|Part of the Solar System|
Mars is the second smallest of the planets in the Solar System, 1.5 AU away from the Sun. Its overall surface area is only slightly less than that of all land on Earth. It exhibits an axial tilt of 25.2°, similar to that of Earth. The length of a day is also similar: 24 hours, 39 minutes, 35 seconds in duration.
The surface of Mars is primarily that of iron oxide (Fe2O3, otherwise known as rust). The martian atmosphere consists primarily of carbon dioxide, with fluctuating polar caps of dry ice (solid CO2). Unlike Earth, Mars does not have a magnetosphere to deflect the solar wind. As a result, the density of the Martian atmosphere is significantly reduced, resulting in an average surface pressure of only 0.6 kPa—not even 1% that of Earth’s atmosphere.
Mars features two tiny moons—Phobos and Deimos—both thought to be captured asteroids. Phobos is especially unusual in that it orbits faster than Mars rotates. It is predicted in 50 million years that the moon will either crash into the planet’s surface or be torn apart by tidal forces when it enters the planet’s Roche limit. Due to the moons’ extreme proximity to Mars, on the surface they each cannot be seen beyond 70.4° (for Deimos) or 82.7° (for Phobos) away from the Martian equator.
As an outer planet, Mars never appears less than 87% illuminated from Earth. On favorable oppositions, it can reach a peak apparent magnitude of -3.0, enough to outshine Jupiter. However, due to the eccentricity of Mars’ orbit, it has a wide range in apparent size at opposition, ranging from 12 to 23 arcseconds. It also features a retrograde motion period lasting 72 days long (centered on opposition), the longest of the outer planets relative to Earth.
Mars can be a very interesting object to observe, a personal Solar System favorite of mine. It features a dynamic surface (but not completely concealed, as in the case of Venus) with numerous albedo features visible at various orientations, not to mention just how humbling of an experience it is to be able to keep watch on the weather of another planet!