Mars Atmosphere Loss Rates – Truth vs Truism

The Internet is a great place for learning new things, but it’s also a breeding ground for persistent truisms – inaccuracies repeated so often they seem true. One of those truisms, heard almost every time the terraforming of Mars is discussed, is that Mars needs a magnetic field to maintain a breathable atmosphere against the steady erosive force of the Solar Wind. Discussions of terraforming then focus on exotic ideas for providing Mars with a new magnetic field. Like all persistent truisms, this one is partially true. Mars has lost substantial atmosphere over the last 4 billion years, and one of the loss processes is the interaction of the Martian atmosphere with the Solar Wind, thus the truism seems plausible.

The MAVEN orbiter mission to Mars, launched in late 2013, has observed the atmospheric loss processes that have long been suspected as the causes of the loss of atmosphere from Mars. Contrary to the truism, the chief loss process is Jeans Escape – the loss of hot atoms and ions from the top of the atmosphere caused by them moving fast enough to escape Mars’ gravity. The relative contribution of the different loss processes is in the Table below. Jeans Escape of Hydrogen (H) is the dominant mass loss process.

 H JeansO IonO Dissociative RecombinationO Sputtering
Present Day Loss Rate (numbers per second)1.6-11 X 10^265 x 10^245 x 10^253 x 10^24
4.2 Gyr @ H2O Rate3.6-25.2 metres0.2 metres2.2 metres0.14 metres
4.2 Gyr @ CO2 Rate6 millibar68 millibar4 millibar

Jeans Escape happens in the very top part of an atmosphere – the Exosphere. The Exosphere where the ions, atoms and molecules no longer collide and instead follow ballistic trajectories – arcs away from the planet. Most fall back into the deeper atmosphere, but there’s a fraction that escape forever. This boiling motion of the Exosphere is energised primarily by short ultraviolet and soft x-rays from the Sun – the so-celled EUV and XUV part of the spectrum. These frequencies aren’t continuously emitted by the Sun, but come from magnetic energy “explosions” on the Sun called Solar Flares. Other Sun-like stars released more EUV & XUV when they were much younger, mostly in their first billion years of life as they converted rotational energy into electromagnetic energy.

At the same time that same magnetic energy release powered a much stronger Solar Wind. The protons and other ions of the Solar Wind cause all the non-Jeans Escape processes listed in the Table above. Collectively several metres of water and perhaps 80 millibars of Carbon Dioxide would be lost over 4.2 billion years – at current rates of loss. As the bare minimum for terraforming is about ~300 millibars of carbon dioxide (equivalent to about 250 millibars of Oxygen) this doesn’t seem like a show stopper for terraforming. If we can supply modern day Mars with ~300 millibars in a few hundred years, then replacing 80 millibars in 4 billion doesn’t seem excessive.

Of course the Sun has changed since its exuberant early days and the total actual loss from Mars is probably somewhere between 2 – 0.5 bars worth of atmosphere and maybe several hundred metres equivalent of water. However the Sun’s output was between 20 – 100 times higher in the very early days of its Main Sequence. This matches the apparent desiccation of Mars about ~3 billion years ago.

So why does the truism persist? I suspect that most people think in verbal terms rather than quantitative terms – we know Mars is losing atmosphere due to the Solar Wind and it doesn’t have a magnetic field, so the two facts seem correlated. People who associate the two don’t take the next step of asking “How quickly is Mars losing atmosphere to the Solar Wind?” Now that the loss rate has been measured, and it’s only significant on billion year timescales, the truism should be seen for the half-truth that it actually is.

Reference:

Jakosky, B.M. et al “Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time”, Icarus, Volume 315, 2018, Pages 146-157.
https://doi.org/10.1016/j.icarus.2018.05.030

5 thoughts on “Mars Atmosphere Loss Rates – Truth vs Truism

  1. I’ve wondered why the notion that a magnetic field is crucial for keeping in a planet’s atmosphere keeps getting repeated.
    The reason I don’t believe it is that Venus has no magnetic field but has a thicker atmosphere than the other rocky planets. It is not only the immense amount of CO2, Earth has about as much CO2 in carbonate rocks, but also Venus has a few times the amount of nitrogen in its atmosphere as Earth.
    If a magnetic field was important to keeping a planets atmosphere, Venus’ atmosphere would be much thinner than Earth’s.

    1. Hi Jim,
      Thanks for commenting. There is another way in which the truism is kind of true even in the present day. One atmospheric loss process which is enhanced without a magnetic field is the loss of hydrogen thanks to Jeans’ Escape and the Solar Wind. Venus lost hydrogen very easily, thus why its CO2 is all in the atmosphere rather than trapped in carbonate rock. But “very easily” is on a geological timescale – a bit less than half a billion years is enough to strip an ocean’s worth of hydrogen away. On a human timescale (thus terraforming timescale) that’s immense, but for a planet, not so much. The truism is really due to a lack of perspective. Even the Earth will lose its oceans in the long run – whether to space or the mantle, we can’t say the exact proportion, but it will happen.

  2. Given that Martian outgassing is, from what I can gather from the NASA site, on the order of 1mb per megayear, something else has to be going on to explain the low atmospheric pressure.

    I have my fingers crossed that it’s clathrate formation. It would be great if Mars turns out to have a substantial frozen atmosphere that can be destabilised and released using orbital mirrors. Even a 25mb atmosphere would be a major improvement over what we have today.

    1. Hi Cererean,
      My hope too is there’s an atmosphere reservoir locked away somewhere. Back in the early 2000s there were a few papers on the ‘White Mars’ scenario, which posited a liquid CO2 ‘aquifer’ in the crust. That would’ve made terraforming significantly easier. Presently the data isn’t pointing that way.

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