How the Earth-Mars Binary Would Affect Each Other

[Jdailey1991]

In my last question, I asked on, in an alternate universe, how far either Venus or Mars would have to orbit Earth as a binary planet system to look the same size in the sky as our view of the moon, which, as I have finally discovered to be measured at half a degree wide.

I have decided to focus the binary question on Earth being orbited by Mars, which is closer in size to the moon at 6600 kilometers wide and almost 11% of Earth's mass.

For Mars to measure half a degree wide from our view of Earth's sky would mean that Mars would orbit Earth from a distance of 350,000 miles away. This poses a list of questions:

1. Would Earth still spin at 24 hours a day, 365 days a year?
2. Would Mars have to spin at the same speed?
3. Would Mars stabilize Earth's axial tilt in place of our OTL moon, thus ensuring that it still has seasons?
4. Will Mars have clearly defined seasons, too?
5. Would both have to rotate in the same direction (counterclockwise)?

 

[Yora]

1a. As I understand it, the day rotation speeds of two objects that orbit a common center of gravity goes down with time because of several mechanism. So as time passes, days gradually get longer. The current speed of their rotiation depends on what their speed was when they formed a pair, and how much time has passed since then. Since both numbers will never come up in any kind of story, you can set the day length at whatever you want, and just assume those two factors are whaever would be needed to get that day length right now.

1b. The time it takes to orbit around the Sun depends directly at the distance from the Sun, completely independent from the size and mass of any orbiting object. At the distance of the Earth and the Moon from the Sun, there is only a single speed that results in an orbit. Any faster and the object will travel farther away, any slower and it will travel closer to the Sun. So if your star is the Sun, and your planet gets the same amount of light and other radiation, the length of a year has to be the same as it is for Earth. Though if you make your days longer, it will of course be less than 365 days.

2. The rotation of the two objects should be independent from each other, unless the smaller one becomes tidally locked or falls into orbital resonance with the larger. Again, this depends on how fast they were rotating originally and how much time has passed since then.

3. I don't think the Moon is required to stabilze any tilt to create seasons. But whatever effect the Moon has on Earth, it would be even stronger with Mars because Mars is much more massive.

4. The axes of both bodies don't have to be aligned with each other and probably could have any orientation. You can set it at whatever you want, and get as strong seasons as you like.

5. They could have the same direction of rotation, and they probably would, as something like 98% of all bodies in the solar system do. But they don't have to. You can have one go in the other direction of the other if you want to.

Basically, you can have almost anything you want, ass long as the masses distances, and ages of the bodies are not specified as being any specific values. There should almost always be one possible configuration in which things on your world look just like you want them to be. The only thing I can think of now that wouldn't work is having multiple large moons around a small planet. But as long as you have only two objects close to each other, or the masses of the objects are very different, almosy anything is possible in some way.