Image: ESA/Marsis
The European Space Agency MARS EXPRESS spacecraft carried a number of experiments including an ionospheric topside sounder called MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding). An example topside ionogram is shown below:
As yet, no ionosonde has been landed on the planet’s surface. However, radio occultation measurements have been taken by a number of spacecraft, and from these, electron density profiles of the Martian ionosphere have been constructed.
In an occultation experiment the radio signal from the spacecraft to the Earth passes through different layers of the planetary atmosphere as the spacecraft orbits the planet. The ionosphere causes a decrease in the velocity of propagation of the radio wave and thus delays the signal slightly. The total free electron content along the propagation path can thus be determined. From a number of passes the electron density height profile may be determined at various locations.
From these profiles it is relatively easy to compute what an ionogram from a Mars surface ionosonde would look like. The figures below show a daytime electron density profile (March 1999 at 74 degrees north) from the Mars Global Surveyor mission together with the computed ionogram:
The critical frequencies for the two layers are indicated on the ionogram, and as expected from the lower Martian peak electron densities, these are lower than their terrestrial counterparts. We have labelled the two layers L and M and these appear to be similar to the terrestrial E and F layers respectively. In the night-time, the lower layer mostly disappears and the higher layer increases in altitude and decreases in density, just as does its terrestrial counterpart. At Mars the most significant ionospheric ion appears to be O2+.
Note that the topside sounder can only 'see' the highest ionospheric layer (M), and can give little indication about any lower layers of ionisation. If, from orbital mechanics, the true height of the satellite above the surface of Mars is known, the difference between this and the apparent range of the surface reflection can allow the total electron content along the line of sight to be determined. This will include the contribution due to lower ionospheric layers, but does not allow any height profiling of these.
As Mars has no global magnetic field, no magnetoionic splitting of the upper trace is shown. However, regional remnant magnetic fields have been detected that are believed to be sourced from the regolith (surface layer), and it is possible that above these areas magnetoionic splitting may occur. This would not be visible in a conventional ionogram because the maximum fields observed are less than 500 nanoTesla, which corresponds to a splitting separation of 7 kilohertz, only one hundredth of that typical for the Earth.
Using an average electron density profile for the nightside of Mars (Haider, 1997) results in the following ionogram:
As with the terrestrial counterpart, the peak electron density has decreased and the layer height has increased. There is also no sign of the layer labelled L on the daytime ionogram suggesting that it might be similar to the E-layer on the Earth. Obviously there are strong enough Martian thermospheric winds so that, together with a sufficiently low collision frequency above 150 km, the night-time electron density is still sufficient to give an ionosonde return in the MF frequency range.
From the above ionograms, both simulated at times of low to moderate sunspot numbers, it would appear that MF and HF communications should be possible between fixed and mobile stations on the surface of Mars. This could provide a lower cost option than Martian orbiting relays, and would probably provide a more continuous relay in some areas (such as deep canyons) than would a satellite link. Such communication has already been considered (Fry & Yowell, 1994). However, some now suspect that strong ionospheric absorption, which could persist into the night sector, might make such radio communication unuseable.
ACRONYMS
REFERENCES
Fry C and R Yowell, “HF Radio on Mars”, Communications Quarterly, 4(#2), 13-23 (1994)
Nielsen E, Mars Express and MARSIS, Space Science Reviews, 111, 1-18 (2004)
Nielsen E, et al, Observations of Vertical Reflections from the Topside Martian Ionosphere, Space Science Reviews, 126, 373-388 (2006)
Haider S, “Chemistry of the Nightside Ionosphere of Mars”, J Geophys Res, A102, 407-416 (1997)
Hinson D, et al,“Initial Results from Radio Occultation Measurement with Mars Global Surveyor”, J Geophys Res, A104, 26997-27012 (1999)
Australian Space Academy