Mars exploration has two distinctly human faces. One is focused on the deeper understanding of our solar system, and our place in it. How is it that the Earth ended up as it did, with features conducive to fostering intelligent life and advanced technologies? Why not on our neighbouring planet, Mars? The other concerns the human drive to investigate and in some cases utilise all accessible places and their resources.
Earlier in modern human history there were new continents to be found, occupied and exploited. In the 20th century these destinations began to spread beyond the Earth with the Apollo missions to the Moon. Now Mars is the ‘new Moon’ as a focal point of human interest and endeavours. Why? Because it is the planet most accessible to our observations, both telescopic and robotic, it’s a particularly good candidate for the study of terrestrial planet and early life evolution, and it is second only to the Earth in terms of its potential for future long-term human presence. These, and a combination of adventure, inspiration and politics, have led to the current onslaught by Mars orbiters, landers and rovers that have made Mars the best-understood planet after the Earth.
Near-Mars space was first visited via spacecraft fly-bys and orbiters. These determined that the surface was cold, dry and barren for the most part, as inferred from close-up images. The ebb and flow of the polar ice cap sizes with the Martian year hinted at a seasonally variable atmosphere. Remote probing, orbital dynamics and radio communications with spacecraft verified the thinness of the gaseous atmosphere, similar in pressure on the surface to that at Earth’s stratospheric altitudes. It was also found that Mars has no radiation belts. Rather, its interaction with its space environment is more like those of comets and Venus, where the planetary atmosphere is directly exposed to whatever comes from the Sun or the galaxy.
Since these early missions first investigated Mars’s surface features and space environment, the surface and shallow subsurface have been probed with high-resolution imaging and radar. Its interior mass distribution and topography have been mapped by use of gravity measurements from orbit and laser altimeter reflections.
In a quest for potentially habitable conditions, landers and surface rovers have carried out up-close, in-depth analyses of soil and rocks in a few locations, while at the same time monitoring local surface weather. These latter observations, together with global remote sensing measurements from orbiters, target the atmosphere and its variations, including dust storms and winds that shape the surface features.
In addition to carrying out the first lifedetection experiments, the Viking landers of the mid-1970s obtained first in-situ measurements of the atmosphere and ionosphere on their descent. However, throughout the decades of Mars exploration, the atmosphere above the surface boundary layer remained less investigated, in spite of numerous attempts to initiate a dedicated mission to fill this knowledge gap.
Missing link
Mars is second only to the Earth in terms of its potential for future longterm human presence
Why does one need to know about the upper atmosphere of Mars? One reason is that it provides information needed to reconstruct the history of Mars’s entire atmosphere and therefore water – a missing link in the search for life elsewhere. Those who investigate solar system formation, and in particular formation of the terrestrial planets Mercury, Venus, Earth and Mars, often consider that at least the latter three formed with similar makeup in the inner solar nebula about 4.5 billion years ago. The original atmospheres of these planets were likely made up largely of gases released from orbiting solids that collided and stuck together, to make larger bodies that eventually underwent internal compression and heating as more and more material accumulated.
In addition, comet bombardment would have delivered water and other ‘volatiles’ in the form of ice, even after the solid planets had formed. Volcanic activity continued to contribute to their atmospheres as well. Hydrogen was likely abundant in these early atmospheres, but the absorption of the young Sun’s enhanced extreme ultraviolet radiation would have driven off much of it by heating and radiation pressure. The remaining original inventory of atmospheric gases probably was similar for Venus, Earth and Mars, which, unlike Mercury, are far enough from the Sun to have retained at least their heavier constituents like carbon dioxide.
Endeavour Crater, with Pillinger Point in the foreground, as viewed in false colour by NASA’s Mars Exploration Rover Opportunity
The story of how Earth ended up with its nitrogen- and oxygen-rich current atmosphere is still unfolding, with formation of organic material releasing most of the oxygen. Much of its original carbon dioxide content ended up as carbonate rocks (limestone for example) because of the presence of liquid water on its surface. A reaction between CO2 gas and liquid water is the key to this process. Venus may not have had sufficient longlived liquid water to allow the same process to proceed because of its closer solar proximity and related higher temperatures. Mars on the other hand could have been too cold, keeping its water in mainly ice form.
