Mars could have lived, even without a magnetic field

When we look at planet Earth, we see a blue world with a mix of oceans and continents, clear and cloudy skies covered in photosynthetic greenery, and many other signatures of life’s presence. Mars, for comparison, is a much smaller planet that’s largely taken on a rusted red color, possessing only a thin atmosphere and with no signs of liquid water on its surface: only frozen ices. While both Earth and Mars may have formed from similar raw ingredients in terms of the atoms that make up these worlds, the tales of their planetary evolution clearly diverged long ago, with Earth becoming life-friendly early on and remaining so, while Mars lost the last traces of its surface liquid water more than 3 billion years ago.

Why are these worlds so different, today?

The standard explanation is that Mars, being much smaller, radiates the heat away from its core far more efficiently than Earth does. This causes the core to cool inside Mars much more quickly than the core inside Earth, causing any magnetic dynamo generated by Mars to fade away swiftly compared to the lifetime of the planet, whereas Earth’s magnetic dynamo persists to the present day. The loss of a planet-wide magnetic field then makes it much easier for Mars’s pre-existing atmosphere to be stripped away by the solar wind and radiation, whereas Earth’s magnetic field continues to protect us (allowing us to keep our atmosphere) all the way to the present.

That line of thinking, accepted for so long, may need to be re-examined when it comes to exoplanetary systems. Here’s the science behind why.

This cutaway view of the four terrestrial planets (plus Earth’s moon) shows the relative sizes of the cores, mantles, and crusts of these five worlds. Despite the fact that the Earth is only 5% larger in diameter than Venus, it has more mass than Mercury, Venus, Mars, and the Moon combined. If you could pass through the Earth’s interior as a projectile that didn’t interact electromagnetically with the Earth, you would see your trajectory change slightly as you transitioned across one internal layer to another.

The science behind the mainstream line of thought makes a lot of sense from a theoretical point of view. When planets form, a lot of heat gets trapped inside of them: partly from the gravitational contraction of the particles that form them, and partly from the decays of radioactive materials that make up the planet’s interior. There’s only one way for that planet’s heat to escape: by radiating away through the planetary surface. Because planets form into spherical shapes, the total amount of heat trapped inside is proportional to the mass of the planet (which scales as its radius cubed), but the rate at which that heat gets radiated away is proportional to the surface area of the planet (which only scales as radius squared).

In other words, the smaller your planet is, the more quickly it radiates its internal heat away. And the more quickly it radiates its internal heat away, the faster it cools. If you have a liquid/fluid core that is simultaneously:

• rotating,
• convecting,
• and electrically conducting,

you can generate a magnetic field that persists over astronomical and/or geological timescales through the dynamo mechanism. This mechanism is thought to power Earth’s magnetic field as well as the magnetic fields of the gas giant worlds in our Solar System. It’s also thought to have once powered Mars’s magnetic field but no longer and not for more than 3 billion years: due to Mars’s tiny size, it cooled, and its core dynamo ceased to exist.

This animation shows Mars’s transition from a wet, ocean-rich world to the dry, barren red planet that we see today. This entire process took roughly 100 million years, and occurred shortly after Mars’s internal core dynamo ceased, leading to the solar wind stripping the Martian atmosphere away, according to the conventional story.

Then, the story goes, without a planet-wide magnetic field to protect it, the history of Mars and Earth began to diverge. The Sun is a constant emitter: not only of radiation, but also of particles in the form of the solar wind. Made up largely of protons, electrons, and heavier atomic nuclei, the solar wind constantly streams away from the Sun, with the direction of the particles determined by the Sun’s magnetic field. Up close, the Sun’s magnetic field is full of switchbacks and reconnections, but at greater distances from the Sun, its magnetic field simply points away from the Sun in all directions. The solar wind particles follow those lines, often orbiting around them in helix-like fashion.

With a planet-wide magnetic field driven by a dynamo, Earth’s magnetism interacts with the solar wind, including the Sun’s large-scale magnetic field and the charged particles it carries. Most of these particles, typically, are diverted away from the Earth, where they fail to strip our atmosphere away. The ones that do fall down onto Earth are largely funneled by our planet’s magnetic field down around the polar areas, where they largely create the harmless features of the Aurora Borealis and the Aurora Australis. This picture of a “protective, planet-wide magnetic field” is generally held to be the underlying cause behind why our atmosphere, unlike that of Mars, hasn’t yet been stripped away.

When charged particles are sent toward Earth from the sun, they are bent by Earth’s magnetic field. However, rather than being diverted away, some of those particles are funneled down along Earth’s poles, where they can collide with the atmosphere and create aurorae. The largest events are driven by CMEs on the Sun, but will only cause spectacular displays on Earth if the ejected particles from the sun have the correct component of their magnetic field anti-aligned with Earth’s magnetic field.

On Mars, the story appears to be quite different, however. We know it maintained a reasonably thick atmosphere for a long time: perhaps a billion or even 1.5 billion years. We see the evidence of a watery past everywhere on geological timescales, from:

• sedimentary rock,
• dried-up riverbeds with features like oxbow bends,
• hematite spheres in basin areas, as found by the Opportunity rover,
• and from tidal rhythmites imprinted on the Martian surface itself.

