In the first unmanned Mars mission, sensors analysing the ground detected components usually found in citrus fruits. Scientists thought that some kind of fruit might grow on Mars. However, they realized afterwards that it was an error: one of the researchers handling the results had eaten a tangerine shortly before. This is just one of the hundreds of disappointments we have bumped into when trying to prove that Mars is our twin. So we’ll not get too excited: however, there are reasons to believe that Mars is our stepbrother, at least.
One of the things we have in common is the existence of water on the planet surface. Both our planet and Mars have large amounts of H₂O, although in the Red Planet it has solid state, i.e. ice.
Another shared feature is atmospheric composition. Both the Martian atmosphere and ours are composed of carbon dioxide, nitrogen, oxygen and argon, although proportions of each gas differ greatly. In the Red Planet, the most abundant atmospheric gas is carbon dioxide (95.32 %), nitrogen being the most abundant in Earth (78.1 %).
The internal structure of Mars and the Earth is also a common point. Both planets are divided into crust, mantle and nucleus. The difference is that Mars’ nucleus is solid, while the Earth’s is molten rock. This means Mars does not have a magnetosphere as the Earth’s: the one of Mars is far simpler and less extensive. In any case, days on Mars last almost as much as they do on Earth: 24 hours… and 40 minutes.