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The martian atmosphere in the region of Hellas basin as observed by the planetary Fourier spectrometer (PFS-MEX)
ARTICLE IN PRESS
Planetary and Space Science 55 (2007) 1346–1357 www.elsevier.com/locate/pss
The martian atmosphere in the region of Hellas basin as observed by the planetary Fourier spectrometer (PFS-MEX) D. Grassia,, V. Formisanoa, F. Forgetb, C. Fiorenzaa,c, N.I. Ignatieva,d, A. Maturillie, L.V. Zasovaa,d a
INAF-IFSI, Via del Fosso del Cavaliere, 00144 Rome, Italy b LMD-IPSL, Paris, France c Universita` degli Studi dell’Aquila, Italy d IKI-RAS, Profsojuznaja 84/32, 117997 Moscow, Russia e DLR, Berlin, Germany Accepted 2 October 2006 Available online 13 January 2007
Abstract This work presents a review of the observations acquired by the planetary Fourier spectrometer (PFS) in the region of the Hellas basin. Taking advantage of the high spectral resolution of PFS, the vertical air temperature profile can be investigated with a previously unexperienced vertical resolution. Extensive comparisons with the expectations of EMCD 4.0 database highlight moderate discrepancies, strongly dependant on season. Namely, the morning observations acquired around Ls ¼ 451 show a series of temperature deficiencies with recurrent spatial patterns in different observations, correlated with the topography profile. Trends of integrated dust loads as a function of the field of view (FOV) elevation are also described. Values are consistent with the retrieval hypothesis of a dust scale height equal to the gas one, even far from the season of main dust storms. r 2007 Elsevier Ltd. All rights reserved.
1. Introduction The Hellas basin represents a major feature in the topography of the Martian surface. Its dimensions made it the most impressive impact structure of the entire Solar system. The depth of the basin with respect to the surrounding terrains (8 km) is comparable with the scale height of the Martian atmosphere, while its diameter has a size similar to the typical length of atmospheric waves. Both factors suggest a likely role of the crater in modifying the circulation of air masses with respect to an ideal flat Mars in a variety of space scales. The location has been identified since a long time as the site of peculiar atmospheric phenomena. Visual observaCorresponding author. Current address: LESIA Observatoire de Paris, Section de Meudon 5, place Jules Janssen 92195 MEUDON Cedex, France. Tel.: +39 06 49934034; fax: +39 06 49934074. E-mail address: [email protected] (D. Grassi).
0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.12.006
tions from Earth recognized Hellas as a preferential location for the beginning of global dust storms, pointing toward slope winds driven by topography profile as a possible explanation (e.g.: Leikin and Zabalueva, 1975). Extensive records from spacecraft dataset have confirmed Hellas to be a special place for silicate aerosol phenomenology, both during the onset (Martin and Richardson, 1993; Murphy, 1997; Liu et al., 2003; Strausberg et al., 2005) as well as in the decay phases (Fenton et al., 1997) of severe dust lifting events. Tamppari et al. (2003) described the frequent occurrence of topographic water ice clouds in the region, while the contents of water vapor reported by Smith (2002) and Fedorova et al. (2004) do not present specific anomalies once the integrated amounts are normalized for the total atmospheric pressure. Several efforts have also been devoted to the numerical modeling of atmosphere dynamics in the Hellas region. Joshi et al. (1997) described a mechanism where the increase of dust loads intensifies the slope winds in the Hellas region, providing a positive feedback for the dust
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
lifting. Siili et al. (1999) investigated the mesoscale interaction of slope winds with ice-edge effects, while Montmessin et al. (2004) invoked a role of regional circulation forced by impact basins in driving the transport of moisture toward the southern pole. Eventually, Colaprete et al. (2005) highlighted the role of Hellas and Argyre basins in forcing the onset of a stationary thermal wave detected in the vicinity of Southern polar region, which eventually led to the observed Southern polar cap asymmetry. This last work provides an important insight on the interpretative help offered in regional atmospheric studies by the analysis of air temperature fields. Despite the very good spatial coverage offered by the measurements of the thermal emission spectrometer (TES) on board of the Mars global surveyor satellite (Smith, 2004, and references therein), no specific analysis of the derived retrievals in the Hellas region has yet appeared in the literature. More recently, the on-going measurements campaigned by the planetary Fourier spectrometer (PFS hereon) on board of ESA Mars express (MEX) has provided further opportunities in this investigation context. This work aims to present the PFS data acquired in the region of Hellas and the observed phenomenology. Unexpected behaviors of the atmosphere are highlighted mainly by comparison against the expectations of the European Martian climate database v4.0 (EMCD hereon, Forget et al., 2006; Lewis et al., 1999).
