The Mars Atmospheric Constellation Observatory (MACO) Concept

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1 The Mars Atmospheric Constellation Observatory (MACO) Concept E. R. Kursinski\ W. Folkne?, C. Zuffada 2, C. Walker 3, D. Hinson 4, A. Ingerso11 5, M.A. Gurwel1 6, J. T. Schofidd 2, S. Limaye 7, A. Stem 8, D. Flittner 1, G. Haj/, J. Joiner 9, H. Pickett 2, L. Romans 2, A. P. Showman 10, A. Sprague 10, C. Young 1, S. Calcutt 11, F. Forgee 2, and F. Taylor 11 1Department of Atmospheric Sciences, University of Arizona, Tucson, AZ, USA atmo.arizona.edu 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA 3Department of Astronomy, University of Arizona, Tucson, AZ, USA 4 Department of Electrical Engineering, Stanford University, Palo Alto, CA, USA 5 Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA 6Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA 7 University of Wisconsin, Madison, WI, USA 8Southwest Research Institute, Boulder, CO, USA 9Goddard Space Flight Center, Greenbelt, MD, USA 10oepartment of Planetary Sciences, University of Arizona, Tucson, AZ, USA 11Department of Physics Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK 12Lab. de Meteorologie Dynamique, Universite Paris, France. Abstract. The Mars Atmospheric Constellation Observatory (MACO) represents an innovative approach to characterizing the present Martian climate from the surface into the thermosphere including the hydrological, C0 2, and dust cycles together with the energy and momentum budgets. The mission concept is based on a constellation of satellites forming counter-rotating pairs for observing satellite-to-satellite microwave occultations to determine vertical profiles of water vapor, C0 2, temperature, pressure, and wind. Satellite radio occultation, used in previous missions such as Mars Global Surveyor (MGS), provides precision, accuracy and vertical resolution typically 1 and sometimes 2 orders of magnitude beyond that of passive radiometers. Furthermore it can measure absolute pressure versus height (which is unobservable by radiometers) and thus remotely determine seasonal C0 2 changes and winds. The microwave observations are supplemented by IR observations by a Dust and Ice Sensor (DIS). With the addition of a UV spectrometer, MACO can characterize the upper atmosphere's composition and thermodynamic structure as well as escape rates. With a three satellite constellation, MACO will sample the Martian atmosphere with more than 80 occultations each day and, with observations from rapidly precessing orbits over at least one Martian year, will characterize the diurnal and seasonal cycles. G. Kirchengast et al. (eds.), Occultations for Probing Atmosphere and Climate Springer-Verlag Berlin Heidelberg 2004

2 394 E. R. Kursinski et al. 1 Introduction In March 2001, NASA held a workshop inviting innovative ideas for a class of Principal Investigator (PI) led missions at Mars called Mars Scout Program. Our mission concept, called MACO, was developed in response to this NASA call, addressing the fundamental science objective of characterizing the Martian climate, according to the objectives and requirements identified in the Mars Exploration Payload Group (MEPAG) and COMPLEX reports (McCleese et al. 2001; COMPLEX 2001). MACO is an innovative mission designed to characterize the present Martian atmosphere and climate from the surface into the thermosphere using satellite-to-satellite microwave occultations supplemented by passive radiometric observations at microwave, IR and UV wavelengths. The combined set of wavelengths observed by MACO provides a wide dynamic range that can penetrate through atmospheric particulates in the Martian atmosphere to sense water vapor, C02 concentrations, pressure and temperature and winds while simultaneously sensing and characterizing atmospheric particulates. Atmospheric water vapor and ice, C0 2 and dust and their respective cycling through the Martian climate system together with the energy and momentum budgets and escape rates will be characterized over a Martian year. The MACO satellites will observe Mars from high inclination orbits providing daily pole-to-pole coverage and rapid precession to sample the entire diurnal cycle every Martian month (-57 days) to separate diurnal and seasonal variability and behavior. Figure 1 shows a 3 satellite configuration with two satellites in the same orbit, and a third one in a different orbit. Fig. 1. MACO constellation, showing two satellites in one orbit (Periapsis of 400 km, Apoapsis of 600 km), the second trailing the first by 5, and the third satellite in a different counter-rotating orbit (Periapsis of 450 km, Apoapsis of 1921 km). First and third satellite as well as second and third satellite form occultation pairs.

