11:00 〜 13:00
[PPS04-P03] フォボス・ディモス分裂説の検討
キーワード：火星衛星、起源、MMX
About the origin of the Martian moons (Phobos and Deimos), there are two competing hypotheses: the first is called the capture hypothesis, in which two small bodies were gravitationally captured by Mars, and started orbiting around Mars. The second one, called the impact hypothesis, is that a body with a few percent of the Mars mass struck Mars obliquely, and the two moons were formed through the re-accumulation of the impact debris. The reflection spectrum of the Martian moons supports the capture theory, but their orbits support the impact hypothesis. The origin of the Martian moons is still unresolved.
Recently, a new study (Bagheri et al. 2021) was reported that Phobos and Deimos may have been split from a single moon. Considering the tidal interaction with Mars, they calculated the orbits of Phobos and Deimos back in time from their current orbits. They found that their orbits got intersected around the corotation radius (~ 6 times Martian radius) 1–3 Gyr ago depending on the tidal parameters. This result suggests that a single parent moon may have been split into two bodies, for example, by an asteroid impact onto it, and they became Phobos and Deimos.
However, it has not been investigated whether the orbits of two split moons were fully separated from each other before two moons collide again. Here, considering the orbits of the two moons just after the separation of a single moon as the initial conditions, we calculate their orbital evolution and evaluate the likelihood of their re-accretion to a single new moon. We consider many initial settings with random separation directions of the two moons. We also study the dependence on the initial separation velocity of the two moons. In addition to the two split moons, we also consider fragments that would be produced during the separation of two moons from a single moon.
As a result, we found that the two moons collide again in 1000 years for all cases with low separation velocity. The results do not change even when fragments are taken into account. When the separation velocity is high, the two split moons have a 70% chance of collision in 1000 years, but for the remaining 30% cases, one of the moon is ejected out of the Martian gravitational field without collision. This time scale of 1000 years is much shorter than the timescale of 1 million years when the orbits of the two moons change due to the tidal interaction with Mars. Therefore, we conclude that it is highly unlikely that one parent moon separated into two to become the present Phobos and Deimos.
The origin of the Martian moons needs further investigation, but it is expected that it will be solved when the samples of the Martian moons are obtained by the MMX (Martin Moons eXploration) mission led by JAXA.
Recently, a new study (Bagheri et al. 2021) was reported that Phobos and Deimos may have been split from a single moon. Considering the tidal interaction with Mars, they calculated the orbits of Phobos and Deimos back in time from their current orbits. They found that their orbits got intersected around the corotation radius (~ 6 times Martian radius) 1–3 Gyr ago depending on the tidal parameters. This result suggests that a single parent moon may have been split into two bodies, for example, by an asteroid impact onto it, and they became Phobos and Deimos.
However, it has not been investigated whether the orbits of two split moons were fully separated from each other before two moons collide again. Here, considering the orbits of the two moons just after the separation of a single moon as the initial conditions, we calculate their orbital evolution and evaluate the likelihood of their re-accretion to a single new moon. We consider many initial settings with random separation directions of the two moons. We also study the dependence on the initial separation velocity of the two moons. In addition to the two split moons, we also consider fragments that would be produced during the separation of two moons from a single moon.
As a result, we found that the two moons collide again in 1000 years for all cases with low separation velocity. The results do not change even when fragments are taken into account. When the separation velocity is high, the two split moons have a 70% chance of collision in 1000 years, but for the remaining 30% cases, one of the moon is ejected out of the Martian gravitational field without collision. This time scale of 1000 years is much shorter than the timescale of 1 million years when the orbits of the two moons change due to the tidal interaction with Mars. Therefore, we conclude that it is highly unlikely that one parent moon separated into two to become the present Phobos and Deimos.
The origin of the Martian moons needs further investigation, but it is expected that it will be solved when the samples of the Martian moons are obtained by the MMX (Martin Moons eXploration) mission led by JAXA.