The solar wind, the main cause of Mars’ atmospheric erosion

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After several campaigns of “deep-dip”, the NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) probe managed to determine the reasons for the escape of the Martian atmosphere into space, and therefore one of the key factors to the transition of a potentially habitable planet to an inhospitable environment. These major results, to which the IRAP researchers (Paul Sabatier University of Toulouse & CNRS) have contributed significantly, are published on Nov. 6, 2015 in the journals Science and Geophysical Research Letters.

MAVEN observing an aurora on Mars. As on Earth, this luminous phenomenon is due to the interaction between energetic particles in the solar wind and molecules in the atmosphere; but since Mars has no internal magnetic field, auroras are not concentrated at the poles.Credit: CU/LASP

In orbit around the Red Planet since September 22, 2014, NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) probe has been carrying eight scientific instruments, including SWEA (Solar Wind Electron Analyser), a detector designed by a team from IRAP. Mission objectives: to better understand the composition and density of the Martian upper atmosphere and ionosphere and their interactions with solar radiation and wind, in order to determine the processes of atmospheric escape and thus to trace the past evolution of the Martian climate. For this purpose, the probe has carried out several deep-dip campaigns to about 120 kilometres from the surface in 2015.

Analysis of the data collected by the onboard instruments indicates that the solar wind has the effect of expelling atmospheric gas at a rate of around 100 grams per second (1). This escape occurs mainly at the intersection between the Martian upper atmosphere and the magnetic tail produced by the interaction of the latter with the solar wind (~75%), to a lesser extent at the poles (~25%) (2) and the gas cloud surrounding the planet.

In addition, it has been shown that this rate of atmospheric erosion increases significantly – perhaps by a factor of 10 – during solar storms, suggesting that it was much higher in the past, when the Sun was younger and much more active. Thus, there is little doubt that Mars was once endowed with an atmosphere dense and warm enough to ensure the presence of liquid water on its surface (3), or even to shelter certain forms of life, and that this solar wind-induced atmospheric escape has had a major impact on the evolution of the Martian climate towards the cold and arid stage we know today.

This discovery raises the subsidiary question: why hasn’t the solar wind also caused the escape of the Earth’s atmosphere? Christian Mazelle, researcher at IRAP and in charge of the SWEA instrument, answers with these few words: “Unlike the Red Planet (4), our Earth is equipped with a powerful magnetic shield that repels most of the solar wind to more than ten planetary rays on the front face. Only a tiny fraction of our atmosphere escapes into space – along the magnetic field lines in the polar regions. At this rate, it will take several times the current age of the Universe to empty out”!

Comparison of field geometry for diffuse and discrete aurora on Earth and Mars. Mars lacks an internally generated global magnetic field due to the cooling of its core. Fields surrounding Mars are a combination of small structures locked in the crust billions ago (lower right) and solar wind field lines draped around the planet.

Notes

(1) The solar wind consists of a flow of energetic particles, mostly protons and electrons, escaping from the Sun at a speed of around 1.5 million km/h. The magnetic field it carries generates an electric field in the vicinity of Mars that accelerates the ions in the Martian upper atmosphere. With sufficient speed, these ions escape from the Martian attraction towards space.

(2) Volutes of ionised gas have been observed in the vicinity of the magnetic poles perpendicular to the induced magnetic tail.

(3) Various Martian regions, valleys in particular, bear traces of erosion by water, others are made up of mineral deposits whose formation requires the presence of liquid water. Recently, the Mars Reconnaissance Orbiter probe detected the seasonal appearance of liquid brackish water on the surface. This suggests the presence of rivers, lakes and even oceans of liquid water on the surface of Mars in the distant past.

(4) Mars has been devoid of a global magnetic shield for at least 3.6 billion years. As a result, even in the regions where fossil magnetic crustal springs are found (mainly in the southern hemisphere), the solar wind can reach about half of the Martian radius intact. It is still present, albeit slowed down at much lower altitudes (a few hundred kilometres) and is only really blocked at the level of the dense ionosphere. The results from Maven (diffuse auroras) show, however, that intense precipitation of energetic particles can occur in the atmosphere down to very low altitudes and even to the ground (for the most energetic particles).

Further Resources

  • Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability, S. Bougher et al., Science, 6 novembre 2015. http://www.sciencemag.org/lookup/doi/10.1126/science.aad0459
  • MAVEN observations of the response of Mars to an interplanetary coronal mass ejection, B.M. Jakosky et al., Science, 6 novembre 2015. http://www.sciencemag.org/lookup/doi/10.1126/science.aad0210
  • Discovery of diffuse auroras on Mars, N.M. Schneider et al.,Science, 6 novembre 2015, http://www.sciencemag.org/lookup/doi/10.1126/science.aad0313
  • Altitude dependence of nightside Martian suprathermal electron depletions as revealed by MAVEN observations – M.Steckiewicz et al., Geophys. Res. Lett., 42, doi:10.1002/2015GL065257
  • Magnetotail Dynamics at Mars – GinaA.DiBraccio et al., Geophys. Res. Lett.
  • Electric Mars: The first direct measurement of an upper limit for the Martian “polar wind” electric potential, Collinson, G., et al. (2015), Geophys. Res. Lett., 42, doi:10.1002/2015GL065084.
  • A hot Flow Anomaly at Mars, Collinson, G., et al. (2015), Geophys. Res. Lett., 42,
  • First Results of the MAVEN Magnetic Field Investigation, J. Connerney et al., Geophys. Res. Lett.
  • The First In-Situ Electron Temperature and Density Measurements of the Martian Nightside Ionosphere, C. M. Fowler et al. (2015), Geophys. Res. Lett.
  • Estimation of spatial structure of detached magnetic flux rope around Mars based on MAVEN plasma and magnetic field simultaneous observations – Takuya Hara et al., Geophys. Res. Lett.
  • Magnetic reconnection in the near-Mars magnetotail: MAVEN observations – Y. Harada et al., Geophys. Res. Lett.
  • Marsward and tailward ions in the near-Mars magnetotail: MAVEN observation – Y. Harada et al., Geophys. Res. Lett.
  • MAVEN observations of solar wind hydrogen deposition in the atmosphere of Mars – J.S.Halekas et al., Geophys. Res. Lett.
  • Implications of MAVEN Mars Near-Wake Measurements and Models – J.G Luhmann et al., Geophys. Res. Lett.
  • Low frequency waves in the Martian magnetosphere and their response to upstream solar wind driving conditions – S. Ruhunusiri et al., Geophys. Res. Lett.
  • Model insights into energetic photoelectrons measured at Mars by MAVEN – S. Sakai et al., Geophys. Res. Lett.
  • Ionopause-like density gradients in the Martian ionosphere: A first look with MAVEN – Marissa F. Vogt et al., Geophys. Res. Lett.

IRAP Contact

  • Christian Mazelle, cmazelle@irap.omp.eu, Tel : 0561557775 / 0661041734

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