Leaky atmosphere linked to lightweight planet

February 9, 2018, European Space Agency
Illustration of ion escape from Mars. As on Earth, solar ultraviolet radiation separates electrons from the atoms and molecules (blue particles), creating a region of electrically charged – ionised – gas: the ionosphere. This ionised layer interacts directly with the solar wind and its magnetic field to create an induced magnetosphere, which acts to slow and divert the solar wind particles around the planet. Credit: European Space Agency

The Red Planet's low gravity and lack of magnetic field makes its outermost atmosphere an easy target to be swept away by the solar wind, but new evidence from ESA's Mars Express spacecraft shows that the Sun's radiation may play a surprising role in its escape.

Why the atmospheres of the rocky in the inner solar system evolved so differently over 4.6 billion years is key to understanding what makes a planet habitable. While Earth is a life-rich water-world, our smaller neighbour Mars lost much of its early in its history, transforming from a warm and wet environment to the cold and arid plains that we observe today. By contrast, Earth's other neighbour Venus, which although inhospitable today is comparable in size to our own planet, and has a dense atmosphere.

One way that is often thought to help protect a planet's atmosphere is through an internally generated , such as at Earth. The magnetic field deflects charged particles of the as they stream away from the Sun, carving out a protective 'bubble' – the magnetosphere – around the planet.

At Mars and Venus, which don't generate an internal magnetic field, the main obstacle to the solar is the upper atmosphere, or ionosphere. Just as on Earth, solar ultraviolet radiation separates electrons from the atoms and molecules in this region, creating a region of electrically charged – ionised – gas: the ionosphere. At Mars and Venus this ionised layer interacts directly with the solar wind and its magnetic field to create an induced magnetosphere, which acts to slow and divert the solar wind around the planet.

For 14 years, ESA's Mars Express has been looking at charged ions, such as oxygen and carbon dioxide, flowing out to space in order to better understand the rate at which the atmosphere is escaping the planet.

The study has uncovered a surprising effect, with the Sun's ultraviolet radiation playing a more important role than previously thought.

"We used to think that the ion escape occurs due to an effective transfer of the through the martian induced magnetic barrier to the ionosphere," says Robin Ramstad of the Swedish Institute of Space Physics, and lead author of the Mars Express study.

"Perhaps counter-intuitively, what we actually see is that the increased ion production triggered by ultraviolet solar radiation shields the planet's atmosphere from the energy carried by the solar wind, but very little energy is actually required for the ions to escape by themselves, due to the binding the atmosphere to Mars."

The ionising nature of the Sun's radiation is found to produce more ions than can be removed by the solar wind. Although the increased ion production helps to shield the lower atmosphere from the energy carried by the solar wind, the heating of the electrons appears to be sufficient to drag along ions under all conditions, creating a 'polar wind'. Mars' weak gravity – about one third that of Earth's – means the planet cannot hold on to these ions and they readily escape into space, regardless of the extra energy supplied by a strong solar wind.

At Venus, where the gravity is similar to Earth's, a lot more energy is required to strip the atmosphere in this way, and ions leaving the sunward side would likely fall back towards the planet on the lee-side unless they are accelerated further.

"We therefore conclude that in the present day, ion escape from Mars is primarily production-limited, and not energy-limited, whereas at Venus it is likely to be energy-limited given the larger planet's higher gravity and high rate of ionisation, being nearer to the Sun," adds Robin.

"In other words, the solar wind likely only had a very small direct effect on the amount of Mars atmosphere that has been lost over time, and rather only enhances the acceleration of already escaping particles."

"Continuous monitoring of Mars since 2004, which covered the change in solar activity from solar minimum to maximum, gives us a large dataset that is vital in understanding the long-term behaviour of a planet's atmosphere and its interaction with the Sun," says Dmitri Titov, ESA's Mars Express Project Scientist. "Collaboration with NASA's MAVEN mission, which has been at Mars since 2014, is also allowing us to study the atmospheric escape processes in more detail."

The study also has implications for the search for Earth-like atmospheres elsewhere in the universe.

"Perhaps a magnetic field is not as important in shielding a planet's atmosphere as the planet's gravity itself, which defines how well it can hang on to its atmospheric particles after they have been ionised by the Sun's radiation, regardless of the power of the solar wind," adds Dmitri.