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procedure is able to compute simultaneously the vertical air temperature profile (in the indicative range 5–45 km above the surface), the surface temperature and the integrated contents for silicate dust and water ice clouds. Numerical experiments on simulated observations, performed taking into account the actual NER values, allowed to estimate the random retrieval error for T(p) to be in the order of 2 K in the region of data highest sensitivity (15–20 km above the surface). A similar approach leads to fix around 0.03 the random errors for the retrievals of both dust and iceintegrated opacities. Retrievals are performed on individual LWC measurements, provided that an adequate absolute radiometric calibration is available. The algorithm takes advantage of a specific subroutine for the computation of synthetic spectra (Ignatiev et al., 2005), able to account for the effects of multiple scattering by atmospheric aerosols. For the purposes of this work, only LWC was considered. The distribution of PFS measurements in terms of latitude, local time and season at the center of the observed FOV derives from a list of operative constraints: not Sun synchronous orbit of MEX satellite, downlink needs and pointing requirements by other experiments in the payload. Namely, the first constraint results in a strong correlation between local time and season, as can be seen in Fig. 1. There, we show the distribution of PFS measurements in the Hellas basin region—as defined by the quadrant [201;701]S, [401;1001]E—currently available for analysis. Fig. 2 shows the geographic location of FOV centers for
2. Dataset 20
Local time (1/24 Sol)
The PFS instrument is a Fourier spectrometer in the payload of MEX mission; it measures the radiation emerging from the atmosphere and surface of Mars in the spectral range between 250 and 8200 cm1. The radiation field is analyzed separately in two channels. The long-wavelength channel (LWC) covers the thermal region between 250 and 1700 cm1. On the other hand, the shortwavelength channel (SWC) measures the incoming photons between 1700 and 8200 cm1, with a thermal part dominating below 2500 cm1 and the Solar-reflected radiation being the most important toward the visible region. For both channels, spectral resolution is in the order of 2 cm1 for apodized spectra, sampled with a 1 cm1 step. The instantaneous field of view (FOV) of LWC has a diameter of 12.2 km at the nominal height of the MEX pericenter (250 km), while the SWC observes a slightly smaller region 7.4 km wide. A complete description of the instrument and its radiometric performances can be found in the papers by Formisano et al. (2005) and Giuranna et al. (2005a, b). Namely, individual PFS measurements in the LWC are affected by a random noise about 0.8 ergs/ (sec cm2 ster cm1) at 500 cm1, leading to a S/N about 100 for most favorable cases. The PFS spectra can be analyzed according to the methods described by Grassi et al. (2005a). Our retrieval
15
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Fig. 1. Distribution of PFS data in the region of Hellas basin as a function of local time, season and latitude (in color code). Representation is limited to the MEX orbit range 10-1600 (data available to the current date for atmospheric analysis).
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time and season corresponding to each individual PFS acquisition.
