Mercury: The Smallest Planet and Its Extreme Days

Mercury is the planet of extremes.

It is the smallest of the eight, the closest to the Sun and the fastest in its orbit,

and its surface swings between some of the highest and lowest temperatures found on any planet.

It is also one of the least familiar worlds, because it is hard to see from Earth and hard to reach by spacecraft.

Here is what we know about this scorched little planet, why its days and years behave so strangely,

and how you can catch it low in the twilight sky.

Mercury by the Numbers

A few basic figures help put Mercury in perspective:

• Size: about 4,900 km (3,000 miles) across, a little over a third of Earth's width and only slightly larger than our Moon.
• Distance from the Sun: about 58 million km (36 million miles) on average, roughly 0.4 times Earth's distance.
• Year: about 88 Earth days to complete one orbit.
• Gravity: about 38 percent of Earth's, so a 100-pound backpack would feel like 38 pounds.
• Moons: none. Mercury and Venus are the only planets without any.

Two moons, Jupiter's Ganymede and Saturn's Titan, are actually wider than Mercury.

Mercury is much denser, though, so it outweighs both of them by a wide margin.

A Year Shorter Than a Day

Mercury races around the Sun, but it turns on its axis very slowly.

One full rotation takes about 59 Earth days, which works out to exactly three spins for every two trips around the Sun.

Astronomers call this a spin-orbit resonance, and it is the result of the Sun's gravity gradually locking Mercury into this rhythm.

The combination creates something odd.

If you stood on Mercury, the time from one sunrise to the next would be about 176 Earth days, which is two full Mercury years.

In other words, a single solar day there lasts longer than its year.

Mercury's orbit is also the most stretched-out of any planet, carrying it from about 46 million km to about 70 million km from the Sun.

Near its closest point, the planet's orbital speed briefly outpaces its spin.

From certain spots on the surface, the Sun would appear to rise, pause, slip back toward the horizon and then rise again.

From Scorching Heat to Deep Freeze

Seen from Mercury, the Sun looks two to three times as wide as it does from Earth, and it pours down far more energy.

On the daytime side, surface temperatures can climb to about 430 °C (800 °F), hot enough to melt tin and lead.

Mercury has almost no atmosphere to hold that heat, so it escapes straight back into space once the Sun sets.

During the long night, the ground can plunge to about −180 °C (−290 °F).

That swing of roughly 600 °C (over 1,000 °F) between day and night is the largest of any planet.

Despite all that heat, Mercury is not the hottest planet. That title belongs to Venus,

which is farther from the Sun but wrapped in a thick blanket of carbon dioxide that traps heat day and night.

An Atmosphere Too Thin to Count

What Mercury has instead of a true atmosphere is an exosphere: a very sparse haze of atoms such as sodium, potassium and hydrogen.

They are knocked loose from the surface by sunlight, the solar wind and tiny meteorite impacts,

and they drift away almost as fast as they are replaced.

Ice on the Planet Closest to the Sun

One of the biggest surprises about Mercury is that it has ice.

Because the planet's axis is barely tilted, the floors of some craters near its poles never see sunlight at all.

These permanently shadowed pockets stay colder than about −170 °C, cold enough for water ice to survive for very long periods.

Radar from Earth first hinted at bright, icy deposits in these craters, and later observations from orbit confirmed that water ice is present.

Scientists think it may have been delivered by comets and water-rich asteroids that struck Mercury over billions of years.

A Shrinking World With an Oversized Core

Mercury is the second densest planet, after Earth, because it is packed with iron.

Its metallic core extends to about 85 percent of the planet's radius, a far bigger share than Earth's core.

Some scientists think a giant collision early in Mercury's history stripped away much of its original rocky outer layers, though the cause is still debated.

As that large core slowly cooled over billions of years,

the whole planet contracted. The crust responded by buckling into long, curving cliffs called scarps,

some hundreds of kilometers long and more than a kilometer high. Mercury is, quite literally, a wrinkled world.

Its surface also records a violent past.

The Caloris Basin, one of the largest impact features in the solar system, is about 1,500 km (900 miles) across.

The impact that formed it was so powerful that it seems to have jumbled the terrain on the opposite side of the planet.

What Spacecraft Have Revealed

Mercury is surprisingly difficult to visit. A spacecraft falling toward the Sun picks up enormous speed,

so it must use complex paths and repeated planetary flybys to slow down enough to stay near Mercury.

Mariner 10 was the first to get a close look,

flying past the planet three times in 1974 and 1975 and mapping part of its surface.

It also discovered that Mercury has a weak global magnetic field, which surprised scientists who had expected such a small planet to have cooled completely.

NASA's MESSENGER became the first spacecraft to orbit Mercury, arriving in 2011.

Over about four years it mapped the entire surface, confirmed the polar ice,

measured the planet's shrinkage and found unexpectedly high amounts of volatile elements such as sulfur and potassium.

The mission ended in 2015 when the spacecraft, out of fuel, was deliberately allowed to crash into the surface.

How to Spot Mercury in the Sky

Because it orbits so close to the Sun, Mercury never appears far from it in our sky.

You will only find it low in the west shortly after sunset or low in the east shortly before sunrise,

and only for a few weeks at a time when it is near its greatest angle from the Sun.

1. Check a sky app or almanac to learn when Mercury is at its best in the evening or morning sky.
2. Find a clear, flat horizon, such as a beach, field or hilltop, since the planet sits low in bright twilight.
3. Look about 30 to 45 minutes after sunset, or before sunrise for a morning appearance, for a small, steady point of light.
4. Use binoculars only after the Sun has completely set, and never sweep them anywhere near the Sun.

Occasionally, Mercury passes directly in front of the Sun as seen from Earth, an event called a transit,

which happens roughly a dozen times per century. Watching one requires a telescope with a certified solar filter.

Never look at the Sun without certified solar filters, even when a tiny planet is crossing its face.

Small Planet, Big Surprises

Mercury may be the smallest planet, but it packs in a remarkable amount of strangeness: days longer than years,

ice beside a blazing Sun and a body that has shrunk as it cooled. Catch it in the twilight once and you will have seen a world that relatively few people ever notice.