A wide range of instrument types would add scientific value to a long-term landed platform on a martian moon. A mission study for such a platform would necessarily trade off the science potential with engineering, safety and programmatic factors, including mass, power, data, budget and science priorities. For justification and context to the visibility calculations we have performed, in this section we summarise selected opportunities for science and operations, and describe the associated instruments and mission elements required.
Building on MMX science results
The origin of the Mars moons remains a compelling question in Mars science. Characterisation by remote sensing instruments of their morphology (Duxbury 1978, 1991; Muinonen et al. 1991; Duxbury et al. 2014; Willner et al. 2014; Witasse et al. 2014), interior (Pätzold et al. 2014, 2016; Cicchetti et al. 2017; Le Maistre et al. 2019), spectral traits (Ksanfomality et al. 1991; Deutsch et al. 2018; Pajola et al. 2018) and dynamical modelling seeking to explain their orbital elements (Rosenblatt et al. 2016) have thus far have not yielded scientific consensus on their origin. The two leading hypotheses entail that they are either captured asteroids, or remnants of a debris disk generated by a giant impact on Mars (Craddock 2011; Rosenblatt 2011; Rosenblatt and Charnoz 2012).
The importance of this question has provoked advocacy for studies and missions with objectives to elucidate the Moons origins, with particular emphasis on sample return (Galeev et al. 1996; Murchie et al. 2014; Usui et al. 2020). The JAXA MMX mission seeks to resolve this uncertainty with in-situ characterisation using remote sensing instruments and analysis of material sampled from Phobos and returned to Earth.
The MMX spacecraft is scheduled to arrive in the Mars system in 2025. During 2026–2027 the mission schedule involves characterisation of Phobos, selection of a landing site for sampling, deployment of a rover (Ulamec et al. 2021) and Phobos sampling before departure from the Mars system in 2028 for return to Earth in 2029 (Campagnola et al. 2018). The potential for a Phobos platform mission to build on findings from the MMX mission can be described in two main areas.
Firstly, remote sensing observations: Prior to landing on the surface of Phobos, the platform’s cruise/deployment stage would be expected to orbit at a low Phobos altitude during a proximity phase, during which remote sensing observations could provide further data to address outstanding questions about Phobos that remain or arise following MMX. This opportunity for observations using science payloads on the platforms’ cruise/deployment stage could offer more coverage (spatial, spectral or temporal), use of next-generation instrumentation, larger resources (time, energy, payload mass, etc.) or different orbital characteristics (for example, to observe at specific emission or solar phase angles), to augment the science return from the MMX mission.
Secondly, observations or measurements to support analysis of returned Phobos samples: Following landing of the platform, payloads for in-situ sensing of local regolith, or sampling and analysis, could be operated to provide observations and measurements to support ongoing analysis of Phobos samples returned by MMX and that would be under analysis in terrestrial laboratories. Based on the outcome of MMX, remaining targets of interest that were not able to be sampled could be investigated further in situ. For example, the giant impact hypothesis for the martian moons implies the moons are composed of a mixture of impactor and martian materials (Hyodo et al. 2015; Rosenblatt et al. 2016). If results from MMX prove this, then the sample mass returned may represent a disproportionate mix relative to the moon’s bulk fractions, and further in-situ analysis may aid to constrain proportions of the progenitors that make up the moon. This example is one of a range of in-situ analysis that could be assessed for their potential to further Phobos science following MMX.
Martian moon surface processes
The long-term operation of a platform on Phobos affords observations with high continuity and long temporal baselines that could be suitable for monitoring surface processes. For example, slow or rare changes in surface properties caused by regolith gardening, mass wasting (Shingareva and Kuzmin 2001) or space weathering (e.g. Nénon et al. 2021) could be observed. Albedo streaks on the slopes of crater rims may be the result of downslope movement and recent geological activity (Basilevsky et al. 2014). Material transport may also be influenced by impact-induced seismicity and thermal cycling. Long-term monitoring of these surface processes and their effects would inform study of surface processes on airless bodies.
Geodesy and fundamental physics
Ranging capabilities on a landed platform could provide opportunities in geodesy and fundamental physics. Study of an active laser transponder presents a case for Phobos as a platform for measuring space curvature (Turyshev et al. 2010). Such a system would allow direct measurement of Phobos libration, deformation, gravity field and interior structure. Study of measurement requirements for such a system (Dirkx et al. 2014) concluded that it could produce order of magnitude precision improvements in geodetic measurements of both Mars and Phobos. Radio science investigations make use of existing spacecraft communications systems, though to enable the needed precision, an ultra-stable oscillator (USO) is also typically required on board.
Remote sensing of the Mars atmosphere and surface
Instruments with the ability to spatially resolve features on surfaces or in atmospheres are typically designed and selected with traceability of their science objectives to their specifications and performance. A landed platform on the Mars-facing side of Phobos would provide a vantage point for instruments to observe both globally, studying the full martian disk for meteorology and climate, and at finer spatial scales for study of regional or local atmospheric phenomena and surface features. The angular resolution of an instrument is visualised in Fig. 2, in which we illustrate the types of scientific study enabled as a function of distance to the target and angular resolution of the observing sensor. Phobos’ mean orbital altitude above Mars provides a basis for a wide range of surface and atmospheric observations. Instrument spatial resolutions may be on the order of 10 s to 100 s of metres, sufficient for resolving most atmospheric phenomena, and local to regional-scale geology, though less suitable for study of fine-detail surface characterisation afforded by high-resolution instruments aboard low-altitude orbiters, such as the High Resolution Imaging Science Experiment (HiRISE) aboard Mars Reconnaissance Orbiter, MRO (McEwen et al. 2007). However, fixed assets observing Mars from Phobos and Deimos could provide frequent, regular observations over long temporal baselines and therefore the potential science return is high for many observation types, e.g. context or survey observations in the visible to far-infrared. The temporal longevity of such an observing platform may be ideal for monitoring seasonal or longer baseline signals of surface change. We do not consider here the technology improvements expected with time, nor the engineering challenges and physical limits in relation to development of high spatial resolution instruments. These may enhance, or limit, science activities that are possible.