Location of PFS observations
3. Retrieved air temperature fields
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Fig. 2. Geographic locations of the PFS measurements given in Fig. 1. Size of the spots is not representative of the actual PFS FOV size at the time of the acquisition.
the same acquisitions. Total number exceeds 4600 individual PFS measurements. Due to the almost polar orbit of MEX satellite (i871), PFS data acquired during a pericenter pass can provide a latitude scan of the atmospheric fields along a given longitude and at a fixed local time (assuming a nadir observation geometry). For the aim of this study, we will focus our attention on the Ls periods around 3401, 451 (in the ‘morning’ data subset in Fig. 1) and 1101. The comparison of PFS retrieval outcomes against the expectations of global circulation models (GCM) of the Martian atmosphere turns out in a powerful investigation tool. GCMs encompass the dynamical factors acting on air masses and demonstrated reliable in providing accurate simulations of different atmospheric fields. In this work, we adopted as reference the EMCD 4.0 database (Forget et al., 2006), a pre-computed collection of outputs from the simulations of LMD-AOPP-IAA GCM (Forget et al., 1999). Noteworthy, EMCD 4.0 takes full advantage of an extensive assimilation of TES retrievals as well as of an accurate numerical description of water cycle (Montmessin et al., 2004). Moreover, several model runs (‘dust scenario’) allow accounting for different dust loads in the atmosphere. In our case, we adopted the reference Martian year 24 (MY24) scenario in which the GCM is forced by the dust column observations retrieved by MGS TES in 1999–2001 (before the global dust storm) in order to mimic the atmosphere fields of this particular year, which is thought to be typical. The comparison PFS retrieval—EMCD expectations was performed on individual spectrum basis. This was possible extracting from the database a set of expected atmospheric quantities for the latitude, longitude, local
The retrieval of vertical air temperature profile from PFS data relies on several preliminary assumptions. The assumed surface pressure value is provided by pres0 subroutine from the LMD team, able to take into account the MOLA topography (Smith et al., 2003) at a spatial scale comparable with PFS FOV size. The direct radiative transfer model used during retrieval includes the dust optical properties by Hansen (2001) and adopts a lognormal dust size distribution, with reff ¼ 1.65 mm and neff ¼ 0.35. Water ice clouds are modeled according the size distribution I described by Clancy et al. (2003) and the complex refractive indices given by Warren (1984). Numerical test demonstrated that other likely options for size distributions and optical constants (for dust: (n,k) from Wolff et al. (2006), size parameters from Hansen (2001) and Pollack et al. (1995), for water ice: (n,k) from Hansen ()) induce systematic variations in the retrieved T(p) with a magnitude less than 3 K even in the cases, not considered here, of very heavy aerosol loads. Absolute values of aerosol opacities are, as expected, more sensitive to these choices but relative trends between different observations remain preserved. Aerosol is considered as uniformly mixed in the atmosphere during the retrieval. Surface emissivity is taken from the maps derived from TES measurements published by Bandfield et al. (2000). Temperature profiles presented in this work were filtered preliminarily in order to accept only the cases where the solution of inverse problem was able to provide a satisfactory fit with observed data, as quantified by a w2 test. 3.1. Southern summer and early autumn Fig. 3a presents the air temperature field observed by PFS during orbit 41, at Ls ¼ 337.91, while Fig. 3b provides the differences between observations and EMCD 4.0 expectations. The initial phase of MEX activities were characterized by a substantial dust load (Grassi et al., 2005b), further confirmed by simultaneous TES-MGS observations (Smith, 2006). In this case, the observed discrepancies can be easily explained taking into account the radiative effects of excess aerosol load with respect to GCM assumption. Actually, the dust tends to raise the temperature of atmospheric layers (exception made for the very lowest altitudes) due to a more effective adsorption of Solar radiation. This behavior was already described in literature for IRIS Mariner 9 (Conrath, 1975) and IRTM Viking (Martin, 1981) data. It is interesting to observe how EMCD expectations for ‘‘warm’’ (dusty) scenario—not shown here—provide differences of the opposite sign, further confirming the role of silicate aerosols. Later during mission, dust tends to settle and the observed aerosol load becomes similar to the values assumed by the model. For
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
a
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Fig. 3. (a) PFS-derived thermal structure of the Martian atmosphere along a latitudinal cross track acquired during MEX orbit 41. Empty squares represent the actual location of PFS acquisitions and (b) difference between PFS air temperature field and EMCD expectations.