In addition to its greater distance from the Sun by 50 per cent, its smaller size and mass and related lower gravity means it is not as effective in retaining a blanket of atmospheric gases as its more massive sisters. Yet, surface features on Mars suggest that liquid water in amounts substantial enough to carve out channels and leave dried-out mudflats once flowed there. That would imply its atmosphere was once more substantial than at present and that there was considerably more water than is currently detectable on either the surface (including in the polar ice caps) or in its atmosphere. The presence of early water is also linked to the possible development of early life as we know it, making this question of Mars’s atmosphere evolution central to the search for life elsewhere in the universe.
The question of Mars’s atmosphere evolution is central to the search for life elsewhere in the universe
Investigating the fate of Mars’s early atmosphere requires knowledge of both existing planetary repositories, still present on or inside Mars, as well as atmosphere loss processes – in other words, escape to space. The evidence of what has been left behind on Mars’s surface and in its shallow subsurface, most evident in polar cap and subsurface ice, suggests that what remains is not enough to explain the early water and atmosphere estimates. Some hypotheses invoke catastrophic episodes of atmosphere escape, with asteroid and/or comet impacts blasting away substantial fractions of the atmosphere toward the end of the epoch when the planets were still forming.
Existing information on its atmospheric gas isotopes, which are enriched at the higher masses, suggest gravity’s role in the puzzle, although chemistry can also affect these abundances. At the same time, we know from previous near- Mars space observations and at a similarly dry Venus that there is indeed ongoing escape of the atmospheres from these planets. The problem is that the inferred present rates of this loss are generally too low compared to what would be required over a few billion years to remove an ocean’s worth of oxygen content, for example.
Earth’s atmosphere is escaping at a similar rate today in outward flows of its ionised gases at high latitudes, where auroral zone acceleration processes are at work. But the volume of Earth’s atmosphere has remained many times greater than that of Mars. Clearly, escape of atmospheres is neither a simple nor constant process, especially when considered over billions of years of solar system history. Speculations include a connection between the presence of a significant global planetary magnetic field, which Mars no longer has today but Earth does, and the retention over time of a planet’s atmosphere and water.
MAVEN’s quest
The MAVEN (Mars Atmosphere and Volatile EvolutioN) mission was motivated by these questions to propose a targeted Mars scout in 2002. Under the leadership of the Principal Investigator, Bruce M. Jakosky, at the Laboratory for Atmosphere and Space Physics (LASP), University of Colorado, and then Deputy Principal Investigator Robert P. Lin, at the Space Sciences Laboratory (SSL), University of California, Berkeley, a successful bid for the mission was realised. A launch date of 2013 was set for arrival in 2014 when Mars was most accessible from Earth. LASP and SSL had been joined by NASA’s Goddard Space Flight Center (GSFC) during the proposal process, together with an instrument team from IRAP (Research Institute in Astrophysics and Planetology) in Toulouse. Lockheed-Martin was to provide the spacecraft, with project development under the guidance of MAVEN Project Manager David.
F. Mitchell and Project Scientist Joseph M.Grebowsky, both of GSFC. NASA’s Jet Propulsion Laboratory participation included navigation and a telecommunications system to provide data relay support for ground assets at Mars. On 18 November 2013, the mission launched from Cape Canaveral, Florida on an Atlas V rocket and was inserted into Mars orbit on 21 September 2014.
Three views of an escaping atmosphere, obtained by MAVEN’s Imaging Ultraviolet Spectrograph. By observing all of the products of water and carbon dioxide breakdown, MAVEN’s remote sensing team can characterise the processes that drive atmospheric loss on Mars. These processes may have transformed the planet from an early Earthlike climate to the cold and dry climate of today
MAVEN’s job is to probe the physics of current atmospheric escape processes by making measurements in the upper atmosphere and space environment where escape occurs. As part of this quest, it will examine what controls the escape rates for important constituents, including hydrogen, oxygen and carbon. This includes the effects of solar activity and ‘space weather’ on escape rates, thought to be a potential missing link in accounting for larger early losses under a more active young Sun. It will also investigate what determines the isotope ratios in the Martian atmosphere, particularly for non-reacting noble gases like argon. Isotope ratios of atmospheric gases have long been used to peer into the past, as ratios that differ greatly in the abundance of the heavier isotope, compared to common cosmic abundances, are thought to be the smoking gun of escape processes.