In order for there to have been liquid water on its surface, the Martian atmosphere must have been much thicker in the past, as its current atmosphere exerts a pressure of just 0.7% of Earth’s atmosphere: below the triple point of water and where a liquid state is impossible.

Therefore, it seems reasonable to conclude that Mars once had a thick, relatively stable atmosphere, protected by a planet-wide magnetic field generated by a core dynamo. As Mars cooled, the dynamo ceased, causing the magnetic field to disappear and enabling the solar wind to slowly but steadily strip the atmosphere of Mars away. After around ~100 million years, give-or-take, the atmosphere was thin enough so that liquid water was now impossible on its surface, and Mars’s surface conditions have roughly been “frozen in” ever since.

Earth (right) has a strong magnetic field to protect it from the Solar Wind. Worlds like Mars (left) or the Moon do not, and they routinely get struck by the energetic particles emitted from the Sun, which continue to strip airborne particles off of those worlds. The solar wind is radiated spherically outward from the Sun, and puts every world in our Solar System at risk of having its atmosphere stripped away. This happened to Mars in the past and could theoretically happen to all the planets with enough solar activity.

This has been the standard story for many decades, and the whole point of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was to make precise measurements of the current rate of atmospheric escape from Mars to better inform our understanding of how it lost its atmosphere in the first place. MAVEN, launched in 2013, arrived at Mars in late 2014 and began taking measurements of its upper atmosphere. It found that Mars was losing between 0.7 and 2.1 kilograms of atmospheric mass every second, primarily due to the solar wind (as expected), and that the rates of atmosphere loss were enhanced every time that solar activity flared.

If you extrapolate this rate of mass loss back to earlier times, when Mars was supposed to have had a much thicker atmosphere, you expect that the mass loss rate would have been greater earlier on: when the atmosphere was denser and contained more particles. If the current mass loss rate of Mars’s atmosphere were to remain constant, Mars would be completely atmosphere-free less than 1 billion years from now. In the past, when solar activity was greater and the mass loss rate of the atmosphere was greater, Mars could have been reduced from a life-friendly world to one where liquid water on the surface was an impossibility in just ~100 million years or so.

Mars, like most rocky worlds in the Solar System, is full of craters and has an impact-rich history. Despite how easily visible the Martian atmosphere is from orbit, it’s very thin, just 1/140th the thickness of Earth’s atmosphere and less than 1/200th of the mass of Earth’s atmosphere.

At least, that’s been the conventional line of thinking for a long time. Sure, Venus doesn’t have a protective magnetic field and it still has an atmosphere, but ongoing volcanic activity — one of the prime producers of atmospheric gases on terrestrial planets — could be replenishing it on an ongoing basis, leaving heavy gases behind while stripping the lighter, more volatile ones away. It would be tempting to believe this story not only applies to our own Solar System and the planets within it, but to exoplanets as well: favoring life on planets with Earth-like sizes and masses, but disfavoring planets with Mars-like sizes and masses. After all, without a magnetic field, how can a planet sustain a life-supporting atmosphere?

That line of thinking received a tremendous challenge in 2018, however, when an impactful paper appeared in the journal Astronomy & Astrophysics with the provocative title, “Why an intrinsic magnetic field does not protect a planet against atmospheric escape.” It flips the narrative around by, instead of assuming that Earth’s atmosphere is well-protected while Mars’s was not, asking the question of whether magnetized planets are more protected against atmospheric escape than unmagnetized ones.

The motivation was simple: if you look at the actual escape rate of particles from the atmospheres of Venus, Earth, and Mars, despite the differences of these planets, the rates are very similar: between 0.5 and 2.0 kilograms-per-second for all three planets, with 0.5 for Venus and 1.4 for Earth.

Discovered for the first time by the Parker Solar Probe, the solar wind exhibits magnetic switchbacks, where the magnetic field bends back on itself as particles stream away from the Sun. This discovery has yet to be fully explained, but particles emanating from the Sun impact all of the planets, and cause similar mass loss rates for the atmospheres of Venus, Earth, and Mars.

There’s a large amount of physics underlying how atmospheres evolve, as many different mechanisms can contribute to the escape of atmospheric particles from a planet’s gravitational pull. The major ones, as identified so far, are:

• Jeans escape,
• dissociative recombination,
• cross-field ion loss,
• ion pickup,
• sputtering,
• and escape through the cusp and the polar cap.

Jeans escape is the simplest: particles in an atmosphere have a temperature to them, and that details the type of energy distribution they have. The most energetic particles will either run into other particles or, at the top of the atmosphere, will have enough energy to gravitationally escape the pull of the planet. This occurs independently of planet magnetization, as it’s solely energy and gravity dependent. Similarly, dissociative recombination happens when positively charged oxygen ions recombine with electrons, producing fast-moving atoms that can escape from a planet’s gravity. Although Mars is low enough in mass that this occurs, Venus and Earth are too massive to permit this; although it is a difference between Venus and Earth vs. Mars, this too is magnetization-independent.

