It sublimes back into a gas during the summer months when the planet moves closer to the Sun again. Mars has two moons, Phobos and Deimos. Both are small rocky bodies which resemble asteroids. A fundamental property of most particles of matter . The amount of mass contained in an object contributes to the strength of its gravitational field.
Every mass attracts every other mass in the universe according to Newton's law of gravity.
Along with length and time, mass is one of the properties which describe anything in the universe. It was this realisation of the significance of mass by Isaac Newton and others in the 17th century that helped usher in the modern scientific era. The identical inertial mass is a measure of the resistance to acceleration when experiencing a force. Mass increases with speed so the rest mass is important. Mass can be de-materialised into energy, for instance the complete annihilation of a particle and its antiparticle produce two massless photons.