MAVEN will carry out the required measurements from an elliptical orbit that precesses to sample the atmosphere above a few hundred kilometres’ altitude at many locations around Mars, and to view it from above and inside with remote sensing. Its in-situ instruments include a neutral gas and ion mass spectrometer (NGIMS) that determines the composition of gases making up the upper atmosphere, including the ionised component – the ionosphere – that is a part of it. It also includes a Langmuir Probe and plasma wave detector to measure electron densities, and also their fluctuations (waves) that can tell us about the physics of some loss processes. Another instrument detects the composition and energies of escaping ions moving outward, telling us what the Mars counterpart to a comet tail consists of, and where the material goes. Several other instruments, including a magnetometer, solar wind analysers, and energetic solar particle detectors, observe the space environment conditions that affect the escape processes. Similarly, the influence of solar extreme ultraviolet flux – the ionising light waves that are known to vary much more with the 11-year solar activity cycle than visible light – is monitored by an EUV detector.
The one imager on MAVEN captures the appearance of the Mars upper atmosphere in scattered, reflected or emitted ultraviolet light, watching for changes in its behaviour that reveal, or even control, the rates of escape. This combination of measurements is unprecedented in its ability to allow us to know whether rates of atmosphere escape, by currently active physical processes, may have been high enough, when integrated over time and changing conditions, to explain the inferred loss of Mars’s once substantial atmosphere.
A standard reference
The business of building a spacecraft and getting it into Mars orbit is full of challenges, but conducting a scientific mission that takes full advantage of that major investment is equally ambitious. MAVEN’s results are going to tell us significantly more about Earth and Earth-like planets. They will help constrain assumptions regarding how combinations of circumstances, such as surface liquid water, solar distance and history, planet mass and size, and possibly planetary magnetic field, can greatly alter the fate of a terrestrial planet. The extrasolar planetary systems harbouring terrestrial planets that are now observed in ever greater numbers are by no means guaranteed to contain Earths. Terrestrial planets in our solar system provide ‘ground truth’ on this topic.
Illustration of the MAVEN spacecraft orbits around Mars during its prime mission year. The view is of the dusk face of Mars, with the coloured areas indicating main solar wind interaction/ space environment features, including the magnetosheath (green), bounded on its outer edge by the bow shock, a boundary region where the solar wind and Mars ionosphere interact strongly (yellow) and the optical shadow (blue). The Sun is off to the right, approximately 1.5 AU away, with the solar wind flowing toward the planet
What will MAVEN tell us that is important for human exploration of Mars? Its measurements will definitively characterise the current upper atmosphere and its variability, as well as Mars’s space environment effects, providing for more specific planning on the part of future mission designers. For example, there is a present synergy between the surface radiation measurements taken by the Mars Science Laboratory Radiation Assessment Detector and the measurements of solar energetic particles in the MAVEN orbit that allows atmospheric transport models to be evaluated. Increased understanding of upper atmosphere modifications by solar activity and surface influences, including dust storms, can improve satellite atmospheric entry, drag and aerobraking calculations. The ionosphere’s variability can affect surface-to-ground, ground-to-surface and surface-to-surface communications. Ionospheric currents generated by solar storms can induce unwanted currents in long conductors on the ground.
First obtaining an understanding, by robotic means, of conditions both along the way and at the destination is a logical approach to human exploration. The textbook-altering knowledge, including model atmospheres and space weather effects, that result from MAVEN mission measurements will provide a standard reference for researchers, from Earth scientists to astrophysicists, and Mars visitors alike for decades to come.
In revealing more about Mars’s past for human explorers to build upon, MAVEN already plays a role in the very human history of robotic solar system exploration – unexpected discoveries notwithstanding.
The MAVEN mission is currently in Mars orbit, making the measurements described above, thanks to the dedicated group of engineers, technicians, support staff and scientists who worked tirelessly toward this goal. NASA support was enabled through the Planetary Science Division. The author thanks PI Bruce Jakosky and MAVEN Project Scientist Joseph Grebowsky for reading and providing comments on this article.
