ExoMars: Searching for Life on Mars
Artist's illustration of the ExoMars Trace Gas Orbiter at Mars.
Credit: ESA/ATG medialab

ExoMars is a multi-part European-led program to explore Mars, both on the surface and from above. The program has two phases in the works. The components of the first phase, the Trace Gas Orbiter (TGO) and Schiaparelli (a landing demonstrator), both arrived at Mars in October 2016. Schiaparelli failed during landing, but TGO remains in excellent health. It is performing science work and will serve as a communications relay for the second phase of the program — a rover and landing surface platform, which are expected to launch in July 2020. 

The ExoMars program took on new significance in June 2018, when NASA's Curiosity rover once again showed evidence of ancient organic molecules — carbon-based chains of molecules that can be associated with life near the surface of Mars. Since the ExoMars rover can dig farther below the surface — where radiation and wind wouldn't destroy delicate samples — it is possible that more organics might be found there. ExoMars has a special set of instruments designed to search for organic molecules, particularly lipids and organics that have chirality (that are left-handed or right-handed, since certain combinations are favorable for life.)

Astronomers are also interested in the methane spikes that Curiosity observed from the surface of Mars. Methane can be a sign of microbial activity, or a sign of geological activity — it's difficult to tell what from a single site. The ExoMars trace gas orbiter is designed to search for methane and other minor constituents of the Martian atmosphere on a global scale. Over time, the orbiter is expected to provide information on the different kinds of gases, their abundances, and if they vary seasonally or by region.

In 2001, ESA started ExoMars as part of the larger Aurora program, which was to have taken humans to Mars. While Aurora was eventually abandoned, ExoMars was formally approved by the ESA member states in 2005. In 2009, NASA and ESA signed a Mars Exploration Joint Initiative that was supposed to meet the goals of both ExoMars and the planned NASA Mars Science Orbiter. 

The NASA/ESA missions were restructured into two phases. The first phase, which would launch in 2016, was supposed to include a European orbiter and landing demonstrator module, which later became the Trace Gas Orbiter (TGO) and Schiaparelli. The second phase initially was planned to include two rovers, but was deemed too complex and scaled back to one European rover with U.S. instruments.

In 2012, however, NASA pulled out of ExoMars, in part due to budget overruns with the James Webb Space Telescope. ESA then made an agreement with Roscosmos to replace the launch vehicles and parts of the payloads that NASA was supposed to provide. TGO and Schiaparelli launched in March 2016 as planned. The European rover and a Russian landing platform were expected to launch in 2018, but delays with industrial activities and the payloads pushed that back two years to July 2020. (Mars launch opportunities from Earth only take place roughly every two years, when the planets are relatively close to each other and a spacecraft can use a minimum of fuel to get there.)

The goal of the Trace Gas Orbiter (TGO) is to search for less-abundant components of the Martian atmosphere. The Martian atmosphere is mostly made up of carbon dioxide, but concentrations of other molecules are poorly understood. For example, methane — a sign of either biological or geological activity — has been measured in different concentrations by different ground-based telescopes. The Curiosity rover has made repeated measurements of methane on the surface, but a global view of Mars would give a better sense of the methane's source or sources.

"Since methane is short-lived on geological time scales, its presence implies the existence of an active, current source of methane. It is not clear, yet, whether the nature of that source is biological or chemical," ESA stated. "Organisms on Earth release methane as they digest nutrients. However, other purely geological processes, such as the oxidation of certain minerals, also release methane."

TGO and Schiaparelli were launched together on March 14, 2016, from a Proton rocket from Baikonur, Kazakhstan. TGO successfully entered orbit at Mars on Oct. 19, 2016, the same day of Schiaparelli's landing attempt, which failed. Two days after the malfunction, NASA's Mars Reconnaissance Orbiter photographed evidence of Schiaparelli's crash site, and additional pictures were sent in over several weeks. In 2017, an ESA investigation showed that a data glitch caused the Schiaparelli crash.

Meanwhile, TGO was inserted into a highly elliptical orbit at Mars that took four Earth-days to complete. To perform its main science mission, it was lowered into a near-circular altitude of about 400 kilometers (250 miles) and had a two-hour orbit. Starting in 2017, mission controllers made a series of controlled skims through the edge of the Martian atmosphere. This technique is called "aerobraking" and has been performed by several other Mars missions, as well as the European Venus Express mission. It finished its aerobraking in February 2018 and sent its first image, of Korolev Crater, that April. More science results are expected to come now that TGO is in its main mapping orbit.