3 The Mars Atmospheric Constellation Observatory (MACO) Concept 395 This paper focuses on the science objectives of the mission, and concentrates on the primary measurement technique, i.e. radio occultation. In particular, the accuracy of absorption occultations is discussed, a unique feature of MACO which allows profiling of water vapor to unprecedented accuracy. 2 Science and Measurement Objectives The overriding goal of MACO is to characterize the climate and weather of Mars focusing on the hydrological, C0 2 and dust cycles as well as the energy and momentum cycles. Our general approach is to generate a global data set as independent from models as possible and use it to develop an objective characterization of the Martian weather and climate system over the duration of the MACO mission. This in tum will allow us to evaluate and improve models of Martian climate and weather. The characterization will provide an excellent test bed for performing analogous evaluations of Earth-based models. Our intent is to create a data set sufficient to write the definitive text on the physics of climate of Mars analogous to the classic "Physics of Climate" text by Peixoto and Oort (1992) describing the climate of Earth. Atmospheric water concen- 1-3% accuracy and km vertical resolution from 0 to tration km altitude -4% accuracy or better and km vertical from 0 to Relative humidity km altitude sub-kelvin precision and accuracy with km vertical Temperature resolution from 0 to 80 km altitude C0 2 density and bulk pres- 0.1% accuracy and km vertical resolution from 0 to sure versus height 80 km altitude -1-2 m/s balanced wind from 0 to ~50 km altitude with 0.4 Winds km vertical resolution, -10 m/s from 15 to ~30 km altitude derived via Doppler Dust and ice concentrations 10-50% with 2 km vertical resolution Isotopic ratios D/H, 13 CI' 2 C and 18 0/ 16 0 to -1% Table 1. MACO measurement objectives Table 1 summarizes the MACO measurement objectives which, as we will discuss, are achievable and provide a better combination of coverage, precision and vertical resolution than any present capabilities at Earth. The MACO suite of instruments and its orbital geometry have been chosen to provide global and regional perspectives much like the space-borne Earth observational systems. Occultation observations near 183 GHz and X-band (7.2 and 8.4 GHz) form the backbone of the MACO observational suite. Satellite radio occultation is a simple and proven technique used in previous missions such as Mars Global Surveyor (MGS) (Hinson et al. 1999; Hinson et al. 2001). The 183 GHz and X-band occul-

4 396 E. R. Kursinski et al. tations will provide very precise and high vertical resolution observations of C0 2, temperature, pressure and the balanced portion of the winds that surpass most capabilities on Earth. The 183 GHz occultations will also yield very precise profiles of water vapor and a second independent estimate of winds. Radio occultations provide precision, accuracy and vertical resolution typically 1 and sometimes 2 orders of magnitude beyond that of passive radiometers. The diffraction limited vertical resolutions of the 183 GHz and X-band observations are approximately 70 m and 400 m respectively. Furthermore, since we control the strength of the signal source, occultations can achieve much higher signal to noise ratios (SNR) than can radiometers. As a result, occultations can achieve much better performance than passive observations in terms of precision and vertical resolution such that the occultation profiles approach the quality of entry probes. To date, the utility of the satellite to Earth X-band occultations has been limited by their coverage which is concentrated near the poles and terminator with sparse sampling elsewhere. With multiple satellites, the MACO satellite-tosatellite occultations will sample the complete range of latitudes and longitudes (see Figure 2) as well as the full diurnal cycle each month. Occultations have another tremendous advantage over passive observations. The relation between radiances and atmospheric temperatures and constituent densities is non-unique. Therefore the derivation of temperatures and constituent densities from radiances is fundamentally ill-posed and requires additional constraints to achieve a unique solution. These constraints, such as an apriori model estimate of the atmospheric state, contribute to a residual bias in the final result. As such, it is difficult to separate observational from model contributions. In marked contrast, the moisture, C02 density, temperature and pressure profiles derived from the occultations are independent of models (e.g. Kursinski et al. 1997, 2002). Therefore occultation observations are quite well suited for deducing and monitoring the climate of Mars (and Earth)... /8 ~ ~ 0~ $ ~ I. ) 0 8 q. G c c 0 ~ ~ c $ <:1 0 0 ~ ~ R ~ 0 1, t 8 "' o ~ lllo lib $ ~.. ~ !10.. I' 0, ~ "' Fig. 2. Daily latitude versus longitude coverage for the constellation in Figure 1, with vertical profiles of H 20 and C0 2 densities, P and T (MACO-MACO satellite occultations: diamonds ); C02 density, P, T profiles (Earth occultations: circles or MACO-orbiter occultations: stars); and passive limb observations

