[P-PS06] Planetary Sciences

[PPS06-11] The orbital evolution of Phobos in the expanded atmosphere of early Mars under an intense solar EUV radiation

*Kohei Fujita¹, Naoki Terada¹, Shotaro Sakai¹, Kaori Terada¹ (1.Tohoku University)

Keywords:Phobos, Satellite formation, Capture scenario

Mars has two moons, Phobos and Deimos, which have the common characteristics of small orbital inclination and eccentricity. The origin of these moons can be explained by the capture scenario, in which asteroids were captured by the Martian gravity, and the giant impact scenario, in which an object crashed into Mars, and then debris accumulated to form the moons. One of the reasons why the capture scenario is supported is that the reflectance spectra of Phobos are consistent with D- and T-type asteroids, and those of Deimos are consistent with D-type asteroids. However, it is difficult to explain the characteristics of the current orbital inclination and eccentricity in both moons. In the capture scenario, we need to consider the dissipation of orbital energy after capture, since the asteroid has velocity at infinity. An example of the orbital energy dissipation process is atmospheric drag. In a stationary atmosphere, the lifetime of the captured asteroid has been found to be short due to atmospheric drag and it cannot survive for a long time (Hunten, 1979). At that time, it is possible to make the orbit circular, but it is difficult to attenuate the orbital inclination. Therefore, in the previous study, Matsuoka and Kuramoto calculated the orbit taking into account the co-rotation of the proto-Martian atmosphere. The results showed that there is a solution with a relatively long lifetime of the prograde satellite in the vicinity of the geostationary orbit, and that eccentricity and orbital inclination attenuate on a short timescale. However, in order to track the orbital evolution of the captured asteroid after its capture, the evolution of the Martian atmosphere has to be considered. The solar EUV radiation has decreased over time, so the early Martian atmosphere would have gradually shrunk. It is possible that the atmosphere was rotating at angular velocity relatively close to that of Mars when it expanded significantly beyond the co-rotation radius. However, as the atmosphere shrunk, wind systems that deviate from the co-rotation, such as day-night flow seen in the upper atmosphere, were expected to intersect the geostationary orbit and affect the orbit of the moon.
In order to take into account for the change in the solar EUV radiation, we deviate the rotation of the atmosphere by several tens of percent from the co-rotation and perform orbit calculations around the geostationary orbit, where the asteroid is expected to have a longer lifetime. As a result, the lifetime of the captured asteroid is shortened by one order of magnitude in the vicinity of the geostationary orbit when the atmosphere is reduced by 10% from the angular velocity of Mars, and the lifetime is shortened as the deviation increases. On the other hand, in order for the captured asteroid to maintain its orbit, the atmosphere needs to shrink more rapidly than the timescale of orbital evolution. We calculate the orbit taking into account the change in the atmospheric to obtain the lifetime of the captured asteroid in different atmospheric structures of the early Mars in addition to the deviation from the co-rotation since the expanded early Martian atmosphere shrunk as the solar EUV radiation evolved. In this presentation, we will discuss the validity of the capture scenario based on the calculation results of the orbital evolution of Phobos in the above scenarios.