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The Mars Atmospheric Constellation Observatory (MACO)
Concept
E. R. Kursinski\ W. Folkne?, C. Zuffada2, C. Walker3, D.
Hinson4, A. Ingerso115, M.A. Gurwel16, J. T. Schofidd2, S. Limaye7,
A. Stem8, D. Flittner1, G. Haj/, J. Joiner9, H. Pickett2, L.
Romans2, A. P. Showman10, A. Sprague10, C. Young1, S. Calcutt11 ,
F. Forgee2, and F. Taylor11
1Department of Atmospheric Sciences, University of Arizona,
Tucson, AZ, USA kursiniski@ atmo.arizona.edu 2Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA, USA
3Department of Astronomy, University of Arizona, Tucson, AZ, USA
4Department of Elec-trical Engineering, Stanford University, Palo
Alto, CA, USA 5Department of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA, USA
6Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
7University of Wis-consin, 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 Ox-ford, UK 12Lab. de
Meteorologie Dynamique, Universite Paris, France.
Abstract. The Mars Atmospheric Constellation Observatory (MACO)
represents an inno-vative approach to characterizing the present
Martian climate from the surface into the thermosphere including
the hydrological, C02, 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 deter-mine
vertical profiles of water vapor, C02, temperature, pressure, and
wind. Satellite radio occultation, used in previous missions such
as Mars Global Surveyor (MGS), provides pre-cision, 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 C02 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 character-ize 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
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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, ad-dressing 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 ra-diometric
observations at microwave, IR and UV wavelengths. The combined set
of wavelengths observed by MACO provides a wide dynamic range that
can pene-trate through atmospheric particulates in the Martian
atmosphere to sense water vapor, C02 concentrations, pressure and
temperature and winds while simultane-ously sensing and
characterizing atmospheric particulates. Atmospheric water va-por
and ice, C02 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.
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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 ac-curacy 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, C02 and dust cycles
as well as the energy and mo-mentum cycles. Our general approach is
to generate a global data set as independ-ent 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 0.1-0.5 km vertical
resolution from 0 to tration 40-60 km altitude
-4% accuracy or better and 0.1-0.5 km vertical from 0 to
Relative humidity 40-60 km altitude
sub-Kelvin precision and accuracy with 0.1-0.5 km vertical
Temperature resolution from 0 to 80 km altitude C02 density and
bulk pres- 0.1% accuracy and 0.1-0.4 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, 13CI'2C and 180/160 to
-1%
Table 1. MACO measurement objectives
Table 1 summarizes the MACO measurement objectives which, as we
will dis-cuss, are achievable and provide a better combination of
coverage, precision and vertical resolution than any present
capabilities at Earth. The MACO suite of in-struments and its
orbital geometry have been chosen to provide global and re-gional
perspectives much like the space-borne Earth observational systems.
Oc-cultation 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-
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396 E. R. Kursinski et al.
tations will provide very precise and high vertical resolution
observations of C02, temperature, pressure and the balanced portion
of the winds that surpass most ca-pabilities 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 sig-nal 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 lim-ited by their coverage which is
concentrated near the poles and terminator with sparse sampling
elsewhere. With multiple satellites, the MACO
satellite-to-satellite 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 den-sities is non-unique. Therefore
the derivation of temperatures and constituent densities from
radiances is fundamentally ill-posed and requires additional
con-straints 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 de-rived 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) .
.. ~ ~ 0~ 0 8 /8 0 0 0 0 • $ ~ • .. .. • • ... • • I.
) • 00 q. G • c c 0 ~ ~ c $ •
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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, preci-sion/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 Mars-orbiting 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
suf-ficient 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 near-continuous horizontal coverage of the
thermal and moisture structure between the occultation profiles
(albeit at lower vertical resolution than the occultation
obser-vations). 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, en-ergetics, 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.
0.01 0.1 1 10 100 1000 Water vapor mixing ratio (ppm)
70
60
50
~ 40
~ 30
20
-Cold dry Mars
10 Water vapor error(%)
Fig. 3. Precision of retrieved moisture profiles. a. possible
Martian water vapor concentra-tions. b. corresponding precisions
from 183 GHz occultations.
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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
hy-drologic cycle over at least one Martian year. This includes
deriving moisture fluxes, sources and sinks as a function of
region, season and diurnal cycle. Mois-ture 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. 2002 for a
more complete discussion of the 183 GHz oc-cultation water
retrieval concept).
