This radar started its science operations on 4 July, the same day as its first commissioning phase ended. Due to the late deployment of Marsis, it was decided to split the commissioning, originally planned to last four weeks, into two phases; the second will take place in December.

It has thus been possible to begin scientific observations with the instrument earlier than initially planned, while it is still Martian night-time. This is the best environmental condition for subsurface sounding, as in daytime the ionosphere is more 'energised' and disturbs the radio signals used for subsurface observations.

As from the start of commissioning, the two 20m-long antenna booms have been sending radio signals towards the Martian surface and receiving echoes back.

"The commissioning procedure confirmed that the radar is working very well and that it can be operated at full power without interfering with any of the spacecraft systems," says Roberto Seu, Instrument Manager for Marsis, of University of Rome 'La Sapienza', Italy.

An excellent test
Marsis is a very complex instrument, capable of operating at different frequency bands. Lower frequencies are best suited to probing the subsurface, the highest frequencies are used to probe shallow subsurface depths, while all frequencies are suited to studying the surface and the upper atmospheric layer of Mars.

"During commissioning we worked to test all transmission modes and optimise the radar's performance around Mars," says Professor Giovanni Picardi, Principal Investigator for Marsis, of University of Rome 'LaSapienza'.

"The result is that since we started the scientific observations in early July, we have been receiving very clean surface echoes back, and first indications about the ionosphere."

The Marsis radar is designed to operate around the orbit 'pericentre', when the spacecraft is closer to the planet's surface. In each orbit, the radar is switched on for 36minutes around this point, spending the middle 26minutes on subsurface observations and the first and last fiveminutes of the slot on active ionosphere sounding.

Using the lower frequencies, Marsis has been mainly investigating the northern flat areas between the 30� and 70� latitudes, at all longitudes.

"We are very satisfied with the way the radar is performing. In fact, the surface measurements taken so far match almost perfectly the existing models of the Mars topography," said Prof. Picardi. Thus, these measurements have proved to be an excellent test.

The scientific reason for concentrating on flat regions with the first data analysis is the fact that the subsurface layers are in principle easier to identify, though the task is still a tricky one. "As the radar appears to work so well for the surface, we have good reason to think the radio waves are also propagating correctly below the surface," added Prof. Picardi.

"The bulk of our work has just started, as we now have to be sure to clearly identify and isolate the echoes coming from the subsurface. To do this, we have to carefully screen all data and make sure that signals which could be interpreted as coming from different underground layers are not actually produced by surface irregularities. This will keep us occupied for a few more weeks at least."

Interesting preliminary findings
The first ionospheric measurements performed by Marsis have also led to some interesting preliminary findings. The radar responds directly to the number of charged particles composing the ionosphere (plasma). This has at times been shown to be higher than expected.

"We are now analysing the data to find out if such measurements may result from sudden increases in solar activity, such as the one observed on 14 July, or if we have to put forward new hypotheses. Only further analysis of the data can tell us," said Jeffrey Plaut, co-Principal Investigator, from the NASA Jet Propulsion Laboratory, Pasadena, USA.

Marsis will carry on sending signals that hit the surface and penetrate the subsurface until the middle of August, when the night-time portion of the observations will have almost ended. After that, observation priority will be given to other Mars Express instruments that are best suited to operating in daytime, such as the HRSC camera and Omega mapping spectrometer.

However, Marsis will continue its surface and ionospheric investigations in daytime, with ionospheric sounding being reserved for more than 20% of all Mars Express orbits, under all possible Sun illumination conditions.

In December, the Mars Express orbit pericentre will enter night-time again. By then, the pericentre will have moved closer to the south pole, allowing Marsis to carry out optimal probing of the subsurface once again, this time in the southern hemisphere.

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