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PFS-EMCD Orbit:00382; Ls: 31.0
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Fig. 4. Same as Fig. 3, for orbit 382.
Ls around 301 some systematic differences between observations and modeling still exist (Fig. 4a and 4b), but they are often in the same order of magnitude of temperature retrieval error.
3. A temperature excess south of the southern rim of the crater, at the same altitude of previous structures. 4. A third weak temperature deficiency close to the surface, at the same longitude range of the previous structure.
3.2. Southern late autumn
Other possible details include a further temperature minima at the northern rim (below 10 km), while the strong gradients observed below 5 km from the surface at 55S fall in an altitude range not effectively sampled by PFS-derived retrievals. The most striking characteristics of these patterns are given by their constant aspect in a relatively broad seasonal range. Possibly, already present since Ls ¼ 301, they are evident at least until Ls ¼ 511. More specifically, it was possible to identify a subset of orbital passages in a relatively narrow range of Ls (36.1–56.7) and local time (9.3–7.7) and to reconstruct the tri-dimensional difference field, with the assumption that differences arise from spatial locations of acquisitions instead of temporal variations. Spatial coverage proceeded systematically from E to W, by means of orbits 422, 444, 466, 488, 510, 521, 532, 554 and 587. Results are presented in Fig. 8. Here, we can appreciate correlations between topography and temperature differences also on the horizontal dimension. In the altitude range 10–20 km, a moderately warm region defines the north, west and south rims of the Hellas depression. Above 30 km, the northern
A more complex behavior becomes evident beyond Ls ¼ 401. Figs. 5–7 show some examples of PFS air temperature fields measured in this period, along with the corresponding differences with respect to EMCD expectations. The general trends observed in the data panels (a) are in good agreement with the global circulation characteristics foreseen by the model. Namely, the temperature inversion observed around 30 km, 60S is consistent with the descending branch of the weak equinox Hadley cell generated by the Solar heating at the equator. The difference panels (b) highlight, however, systematic patterns, well outside the retrieval error bars. These characteristics can be listed as follows: 1. A first temperature deficiency, of moderate amplitude, centered just north of the north rim of the basin, at 40 km altitude. 2. A second, stronger, temperature deficiency, located almost at the center of the basin at the same altitude of the previous one, more extended toward the surface.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
PFS-EMCD Orbit:00466; Ls: 41.6 50
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Fig. 5. Same as Fig. 3, for orbit 466.
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Fig. 6. Same as Fig. 3, for orbit 510.
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Fig. 7. Same as Fig. 3, for orbit 543.
rim becomes colder, to eventually lead to the situation observed at 40 km, with the northern rim of crater quite well defined by the temperature deficiency. The central part of basis remains colder than model expectations in the entire range of altitudes. Very peculiar behavior is observed later in the eastern part of the basin. Orbits 583, 594 and 605 present trends generally consistent with the ones described above for the western side of crater (Fig. 9a). Since orbit 616, the deep temperature deficiency suddenly disappears (Fig. 9b), and is absent also in following orbits 627 and 638. Panels c–f of Fig. 9 demonstrate how this change should be ascribed to a sudden change in the PFS derived fields. The wide spatial scale involved, the consistently homogeneous trends of the two orbit groups and the rejection of poor-fit
cases make us confident on the real occurrence of this phenomenon. 3.3. Southern winter Another spatial contiguous group of orbits encompass the 988, 1032, 1043, 1065, 1076 and 1109th passes. In this case, the Ls and local time ranges covered are, respectively, (106.1–121.7) and (16.0–14.8). Despite a limited latitude coverage due to MEX orbital evolution, a clear temperature excess could be observed on the northern rim of the basin around 40 km altitude (Fig. 10). Magnitude of differences tends to vanish at the center of the crater in the entire range of altitudes effectively probed by PFS measurements, suggesting, once again, a relation with topography.
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Fig. 8. Tri-dimensional air temperature difference field derived from PFS observations for Ls ¼ 451 and LT ¼ 8.5.
