Methane (CH4) is the second most important greenhouse gas (GHG) after carbon dioxide (CO2). It has a global warming potential (GWP) of 28 over 100 years, which makes it more efficient than CO2 at lower concentrations. The main anthropogenic sources of CH4 are emissions from the production of energy, landfills, waste treatment, cattle, rice paddies and the incomplete combustion of biomass. There are also major natural sources of CH4, such as wetlands. Due to human activities, the atmospheric mixing ratio of CH4 has increased by over 2.5 since 1750. Furthermore, methane also increases the radiative forcing of ozone and water vapour in the stratosphere. Taking into account these indirect effects, the radiative forcing of CH4 attains about 60% of that of CO2 despite a much lower atmospheric mixing ratio.
The methane cycle is largely imperfect. Even on a global scale, the different types of emission have never been accurately quantified. CH4 emissions may be estimated either through a bottom-up approach, by directly measuring fluxes constraining models representing the processes at work and GHG emissions inventories, or through a top-down approach by inverting atmospheric methane observations using a chemical transport model. This second approach relies on surface observation networks which are very accurate but unequally distributed worldwide, and on reconstructions of methane columns by spaceborne passive remote sensing instruments. Even though these passive methods supply many additional data to constrain the methane cycle, the data lack precision, are subject to bias (such as aerosols), and do not offer sufficient coverage of extensive areas of key importance for the methane cycle (upper latitudes and cloud-covered areas, for example).
The CH4 Lidar on Merlin is designed to improve this situation by providing methane data collected over the whole Earth, and particularly areas of vital importance for the methane cycle not currently covered, or insufficiently covered, i.e. the Arctic, Eurasia and tropical continents. An active instrument such as the Merlin Lidar should offer more accurate data than current passive instruments because it is less influenced by the bias associated with the presence of particulate layers in the atmosphere and offers observation of upper latitudes. Its small 100-metre footprint and the absence of bias caused by particulate layers should allow reconstruction of a methane column even in complex meteorological situations. This relative insensitivity of active instruments to bias is fundamental for atmospheric inversion systems, which generally assume zero bias.
Global warming -- particularly in the boreal regions -- could lead to a thawing of the permafrost, unlocking the substantial quantities of methane it contains. There are also methane hydrate deposits in oceanic continental shelves that could be sensitive to ocean warming. Another important example is the alteration of tropical or boreal precipitation patterns associated with climate change, which could significantly impact the emission of methane in flooded areas, wetlands constituting the primary source of methane on a global scale.
The development and implementation of an observation and monitoring system to detect CH4 in these vulnerable areas is therefore of capital scientific importance. Merlin's CH4 Lidar aims to demonstrate the feasibility of providing a relevant response to meet these major scientific challenges.
Merlin main programmatic goal is to develop, launch and operate an earth observation satellite in French-German cooperation to provide relevant information for the climate change forecast.
CNES, the French space agency, and DLR, the German Space Administration, in order to strengthen their cooperation in space activities, decided to develop an Earth-Observation mission, dedicated to methane (CH4) monitoring. Methane is an important component of the global carbon cycle; it contributes significantly to the warming of the Earth's climate. The main science objective is to bring a significant improvement on the retrieval of CH4 fluxes which requires 1% accuracy on CH4 column averaged dry-mixing ratio, at a 50 km horizontal resolution.
The Merlin (MEthane Remote sensing LIdar missioN) mission is based on a small satellite for space-based measurement of spatial and temporal gradients of atmospheric methane columns on a global scale. The space segment consists of the new platform product line named Myriade Evolutions (range of 400 kg) developed under CNES control, and the first IPDA (Integrated Path Differential Absorption) lidar (Light Detecting And Ranging) instrument under DLR responsibility.
The Merlin satellite will be operated at an altitude of around 500 km, on a sun-synchronous orbit, either at 06:00 or 18:00 of the local time of the ascending node, depending on launch opportunities.
The IPDA technique enables measurements in all seasons, at all latitudes. The IPDA technique relies on DIAL (Differential Absorption Lidar) measurements using a pulsed laser emitting at two wavelengths, one wavelength accurately locked on a spectral feature of the methane absorption line, and the other wavelength free from absorption to be used as reference.
Merlin viewing concept
© CNES/Illustration D. Ducros
Aside from the space-segment, the other key part of a mission is its ground segments consisting of a Control Ground Segment and a Payload Ground Segment.
- The Merlin Control Ground Segment is in charge to command and control the space segment. It uses the CNES Earth terminal network.
- The Merlin Payload Ground Segment is devoted for commanding and monitoring the payload, and it performs the payload data processing which ensures availability of science data according to science requirements.
The mission data products are divided into different levels (from 0 to 3), and distributed in accordance with the data policy defined by the instances which contribute to the mission.