Here are two claims:
Mars has living organisms on it.
Mars has Earth-like plate tectonics.
The first claim is supported by good evidence. The second is completely bogus.
In this post I explain why Mars is probably alive with some primitive life, but is not a very exciting or hospitable place for it when compared to Earth. I also make the argument that learning about Mars is useful because it will help us to avoid visiting such places in the future.
One might say that the main difference between Earth and Mars is that the former has life and the latter doesn’t. This is incorrect (more on Martians below).
Mars and Earth occupy different orbits, but Mars is only about 50% farther from the Sun than Earth. By contrast, the next planet out, Jupiter, is 550% farther from the Sun than Earth. So in this respect they are quite similar and both occupy the so-called “habitable zone” of our solar system where liquid water is possible.
When searching for “Earth-like” planets in other solar systems, the distance from the star is the primary criteria. However, should a spaceship of humans flee Earth to an unknown planet in another solar system that happened to be at the right distance from its star, I am sure they would be very disappointed to find something resembling Mars. We would be wise to determine what makes a planet in the habitable zone full of life like ours as opposed to a (mostly) dead, dry rock like Mars.
With the exception of its atmosphere, which on both planets represent less than .0001% of the total mass, both planets have the same composition inside and out (iron core, aluminum-silicon-oxygen mantle and crust). The main difference between Earth and Mars is their size! The radius of Mars is only half that of Earth. As a result, Earth is 10 times more massive than Mars. This has enormous consequences for the evolution of the planet.
The most important consequence of the size difference is that the interior of Earth is relatively hot compared to Mars. Earth has much more heat energy still trapped inside because, speaking simply, big bodies cool down slower than small bodies. Specifically, the time it takes for a solid body to cool down to a given temperature is proportional to the radius squared. The radius of Earth is twice that of Mars. Therefore, in terms of pure conduction, Mars cooled down four times faster than Earth, and lost most of its heat energy long ago.
The internal heat of Earth has two effects which are very important for maintaining conditions favorable to life. The existence of a molten iron outer core surrounding a solid inner core in Earth’s interior produces a magnetic field that shields our atmosphere from the barrage of high energy particles streaming from the sun (the solar wind).
The solar wind excites molecules in the atmosphere and the lighter ones (such as hydrogen) tend to escape. The lower gravity of Mars also means that the energy required for a hydrogen atom to reach escape velocity is significantly lower than on Earth. Mars has a very weak magnetic field, because the dynamics of the molten and solid core are different than on Earth, also because of size. The exact nature of the Martian core is unknown, but in any case, the result of not having a magnetic field was that, over time, any hydrogen in the surface environment of Mars (including water) was lost to space. Any oxygen in the atmosphere oxidized the iron rich minerals of the surface, resulting in its signature red color. All that’s left is a very thin atmosphere composed mainly of carbon dioxide. On Earth, the massive quantities of photosynthetic organisms living in the oceans keep the oxygen levels high.
The loss of hydrogen to space occurs on Earth, so why does Earth still have oceans? The answer is plate tectonics.
Plate tectonics is the consequence of a global convection process, whereby the interior of the planet is slowly overturning, carrying heat to the surface like a (very) slowly boiling pot. On Earth, the hot material mainly comes up at the mid-ocean ridges, where it forms the ocean plates. As the ocean plate cools, it becomes denser, and begins to sink, which happens at subduction zones. At mid-ocean ridges, as well as all other volcanoes on Earth, in addition to the solid rocks that solidify from the magma, great quantities of gas are released. The most important of these gases (about 70-90% of the volatile content) is water vapor.
The oceans of Earth were formed from and still are forming from volcanic gases. By looking at the water content of lavas at mid-ocean ridges, we estimate that the interior of Earth contains as much as 500 ppm of water. While this is a very small concentration, when rocks melt, most of the volatiles, including water, escape to the surface. The mantle is so much more massive than the oceans themselves, that despite the low concentrations, a back-of-the-envelope calculation shows that there is enough water in Earth’s mantle to fill the oceans 30 times over. It is possible that the volume of water in the oceans has been growing steadily since the formation of Earth, released from volcanoes tapping the deep reserves locked in minerals. Volcanism is driven by convection. Convection of a planet’s interior results in the phenomenon we call “plate tectonics”.