5 The Mars Atmospheric Constellation Observatory (MACO) Concept 397 A 3 satellite constellation like that in Figure 1 will yield 80 MACO to MACO satellite occultation profiles per day providing global coverage that extends from the surface into the upper atmosphere with very high vertical resolution, precision/accuracy under clear and cloudy/dusty conditions. The MACO satellites will also be able to receive X-band signals from Earth and other non-maco Marsorbiting satellites. The X-band occultations from Earth and one non-maco Mars orbiting satellite carrying Ultra-Stable Oscillators (USOs) and transmitting on its low gain antenna will roughly double the number of MACO-MACO occultations (see Figure 2). We also note MACO' s built in redundancy in that any one of the 3 satellites can fail and MACO can still perform satellite to satellite occultations. In the event the green satellite in Figure 1 fails, the other two satellites will carry sufficient fuel to change altitudes to precess differentially for approximately 6 months to separate by 180 degrees at which point they will return to the altitudes at which they make occultation observations precessing together. To supplement the microwave occultations, MACO will carry passive IR and microwave radiometers to characterize atmospheric dust and ice and provide nearcontinuous horizontal coverage of the thermal and moisture structure between the occultation profiles (albeit at lower vertical resolution than the occultation observations). To complement MACO's extensive radio occultation mapping of the ionosphere, a UV spectrometer would extend the MACO altitude coverage through the thermosphere and allow MACO to characterize the composition, energetics, dynamics and rates of escape of the upper atmosphere and how they vary as a function of the seasonal, diurnal and solar cycles (Slater et al. 2001). This would complement very nicely Earth based contributions to NASA's 'Living with a Star' initiative. 70 -Cold dry Mars ~ 40 ~ Water vapor mixing ratio (ppm) 10 Water vapor error(%) Fig. 3. Precision of retrieved moisture profiles. a. possible Martian water vapor concentrations. b. corresponding precisions from 183 GHz occultations.

6 398 E. R. Kursinski et al. 2.1 Atmospheric Water MACO will characterize atmospheric water on Mars to unprecedented levels of precision, accuracy, resolution and coverage. The goal is to develop a global, four-dimensional characterization of the atmospheric portion of the Martian hydrologic cycle over at least one Martian year. This includes deriving moisture fluxes, sources and sinks as a function of region, season and diurnal cycle. Moisture will be characterized at high vertical resolution by the 183 GHz occultations and at medium vertical resolution via the 183 GHz radiances. Expected precision of water vapor retrievals derived from the 183 GHz occultations is given in Figure 2. (See Kursinski et al for a more complete discussion of the 183 GHz occultation water retrieval concept). Atmospheric moisture will be characterized precisely in terms of both specific and relative humidity. While relative humidity measurements are crucial in determining the role of condensation and evaporation, specific humidity measurements are extremely important in deducing sources and sinks and acting as a tracer in regions where no phase changes occur. Relative humidity will be determined by combining the observed moisture concentrations and temperatures. The vertical resolution of the MACO water profiles is quite important. While the MEPAG water vapor 5 km vertical scale requirement (consistent with thecapability of Mars Climate Sounder [MCS] to fly in 2005) represents a significant improvement over the vertical resolution of the ongoing MGS-TES water observations, significantly finer vertical resolution will likely be crucial for determining the processes controlling the moisture in the atmosphere, particularly the lower atmosphere. For instance, the water saturation scale height in relatively rapidly rising air is -3 km. Furthermore, radio occultations have imaged near-surface thermal inversions that extend only 1 to 2 km above the surface (e.g. Figure 4). These inversions very likely have sharp vertical moisture variations associated with them. Since the water ice cloud fraction and amount vary widely on Mars, the vertical water distribution is almost assuredly not characterized by a single scale height. As COMPLEX points out, space-borne and Earth-based observations indicate that water vapor on Mars varies widely in space and time just as it does on Earth. If terrestrial behavior is any indication, the Martian water vapor scale height will also vary dramatically with time and region, with scale heights ranging from 50 m to 10 km. MACO's ability to resolve such scales will be crucial in identifying and understanding the underlying processes controlling water and dust in the Martian atmosphere. The desire for finer resolution here is analogous to the desires of planetary geologists seeking higher resolution to reveal finer and finer imaging details of the surface properties to use as fingerprints to infer the geological processes responsible for the observed properties. Measurements of atmospheric moisture to date have revealed significant hemispherical and seasonal asymmetries. Arguments have been made that the asymmetries are due either to the present orbital geometry of Mars (Clancy et al. 1996) or to the dynamical and thermodynamical consequences of the large hemispherical differences in topography (Richardson and Wilson 2002). In the first scenario, the