Atmospheric moisture will be characterized precisely in terms of
both specific and relative humidity. While relative humidity
measurements are crucial in de-termining the role of condensation
and evaporation, specific humidity measure-ments 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 theca-pability of Mars Climate Sounder
[MCS] to fly in 2005) represents a significant improvement over the
vertical resolution of the ongoing MGS-TES water observa-tions,
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 ris-ing air is
-3 km. Furthermore, radio occultations have imaged near-surface
ther-mal 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 verti-cal water distribution is almost assuredly not
characterized by a single scale height. As COMPLEX points out,
space-borne and Earth-based observations in-dicate 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 geologi-cal processes responsible
for the observed properties.
Measurements of atmospheric moisture to date have revealed
significant hemi-spherical and seasonal asymmetries. Arguments have
been made that the asym-metries 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
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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 character-ize 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 sea-sonal, diurnal and spatial dependencies. To
characterize the complete Martian hy-drological 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 concentra-tions 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 dis-tinguish
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
hy-drophilic or hydrophobic and could place significant constraints
on the makeup of the dust.
o~~--~---L~~~~~
200 210 220 230 240 250 260 ~~~~----~----~--~~ 170 190 210
230
Tcmpcnuurc.K Temperaturc.K
Fig. 4. Atmospheric profiles from MGS radio occultations
characterizing the lower atmos-phere of Mars. a. Two profiles
reveal nocturnal near-surface inversions and several inter-vals 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).
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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 C02 ice
clouds by precisely meas-uring the H20 and C02 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 indi-cate 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 be-tween the polar caps
and the atmosphere as well as any significant interactions with the
regolith. The large number of occultation profiles and passive
observa-tions will help us separate dynamic and thermodynamic
variations. With high ac-curacy 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
forma-tion such as the possible role of water ice in providing
nucleation sites for forma-tion of C02 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 combi-nation
of spatial and temporal coverage, accuracy and resolution far
surpasses anything planned for Mars. These observations will
continually measure the in-ternal energy of the Martian atmosphere,
determine the source function of the ra-diative transfer and how
the energy states are populated. Combined with the H20 and C02
concentration estimates, MACO will determine their relative
proximity to saturation. MACO will very accurately measure the
vertical gradient of tem-perature 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 pas-sive 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
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The Mars Atmospheric Constellation Observatory (MACO) Concept
401
MACO will allow us for the first time to separate out the
signatures of atmos-pheric tides from other modes of variability
and how they vary with season, loca-tion and dust loading. The
coverage, precision and resolution of MACO will pro-vide 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
varia-tions 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
understand-ing 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 de-termining winds, namely
(a) deriving the balanced portion of the wind from the
horizontal pressure gra-dients obtained from occultation pressure
versus height profiles
(b) deriving the thermal wind from the passive temperature
versus pressure ob-servations
(c) deriving the line-of-sight wind from the Doppler shift of
the water absorp-tion profiles
(d) inferring winds from the motion of atmospheric tracers such
as clouds, wa-ter vapor and other constituents.
Horizontal differences between occultation-derived observations
of the abso-lute 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
gra-dient 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 pro-files 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.
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402 E. R. Kursinski et a!.
f j a.oo l J 4.00 J
2.00
0 20 80
Fig. 5. Precision of wind estimates. a. Precision of gradient
wind versus latitude esti-mated 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 resolu-tion 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 bal-anced 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 atmos-phere and surface are high priorities in both the
MEPAG and COMPLEX reports. Understanding the exchange of heat,
moisture and momentum between the Mar-tian atmosphere and the
surface requires spatially and diurnally resolved observa-tions 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
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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 obser-vations sample the boundary layer only two times
of day over a very limited por-tion of the globe. The high
inclinations and rapid precession of the MACO
satel-lite-to-satellite occultations will provide global coverage
and sampling of the full diurnal cycle each Martian month. Using
these observations, we will assemble re-gional pictures of the
diurnal cycle of PBL structure as a function of season in-cluding
temperature, atmospheric moisture and winds. We will combine these
ob-servations with estimates of the surface thermal inertia
(Jakosky et al. 2000) to estimate and understand the PBL energy and
constituent cycles and derive the tur-bulent 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.
2. 7 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 12C to 13C 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 hydro-gen, carbon and
oxygen from orbit using one instrument near 120 GHz measuring the
concentrations of HDO, 13C160 and 12C180 and 12C160 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 ra-tios.
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, pres-sure, and winds to unprecedented
levels of precision, vertical resolution and cov-erage. The
vertical resolution provided by the occultations will be 100 to 400
me-
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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 al-low 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 approxi-mately 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 Mar-tian 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
occul-tation. Second, the 183 GHz occultation sensor is essentially
a multi-tone spec-trometer 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 fund-ing for developing Mars Scout Mission
concepts.
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