Explore further: Mars atmosphere well protected from the solar wind

More information: Robin Ramstad et al. Global Mars-solar wind coupling and ion escape, Journal of Geophysical Research: Space Physics (2017). DOI: 10.1002/2017JA024306

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Mark Thomas
1 / 5 (1) Feb 09, 2018
"Perhaps a magnetic field is not as important in shielding a planet's atmosphere as the planet's gravity itself, which defines how well it can hang on to its atmospheric particles after they have been ionised by the Sun's radiation, regardless of the power of the solar wind,"


Given that Venus's atmosphere is about 93 times thicker than Earth's under tougher conditions, this makes a lot better sense to me that gravity is key. It also supports a long-time hypothesis of mine that Earth's atmosphere is on the thin side for a planet with such strong gravity and magnetism. Astronomers look for Earth mass planets in Earth-like orbits, but the planets with the most Earth-like atmospheres may actually be between Mars and Venus in size. Time will tell.

BTW, it seems possible our atmosphere is on the thin side because much of it was sheared off when the moon was formed.
rrwillsj
3 / 5 (2) Feb 09, 2018
MT,, at first I disagreed with your supposition. But after rereading the article and considering the differing possibilities? I think you may be correct.

Just not sure how to prove it yet. By whatever means our Moon was formed. I am guessing that the earliest Earth atmosphere was mainly hydrogen and helium. light weight elemental gasses, gravitationaly bound.

Whatever means produced Luna? The energies involved could have stripped off the light mass hydrogen and helium from the Earth. Leaving a predominance of nitrogen and maybe even some oxygen?

"When Chaos rules, all possibilities may be probable. And always leave us with an awkward uncertainty!"
wduckss
not rated yet Feb 10, 2018
Taking two examples from a small set is not scientific. When we add Titan Moon, Pluto, Earth, Mercury, and most of the exoplanets. we see that the article was written by (almost) lay people.
I try to avoid common comments, but such articles deserve it.
Each body has its own conditions that contribute to the size of the atmosphere. Mars, low temperatures remove CO2 from the atmosphere and will not change until the temperature rises. Titan moon, low temperatures remove CH4, leave N2. Pluto, N2 is deposited on the surface due to low temperatures etc.
Where would the atmospheric particles escape? Which source of gravitation exists near the body?
rrwillsj
1 / 5 (1) Feb 11, 2018
wd, you are comparing pineapples and pumpkins.

This article was specific in discussing events pertaining to the planet Earth. Several billion years ago.

You haven't noticed the changes since that chthonic era compared to today's planets ?

Really? Aren't you paying attention?

These researchers are offering suppositions based on what evidence they are still accumulating. As to what may have caused the differences we see between this Earth and other, similar massed rocky planets.

If you want different answers? You are going to have to devise and run repeatedly verifiable experiments to prove your contentions.
wduckss
not rated yet Feb 12, 2018
@ rrwillsj
Maybe matter is unknown to you and to the authors. If you follow the research then you should be familiar with the matter. I'd like to give you links, but you do not care. Anyone who does not understand the comment and the evidence, provides the story and the denial of evidence. Check at which temperatures, these compounds and elements (CO2, CH4, N2, SO2 - Io Moon) go into solid state. Compare it to the temperatures on planets and satellites.
Mark Thomas
not rated yet Feb 19, 2018
I think you may be correct. . . . Just not sure how to prove it yet.


It is my hope that with more advanced telescopes we will be able to prove (or refute) this statistically. If we had a large enough sample size of exoplanets, we should be able to figure out what is statistically normal for a planet with Earth-like gravity and sunlight levels (isolation). My guess is that our atmosphere is well below average because of the way the Earth was formed by the off-angle giant impact that lead to the creation of the Moon. Regardless of how it happened, just looking at our own solar system, especially Venus and Saturn, it seems clear Earth's gravity could easily support a much larger atmosphere.
Mmanx
not rated yet Feb 27, 2018
Interesting theory Mark Thomas and maybe Earth's atmosphere is thinner than the norm for its size because of an impact that perhaps created the moon. But, if Earth was suddenly moved inwards to where Venus is orbiting the Sun, then maybe, as a result of the nearer Sun's increased heating effect, the Earth's oceans would boil and much hydrogen would then escape into space, leaving several hundred atmospheres of pure oxygen about the Earth. That oxygen would probably react with carbon in the crust resulting in the Earth ending up with several hundred, or even several thousand atmospheres of Carbon Dioxide. So, maybe Venus' atmosphere is only thicker than Earth's because its oceans have boiled away.

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