The Value of Landed Meteorological Investigations on Mars

The Value of Landed Meteorological Investigations on Mars: The Next Advance for Climate Science

For Consideration by the National Research Council Space Studies Board in the Development of the Next Planetary Science Decadal Survey September 2, 2009

Scot C. R. Rafkin Jim Murphy, New Mexico State University Southwest Research Institute Jon Weinberg, Ball Aerospace Corporation Department of Space Studies, 1050 Walnut Street, Suite 300 Boulder, CO, USA 80302 The International Space Science Institute Mars (720)240-0116 (O) Planetary Boundary Layer Study Team Members: (303)546-9687 (F) Arakel Petrosyan IKI, MOSCOW (Russia) [email protected] [Chair] and Boris Galperin Univ. South Florida (USA) Robert. M. Haberle, NASA Ames Research Center Kurt Gunderson Univ. BERN (Switzerland) Don Banfield, Cornell University Soeren Larsen RISØ National Laboratory Jeff. Barnes, Oregon State University (Denmark) Stephen Lewis Open University (UK) With contributions by: Anni Määttänen, LATMOS, UVSQ, CNRS/INSU, John Andrews, Southwest Research Institute IPSL (France) Jill Bauman, NASA Ames Research Center Peter Read Univ. OXFORD (UK) Michael Bicay, NASA Ames Research Center Nilton Renno Univ. MICHIGAN (USA) Anthony Colaprete, NASA Ames Research Center Mark Richardson CalTech (USA) Rich Dissly, Ball Aerospace Corporation Peter Rogberg Univ. Oxford (UK) Ronald Greeley, Arizona State University Hannu Savijarvi Univ. Helsinki (Finland) Robert Haberle, NASA Ames Research Center Walter Schmidt FMI, Helsinki (Finland) Jeffrey Hollingsworth, NASA Ames Research Center Karsten Seiferlin Univ. Bern (Switzerland) Conway Leovy, University of Washington Aymeric Spiga Univ. Paris VI (France) and Marc Murbach, NASA Ames Research Center Open University (UK) Adam McKay, New Mexico State University Nick Thomas Univ. Bern (Switzerland) John McKinney, The Boeing Corporation Anthony Toigo Cornell Univ. (USA) Timothy Michaels, Southwest Research Institute Luis Vazquez Univ. Complutense of Franck Montmessin, Service D’aéronmie Madrid (Spain)

Additional XX supporting signatories may be found at: http:://mepag.jpl.nasa.gov

1 The Case for Mars Surface Climate Investigations Complementary whitepapers submitted for consideration by the Decadal Survey Committee make the case for Mars as a high- priority exploration target1 and for Mars climate investigations in particular2. In this paper, we argue that major advances in the understanding of the present and past Mars climate system are most likely to be accomplished by in situ meteorological surface measurements operating from both a network configuration and individual stations. Support for this position is based on

1 the scientific output from past and ongoing The processes operating at the surface Mars atmosphere measurements, and within the planetary boundary layer expectations of future orbital information, the (PBL) are the engines that drive the climate unique science enabled only by surface and constantly modify the geological measurements, and the nature of the key landscape. To understand how the current outstanding climate science objectives. climate system operates, it is necessary to Therefore, the Space Studies Board is urged to understand the climate engine. And, to recommend that all future landed Mars understand why Mars’ past climate may have missions carry a capable meteorological been warm and wet, or to understand how it investigation, and that the recommendation evolved to its present state, it is necessary to for a Mars meteorological network as a high- understand how the present state works. In priority mission within NASA’s Mars situ surface measurements are the Exploration Program be retained from the underpinning to this understanding, and for previous Decadal Survey. the most part, none of these measurements A great deal was learned3,4,5,6 from the can be made from orbit. Looking holistically meteorological observations of the Viking at the history of Mars atmospheric Landers over 30 years ago, including the measurements both from orbit and from the identification of a strong seasonal cycle of surface, it is clear that there is a significant

