Studies of the Martian Ionosphere

The ionosphere of Mars has been studied extensively and our understanding of the physical and chemical processes at play have advanced not least due successful space missions such as Viking, Mars Express and MAVEN.

Through the missions, scientists have for instance learned more about Mars’ induced magnetosphere, current systems in the ionosphere, how crustal magnetic fields influence the ionosphere, and what processes drive the (by now rather well quantified) atmospheric escape.

Curiously, as much has been refined over the years, little has changed over the past few decades in what concerns the view on Mars’ basic ion-chemistry and ionization balance.

In comparison to ionospheres like those of the Earth, or Saturn’s moon Titan, Mars’ ion-chemistry is relatively simple.

Modellers need only to consider a handful of reactions in order to relate the background neutral atmosphere and impinging solar extreme ultraviolet irradiation spectra, to dominating ionospheric features (altitude profile for number densities of free electrons and the most abundant ion species).

An ion-chemistry network relevant for Venus has significant overlaps with one relevant for Mars, as both atmospheres are largely dominated by CO2.

As a result, the somewhat stagnant nature in what concerns new insights about Mars’ basic ionization balance naturally implies a similar situation with respect to Venus.

One of the (historically) perceived most critical reactions in respective ion-chemistry network is that between CO2+(a direct product of photoionization of CO2) and atomic oxygen, O.

Along with interactions of O+with CO2 it has been viewed as a main process in the production of O2+, which is the most abundant ion both in the ionosphere of Mars and in the ionosphere of Venus.

Only a few years ago it was shown experimentally that the reaction is far less efficient than had been reported from an experiment conducted back in 1970.

The new experimentally derived rate coefficient has thus far not been picked up by the major chemical databases and effects on model outputs and model-observation comparisons remain to be thoroughly explored.

It can at least be said that incorporation of the new rate constant into a photochemical model removes a conundrum of calculated electron temperatures falling well below the neutral temperature in Mars’ ionosphere.

In fact, electron temperatures derived through a photochemical model now not only appear feasible, they fall better in line with predictions from electron energetics model than with those inferred from Langmuir probe sweep analysis. Reasons for the model-observation discrepancy have been speculated on, but remains to be established.

Also, it remains to study further consequences of using in Mars ionosphere chemical models the new rate coefficient for the reaction CO2++ O -> O2++ CO.

Is data collected in Mars’ ionosphere in general supporting the new rate coefficient or are there comparative tests lending support to the “outdated” value?

Finally, through the MAVEN mission came the first confirmed detection of doubly charged ions in the Martian ionosphere; CO2++.

We seek, through modelling, to constrain its lifetime against spontaneous dissociation, expecting/hoping to find evidence for metastable states with lifetimes longer than those observed experimentally.

If so we shall promote re-examination of lifetimes in a modern facility offering better vacuum conditions than in older experiments wherein interactions with the residual gas may have contributed to the loss of the doubly charged ions.