7 The Mars Atmospheric Constellation Observatory (MACO) Concept 399 difference in northern and southern summertime temperatures due to the present orbital geometry and eccentricity causes a net annual flow of moisture from the south to the north that is absent in the second explanation. MACO will characterize in detail the hemispherical and seasonal asymmetries in the present moisture and temperature distribution and determine whether there is significant net flow of moisture between hemispheres over the seasonal cycle. 2.2 Atmospheric Water Ice Moisture in the Martian atmosphere condenses into clouds that exhibit strong seasonal, diurnal and spatial dependencies. To characterize the complete Martian hydrological cycle and moisture sources and sinks, MACO will track atmospheric moisture in both its vapor phase and its ice phase via its IR emission. There may be a substantial diurnal component to the hydrological cycle. For instance, given the observed changes in cloudiness over the diurnal cycle, water vapor concentrations may decrease as the clouds form overnight. Since the cloud particles appear to be near 1 micron in size (e.g. Pearl et al. 2001), they may not precipitate out such that the moisture remains in place in the atmosphere simply moving back and forth between phases. Alternatively significant frost formation on the surface as seen by the Viking landers can cause a significant diurnal variation in total and vertical distribution of atmospheric moisture. Observations of cloud ice can distinguish between these two scenarios. The diurnal variations of water vapor, ice and dust may be linked substantially such that simultaneous observations of all three are required to understand the formation and evolution of water ice clouds. The threshold relative humidity at which clouds form may vary with the amount of atmospheric dust which would indicate whether the condensation nuclei are hydrophilic or hydrophobic and could place significant constraints on the makeup of the dust. ~~~~----~----~--~~ o ~~--~---L~~~~~ Tcmpcnuurc.K Temperaturc.K Fig. 4. Atmospheric profiles from MGS radio occultations characterizing the lower atmosphere of Mars. a. Two profiles reveal nocturnal near-surface inversions and several intervals of adiabatic lapse rates associated with an atmospheric wave propagating above 25oN. b. Several temperature profiles between 30 S and 64 S revealing large diurnal temperature variations near the surface. Local times from left to right are 03:53, 01:13, 23:21, 21 :16 and 18:40. Dashed lines are a dry adiabat (from Hinson eta!. 1999).

8 400 E. R. Kursinski et al. Because atmospheric ice can be either water or C02, it is important that MACO can distinguish between water ice and C0 2 ice clouds by precisely measuring the H 20 and C0 2 densities and temperatures to determine which of the two constituents is at or close to saturation. Changes in water isotopic ratios before and after cloud formation would indicate isotopic fractionation processes and the presence of precipitation/virga. MACO will measure the HDO to H20 ratio for this purpose. 2.3 Atmospheric Mass and C02 The MACO radio occultations can measure the atmospheric C02 very accurately ( -0.1%) and will monitor the amount of C02 in the Martian atmosphere and its variations over the Martian annual cycle. The C02 variations include exchange between the polar caps and the atmosphere as well as any significant interactions with the regolith. The large number of occultation profiles and passive observations will help us separate dynamic and thermodynamic variations. With high accuracy and vertical resolution C02 density and temperature information we will determine where and when the C02 reaches saturation, condenses and precipitates out. We can also investigate the interrelationship between C02 and H20 ice formation such as the possible role of water ice in providing nucleation sites for formation of C0 2 ice (e.g. Pearl et al. 2001). By combining our estimates of atmospheric mass with the saturation information and our estimates of the wind field, we will refine the energy balance at the poles. 2.4 Temperatures, Waves and Fronts With 3 satellites, MACO will provide at least 80 globally distributed temperature profiles daily, with -0.5 K accuracy from the surface to 60 km or higher in clear and dusty/cloudy conditions with vertical resolutions of 0.5 km or better. The MACO satellites will sample the diurnal cycle each Martian month. This combination of spatial and temporal coverage, accuracy and resolution far surpasses anything planned for Mars. These observations will continually measure the internal energy of the Martian atmosphere, determine the source function of the radiative transfer and how the energy states are populated. Combined with the H20 and C0 2 concentration estimates, MACO will determine their relative proximity to saturation. MACO will very accurately measure the vertical gradient of temperature and atmospheric stability providing an indication of the role and vigor of convection and vertical overturning and mixing and the drag on near-surface winds. Figure 4 shows significant variations in atmospheric stability observed by MGS occultations which cannot be observed with present or planned orbiting passive sensors. The quasi-periodic intervals of adiabatic behavior reveal both the presence of a wave and the vertical intervals over which the wave is breaking and transferring momentum into the background flow. The diurnal sampling of