CO2, the existence of extratropical storm and substantial lack of information about systems similar to those on Earth, and the surface meteorology and the processes documentation of a large amplitude thermal operating at the surface-atmosphere interface atmospheric tide. Although surface and within the PBL. meteorology investigations were later Surface measurements are advantageous conducted by the Pathfinder7 and Phoenix8 compared to satellite-derived data. Geo- missions, these data lack the time coverage asynchronous satellites are unable to provide needed to document the strong seasonal a continuous time series of measurements for cycles and climate of Mars, and many of these a given location, and past, present and data suffered from calibration problems, planned Mars orbiters are unable to obtain limited sampling, or sensitivity. anything close to continuous full time of day Numerous orbiting spacecraft have also coverage. Neither nadir nor limb retrievals provided a wealth of information on (even through multiple orbiter occultations) atmospheric state. The most groundbreaking satisfactorily provide local information of information was obtained by the nearly three surface meteorology and processes, and Mars years of vertical temperature profiles neither method provides synoptic coverage. and column water vapor, ice, and dust opacity Satellites cannot provide the continuous, data acquired with the Thermal Emission long-term measurements at a single point Spectrometer (TES)5. needed to achieve seasonal, diurnal or Meteorological measurements at the turbulent statistics. Further, satellite surface of a planet provide information that is retrievals often require complex inversions of difficult, if not impossible, to obtain from measured radiance. Surface measurements orbit. It is at the surface and within the provide both a convenient lower boundary planetary boundary layer (PBL) immediately condition for atmospheric retrievals and a above where there are large exchanges of validation point for satellite retrievals. heat, momentum, dust, water, CO2, CH4 and To date, emphasis has been on orbital other volatiles. It is at the surface where the measurements that cannot adequately weather shapes, through aeolian processes, address the surface-atmosphere interaction. the surface of contemporary Mars. Therefore, landed missions are required to complete the characterization of the climate

