What if Earth had no magnetic field?

Hold a compass, and its needle swings to point north. Earth's magnetism pulls it.

What if Earth had no magnetism at all? Let's find out what it does for us.

A compass needle is a tiny magnet. It swings to line up with Earth's own magnetic field, which is why sailors and hikers can use compasses to find their way. But the field does much more than move needles.

So what if Earth had no magnetic field at all? What would happen to compasses, to animals that sense magnetism, and to the air we breathe? This is a thought experiment: Earth does have a strong field, and NASA says it does not disappear even when its poles swap. Choose a path to explore what the field does and what we can learn from Mars, a planet without one.

NOAA's National Centers for Environmental Information describe Earth's field as very similar to that of a bar magnet at the planet's center, tilted from the rotation axis. NASA explains that this field, extending into space as the magnetosphere, shields the atmosphere from erosion by the solar wind, deflects coronal mass ejection particles and cosmic rays, and provides the basis for compass navigation.

This thought experiment removes the field. Real evidence comes from compass physics, animal navigation studies, and Mars, which lacks a global field today. Statements about a field-free Earth are ideas, not predictions; the lab does not model the magnetosphere, and NASA notes that even during reversals the field weakens without disappearing.

Deep inside Earth, hot melted metal swirls around. This makes Earth a giant magnet.

Its magnetism reaches far out into space, like a big invisible bubble.

NASA explains that Earth's magnetic field is made in its outer core, a layer of melted iron and nickel deep underground. Heat makes the metal churn, and Earth's spin swirls it, creating huge electric currents. Those currents make the field. Scientists call this machine the geodynamo.

The field reaches far into space, forming a bubble called the magnetosphere. The solar wind, a stream of particles from the Sun, squashes the bubble on the Sun's side and stretches it into a long tail on the night side. Most solar particles are kept a safe distance from the ground, many trapped in two doughnut-shaped zones called the Van Allen Belts.

Nearly all of the geomagnetic field originates in the fluid outer core. NASA describes convection churning the molten metal while Earth's rotation organizes it into swirls, generating electric currents hundreds of miles wide: the geodynamo. At the surface the field is close to a dipole, with lines flowing into Earth near the north magnetic pole and out near the south.

The solar wind compresses the magnetosphere on the dayside and draws it into a long magnetotail. NASA notes that most of the incoming energy is trapped far from the surface in the Van Allen Belts. The shield is not perfect: storms cause space weather and aurora. NOAA gives surface intensities of roughly 25,000 to 65,000 nanotesla.

With no Earth magnet, a compass needle might not point north anymore.

Turtles that use magnetism might need other clues to find their way.

A compass works because its needle lines up with Earth's field where you are standing. With no field, the needle would have nothing to line up with, so it might just stay pointing wherever you left it.

Animals that navigate with magnetism might need other clues. And the shield around our air would be gone. NASA's MAVEN spacecraft measured the solar wind stripping gas from Mars, which has no global magnetic field today, so Earth's air might slowly be worn away too, over a very long time. These are ideas built on real evidence, not predictions.

NOAA notes that a compass aligns with the horizontal component of the local field, not with a point; with no field there would be nothing for the needle to align with, so compasses might give no direction at all. Animals with magnetic senses might have to rely on other cues.

The more consequential change might be to the atmosphere. NASA says the magnetosphere shields Earth's air from erosion by the solar wind, and MAVEN measured solar-wind stripping at Mars. By analogy, Earth's upper atmosphere might erode faster without a field. The lab does not model the rate, so treat this as an idea, not a forecast.

The Sun sends out a wind of tiny bits. Earth's magnetism pushes most of them away.

It is like an umbrella that keeps the rain off.

The solar wind is a stream of particles, mostly protons and electrons, that NASA says flows from the Sun at about one million miles per hour. Earth's magnetosphere acts like a gatekeeper, turning most of these particles away before they reach our air.

Mars shows what can happen without that shield. NASA's MAVEN spacecraft measured the solar wind stripping gas from the top of Mars's air at about 100 grams every second, and faster during solar storms. MAVEN's lead scientist, Bruce Jakosky, compared it to a few coins taken from a cash register every day: the loss adds up over time.

The solar wind, mainly protons and electrons at about one million miles per hour, carries a magnetic field. At Mars, NASA explains, that field can generate an electric field which accelerates ions in the upper atmosphere and flings them into space. MAVEN measured a loss of about 100 grams per second, rising during solar storms such as those of March 2015; almost 75 percent of escaping ions left down the tail and nearly 25 percent through a polar plume.

NASA concludes that atmospheric loss to space was likely a major process in changing Mars from warm and wet to cold and dry. On Earth, NASA describes the magnetosphere as a gatekeeper that shields the atmosphere from erosion by the solar wind.

Earth's magnet changes slowly over a very long time. Its north and south have even swapped places before!

When that happens, Earth still keeps a magnetic field. Plants and animals carried on.

Earth's field is always changing a little. NASA says the north magnetic pole has drifted more than 1,100 kilometers since it was first located in 1831, and its speed has risen to about 55 kilometers a year.

The north and south magnetic poles have even swapped places, which scientists call a reversal. It has happened 183 times in the last 83 million years, most recently about 780,000 years ago. During a reversal, the field weakens and gets jumbled, but it does not disappear. Fossils from the last big reversal show no big changes in plants or animals.

The field varies on every timescale. Since James Clark Ross located the north magnetic pole in 1831, NASA says it has drifted north-northwest more than 1,100 km, speeding up from about 16 to about 55 km per year. Over the past 200 years the field has weakened about nine percent, yet it is twice as intense as its million-year average.

Paleomagnetic records show 183 reversals in the last 83 million years, spaced on average about 300,000 years apart, the last about 780,000 years ago. NASA notes reversals take hundreds to thousands of years, the field weakens but does not vanish, and geologic and fossil records from past reversals show no major extinctions. NOAA adds that during a reversal compass navigation would be difficult and migratory animals might have problems.