Early on, even small-mass planets like Earth had a large hydrogen and helium envelope. Due to their low gravity, solar wind and radiation swiftly stripped those primitive atmospheres away, with only the gas giant worlds of the outer Solar System maintaining the lightest volatile elements in abundance.

Cross-field ion loss happens where ions slowly migrate across magnetic field lines due to energy transport and is extremely weakly dependent on magnetization; on average, the escape of particles through this process is neither aided nor hindered by a magnetic field’s presence or strength. When atoms making up the outermost extent of a planet’s atmosphere are ionized and picked up by the solar wind, that’s known as ion pickup. It happens for unmagnetized planets more easily than for magnetized ones, but even if Earth were unmagnetized, it would only allow the lightest species of particle — hydrogen — to be stripped away. For heavier atmospheric constituents, the difference between a magnetized and unmagnetized planet plays virtually no role.

The final two contributions, however, sputtering and polar cap and cusp escape, are very much dependent on a planet’s magnetization. If an ion in a planet’s ionosphere gets swept up and accelerated by the solar wind, it can then re-impact the ionosphere, in which it can hit oxygen atoms and give them enough speed to escape the planet’s gravity. These ions are swept to larger distances away from the planet if the magnetization is weaker, meaning that sputtering is greater for planets with smaller (or no) magnetic fields. However, the overall effect of sputtering is generally small compared to all other processes, even at maximum. Therefore, everything depends on that final factor: escape from the polar caps and cusp regions.

This schematic shows the difference between the magnetic configurations of magnetized (top) and unmagnetized (bottom) planets. The magnetopause (MP), the polar caps (PC), and the cusp region (in red) are all shown for a magnetized planet. For an unmagnetized planet, there is only the boundary of the induced magnetosphere (IMB).

If your planet is magnetized, the polar caps are the regions surrounding the magnetic poles: where the field lines connect the ionosphere to interplanetary space. A polar wind, where the light electrons are faster than the heavier ions, sets up an electric field, accelerating ions away from the planet. Unmagnetized planets have no polar caps; severely magnetized planets have smaller, narrower caps. It’s that in-between area, where a planet is “barely magnetized,” that leads to the largest polar caps and the largest escape fraction of particles.

The cusp regions are small: the part of the polar cap that’s connected to the dayside part of the magnetopause. However, because it’s in contact with the solar wind plasma, external kinetic energy can be carried into the cusp region where it then heats ionospheric particles, leading to their escape. For barely magnetized planets, this causes only a fraction of the polar cap effect. But for severely magnetized planets, there’s a peak value for the escape rate of particles that can exceed the polar cap escape rate. For planet Earth, the escape rate would be maximized if its magnetic field were about 30 times its present strength.

If we want to compare atmosphere loss for magnetized and unmagnetized planets, we have to compare the total rates from all of these mechanisms combined. You might think — based on the known stories of Mars and Earth — that unmagnetized planets lose their atmospheres more quickly, but this isn’t necessarily true.

This graph shows the hydrogen (H) and oxygen (O) escape rates for planets like Venus, Earth, and Mars, dependent on magnetization (x-axis). The HDR and LDR curves represent the high and low estimates for dissociative recombination on Mars. Note that at very low or very high magnetizations, the rates for escape are smaller than at intermediate values.

It is true that lower-mass planets have a more difficult time holding onto their atmospheres, just as planets closer to the Sun have a harder time maintaining substantial ones. But putting those factors aside and looking only at the cases of a magnetized versus an unmagnetized planet, the mass escape rate isn’t always greater for unmagnetized planets. In fact, that rate is similar for unmagnetized planets and for highly magnetized planets, and that for in-between magnetizations, there are two peaks where the escape rate is actually greatest: polar cap escape, which is dominant for hydrogen, and cusp escape, which dominates for significant magnetizations before falling off at very high magnetizations.

Compared to the unmagnetized case, Mars, Venus, and Earth-like planets could lose their mass up to 5 times as quickly with the right levels of intermediate-to-high magnetizations. It’s worth noting that even the maximum escape rate for a terrestrial planet, ~3 kg/s, could not have removed even one Earth atmosphere’s worth of mass (about 5 × 10¹⁸ kg) over the history of the Solar System. While planetary magnetic fields protect the atmosphere from the effects of sputtering and ion pickup, these effects are small compared to the polar cap and cusp escape rates that they enable. On the other hand, even unmagnetized planets gain induced magnetospheres from interactions with the Sun, which can protect them from sputtering and ion pickup as well.

Mars may not have died because it lost its magnetic field; it may have died solely because it lost its atmosphere from being small and low in mass. When we look to exoplanets, we shouldn’t assume that a planet-wide magnetic field is essential for life. As long as you have a substantial but not-too-thick atmosphere, which can happen with any or even no magnetization, life, even as we know it, remains plausible.