On Friday, a UCLA scientist, adding to the current hubbub over Mars, made the misleading claim that the Red Planet has plate tectonics as well. The claim is based on observations that classic, normal-fault scarps exist on Mars. Normal (sense of motion is extensional) faults can occur on any body which is experiencing differential contraction. The most familiar example of this are the cracks that form in clay when it drys, or for that matter, a leather belt as it dries out. In the case of Mars, small differences in the cooling of the planet resulted in tensional stresses that opened cracks. These faults are probably no longer active, and haven’t been for billions of years. The fact that we can still see them is that, without running water, erosion and sedimentation has not erased their expression at the surface.
It is distressing to hear someone claim that Mars has “plate tectonics”. It couldn’t be further from the truth. Mars has a few, very old, extinct volcanoes. There hasn’t been any volcanism on Mars for billions of years. Tectonically, it is a dead planet. Let me say that again. Tectonically, Mars is a dead planet. That, combined with the loss of its original hydrogen means that the surface of Mars is completely dry save for trace amounts of water still locked in minerals and tiny ice caps at the poles.
There is really only one reason why Mars does not have the replenishing internal convection that Earth does. Once again, it’s the size of the planet. The mantle of Mars is similar in composition to Earth’s and likely has a strong temperature difference with the surface. However, Mars’ mantle is not as thick as Earth’s. For thermal convection to occur, the non-dimensional “Rayleigh number” must be greater than ~1000. As Rayleigh number is proportional to temperature but proportional to the depth of the convecting layer cubed, it is once again Earth’s large size that gives it a higher Rayleigh number than Mars, and allows for a convecting mantle, plate tectonics, and all the wonderful benefits of that process.
Besides providing life sustaining water, and helping to regulate the temperature of Earth’s atmosphere by supplying greenhouse gases (Earth would be an ice cube without at least some greenhouse gases, including water vapor), plate tectonics is also implicated in providing the changing environmental conditions that drive rapid biological evolution, leading to more complex and fit organisms and eventually us!
Now for the real eye popping non-news: Mars is alive!
Despite the massive disadvantage that life on Mars has, there is strong evidence that it is still there! In fact, we’ve known this for several decades, though the original result, which was denied by NASA and largely hidden from the public, was recently confirmed using new statistical techniques. In 1976, the Viking rover collected soil samples from the martian surface and put an aqueous solution of organic nutrients in it that had been produced using a high concentration of carbon 14 (which typically is in very small proportion to the stable isotopes). They then detected carbon 14 – rich CO2 coming off the soil. The only reasonable explanation for the data, again confirmed by scientists in the past year, is that viable, living microogranisms in the Martian soil metabolized the nutrients and released CO2!
It is to be expected that if Mars or Earth ever had life, that some micro-organisms survived the journey across space on pieces of ejecta from impact events and seeded the other planet. Whether life originated on Mars or Earth is inconsequential. What is important is that only Earth was able to fill the oceans through plate tectonic driven volcanic gassing and retain them with its strong gravitational and magnetic fields. This is because of its relatively larger size! So while climbing a flight of stairs is easier on a smaller planet, it is less likely that a small planet can sustain complex life.
If we were serious about finding an Earth-like planet, not only would we be concerned with the orbital radius, which determines the temperature at the surface, we would also consider its planetary radius, which determines whether it can sustain a magnetic field, a water-rich surface environment, and plate tectonics, without which we may never have existed.
Plate tectonics can also be influenced by the volatile content of the planet’s mantle. Earth is larger, so its gravity may have helped to retain more water in the interior during accretion. Retaining the oceans also provides the subducting plates with “lubricating” material, namely sediments.
Does venus have plate tectonics?
Venus shows very good evidence for recent (within the last billion years) and probably ongoing convection in its mantle. Great ridges and troughs are expressions of this motion, but Venus does not seem to have Earth-like plate tectonics, and is also lacking a magnetic field. Venus is very interesting because it is probably more “habitable” than Mars is (for example, a balloon filled with Earth air would float in the Venusian atmosphere where the ambient temperature is about 70 deg). Why Venus doesn’t have plate tectonics may have something to do with it’s composition (not enough water when it formed, being closer to the sun) or too much carbon dioxide. Perhaps the lithosphere is too hot to subduct or too strong from lack of water, but the lack of a magnetic field is probably the reason why water is not present in the atmosphere. It would be great to understand why Venus (being Earth-like in size) lacks a magnetic field.
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