Mars: Red Deserts, Ancient Water and Giant Volcanoes

Mars has fascinated skywatchers for as long as people have looked up. Its reddish glow stands out among the stars,

and for more than a century it has been the planet most often imagined as a home for life beyond Earth.

Spacecraft have turned that fascination into real knowledge. Today we know Mars as a cold, dusty desert with record-breaking volcanoes,

a canyon that dwarfs any on Earth and clear signs that water once flowed across its surface.

Here is what makes the Red Planet so compelling.

Mars at a Glance

• Size: about 6,800 km (4,200 miles) across, roughly half the width of Earth.
• Distance from the Sun: about 228 million km (142 million miles) on average, or about 1.5 times Earth's distance.
• Day: just over 24 and a half hours, remarkably close to our own.
• Year: about 687 Earth days, so each Martian season lasts roughly twice as long as ours.
• Gravity: about 38 percent of Earth's.
• Atmosphere: mostly carbon dioxide, with a surface pressure less than 1 percent of Earth's.

Because its axis is tilted by a similar amount to Earth's, Mars has seasons too.

Its thin air holds little heat, though, so temperatures average around −60 °C (−80 °F).

A summer afternoon near the equator can reach about 20 °C (70 °F), while winter nights near the poles plunge below −120 °C (−190 °F).

Why Mars Is Red

The color comes from iron.

The Martian surface is rich in iron minerals, and over time they have reacted to form iron oxides, the same family of compounds as rust.

Fine, rusty dust coats much of the planet and hangs in the air, tinting the whole landscape red and orange.

That dust is always on the move. Swirling dust devils scour tracks across the ground,

and in some years regional storms merge into dust storms that wrap around the entire planet for weeks.

A planet-wide storm in 2018 blocked so much sunlight that the solar-powered Opportunity rover lost power,

ending a mission that had lasted more than fourteen years.

Giant Volcanoes and a Colossal Canyon

Olympus Mons

Mars is home to Olympus Mons, the largest known volcano in the solar system.

It rises about 22 km (nearly 14 miles),

roughly two and a half times the height of Mount Everest above sea level,

and it is about 600 km (370 miles) wide.

Its slopes are so gentle and broad that someone standing on them would not see the summit, which would lie beyond the horizon.

Olympus Mons sits at the edge of the Tharsis region, a vast volcanic plateau with several other giant volcanoes.

They grew so large partly because Mars lacks moving tectonic plates.

On Earth, plates slide over hot spots and create chains of smaller volcanoes, like the Hawaiian Islands.

On Mars, the crust stayed put, and lava piled up in the same place for an immensely long time.

Lower gravity also helped the mountains grow tall without collapsing.

Valles Marineris

Next to Tharsis lies Valles Marineris, a system of canyons about 4,000 km (2,500 miles) long and up to about 10 km (6 miles) deep.

On Earth, it would stretch roughly across the continental United States.

Unlike the Grand Canyon, it was not carved mainly by a river;

it is largely a great crack in the crust that formed as the Tharsis region bulged upward,

later widened by landslides and erosion.

The Evidence for Ancient Water

Mars today is far too cold, and its air far too thin, for liquid water to last long on the surface.

Yet orbiters and rovers have found overwhelming evidence that water was once common. Key clues include:

1. Dry river valleys and channels that branch like river systems on Earth, some carved by enormous ancient floods.
2. River deltas and lakebeds. NASA's Curiosity rover, which landed in Gale Crater in 2012, found layered rocks laid down in an ancient lake. Perseverance landed in Jezero Crater in 2021, beside the remains of a river delta.
3. Water-formed minerals, such as clays and sulfates, which form only in the presence of liquid water.
4. Rounded pebbles like those found in streambeds on Earth, smoothed by flowing water.
5. Ice that is still there. The polar caps contain large amounts of water ice, and the Phoenix lander dug into frozen ground just below the surface in 2008.

Together, these clues suggest that billions of years ago Mars was warmer and wetter, with rivers,

lakes and perhaps even a shallow sea in its northern lowlands.

How long those conditions lasted, and whether the climate was warm for long stretches or only in bursts, is still debated.

Where Did the Water Go?

Mars likely lost much of its water along with its atmosphere.

Early in its history, the planet's global magnetic field faded. Without that shield,

the solar wind gradually stripped gas from the upper atmosphere and carried it into space,

a process spacecraft have measured directly.

As the air thinned, the surface grew colder and drier. Some of the remaining water escaped to space as well,

while a significant amount appears to be locked away as ice in the polar caps and beneath the surface.

Just how much water is still hidden underground is an active area of research.

Two Tiny Moons

Mars has two small, lumpy moons named Phobos and Deimos.

Phobos is only about 22 km (14 miles) across, and Deimos is even smaller.

Their origin is uncertain: they look like captured asteroids,

but their orbits fit better with the idea that they formed from debris thrown up by a large impact on Mars.

Phobos orbits so close to Mars that it circles the planet about three times a day, and it is slowly spiraling inward.

In tens of millions of years, it is expected to either break apart into a ring or crash into the surface.

Robots on the Red Planet

Mars has been explored more thoroughly than any planet besides Earth.

Mariner 4 sent back the first close-up pictures in 1965, revealing a cratered world.

The two Viking landers touched down in 1976 and ran the first experiments designed to search for life.

Sojourner became the first rover in 1997, followed by Spirit and Opportunity in 2004.

In 2021, a small helicopter called Ingenuity, carried by Perseverance, made the first powered, controlled flight on another planet.

Seeing Mars for Yourself

Mars is easy to recognize by its steady orange-red color.

Its brightness changes a great deal because the distance between Earth and Mars varies so much.

It looks best around opposition, when Earth passes between Mars and the Sun, which happens about every two years and two months.

Near opposition, Mars rises around sunset and stays up all night, often outshining nearly every star.

A modest telescope may then show a white polar cap and dark surface markings, though the planet's small disk rewards patience and steady air.

Final Thoughts

Mars is a world caught between past and present: a frozen desert that still carries the fingerprints of rivers and lakes.

Whether life ever took hold there remains one of the biggest open questions in science.

The next time that orange point of light catches your eye, you will know just how much history it holds.