A neighbor worth studying
Mars is the fourth planet from the Sun, and only Venus comes closer to Earth at its nearest approach. But distance is only part of why Mars draws so many missions. It is also the planet whose surface is easiest to study. Unlike Venus, which has crushing pressure and extreme heat at the ground, or the gas giants, which have no solid surface, Mars has a rocky surface that spacecraft can land on and explore with wheels, cameras and instruments.
Some familiar features
Mars is about half the diameter of Earth, with a surface gravity around 38 percent of ours. A Martian day, called a sol, lasts about 24 hours and 37 minutes, close to an Earth day. Its axis is tilted at about 25 degrees, similar to Earth’s, so Mars has seasons, although they are longer because a Martian year lasts roughly 687 Earth days. It has polar caps made of water ice and frozen carbon dioxide, clouds, dust storms, volcanoes, canyons and dry riverbeds.
It would be wrong to call Mars Earth-like in an everyday sense. The atmosphere is very thin, mostly carbon dioxide, with a surface pressure well under one percent of Earth’s. Temperatures are typically far below freezing, though they vary by place and season. There is little protection from radiation, and liquid water is not stable on the surface today. The thin air also means there is only a weak greenhouse effect to hold in heat.
Signs of a wetter past
The main scientific draw is evidence that Mars was once different. Orbiters have photographed branching valleys and channels that look carved by flowing water. Rovers have found rounded pebbles, layered sediments and minerals that normally form in the presence of water. Some regions show features resembling ancient lake beds and river deltas. Researchers generally conclude that Mars had liquid water on its surface billions of years ago, although how long it lasted and how warm and wet the planet was are still debated.
This matters for a basic question: if conditions were once suitable, did anything ever live there? No evidence of life on Mars has been confirmed. Scientists therefore look for ancient habitable environments and for chemical traces that life could have left behind. The wider context is discussed in our look at the question of alien life.
Understanding how planets change
Even without the life question, Mars is a natural laboratory. It started out with some similarities to early Earth, yet it ended up cold, dry and thin-aired. Learning how its atmosphere was lost, perhaps in part because it has no global magnetic field today and has weaker gravity, helps scientists understand how planets evolve. Mars also seems to lack the plate tectonics that constantly reshape Earth’s crust, so its surface preserves a very old geological record, including some features that are billions of years old.
Mars has remarkable landforms. Olympus Mons is the tallest known volcano in the solar system, and Valles Marineris is a canyon system that stretches thousands of kilometers. These help researchers compare geological processes across worlds.
What the missions have done
Mars has been visited by many spacecraft, from flybys to orbiters to landers. Mariner 4 sent the first close-up pictures in 1965. The Viking landers of 1976 ran experiments to test for microbes in the soil, and the results were not seen as clear evidence of life.
Rovers have since become the central tool. Sojourner (1997) was small and short-lived. Spirit and Opportunity landed in 2004 and found evidence of past water, and Opportunity kept operating for many years beyond its planned lifetime. Curiosity, which arrived in 2012 in Gale Crater, found that the area once had conditions that could have supported microbial life. Perseverance landed in 2021 in Jezero Crater, an old lake basin with a delta, and it is collecting rock samples in sealed tubes for possible return to Earth. It carried a small helicopter, Ingenuity, which showed that powered flight is possible in the thin air. China’s Zhurong rover landed in 2021 as well. Several orbiters from the United States, Europe, India, the United Arab Emirates and China study the surface, atmosphere and weather from above, and they also relay data from the rovers.
Why bring samples back?
Instruments on a rover are limited by size and power. Laboratories on Earth can use larger and more sensitive equipment and can repeat tests. That is why sample return is a long-sought goal, though it is technically complex and expensive, and plans for it have been repeatedly reviewed and changed.
Practical reasons
Mars is also reachable. The best launch opportunities come about every 26 months, when Earth and Mars are well placed relative to each other, and a one-way trip typically takes around seven months. Its atmosphere, while thin, can help slow a spacecraft, but it is not thick enough for parachutes alone, so landers combine heat shields, parachutes and rockets. Landing is hard, and not every attempt in history has succeeded. Weather conditions, dust and the solar energy available also affect how long machines last. Solar panels can be coated by dust, which limits some missions, while others use nuclear-powered generators.
Could people go?
Crewed missions are often discussed, but they involve major unsolved challenges: a long journey, exposure to radiation, life support, landing a heavy vehicle, and return. Scientists and engineers disagree about timelines, and no crewed Mars mission is scheduled with certainty. Robotic missions continue to provide information that would help plan any future human visit, such as where resources like water ice might be found.
Why it keeps being a favorite
Mars combines a surface we can reach, a record of dramatic change, and the intriguing possibility that it once held habitable places. It is close enough to visit regularly and different enough to teach us something about our own planet. The open questions about its past water and potential life will likely keep it on mission lists for a long time.