4. Retrieved dust load fields PFS data can provide an estimate of the dust opacity integrated along the entire atmospheric column on single spectrum basis. Further insights on the vertical distribution of these aerosols can possibly be gained from nadir measurements once we assume that: 1. the number of dust particles per unit volume ndust obeys to an exponential decay dustðzÞ ¼ ndust ð0Þ ez=hdust
(1)
being z the altitude above the reference geoid 2. the parameters ndust(0) and hdust can be considered as constants in a limited spatial range. For the former hypothesis, Z 1 tdust / ndust ðzÞ dz ¼ hdust ndust ð0Þ ezsurf =hdust (2) zsurf
therefore, for the latter hypothesis, hdust can be estimated by a best-fit procedure from a series of (tdust, z) measurements over a region of rapidly varying topography. Latitudinal cross sections derived from PFS data over the Hellas rims represent a suitable dataset for this study. Fig. 11 provides the outcomes from two different orbital passes during Southern late autumn. Measurements are classified as north or south according to the crater rim on which they were acquired. For orbit 466, northern data follows quite strictly Eq. (2). The values of hdust are very close to the effective scale heights of the atmosphere1 (8165 vs. 8170 m), suggesting an effective mixing of the dust in the lowest 1 A figure of merit for atmospheric scale height is given by H ¼ RT/g, being R the gas constant, g the Mars gravity acceleration and T the ‘‘mean’’ atmospheric temperature. From Figs. 5 and 6, this mean value can be set to 160 and 180 K, respectively, on the Southern and Northern rims of the crater.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
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Fig. 9. Air temperature fields from orbit 605 and 615. Panels a and b PFS measurements—EMCD expectations, panels c and d. PFS measurements, panels e and f EMCD expectations.
levels. Southern data are much more sparse, but still provides hdust values similar to atmospheric scale height (9010 vs. 9190 m). Agreement is not so good for orbit 511 (9482 vs. 8170 m on south, 12049 vs. 9190 m on north), but still compatible once the not negligible errors related to data spread (2–3 km for the cases presented here) and limited population is taken into account. PFS-derived measurements of tdust can be compared against the EMCD assumptions (current version of parent GCM does not provide forecast for dust load; this parameter is initialized to the values observed by TES instrument and given by Smith, 2004). For an effective comparison, dust loads are scaled to the reference pressure level of 610 Pa (Fig. 12) assuming uniformly mixed aerosols. The period of southern late autumn is characterized by a general low dust content in the atmosphere.
Nevertheless, PFS data remain usually smaller than model assumptions in almost the entire range of latitudes covering the Hellas basin. Also relative trends differ. While EMCD assumes a local dust maxima over the crater (even after scaling to reference level), PFS data point toward a decrease toward the South pole, with a discontinuity between 40 and 50S. In any case, no correlation appears evident between the discrepancies in the dust fields and those in air temperatures, even once a bidimensional map is taken into account. More specifically, the warm zone on the southern rim of the carter observed in this season (Section 3.2, point 2), if related to dust radiative effects, would require an higher aerosol content with respect to model expectations, while the opposite is observed here. Low amounts of observed (and expected) dust rule out aerosols effects in the explanation of the observed air temperature phenomenology.
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5. Retrieved water ice load fields
6. Possible ambiguities in data interpretation
Thermal IR spectra measured by PFS can also effectively constraint the column-integrated content of water ice in the atmosphere of Mars. During the retrieval, we assume the ice as well mixed in the atmosphere, even if the vertical trends of air temperature as well as the likely vertical variations of water vapor mixing ratio allow the existence of stable water ice only in a limited range of altitudes in the atmosphere. This fact leads us to avoid any normalization to a reference pressure level while producing Fig. 12b. This map refers to the southern late autumn, and a correlation could be observed with air temperature deficiency on the Northern rim of the crater and local ice enrichment. Noteworthy, EMCD parent GCM is able to foresee this enhancement (Fig. 12c), but not to take into account the radiative effects of clouds in the energy budget of the atmosphere.
