Introduction to special issue

MAVEN Explores the Martian Upper Atmosphere

Science  06 Nov 2015:
Vol. 350, Issue 6261, pp. 643
DOI: 10.1126/science.aad3443

After decades of exploration by landers, rovers, and orbiting spacecraft, Mars continues to capture imaginations and reveal surprises. Indeed, the more we learn about Mars, the more we see the connections between its interior, surface, and atmosphere. Answering questions about the potential for habitability, presence of water, and history of its climate requires understanding Mars as a connected system over time.

Artist's rendition of the MAVEN spacecraft over Mars.

CREDIT: NASA/GSFC

It was in this context that NASA flew the orbiting Mars Atmosphere and Volatile Evolution (MAVEN) mission (1). Its goal was to explore the modern martian upper atmosphere in unprecedented detail. MAVEN carries a suite of instruments that take measurements while directly traveling through the atmosphere as it orbits the planet, from ~125 to ~6000 km above the rocky surface. MAVEN has been operating in orbit around Mars since 18 September 2014, and the data now allow us to start addressing key science questions that had otherwise been unanswerable.

Previous Mars missions have provided compelling evidence for abundant liquid water on ancient Mars. This would seem to require a thicker CO2 atmosphere to provide the necessary greenhouse warming. Where did the CO2 go? Where did the water go? There do not appear to be sufficient carbon-bearing minerals on the surface or in the subsurface to account for a thick early atmosphere (2), but loss to space is thought to be a viable mechanism for producing the observed changes in climate (1). Measurements by MAVEN shed light on how the upper atmosphere interacts with the Sun and solar wind, and the ability of gases to escape to space—both important processes for understanding planetary atmospheres. Four papers in this issue report some of the initial discoveries and are complemented by a series of companion papers in Geophysical Research Letters (3).

Jakosky et al. examine measurements made during the impact of an interplanetary coronal mass ejection onto the Mars system. The sudden and dramatic increase in solar wind density, velocity, and magnetic field intensity lead to enhancements in the production and energization of ions picked up by the solar wind as it streams by, and to an increase in the rate of gas escape to space. Solar storms might have been more abundant and more intense early in solar system history, so this increase may have implications for the early loss of Mars' atmosphere to space.

Bougher et al. describe in situ measurements of the Mars thermosphere; the only previous measurements came from profiles obtained during the entry of the two Viking landers in 1976. The thermosphere, spanning roughly 120 to 200 km, connects the lower atmosphere that controls the martian climate to the exosphere that controls gases' escape to space. Its properties are central to understanding the long-term evolution of the atmosphere. The measurements provide detailed information on the composition of the neutral atmosphere and ionosphere, the variability of this region, and the energetics of ion acceleration that leads to loss.

Schneider et al. report the discovery of a “diffuse” aurora produced by a solar storm. The dif use aurora was widespread geographically, as opposed to previously observed “discrete” auroras. The solar electrons responsible for the aurora probably entered the martian atmosphere directly from the solar wind. The auroras reflect active processes that produce ionization and dissociation, which may affect the overall atmospheric escape rate. The lack of a global magnetic field on Mars probably causes the martian auroras to be more a global phenomenon than they are on Earth.

Andersson et al. describe the discovery of dust at orbital altitudes surrounding Mars, detected by the effect a dust grain has when it strikes the spacecraft. Dust impacts were detected from the lowest altitudes visited by the spacecraft, around 125 km above the surface, up to ç1000 km altitude. The distribution of the dust points to interplanetary debris in the solar system as the likely source. Such debris is thought to be responsible for producing discrete layers in the ionosphere and for affecting the chemistry and energetics in that region.

Together, these measurements—along with those reported in the companion papers—are beginning to shape a new view of both modern and past processes on Mars. The upper atmosphere of Mars was probably a major contributor to the evolution of Mars' climate, and thereby influenced the stability of liquid water and the potential habitability of Mars by microorganisms. MAVEN is continuing to make measurements as it orbits the Red Planet and will be able to explore the upper atmosphere over a full Mars year and during the changing phases of the solar cycle. The measurements being reported here only scratch the surface of what remains to be learned about the connected Mars system.

References

  1. B. M. Jakosky and 70 others, The MAVEN mission to Mars, Space Sci. Rev. 10.1007/s11214-015-0139-x (2015).

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