9 The Mars Atmospheric Constellation Observatory (MACO) Concept 401 MACO will allow us for the first time to separate out the signatures of atmospheric tides from other modes of variability and how they vary with season, location and dust loading. The coverage, precision and resolution of MACO will provide a tremendous data set for characterizing the morphology of atmospheric waves, how they vary with season and location, where and how they are generated and where and how they break and transfer their momentum into the circulation in the middle and upper atmosphere. The MACO occultations also provide the vertical resolution and sensitivity necessary to identify and characterize frontal surfaces providing for the first time the ability to characterize the vertical structure of weather fronts on a planet other than Earth. Such fronts are believed responsible for the day-to-day pressure variations observed by the Viking landers and lines of dust seen from orbit. With such identifications we can scale and apply the Earth-based frontogenesis theory to Mars and evaluate its success on another similarly rotating and inclined terrestrial planet. 2.5 Winds, circulation and momentum budget Knowledge of winds is fundamental to defining, characterizing, and understanding weather and climate. Knowledge of winds is a fundamental to achieving our goals of estimating the sources and sinks of moisture, dust and other constituents. Winds are also responsible for modifying the surface properties, an accurate understanding of which is required for separating aeolian and water related modification of the Martian surface. As on Earth, remotely characterizing wind is challenging. The MACO satellite constellation and instrument suite offers at least four different approaches to determining winds, namely (a) deriving the balanced portion of the wind from the horizontal pressure gradients obtained from occultation pressure versus height profiles (b) deriving the thermal wind from the passive temperature versus pressure observations (c) deriving the line-of-sight wind from the Doppler shift of the water absorption profiles (d) inferring winds from the motion of atmospheric tracers such as clouds, water vapor and other constituents. Horizontal differences between occultation-derived observations of the absolute geopotential of pressure surfaces directly determine the balanced portion of the wind via the gradient wind equation. With occultation profiles separated by a few hundred kilometers, these observations will yield mid and higher latitude gradient winds estimates to approximately 1 to 2 rn/s (see Figure 5a). Hinson et al. (1999) showed that gradients between nearby high vertical resolution density profiles reveal regions of low level jets and high wind shear. These jets are crucial in the momentum coupling between the surface and atmosphere and transport of constituents and are likely important in the genesis and evolution of dust storms.

10 402 E. R. Kursinski et a!. f j a.oo l J 4.00 J Fig. 5. Precision of wind estimates. a. Precision of gradient wind versus latitude estimated from occultation pressure vs. height profiles separated by 300 km. b. precision of winds derived from the Doppler shift of the 183 GHz water line as a function of altitude. The MACO estimates will have several hundred meter or better vertical resolution which is crucial for characterizing near-surface jets. In the second wind estimation approach, the horizontal wind field is inferred from the passive observations via the thermal wind equation providing important horizontal coverage between the occultations. In the third wind estimation method, winds are derived by measuring the Doppler shift of the absorption line such as the 183 GHz water line yielding estimates independent from assumptions unlike the first and second estimation methods. Because the Doppler-derived wind component lies along the occultation line whereas the component of the balanced wind estimate tends to be orthogonal to this line, the two estimates provide both horizontal components of the wind. Figure 5b shows how the accuracy to which wind can be estimated from the 183 GHz line depends on altitude and water concentrations. 2.6 Boundary layer Characterization of boundary layer processes and exchange between the atmosphere and surface are high priorities in both the MEPAG and COMPLEX reports. Understanding the exchange of heat, moisture and momentum between the Martian atmosphere and the surface requires spatially and diurnally resolved observations of the boundary layer. MGS occultation results from Hinson et al. (1999) in Figure 6 reveal large diurnal variations in near-surface temperatures. The diurnal

