When talking about the exploration of Mars, our mind often jumps to the ground missions like Curiosity and Perseverance. However, a lot of valuable data is being collected from orbit by a fleet of spacecraft from different countries. Two of them belong to the European Space Agency and are at the forefront of the study of the Red Planet.
Mars Express is the first European mission to study Mars. The spacecraft was launched on June 2, 2003, on a Russian Soyuz-FG/Fregat rocket. This was not the first time the two agencies cooperated on Mars exploration, as many scientific instruments on the failed Mars 96 spacecraft were European. By capitalizing on that experience and the one from Rosetta, the mission was put together very quickly and efficiently.
The spacecraft bus is a cube, with sides of 1.5 m, 1.8 m, and 1.4 m. It houses most of the scientific instruments, computers, and propellant tanks. The latter contains hydrazine and nitrogen tetroxide and feeds the 400 N main engine used for orbital insertion. Eight 10 N thrusters and four reaction wheels provide attitude control. A 1.6 m dish mounted on the side provides communication to Earth from interplanetary distances. A few objects stick out of the main body: two solar panels, three radar booms, and two small omnidirectional antennas for communications, released shortly after launch.
Mars Express’ goals are to study the atmosphere and climate of Mars, the surface composition, and the planet’s structure. It also looks for traces of water. To do so, it carries a variety of instruments. Three spectrometers (OMEGA, PFS, and SPICA) study the detailed chemical composition of the surface and of the thin atmosphere. The surface is also imaged by a high-resolution color camera, HRSC, capable of detecting details as small as 2 m. The MARSIS radar is able to go beneath the surface, where it maps the structure of the crust and the distribution of ice. The Mars Radio Science experiment uses the existing communication equipment to investigate atmospheric density and pressure, gravity field, and the Solar corona by measuring the perturbations these objects inflict on the communication signals. Lastly, ASPERA detects the interactions of the solar wind with the Martian atmosphere.
Mars Express’ instruments have provided a lot of valuable data. They allowed the creation of incredibly detailed maps of the surface, even in 3D. The spectrometers found evidence of atmospheric compounds that may be linked to past life. The spacecraft detected signs of past volcanism and water flow, reinforcing the idea that Mars may have been more hospitable and active in the past. The probe has also conducted flybys of Phobos, with the closest one getting within 45 km. This allowed for unprecedented measurements of its gravity, which indicates it might be porous rather than a solid chunk of rock. Perhaps most excitingly, it found evidence of underground polar lakes containing liquid water.
Trace Gas Orbiter
The second European mission to Mars is ExoMars 2016, once again in cooperation with Russia. The international partner provided a scientific instrument and the Proton-M/Briz-M launch vehicle, which launched the spacecraft on March 14, 2016.
Once again, the spacecraft body is a cube, with dimensions of 3.2 m x 2 m x 2 m, containing most major systems. Power is provided by two solar panels, spanning 17.5 m tip-to-tip. They provide 2000 W of electricity, which can be stored in two Li-ion batteries with a total capacity of 5100 Wh. A 2.2 m dish ensures communication with Earth, and two NASA-provided Electra UHF transceivers allow the orbiter to serve as a relay for surface missions. A 424 N pressure-fed engine provides propulsion for Mars orbit insertion. The propellants used are monomethylhydrazine and Mixed Oxides of Nitrogen. Twenty 10 N thrusters use the same propellants and provide attitude control and precise course correction capability. The orbiter has also four reaction wheels.
As the name of the orbiter suggests, the objective of the mission is to study the most rare gases present in the Martian atmosphere. In particular, scientists hope to find out more about the formation of tiny quantities of methane, one possible explanation being microbial activity. To do so, the spacecraft is equipped with two spectrometers (NOMAD and ACS), a high-resolution camera (CaSSIS), and a neutron detector (FREND).
The spectrometers are able to provide extremely detailed analysis of the atmospheric composition. They detect trace gases and provide maps of their distribution. Analyzing the whole spectrum of rare components of the atmosphere is key to understanding the processes that lead to the formation of methane. For example, detecting other hydrocarbons may suggest a biological origin, while sulfur compounds would hint at active volcanism. The high-resolution camera investigates sites of interest to further corroborate the spectrometers’ findings.
Meanwhile, the neutron detector measures the energies of the neutron that escape from the surface after being produced by incoming cosmic rays. The energy of these neutrons varies depending on how much the different atoms in the soil have slowed them down. This way, the chemical composition of the soil can be assessed, and the presence and distribution of hydrogen can be precisely measured. One compound that can explain the presence of hydrogen is of course water.
So far TGO has generated the most detailed maps of hydrogen on Mars. The probe found evidence of it not only in the polar ice caps but also in other places like Valles Marineris. This might indicate the presence of both hydrated minerals and water ice permafrost. As far as methane is concerned, TGO detected much lower levels than previously thought. Methane was found to be present in concentrations of up to 20 parts per trillion. This result is in disagreement with both previous observations from Mars Express, which found 10 parts per billion, and from Curiosity. Work is ongoing to try and understand what is happening. Meanwhile, new theories have emerged for the formation of organic substances on the planet. These involve the sunlight-induced breakup of carbon dioxide into carbon monoxide, which then goes on to form the compounds. This is hinted by the greater isotopic abundance of the lighter 12-C carbon isotope in the organic molecules since sunlight more easily breaks apart the CO2 containing the lighter isotope.
The unfortunate companions
Both probes also carried surface landers, neither of which succeeded in its mission. Mars Express carried the British-made Beagle 2 lander. It was supposed to analyze surface samples looking for signs of past life. The lander was a relatively thin disc with a diameter of 1 m. Once on the surface, the lid of the disc would have opened, revealing the experiments, and then four solar panels would have unfolded from the lid. The lander successfully landed in December 2003, but contact was never established. The most likely theory is that some solar panels did not open, blocking the UHF antenna placed underneath them. This theory is supported by pictures of the lander taken by NASA’s Mars Reconnaissance Orbiter in 2015.
Trace Gas Orbiter had a companion too. The Schiaparelli lander was supposed to demonstrate landing techniques, and also to carry out analysis of the weather once on the ground. It was a platform 2.4 m in diameter, with the various instruments and subsystems mounted on top. The landing, however, failed due to major deficiencies in the control logic.
After the parachute opened, unpredicted vibrations caused the Inertial Measurement Unit to saturate, and improper handling led to the calculated attitude being upside down. Once the radar altimeter was turned on, this led the lander to think the measured altitude was negative. The software recognized this as being below the threshold for cutting the parachute and activating the thrusters to slow to a stop. However, the negative altitude meant the lander thought its mechanical energy was low enough since this is the sum of kinetic and potential energy. Schiaparelli shut down the thrusters and crashed on the Red Planet at 150 m/s around half a minute later. This reconstruction was made possible by the telemetry the lander sent back during parts of the descent.
While the surface landers have had little luck so far, the orbiters continue to provide valuable data. A few months ago, ESA celebrated the 20th anniversary of Mars Express’ launch by streaming live images from the Red Planet. The live coverage featured one new image roughly every minute, something that had never been attempted before. In the future, we can look forward to even more science, hopefully solving the mysteries related to methane.
ESA’s Martian exploration was supposed to continue with the long-delayed ExoMars 2022. The mission would have brought to the surface the Rosalind Franklin rover, once again in strong partnership with Russia. After the outbreak of the war in Ukraine, the collaboration effort fell apart, and the future is now uncertain for the European rover. Only time will tell how, or if, it will get to Mars.