6.1. Air temperatures
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EMCD 4.0 derives from GCM runs performed on a moderate resolution topography grid (3.751latitude by 5.6751 longitude), adopted to achieve acceptable computational times and dataset distribution size. The comparison of model expectations against the PFS-derived data on an absolute altitude scale (i.e. referred to the standard MOLA geoid) becomes therefore ambiguous. For a given observation, strong surface elevation differences between the 51 map and the average actual surface elevation in the area encompassed by PFS FOV cannot be ruled out. Three different approaches can be adopted to overcome the problem:
1. Relative altitude: Data are compared considering instead of the absolute altitude above the geoid, the relative altitude above the corresponding adopted surface, where expectations and observations have possibly two different absolute surface elevations. Eventually, the differences can be referred back to the surface elevation of PFS data. This approach was used for the analysis presented above. 2. Comparison on pressure grid: PFS-derived profile are computed assuming the surface pressure returned by the pres0 subroutine on the basis of an high-resolution topography map, not the surface pressure expected by EMCD 4.0. The difference in surface pressures in PFS data and EMCD expectation makes, once again, a direct comparison ambiguous. 3. Comparison on s coordinate: The quantity s ¼ log10 (p/psurface) allows a comparison of PFS data and EMCD expectations, as long as each dataset maintains its own value of psurface. Direct tests demonstrated that the structures described in Section 3 for air temperature
b
Fig 11. Observed integrated dust opacities as a function of surface elevation for two PFS orbits inside the Hellas basin. Straight lines represent best-fit curves according Eq. (1). The measurements are classified in two populations according to latitudes.
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-5 0.1
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-50 -40 Latitude (deg.)
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Norm. dust opacity @ 1100 cm -1
0
0.2
0.0 -70
b
Orbit: 0510 5
Altitude (km)
Norm. dust opacity @ 1100 cm -1
Orbit: 0466 0.3
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0.2
0
-5 0.1
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a
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c
Fig. 12. (a) Comparison between the dust loads observed by PFS (solid line) and those assumed by EMCD (dot-dashed line) for two orbits during southern late autumn. Red line provides the topography profile. Values are referred to the standard 610 Pa level. (b) Water ice load field derived from PFS observations for Ls ¼ 451 and LT ¼ 8.5. Unit is column integrated optical depth at 830 cm1. (c) Water ice volume mixing ratio expected by EMCD 4.0 at around 8am LT for Ls ¼ 30–60 at Longitude ¼ 61.9E.
differences are still present at the same locations even adopting the comparison on s grid. Therefore, they cannot be explained as artifacts due to ambiguity in the definition of reference vertical grid. The horizontal trends shown in Fig. 8 are made ambiguous by the joint contribution of spatial and temporal variations. This source of ambiguity can be investigated considering three orbits closely spaced in time, covering the regions respectively east, in the middle and west of main basin. Fig. 13 presents data from three orbital passages with these characteristics. Panel a shows the air difference field derived from orbit 422, at the center of the basin. Panel b provides the same quantity for orbit 429, on the far east side of the basin. In the latter case, magnitudes of differences northward of 50S are negligible. Noteworthy, just a weak trace can be found of the (45S,
40 km) deficiency observed in panel a at the center of the basin. This suggests that the feature shall be ascribed mainly to topography effects instead of latitudinal variations. The same holds true for the northern rim deficiency (at 25S), virtually absent in Fig. 13b. On the other hand, the temperature excess southward of 60S at 40 km is also evident outside the basin, pointing toward an origin not immediately related to the basin presence. Panel c presents the difference field derived from orbit 426. This panel is more similar to a with respect to b, but the magnitude of the deficiency at 40S remains relatively small. Noteworthy, the southern temperature excess has almost disappeared, highlighting possibly a longitudinal dependence. Note also the generally different trends observed in the eastern and western parts of the basin. These comparison allows us to state that, despite the unresolved ambiguities related to observational biases,
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6.2. Dust loads
PFS-EMCD Orbit:00422; Ls: 36.1 -2.
2.
-2.
