Population: 5.6 Billion
Capitol: UN Mars
While Luna's terraformation had begun almost immediately before the relocation of Old Earth, Mars' terraformation had been in planning for nearly 100 years before, and expeditionary ships had already been set up to slowly begin the process. When the relocation had undergone its planning, Mars' terraformation had sped up to full capacity and by the time the ships came from Old Earth (it took 1 UY for the ships to start arriving), Mars was nearly fully terraformed.
Mars is seen as a desirable place to go, unlike Luna, which had gotten a bad reputation since the start of the relocation as being a terribly busy port planet. By UY 10, that isn't as much the case anymore as Luna's population evened out and nearly all of them came to Mars. As Mars is much further along in its terraformation than Luna is, plus the fact that there is more area to occupy, Mars is seen as the new home for the majority of the human population.
The government is akin to a parliamentary system. An appointed group of leaders from Old Earth convene in a diplomatic manner and create policies. There are fifteen head legislators of the planet who also administer specific parts of the government. There is not one figurehead in this form of government, and decisions are always seen as a group effort in deliberation. The government is established and authorized by the United Nations of the Old Earth. Mars' sister planet, Luna, is also ran in an identical fashion, however the people that run each individual planet are different.
Mars imports many of the resources that are mined from Old Earth. Many of the businesses that had been established on Old Earth re-established themselves on Mars, and began exporting their various goods on Mars. Luna, by contrast, is a fraction of Mars in terms of business exports.