2 system. Without in situ measurement, the FINDING Mars climate picture of both past and present Orbital retrievals are valuable, but are will remain incomplete. The need for surface measurements remains as strong as it did not a substitute for in situ measurements. There is high priority prior to the last Decadal Survey. 2 Traceability of Surface Science science that is best achieved or can only be achieved from the surface. The Mars Exploration Program Analysis Group (MEPAG) has identified a prioritized The role of surface measurements in list of science objectives and investigations relation to the understanding of climate 12 under the Mars Climate Goal : processes is through investigations of the CO2, H2O and dust cycle, which involve the MEPAG Climate Goal: Understanding the Processes and History of Climate exchange of heat, mass, and momentum between the atmosphere and surface, and the on Mars. variation of these processes around the Highest Priority Objective planet and over time. MEPAG has explicitly Characterize Mars’ Atmosphere, Present recognized the importance of these cycles and Climate, and Climate Processes Under the unique and necessary contribution from Current Orbital Configuration in situ measurements at the surface. Highest Priority Investigation 3 Implementation Strategy The only way to address the highest Determine the processes controlling the priority investigation with a single mission is present distributions of water, carbon to establish a long-lived global network dioxide, and dust by determining the capable of measuring a variety of short- and long-term trends (daily, fundamental parameters (e.g., T, p, relative seasonal and solar cycle) in the present humidity, winds, dust) and fluxes of these climate… quantities with the global monitoring support There has yet to be a mission dedicated to the of one or more orbital assets. first half (lower atmosphere) of the highest FINDING priority investigation13. None of the previous landing sites (VL1, VL2, Pathfinder, Phoenix) Regardless of the mission architecture, were driven by climate science or maximizing the dynamic range of the climate system climate science return. The same is true for mandates that the full achievement of the upcoming MSL mission. Of all the the highest priority MEPAG Climate recommended missions within the last Science Goal and Objective will require Decadal Survey14, the Mars Meteorology long-term, global measurements. Network mission, which would have The CO2, water, and dust cycles are all of substantially addressed the MEPAG goal, has global extent, and monitoring requires global gone mostly ignored. coverage. The minimum number of stations Climate is the long-term value, trend, and required to accurately determine the global variance of weather, and has properties that mass of the atmosphere is ~16 stations, vary in space and time on scales that range roughly equally spaced over the planet15. from local (microscale) to global (planetary- FINDING scale) and from seconds (e.g., turbulent eddies) to years (e.g., climate change). Thus, A global meteorological network designed to address the global MEPAG climate to capture the climate, it is necessary to measure the weather at appropriate spatial objective requires at least 16 nodes. and time scales. Long-term, global, and high- Given the reality of limited available frequency measurements are, therefore, a resources and the state of technology, the necessity. best strategy for achieving the MEPAG science 3 goals with surface investigations consists of mesoscale phenomena such as frontal two parts. The first part is to ensure that structure16, local dust storms17,18, polar lows19, every future lander carry meteorological gravity wave excitation20,21,22, and bore payloads that can begin to address the highest waves22. Thus, payload sophistication is an priority MEPAG climate investigation. advantage for non-network missions that can Operating under this strategy, the payload enable fundamental new and important should be sufficient to fully characterize measurements. exchange processes (e.g., fluxes) at a limited Recognizing that resources can be limited number of sites. The resulting data can then even on single lander missions, there is a be used to infer exchange processes from a hierarchy of possible payloads each of which global, but likely less sophisticated and can contribute to our understanding of capable network investigation. The second atmospheric science at Mars. The simplest part is to work toward the implementation of useful investigation that could be addressed such a global network. This strategy is with a single lander is the global mass realizable in the next decade. balance, which requires the measurement of Embedded within this strategy is the only one parameter: surface pressure. assumption that under realistic scenarios, the Surface pressure varies not only because of trade between a few meteorological stations meteorology, but also because the main versus a network is likely to be one of atmospheric constituent of the Martian sophistication versus number. Non-network atmosphere (CO2) alternately condenses and missions, because they have fewer landers, sublimes in the polar regions. Thus, surface could carry more sophisticated payloads, pressure records the heartbeat of the climate measuring parameters beyond the core system and, because pressure sensors are pressure, temperature and winds. A network, light, require little power, and do not need even though potentially less capable, provides orientation or deployment, they should form information about the variability and the core of any landed meteorology package. diversity of climate processes for which even Pressure is the highest priority measurement. a fully equipped single lander is not capable. Air temperature is relatively easy to Network and non-network missions are measure, and provides information about highly complementary and both are required water and CO2 volatility, and even to achieve the high-priority science goals atmospheric dust loading, which modulates identified by the community. the diurnal cycle. Thus, air temperature There are many types of meteorological should also be included as part of a basic payloads worth flying on single or multiple meteorology package for all future Mars lander non-network missions. Investigations landers. addressing the global mass balance, local However, the next major increase in our circulations, the planetary boundary layer, understanding of the near-surface aeolian processes, and the exchange of heat, environment will come from high-quality momentum, and water between the surface systematic measurements of winds. For this and atmosphere are all feasible from a single reason, winds are the next highest priority for station. We presently have limited any landed meteorology payload after meteorological data from four sites (VL1, VL2, pressure and temperature. Surface fluxes are Pathfinder, and Phoenix) widely separated in major forcing functions of atmospheric time and space, and these have provided a motions, yet very little is known about their glimpse at a small sample of the rich diversity magnitude and variability; fluxes require the of meteorological regimes that surely exists high-frequency measurement of the vertical around the planet. Missions with a limited set wind23. To maximize return, these of landers could, in principle, address measurements should be made at three or

