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13

Uranus rotates once every 0.718 days. 11 of its satellites have a shorter orbital period. These are inner satellites of Uranus which are roughly in the equatorial plane of Uranus. I don't quite understand what direction they orbit in compared to the direction in which Uranus spins. Neptune rotates once every 0.671 days. 5 of its satellites have a shorter ...


7

Jupiter has a sidereal rotation period or day of 9.925 hours. https://en.wikipedia.org/wiki/Jupiter The two innermost moons of Jupiter, Metis and Adrastea, have orbital periods of 7 hours 10 minutes 16 seconds and 7 hours 15 minutes 21 seconds respectively. https://en.wikipedia.org/wiki/Moons_of_Jupiter#List Saturn has a sidereal rotation period or day of 10 ...


6

Taking Mars' average distance from the sun of 1.52 AU, the sun would be 43% as bright. Phobos has an albedo of about 0.071 which is pretty dark. Darker than the Moon with an average albedo of about 0.12, so it reflects about 59% as well as the Moon does. That gives it a brightness to area of just over 25% the brightness of the moon. Phobos is an odd ...


5

No. Phobos is small - just 11 Km across - the size of a small city. Mars (and Phobos) is so far away that a Phobos impact will not affect Earth much. (Mars ranges from 100 billion meters away to nearly 400 billion meters depending on its and Earth's position in their orbits.) When Phobos hits the Roche limit as it will break up and become a thin light-grey ...


4

I'd caution that absolute statements are not good things in science. When Sagan says a natural satellite cannot be hallow, it is an absolute statement, but there's no way to 100% prove it true. That being said, everything we know about planet/satellite formation tells us that the chances of a satellite forming as a hollow structure (or becoming hollow ...


3

Sometime before now and "tens of millions of years". Phobos currently orbits at a radius of about 9400km The theoretical Roche limit is different for rigid and fluid bodies. If Phobos were a fluid body, it would already have passed the Roche limit at 10500km. This is because fluid bodies will deform into an ellipse pointing towards the planet, ...


3

You're right, Mars' moons likely did not form with from the same dust mass as it; their spectra, albedo, and density are suspiciously similar to those of C- or D-type asteroids. Thus, Mars presumably captured Phobos and Deimos from the asteroid belt. However, the origin of the Martian moons is quite controversial. Burns (1992) notes that capture requires ...


3

A large moon (the size of a planet) would be modeled by the fluid model, meaning friction and tensile strength are too weak to modify the shape of the moon significantly, shape is determined by rotation, vortices, self-gravitation, and tidal forces. Smaller moons, like Phobos would likely follow a rubble pile model. On circular orbits tidal heating doesn't ...


2

About Mars moons, eclipses and transits (from NASA - Curiosity Captured Two Solar Eclipses on Mars: Phobos doesn't completely cover the Sun, so it would be considered an annular eclipse. Because Deimos is so small compared to the disk of the Sun, scientists would say it's transiting the Sun. Phobos does pass in front of Deimos but the current models of ...


2

Bodies typically progress outward rather than inward. (See Why is the Moon receding from the Earth due to tides? Is this typical for other moons? .) The only orbiting bodies that might approach are ones that orbit faster than the main object spins, IOW, closer than synchronous orbit. Even then they could recede if locked into resonance w/ other bodies, eg, ...


1

Astronomers are not still completely sure that these satellites are captured asteroids (see for example this paper). However, in case they are, it's true that their eccentricity should be much higher. There are several theories that provide scenarios for the circularization of the orbits (the most recent I found is this one, which invokes impacts on the ...


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