How to Sample an Alien World? Lessons for Europa from Apollo 16

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Planetary topography measurement by descent stereophotogrammetry

Digital Terrain Models (DTMs) provide valuable insights into the nature of solar system surfaces, facilitating geological analysis, landing site selection and characterisation, and contextualising in situ measurements. For missions to solar system bodies for which orbiters and soft landed platforms are technologically or financially challenging to achieve, low mass descent or ascent probes (e.g. planetary penetrators) provide an alternative means by which to access the atmosphere and/or surface, and a platform from which to image the surface from a range of altitudes and perspectives. This paper presents a study into the concept of large-coverage descent stereophotogrammetry, whereby the stereo geometry of vertically offset wide-angle descent images is used to measure surface topography over a region of large extent. To do this, we simulate images of Mars' Gale Crater using a large coverage, high resolution DTM of the area, and derive topographic measurements by stereo matching pairs of simulated images. These topographic measurements are compared directly with the original DTM to characterise their accuracy, and dependence of elevation measurement accuracy on stereo geometry is thus investigated. For a stereo pair with a given altitude (corresponding to the altitude of its lower image), error in elevation measurement is found to have its minimum value for surface at a horizontal distance between 1 and 3 times the altitude. For a point on the surface with given horizontal distance from the imaging location, a stereo imaging altitude between 0.2 and 0.5 times this distance is found to achieve best elevation measurement accuracy. Surface appearance, and its change between two images of a stereo pair, is found to have a significant impact on stereo matching performance, limiting stereo baseline length to an optimum value range of 0.2–0.4 times the lower image's altitude, and resulting in the occurrence of occlusions and blind spots, particularly at oblique viewing angles

Theory of the electron and nuclear spin coherence times of shallow donor spin qubits in isotopically and chemically purified zinc oxide

In this article, I present a theoretical study of the electron and nuclear spin coherence times of shallow donor spin qubits in zinc oxide (ZnO) at low temperature. The influence of different spin-phonon processes as well as different spin-spin processes on the spin coherence time of shallow donors in ZnO is considered, both in the case of an electron spin qubit and in the case of a nuclear spin qubit encoded on a shallow donor. It is estimated that the electron spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of microseconds, limited by the nuclear spectral diffusion process. The electron spin coherence time of an isolated indium shallow donor can be extended to few milliseconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the electron spin coherence time of an isolated indium shallow donor is only limited by a spin-lattice decoherence process. It is also estimated that the nuclear spin coherence time of an isolated indium shallow donor in natural quasi-intrinsic ZnO is on the order of hundreds of milliseconds, limited by the nuclear spectral diffusion process. The nuclear spin coherence time of an isolated indium shallow donor can be extended to few seconds in isotopically and chemically purified quasi-intrinsic ZnO. In this optimal case, the nuclear spin coherence time of an isolated indium shallow donor is only limited by the cross relaxation decoherence process. This study thus shows the great potential of electron and nuclear spin qubits encoded on shallow donors in isotopically and chemically purified quasi-intrinsic ZnO for the implementation of quantum processor and/or quantum memories