4 more heights. Thus, even a single lander Given the mature state of measuring pressure, temperature, and winds meteorological instruments, their would acquire fundamentally new and technical readiness (most are at or important data. above TRL 5), low cost, relative ease of After the core parameters of pressure, implementation, and high value to temperature, and wind, are simultaneous science and engineering, credible measurements of dust and moisture meteorological instruments must be concentrations that permit the direct in situ part of every future landed package to determination of local sources and sinks of Mars. these quantities. The seasonal dust cycle has There has been much confusion over what a strong influence on atmospheric circulations and therefore controls how constitutes a meteorology network. A network must have a sufficient number of material is transported around the planet;24,25 yet, the conditions that enable lifting off the nodes so as to conduct network science. Anything less is not a network, but a surface and injection into the atmosphere are poorly understood. Wind measurements collection of simultaneously operating stations. At a recent Mars Meteorology coupled with simultaneous particle 29 concentration measurements would enable Workshop , consensus was reached among stakeholders using a practical definition for a the determination of the threshold stress required for lifting26. This has yet to be done network: a network provides information and science not attainable by the measurements for Mars and it can only be done from the surface. of the nodes taken individually. Network science is achieved by combining node For water, there is still uncertainty about the role of adsorbed water in the regolith measurements to create information that could not otherwise be attained through and/or ice beneath the surface27,28. Near- surface vapor measurements would enable individual measurements, by analyzing network measurements to identify patterns the determination of the direction, and magnitude of the vapor flux exchange . and spatial distributions, or by acquiring simultaneous measurements of similar Other important measurements yet to be made at the surface are the downwelling quantities in order to improve the signal-to- noise ratio. infrared radiation and total solar flux. The net radiative flux at the surface is the primary As previously indicated, the ideal network would consist of dozens of long-lived nodes driver of the climate system, and when coupled with surface turbulent fluxes would with highly capable instruments similar to the payload desired on a single station lander. allow for full characterization of the boundary forcing. Realistically, a network will likely be much less capable. Thus, the role of the individual It is clear that there is great value in surface meteorological measurements from a stations is to provide details on the complex processes operating at a limited number of single lander, or from a multiple set of landers too few in number to constitute a true locations on the planet, while the role of the network is to obtain a diversity of meteorological network. Furthermore, these measurements address fundamentally simultaneous measurements to obtain context for the surface (and orbital) important processes that are active on Mars today, have not yet been measured, and are measurements, to extend the local information to the global scale, and to obtain not merely a repeat of earlier weather observations. The Martian climate system is information meaningful only on a global scale. process driven and spatially diverse. FINDING FINDING 5 Meteorology should remain as a high- the water cycle operated thousands, millions priority Mars network investigation, as it or billions of years ago. Observations from was in the previous Decadal Survey. present-day Mars permit the development and testing of hypotheses that can explain the The global CO2 cycle produces the behavior of the global water cycle. Armed strongest climate signal on Mars in the form with this knowledge, it becomes a more of the large seasonal variation of surface tractable problem to determine how these pressure. Pressure is the primary quantity of same processes may have operated in the interest in studying the CO2 cycle, since it is past, or how the relative importance of tied directly to the mass of the atmosphere, processes may have changed over time. which is predominately CO2. One of the Atmospheric dust, through its radiative outstanding questions about the CO2 cycle is effects, is a primary driver of the circulation whether the atmosphere is undergoing on contemporary Mars. Furthermore, for at secular climate change and what the least the last several billion years, the magnitude of interannual variability of the dominant process shaping the surface of Mars climate is. Accurate, networked has likely been erosion through aeolian measurements of pressure over several processes. Therefore, a greater understanding seasonal cycles would be able to make this of the dust cycle and dust lifting processes is a determination. A single station could also critical element to understanding the present- make strides in constraining the atmospheric day climate and to understanding how Mars mass balance, but the seasonal variation of evolved to the present state. A well-dispersed pressure will differ from one site to another global network of stations would provide due to variations in the general circulation15. information on the disturbances that lift dust. Therefore, a collection of simultaneous And, a modestly equipped network might measurements are needed to truly separate finally provide insight into what produces out the global effects from the local effects. global dust storms. A network might also “ Follow the water” has been the theme identify any precursor signatures to the onset driving Mars exploration for well over a of these storms33. decade. It is not known whether the current 4 Support for future missions water cycle is in equilibrium. Modeling The benefits of meteorological surface studies are unable to produce a balanced measurements