity underneath the Barents sea, for which I found a minimum value of 4x10²⁰ Pas, regardless of the ice loading history. The GRACE gravity-change should be used as a constraint in any future GIA modelling of the Barents Sea, because it is the only measurement that captures the signal of maximum GIA.

The high resolution and accurate global gravity field models do give new insights in the structure and density distribution of the upper mantle. The presented studies in this dissertation demonstrate that analysing the spectral signature of gravity data is very useful. Medium-to-short-scale features, like lateral density variation in the lithosphere and GIA gravity-change in the Barents Sea can be separate from other gravity-change sources by applying spectral filters. For longer wavelength signals, such as the GIA static gravity signal in Fennoscandia, this proves to be more difficult due to the overlap in the long-wavelength region by mantle convection signals and other deep mantle signals. On the whole, the global gravity field models and their spectral signature play an important part in building a global density model of the Earth, in which lithosphere, GIA, but also mantle convection and core-mantle boundary effects need to be combined to explain the gravity field.

Sometime late in the fall of 2007, the NetLander mission will land four probes on the surface of Mars containing geodetic and seismic experiments, thereby establishing the first network of geophysics stations on the surface of a terrestrial body other than our own Earth. This mission will yield a tremendous amount of information pertaining the internal structure and orientation of Mars. Thanks to such missions, the data available for the terrestrial bodies will increase and with it our understanding of the rotational instabilities.
Although this was not always the case, it is now known that forces and deformations due to the non-rigid characteristics of the Earth constantly perturb the motion of the planet to various degrees. The fact that our planet, and all realistic bodies for that matter,is not wholly solid, that it has oceans, an atmosphere and a visco-elastic crust, mantle and core, means that the actual position of the rotational axis and rotation rate of the Earth vary from the idealized rigid body motion on virtually every time scale. The Earth constantly reshapes itself to cope with the ever changing loads and other geo-dynamic forces that act upon it. This deformation in turn leads to shifts in the position of the rotation axis with respect to the Earth’s surface, or polar motion, and to a change in rotation rate, also known as a change in length-of-day. This reshaping of a body due to geo-dynamic forces is dependent on the rheology of that body, since material properties such as rigidity and viscosity determine how a body deforms and flows under certain stresses. Although their regularities in the rotation of the Earth complicate astronomical research, for the geophysicist they are a gift. The rotational perturbations must have sources and thus provide information on the internal structure of the Earth and the geophysical processes acting on and within it. The main objective of this thesis is to examine the influences of some of the parameters that determine the polar motion of a terrestrial body, without adhering to the constraints put on them by the application to the Earth. For instance, the influence of the absolute size of a body as defined by its radius has never been examined since the radius of the Earth is known very accurately. This leads to more general and more widely applicable results as the driving parameters are examined in wide ranges.

To this end, a linearized formulation of the polar motion was used in conjunction with the Normal Mode technique, which uses the Laplace domain to calculate the elastic equivalence of the visco-elastic problem in the time domain.