. -4-6. . -8
-6 .
-8-1. 0.
4.
-10.
30
0. 2.
-4 .
-2 .
-6.
-8.
-8-6 . .
20
-1
2. . 68.
40
10
0.
Altitude (Km)
-2. 4. -
-6 . -8.
0.
50
-1 ---14. 1 12 6.. 0.
0
. -4
-8--26. .0 ..
-10 -70
-60
2.
0.
-50 -40 Latitude (deg)
-2.
0.
-30 -20 LT: 9.3
12.
30
The retrieval of dust scale height is made ambiguous by the validity of the hypothesis listed in Section 4. Namely, ndust(0) has to vary with latitude along the basin, as demonstrated by Fig. 12. Also Fig. 11 shows that, at a fixed altitude, retrieved southern dust loads tend to be much smaller than northern counterparts (i.e. best-fit lines in Fig. 11 are parallel, not overlapped). However, our computations for the retrieval of hdust were performed including just the steeper parts of orbital passages (slope 4300 m/deg latitude). This resulted in individual subset of measurements (north and south) extended for 5–81 in latitude, to be compared with the 351 diameter of the impact basin. The ratio between these two figures makes us confident that retrieved hdust are really representative of vertical distribution of dust and not of horizontal variations. Even more important, PFS tdust reported in Fig. 12 shows negligible variations on the steeper parts of orbital passages, where values remain consistent with those observed outside the main basin.
2. 0.
2.
7. Possible interpretations
20
4.
Altitude (Km)
8.
40
4. 2.
2. 0. -2. 6.4.
0.
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10
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-6.
0
-4
.
-10 -70
-60
-50 -40 Latitude (deg)
-30 -20 LT: 8.5
A complete and self-consistent mesoscale modeling of the regional circulation in the Hellas basis is beyond the purposes of this paper. Nevertheless, some qualitative interpretations can be attempted:
PFS-EMCD Orbit:00426; Ls: 36.6 50
-4.
0.
-6.
-8.
0 -10 -70
-4.
-2.
-8. -6. -4.
30
10
0.
-2.
0.
Altitude (Km)
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-4.
-2.
40
20
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-60
-6. -4.
-2.
20..
-50 -40 Latitude (deg)
-2.
-30 -20 LT: 9.3
Fig. 13. Air temperature difference fields derived from three orbits closely spaced in time, covering respectively the middle (panel a, orbit 422), eastern (panel b, orbit 429) and western (panel c, orbit 426) regions of Hellas region.
some features in the air temperature difference fields seems to be related to topography of the considered region. Among them, temperature deficiencies at the northern rim and at the center of the basin are well evident in the morning hours around LsE451.
the main reason of disagreement between GCM expectations and observations is usually represented by the dust load assumed during model computations. A possible explanation of late southern autumn observations could consists in a less dusty atmosphere around 40 km high over Hellas, because of the low altitude of the surface. This simplistic picture, however, does not consider that stretching of vortex tubes as air passes over the Hellas topographic depression tends to create a ‘‘low-pressure zone’’ over and an ascendance of air. Thus, we would expect dust to be lifted higher there, and this dust to be transported eastward in the zonal flow. Since we observe the opposite (i.e. cooler temperature on the east side), the dust distribution does not represent a likely cause. Moreover, the simultaneous PFS dust estimation does not highlight any meaningful correlation between the dust and temperature discrepancies in EMCD and actual Martian conditions. Conversely, the previous scheme could work for water and water ice. Unlike dust, water ice induces a cooling. However, the model foresees an ice-rich zone near the surface (Fig. 13 ter), which possibly would represent an explanation for the doubtful low-altitude temperature deficiency on the northern rim but does not explain the 40 km discrepancy. A second ice layer at that altitude could not be ruled out a priori, but appears as an ad hoc model. The PFS-derived cloud coverage and its correlation with temperature deficiencies points toward a role of water ice but, unfortunately, PFS dataset alone does
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not allow to constraint directly the vertical location of the observed cloud coverage on the eastern part of the basin, precluding firmer interpretation. Dynamic of the atmosphere represents a further hypothesis. Observed structure could be related to the strength of the adiabatic cooling and warming resulting from the Meridian circulation. Unfortunately, this scheme is highly model sensitive and possible consistencies between computations and data could be suspected to be due to ad hoc initial constraints. Nevertheless, this hypothesis is consistent with the fact that PFS observes warmer temperature at 40 km near the pole and colder temperature around 40S–50S.