TGO has four principal instruments:

  • NOMAD (Nadir and Occultation for Mars Discovery) — a package of three spectrometers (two infrared, one ultraviolet) to identify methane and other parts of the atmosphere. Some elements will be found by looking at the atmosphere with the sun behind it, while others will be examined by direct reflected-light observations.
  • ACS (Atmospheric Chemistry Street) — three infrared instruments will provide information on the Martian atmosphere's chemistry and structure.
  • CaSSIS (Colour and Stereo Surface Imaging System) — provides high-resolution images of the surface that will give geological context — and the possible sources or sinks — for trace gases found by NOMAD and ACS.
  • FREND (Fine Resolution Epithermal Neutron Detector) — maps potential deposits of water ice by looking for hydrogen on the surface to depths of up to one meter (3 feet).

Besides its science mission, TGO is expected to serve as a communications relay for the ExoMars 2020 rover when it reaches the Martian surface. (TGO was also supposed to send communications from the failed Schiaparelli lander to Earth, but that part of the mission was never realized.)

The ExoMars 2020 mission will include a European rover and a Russian surface platform. The rover will provide information about potential signatures of life on Mars, specifically by looking at environments where water could have flowed. It also will carry a drill that can penetrate up to 2 meters (6 feet) below the surface. 

The rover and surface platform will travel together to Mars inside a European carrier module. Shortly before descent, a Russian-led descent module will separate from the carrier and bring the rover and surface platform to the surface, using elements such as parachutes and thrusters to reduce the speed of landing. 

After landing, the rover will leave the landing platform behind to move around Mars and look for organic material from the planet's past. Meanwhile, the surface platform (which is stationary) is expected to operate for about one year. The platform will take pictures of the landing site, watch the local weather, probe the internal structure of Mars, and do investigations of the atmosphere. It also will look at subsurface water distribution and radiation around the landing site, in comparison with measurements from TGO.

"The primary objective is to land the rover at a site with high potential for finding well-preserved organic material, particularly from the very early history of the planet," ESA said

"The rover will establish the physical and chemical properties of Martian samples, mainly from the subsurface. Underground samples are more likely to include biomarkers, since the tenuous Martian atmosphere offers little protection from radiation and photochemistry at the surface. The drill is designed to extract samples from various depths, down to a maximum of two meters."

In January 2018, the life-detection package successfully found microbes in the Canadian Arctic, an analog environment to Mars. Checkouts are continuing on the rover as its instruments are completed; integration is expected in 2019 ahead of the 2020 launch date.

Instruments on the rover include:

  • PanCam (Panoramic Camera)
  • ISEM (Infrared Spectrometer for ExoMars)
  • CLUPI (Close-UP Imager)
  • WISDOM (Water Ice and Subsurface Deposit Observation On Mars)
  • Adron (which will look for subsurface water and hydrated minerals, in combination with WISDOM)
  • MA_MISS (Mars Multispectral Imager for Subsurface Studies)
  • MicrOmega (a visible and infrared imaging spectrometer)
  • RLS (Raman Spectrometer)
  • MOMA (Mars Organic Molecule Analyser)

Instruments on the surface platform include:

  • LaRa (Lander Radio-science experiment)
  • HABIT (Habitability, Brine, Irradiation and Temperature Package)
  • METEO-M (a meteorological package). This includes sensors to measure pressure (METEO-P), humidity (METEO-H), radiation and dust (RDM) and magnetic fields (AMR).
  • MAIGRET (a magnetometer), including a Wave Analyzer Module (WAM)
  • TSPP (cameras)
  • BIP (instrument interface and memory unit)
  • FAST (IR Fourier spectrometer to study the atmosphere)
  • ADRON-EM (active neutron spectrometer and dosimeter)
  • M-DLS (Multi-channel Diode-Laser Spectrometer for atmospheric investigations)
  • PAT-M (radio thermometer for soil temperatures, including below the surface)
  • A dust suite to look at dust particle size, impact and atmospheric charging
  • SEM (a seismometer)
  • MGAP (gas chromatography-mass spectrometry for atmospheric analysis).

A landing site will be selected about six months before the mission launches in 2020. When the rover was originally expected to launch in 2018, ESA's landing site selection working group recommended landing in Oxia Planum. ESA reopened the site selection due to the two-year mission delay, and in 2017 Oxia Planum and Mawrth Vallis were selected as semi-finalists. Both regions have old rocks and possible regions where ancient microbes would have flourished.