go beyond science. In situ water cycle, and instead tend to show a net surface information is vital to the validation accumulation of water at the south pole at the 30,31 and improvement of the atmospheric models expense of the north water ice cap . that are used to predict the environment for Variations in obliquity almost certainly Mars spacecraft. And, as described in the provide a forcing mechanism that drives the MEPAG Mars Human Precursor Measurement water cycle towards different equilibrium document, monitoring of the lower states that may include water ice stability in atmosphere is essential for the safety of the tropics32. Therefore, the present humans, and for safe operation of the robotic reservoirs of water on Mars are time capsules, components. containing information about previous orbital Beginning with the Mars Exploration configurations, and, the atmosphere, being Rovers, substantial effort has gone into the primary reservoir exchange mechanism at characterizing the lower atmospheric present, controls the rate at which the environment for the entry, descent, and reservoirs respond to the obliquity changes. landing for spacecraft. Due to lack of data, The lack of information and these efforts have relied almost exclusively on understanding about the contemporary water the use of models34,35. Surface measurements cycle on Mars makes it extremely difficult to are needed to validate the models. If the rewind the clock and then understand how 6 atmospheric environment were better known, FINDING costs would be reduced by eliminating over- Additional network technology engineering, and additional resources could be made available to the scientific payload so development is needed, primarily in the areas of power, EDL, and communication. as to increase the scientific return per dollar. FINDING NASA has provided little to no focused support of projects that would help to reduce Networks provide a major risk reduction the risk of, and advance the technology for, and cost reduction benefit to future network missions. Nuclear power sources in missions by better constraining the the 1-10 W range are needed for long-term environment and improving and high-latitude meteorological stations, as environment predictions. are small probe EDL and deployment 5 Current and Needed Technology technologies. A commitment to a network by The instrument technology for measuring NASA should include resources to advance basic meteorological parameters is mature these technologies. (TRL 6-9). New, more advanced, accurate and 6 International Cooperation robust methods that enhance these existing The history of Mars network science measurement techniques are currently under efforts demonstrates strong interest by the development. Compared to typical Mars European community. A network mission is instrumentation, all of these sensors are well suited to international cooperation, relatively low power, low mass, low risk, low because the mission elements can be easily cost, and are relatively easy to accommodate separated. A partnership that makes use of with low data rates. the strengths of the global community is a FINDING cost-effective approach and might accelerate network mission endeavors. Leveraging Core instrumentation for a global technology would almost certainly meteorological station is mature and reduce the cost of a network mission to NASA. ready for flight. Advanced instrumentation is relatively advanced FINDING and can be credibly proposed. NASA should engage with foreign space There are numerous supplementary, agencies to enhance scientific expertise, modest TRL payloads that would enhance a leverage mutual technology mission. These include electromagnetic development, and to reduce the overall sensors, flux radiometers and dust optical cost to any one agency. depth sensors, and instruments that 6 Summary characterize the size distribution of dust. Surface meteorology measurements have The major challenges with surface not been given priority in the exploration of measurements are not primarily in the Mars over the last several decades. However, instrumentation, but in the implementation of there are high-priority science objectives and the architecture. How are the probes investigations that have been identified by the properly oriented for measurement upon community for which such measurements are landing? How is power provided at high essential. A long-lived, highly capable, global latitudes and over long, perhaps multiannual, network is needed to achieve the science periods? In the case of networks, how are within a single mission. However, individual over a dozen probes launched, deployed, and highly capable meteorological stations flown successfully landed, and how is on every future mission, coupled with a global communication between numerous nodes network of core meteorological and an orbiter achieved? measurements would also make great strides

7 toward the scientific objectives, and this  All future landed missions should carry a implementation strategy is realistic. capable meteorological payload to address FINDING MEPAG climate objectives, to serve as a pathfinder for a network, and to reduce risk The National Research Council Space and cost of future missions. Studies Board Committee is urged to  NASA should commit resources to network recommend that all future landed Mars technology development and commission a missions carry a capable meteorological meteorology network concept study. investigation and that the  A meteorology network mission should be recommendation for a Mars flown as a precursor to human exploration. meteorological network as a high-  NASA should engage with foreign space priority mission within NASA’s Mars agencies to enhance scientific expertise, Exploration Program be retained from leverage mutual technology development, and the previous Decadal Survey. to reduce the overall cost to any one agency. In particular, the NRC is urged to make the 7 Notes and References following recommendations: Available at:  Meteorology should remain as a high- . priority Mars network investigation, as it was in the previous Decadal Survey.