MARSIS, the sounding radar on board ESA's Mars Expressspacecraft, is collecting the first data about the surface and theionosphere of Mars.
The radar started its science operations on4 July 2005, after the first phase of its commissioning was concludedon the same day. Due to the late deployment of MARSIS, it was decidedto split the commissioning, originally planned to last four weeks, intotwo phases, one of which has just ended and the second one to bestarted by December this year.
This has given the instrument thechance to start scientific observations earlier than initiallyforeseen, while still in the Martian night. This is the environmentalcondition favourable to subsurface sounding, because the ionosphere ismore 'energised' during the daytime and disturbs the radio signals usedfor subsurface observations.
From the beginning of thecommissioning, the two 20-metre long antenna booms have been sendingradio signals towards the Martian surface and receiving echoes back."The commissioning phase confirmed that the radar is working very well,and that it can be operated at full power without interfering with anyof the spacecraft systems," says Roberto Seu, Instrument Manager forMARSIS, from the University of Rome 'La Sapienza', Italy.
An excellent test
MARSISis a very complex instrument, capable of operating at differentfrequency bands. Lower frequencies are best suited to probe thesubsurface and the highest frequencies are used to probe shallowsubsurface depths, while all frequencies are suited to study thesurface and the upper atmospheric layer of Mars.
"During thecommissioning we have worked to test all transmission modes andoptimise the radar performance around Mars," says Prof. GiovanniPicardi, Principal Investigator for MARSIS, University of Rome 'LaSapienza'. "The result is that since we have started the scientificobservations in early July, we are receiving very clean surface echoesback, and first indication about the ionosphere."
The MARSISradar is designed to operate around the orbit 'pericentre', when thespacecraft is closer to the planet's surface. In each orbit, the radarhas been switched on for 36 minutes around this point, dedicating thecentral 26 minutes to subsurface observations and the first and lastfive minutes of the slot to active ionosphere sounding.
Using thelower frequencies, MARSIS has been mainly investigating on the northernflat areas between 30° and 70° latitudes, at all longitudes. "We arevery satisfied about the way the radar is performing. In fact, thesurface measurements taken so far match almost perfectly with theexisting models of the Mars topography," said Prof. Picardi. Thus,these measurements provided an excellent test.
The scientificreason to concentrate the first data analysis on flat regions lies inthe fact that the subsurface layers here are in principle easier toidentify, but the question is still tricky. "As the radar is appearingto work so well for the surface, we have good reasons to think that theradio waves are correctly propagating also below the surface," addedProf. Picardi.
"The biggest part of our work just started, as wenow have to be sure that we clearly identify and isolate those echoesthat come from the subsurface. To do this, we have to carefully screenall data and make sure that signals that could be interpreted as comingfrom different underground layers are not actually produced by surfaceirregularities. This will keep us occupied for a few more weeks atleast."
Interesting preliminary findings
The firstionospheric measurements performed by MARSIS have also revealed someinteresting preliminary findings. The radar responds directly to thenumber of charged particles composing the ionosphere (plasma). This hasshown to be higher than expected at times.
"We are now analysingthe data to find out if such measurements may result from suddenincreases of solar activity, like the one observed on 14 July, or if wehave to make new hypotheses. Only further analysis of the data can tellus," said Jeffrey Plaut, Co-Principal Investigator, from NASA JetPropulsion Laboratory, Pasadena, USA.
MARSIS will continue sendsignals to hit the surface and penetrate the subsurface until themiddle of August, when the nighttime portion of the observations willhave almost ended. After that, observation priority will be given toother Mars Express instruments that are best suited to work duringdaytime, such as the HRSC camera and the OMEGA mapping spectrometer.
However,MARSIS will continue surface and ionospheric investigations duringdaytime, with the ionospheric sounding being reserved for more than 20%percent of all Mars Express orbits, in all possible Sun illuminationconditions.
In December 2005, the Mars Express orbit pericentrewill enter the nighttime again. By then, the pericentre will have movedcloser to the South pole, allowing MARSIS to restart optimal probing ofthe subsurface, this time in the southern hemisphere.
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