For atmospheric studies, Phobos offers a preferential position for a long-term monitoring of the Martian atmosphere on both Mars' dayside and nightside. Additionally, it could permit monitoring auroras (e.g. via UV instrument) and meteor flux (e.g. Christou et al. 2012) on the nightside. On the dayside, observations and retrieval of atmospheric conditions would provide data supporting study of climate and atmospheric processes, as well as monitoring of clouds or dust storms affecting surface missions.
Typical atmospheric parameters of interest include temperature, dust, CO2 ice, H2O vapour and ice, CO or CH4. A summary of the wavelength ranges (from UV to mid-IR) that allow retrieval or detection of these parameters is summarised in Cardesín-Moinelo et al. (2021), with reference to the OMEGA, SPICAM and PFS spectrometers aboard Mars Express, and the NOMAD and ACS spectrometers aboard TGO.
Next-generation instruments may offer improved capabilities for retrievals from Phobos’ altitude above Mars. The focus may be on long-term reliable survey and monitoring of atmospheric parameters, though application of cutting edge techniques, such as retrieval of wind speed profiles by measurement of Doppler shift of aerosol backscatter profiles (Cremons et al. 2020). Phobos’ orbit would allow atmospheric retrievals at nadir in equatorial regions, with the emission angle of observations increasing with latitude, leading to opportunities for limb observations in polar regions.
A visible wavelength atmospheric imager would follow and build on the datasets provided by The Mars Color Imager (MARCI) aboard MRO (Malin et al. 2008) and the Visual Monitoring Camera aboard Mars Express (e.g. Sánchez-Lavega et al. 2018). Whole Mars-disk observations would be common at Phobos’ altitude, and could provide a basis for regular ‘weather reports’ and a compelling opportunity for public outreach (Ormston et al. 2011).
Radio occultation of the Martian atmosphere, and most pertinent to this technique—the ionosphere, could be performed between Earth and Phobos, between Phobos and Deimos (assuming a receiver or transmitter on Deimos), or between Phobos and Mars orbiters, to retrieve atmospheric and ionospheric properties. The approach has been successfully demonstrated between the Mars Express and ExoMars Trace Gas Orbiter (TGO) spacecraft (Nava et al. 2020, 2021; Cardesín-Moinelo et al. 2021).
Mars space environment monitoring
The equatorial orbits of Phobos and Deimos offer opportunities for monitoring the space environment and its interaction with the moons. Investigation of the martian magnetotail is of particular interest, since both moons cross the current sheet a few times per day, and Phobos’ ~ 8 h orbital period permits frequent sampling of the martian bow shock, magnetosheath, magneto pileup boundary and terminator-nightside ionosphere current sheet. These are regions of special interest for important plasma processes, such as atmospheric escape, solar wind particle precipitation, solar wind interaction with the induced magnetosheath or crustal field reconnection with the solar wind (e.g. Halekas et al. 2016; Hall et al. 2016, 2019; DiBraccio et al. 2018; Sánchez-Cano et al. 2019). At the time of writing, there are no observations made from an equatorial orbit of these regions of the martian tail. The Indian Space Research Organisation’s (ISRO) Mars Orbiter Mission, ‘Mangalyaan’, has an equatorial orbit, but it does not host sufficient plasma instrumentation. The moon’s equatorial orbits around Mars provide an opportunity to investigate the horizontal extent of the martian tail for the first time, and in combination with measurements by polar orbiters. Investigations could focus on energetic particles, magnetic field strength, plasma properties and dynamics of space dust flux/micrometeorite impacts (Zakharov et al. 2014) on the moons. Long-term monitoring investigations of the surface environment could be conducted, for example, on surface charging and sputtering products in response to solar irradiance and solar wind cycles (Dubinin et al. 1991; Cipriani et al. 2011). Measurement of the solar wind at Mars would also provide value as an upstream monitor of space weather conditions to outer solar system spacecraft. Instrument types and investigations could include plasma packages designed for ionospheric and magnetosheath regions (including atmospheric escape), solar wind and cosmic ray monitoring, seismometers, dust detectors or electric field sensors.
Data relay and navigation
A platform on Phobos could provide relay functionality for spacecraft telemetry and telecommands. Communication would be possible with assets on the surface of Mars, such as rovers, landers or future human missions. Communications capacity similar to or exceeding that of present orbiters (MRO, ExoMars TGO) could be desirable, and could use Ka, X and UHF bands. Data downlinks, e.g. ExoMars TGO, are on average on the order of 10Gib per day, though the link budget varies with Earth–Mars distance.
Relay of data from and to interplanetary spacecraft outside the Mars system, including in the outer solar system (for example ESA’s Jupiter Icy Moons Explorer, JUICE), would also be desirable. This would work to increase the data rates of individual hops and hence the aggregate bandwidth for downlink of science data. Time for mission operations usually lost due to solar conjunction could be regained by the presence of a relay in the Mars system. Risks to spacecraft operations could be reduced by the ability to check spacecraft health and navigation during solar conjunctions.
To fully understand the requirements and feasibility for a communications relay system on a landed Phobos platform, a study of the link architecture should be performed. Future Mars exploration may require or benefit from orbital positioning systems, for which one or more elements could be provided by equipment on Phobos or Deimos.