11 The Mars Atmospheric Constellation Observatory (MACO) Concept 403 changes in vertical temperature structure are a measure of the integrated heat flux between the atmosphere and surface over the diurnal cycle. The figure reveals both the need for combined sub-kilometer vertical resolution and high precision as well as the ability of radio occultation observations to profile boundary layer structure and its large diurnal variations. Unfortunately the MGS geometry is such that MGS to Earth occultation observations sample the boundary layer only two times of day over a very limited portion of the globe. The high inclinations and rapid precession of the MACO satellite-to-satellite occultations will provide global coverage and sampling of the full diurnal cycle each Martian month. Using these observations, we will assemble regional pictures of the diurnal cycle of PBL structure as a function of season including temperature, atmospheric moisture and winds. We will combine these observations with estimates of the surface thermal inertia (Jakosky et al. 2000) to estimate and understand the PBL energy and constituent cycles and derive the turbulent energy and constituent fluxes involved as a function of time of day, season and region and the exchange of energy, momentum and constituents between the atmosphere and surface Isotopes The isotopic ratios of certain elements are extremely interesting because they place key constraints on the past formation and evolutionary history of planets as well as any fractionation processes operating in their present climate systems. For instance, on Earth, the isotopic ratios of ice cores tell us a great deal about the ice ages and isotopic ratios of rainwater are quite revealing in determining the source and past history of the water in the air parcels. Based on our Earth experience, any changes in the 12 C to 13 C ratio over the seasonal cycle could be the signature of Martian life. Due the combined kindness of nature and sensitivity of the radio occultation observations, we can measure precisely the isotopic ratios of hydrogen, carbon and oxygen from orbit using one instrument near 120 GHz measuring the concentrations of HDO, 13 C 16 0 and 12 C 18 0 and 12 C 16 0 in combination with the 183 GHz water measurements. The isotopic ratio of nitrogen cannot be measured analogously because the dominant reservoir of nitrogen, N 2, has no significant lines in the microwave band. Upon averaging, the achievable accuracies should be 10 per mil or better for each of the hydrogen, carbon and oxygen isotopic ratios. 3 Summary and Conclusions MACO offers an unprecedented capability for characterizing the key variables in the climate of Mars, namely water vapor, water ice, dust, C02, temperature, pressure, and winds to unprecedented levels of precision, vertical resolution and coverage. The vertical resolution provided by the occultations will be 100 to 400 me-

12 404 E. R. Kursinski et al. ters. Water will be characterized to precisions near 1% and C02 to 0.1 %. MACO can characterize winds via several methods, two of which will determine the winds with sub-kilometer vertical resolution. The MACO coverage is global spanning at least one complete annual cycle. The rapidly precessing orbits will allow us to separate the seasonal and diurnal cycles and atmospheric tides from other dynamical effects. MACO's observations of atmospheric water represent a substantial improvement over the MCS observations on MRO covering approximately the same vertical range as MCS with significantly higher accuracy and 10 to 50 times better vertical resolution. The MACO microwave observations are also completely insensitive to dust. MACO will characterize the boundary layer and exchange between the surface and atmosphere. A combination of atmospheric stability, specific humidity, dust and water ice will be used to determine the heights to which convection penetrates. MACO will also characterize aspects of the synoptic and mesoscale meteorology including the structure and roles of Martian weather fronts. MACO will also illuminate and quantify the factors involved in dust storm genesis. We also note that MACO promises not only unprecedented precision and high vertical resolution but absolute level of accuracy as well. The accuracy is achieved through two factors. First, occultations are inherently self-calibrating because the signal source is viewed either immediately before or after each occultation. Second, the 183 GHz occultation sensor is essentially a multi-tone spectrometer with which we will perform a spectroscopic calibration and determine the line shape and displacement parameters while in orbit around Mars. Therefore the MACO data will likely become the standard against which other observations are calibrated. Acknowledgement. This work was supported in part by NASA Mars Program Office funding for developing Mars Scout Mission concepts. References Clancy T et al. (1996) Water Vapor Saturation at Low Altitudes around Mars Apehelion: A Key to Mars Climate?. Icarus 122: COMPLEX (2001) The National Research Council (NRC) Committee for Planetary Exploration Report, Assessment of Mars Science and Mission Priorities, Hinson DP, Simpson RA, Twicken JD, Tyler GL, Flasar FM (1999) Initial results from radio occultation measurements with Mars Global Surveyor. J Geophys Res 104: Hinson DP, Tyler GL, Hollingsworth JL, Wilson RJ (2001) Radio occultation measurements of forced atmospheric waves on Mars. J Geophys Res 106: Jakosky BM, Mellon MT, Kieffer HH, Christensen PR, Varnes ES, Lee SW (2000) The thermal inertia of Mars from the Mars Global Surveyor Thermal Emission Spectrometer. J Geophys Res 105:

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