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Register a new userThe Sands of Phobos: The Martian moons eccentric orbit refreshes its surface
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Added by
Ronald-Louis Ballouz
Publication date
2018
fields
Physics
and research's language is
English
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The surface of the Martian moon Phobos exhibits two distinct geologic units, known as the red and blue units. The provenance of these regions is uncertain yet crucial to understanding the origin of the Martian moon and its interaction with the space environment. Here we show that Phobos orbital eccentricity can cause sufficient grain motion to refresh its surface, suggesting that space weathering is the likely driver of the dichotomy on the moons surface. In particular, we predict that blue regions are made up of pristine endogenic material that can be uncovered in steep terrain subject to large variations in the tidal forcing from Mars. The predictions of our model are consistent with current spacecraft observations which show that blue units are found near these regions.
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The Martian Moons Exploration (MMX) spacecraft is a JAXA mission to Mars and its moons Phobos and Deimos. MMX will carry the Circum-Martian Dust Monitor (CMDM) which is a newly developed light-weight ($mathrm{650,g}$) large area ($mathrm{1,m^2}$) dust impact detector. Cometary meteoroid streams (also referred to as trails) exist along the orbits of comets, forming fine structures of the interplanetary dust cloud. The streams consist predominantly of the largest cometary particles (with sizes of approximately $mathrm{100,mu m}$ to 1~cm) which are ejected at low speeds and remain very close to the comet orbit for several revolutions around the Sun. The Interplanetary Meteoroid Environment for eXploration (IMEX) dust streams in space model is a new and recently published universal model for cometary meteoroid streams in the inner Solar System. We use IMEX to study the detection conditions of cometary dust stream particles with CMDM during the MMX mission in the time period 2024 to 2028. The model predicts traverses of 12 cometary meteoroid streams with fluxes of $mathrm{100,mu m}$ and bigger particles of at least $mathrm{10^{-3},m^{-2},day^{-1}}$ during a total time period of approximately 90~days. The highest flux of $mathrm{0.15,m^{-2},day^{-1}}$ is predicted for comet 114P/Wiseman-Skiff in October 2026. With its large detection area and high sensitivity CMDM will be able to detect cometary meteoroid streams en route to Phobos. Our simulation results for the Mars orbital phase of MMX also predict the occurrence of meteor showers in the Martian atmosphere which may be observable from the Martian surface with cameras on board landers or rovers. Finally, the IMEX model can be used to study the impact hazards imposed by meteoroid impacts on to large-area spacecraft structures that will be particularly necessary for crewed deep space missions.
Potential microbial contamination of Martian moons, Phobos and Deimos, which can be brought about by transportation of Mars ejecta produced by meteoroid impacts on the Martian surface, has been comprehensively assessed in a statistical approach, based on the most probable history of recent major gigantic meteoroid collisions on the Martian surface. This article is the first part of our study to assess potential microbial density in Mars ejecta departing from the Martian atmosphere, as a source of the second part where statistical analysis of microbial contamination probability is conducted. Potential microbial density on the Martian surface as the source of microorganisms was estimated by analogy to the terrestrial areas having the similar arid and cold environments, from which a probabilistic function was deduced as the asymptotic limit. Microbial survival rate during hypervelocity meteoroid collisions was estimated by numerical analysis of impact phenomena with and without taking internal friction and plastic deformation of the colliding meteoroid and the target ground into consideration. Trajectory calculations of departing ejecta through the Martian atmosphere were conducted with taking account of aerodynamic deceleration and heating by the aid of computational fluid dynamic analysis. It is found that Mars ejecta smaller than 0.03 m in diameter hardly reach the Phobos orbit due to aerodynamic deceleration, or mostly sterilized due to significant aerodynamic heating even though they can reach the Phobos orbit and beyond. Finally, the baseline dataset of microbial density in Mars ejecta departing for Martian moons has been presented for the second part of our study.
Measuring the obliquity distribution of stars hosting warm Jupiters may help us to understand the formation of close-orbiting gas giants. Few such measurements have been performed due to practical difficulties in scheduling observations of the relatively infrequent and long-duration transits of warm Jupiters. Here, we report a measurement of the Rossiter-McLaughlin effect for K2-232b, a warm Jupiter (M_P=0.39 M_Jup) on an 11.17-day orbit with an eccentricity of 0.26. The data were obtained with the Automated Planet Finder during two separate transits. The planets orbit appears to be well-aligned with the spin axis of the host star, with a projected spin-orbit angle of lambda = -11.1+/-6.6 deg. Combined with the other available data, we find that high obliquities are almost exclusively associated with planets that either have an orbital separation greater than 10 stellar radii or orbit stars with effective temperatures hotter than 6,000K. This pattern suggests that the obliquities of the closest-orbiting giant planets around cooler stars have been damped by tidal effects.
Each of the giant planets within the Solar System has large moons but none of these moons have their own moons (which we call ${it submoons}$). By analogy with studies of moons around short-period exoplanets, we investigate the tidal-dynamical stability of submoons. We find that 10 km-scale submoons can only survive around large (1000 km-scale) moons on wide-separation orbits. Tidal dissipation destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar Systems moons. A handful of known moons are, however, capable of hosting long-lived submoons: Saturns moons Titan and Iapetus, Jupiters moon Callisto, and Earths Moon. Based on its inferred mass and orbital separation, the newly-discovered exomoon candidate Kepler-1625b-I can in principle host a large submoon, although its stability depends on a number of unknown parameters. We discuss the possible habitability of submoons and the potential for subsubmoons. The existence, or lack thereof, of submoons, may yield important constraints on satellite formation and evolution in planetary systems.
When, in the course of searching for exoplanets, sparse sampling and noisy data make it necessary to disentangle possible solutions to the observations, one must consider the possibility that what appears to be a single eccentric Keplerian signal may in reality be attributed to two planets in near-circular orbits. There is precedent in the literature for such outcomes, whereby further data or new analysis techniques reveal hitherto occulted signals. Here, we perform suites of simulations to explore the range of possible two-planet configurations that can result in such confusion. We find that a single Keplerian orbit with $e>$0.5 can virtually never be mimicked by such deceptive system architectures. This result adds credibility to the most eccentric planets that have been found to date, and suggests that it could well be worth revisiting the catalogue of moderately eccentric confirmed exoplanets in the coming years, as more data become available, to determine whether any such deceptive couplets are hidden in the observational data.
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