8. Conclusions and guidelines for further investigations PFS data allowed a first direct detection of anomalies in the air temperature fields in the region of Hellas basin. These features are likely related to the dynamical effects of the topography over the global air circulation. The dust remains well mixed in the atmosphere despite the sedimentation expected away from main dust rise events, suggesting a persistent effective vertical mixing of the atmosphere. Future work will be firstly devoted to the analysis of later PFS measurements. The joint study of temperature profiles derived from TES spectra and MaRS radio occultations could possibly complement our experimental scenario. Nevertheless, an effective understanding of the described phenomenology will require an intensive modeling work by means of mesoscale circulation codes, with possible inclusion of radiative effects of water ice clouds and a dust content directly initialized on the basis of PFS retrievals. Acknowledgments For funding: ASI, ESA. For code development: R. Haus and D. V. Titov, G.A. Bianchini and CISAS. For calibration and SC operations: M. Giuranna, A. Mattana and F. Nespoli. Preliminary results of this study were presented at the 37th annual meeting of the Division for Planetary Sciences of the American Astronomical Society, Cambridge, UK, September 4–9 2005. A sponsorship kindly agreed by Dr. A. L. Graps allowed D.G. to attend to the meeting. The members of PFS team wish to thank the LMDAOPP-IAA teams for their excellent work in developing, testing, distributing and supporting the usage of the results of their global circulation model, for the benefit of the entire scientific community. References Bandfield, J.L., Hamilton, V.E., Christensen, P.R., 2000. A global view of Martian surface compositions from MGS-TES. Science 287 (5458), 1626–1630.
Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. Mars aerosol studies with the MGS TES emission phase function observations: optical depths, particle sizes, and ice cloud types versus latitude and solar longitude. J. Geophys. Res. 108 (E9), 2-1 CiteID 5098. Colaprete, A., Barnes, J.R., Haberle, R.M., Hollingsworth, J.L., Kieffer, H.H., Titus, T.N., 2005. Albedo of the south pole on Mars determined by topographic forcing of atmosphere dynamics. Nature 435 (7039), 184–188. Conrath, B.J., 1975. Thermal structure of the Martian atmosphere during the dissipation of the dust storm of 1971. Icarus 24, 36–46. Fedorova, A.A., Baklanova, I.V., Rodin, A.V., 2004. Mars atmospheric water vapor in the southern hemisphere: MAWD observations revisited. Adv. Space Res. 34 (8), 1677–1682. Fenton, L.K., Pearl, J.C., Martin, T.Z., 1997. Mapping mariner 9 dust opacities. Icarus 130 (1), 115–124. Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S.R., Read, P.L., Huot, J., 1999. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. 104 (E10), 24155–24176. Forget, F., Millour, E., Lebonnois, S., Montabone, L., Dassas K., Lewis, S.R., Read, P.L., Lopez-Valverde, M., Gon-zalez Galindo F., Montmessin, F., Lefevre F., Desjean, M.C., Huot, J.P., 2006. The new Mars climate database. In: Second Workshop on Mars Atmosphere Modelling and Observations Abstract Book, Granada, Spain, http://wwwmars.lmd.jussieu.fr/granada2006/abstracts/Forget2_Granada2006.pdf. Formisano, V., Angrilli, F., Arnold, G., Atreya, S., Bianchini, G., Biondi, D., Blanco, A., Blecka, M.I., Coradini, A., Colangeli, L., Ekonomov, A., Esposito, F., Fonti, S., Giuranna, M., Grassi, D., Gnedykh, V., Grigoriev, A., Hansen, G., Hirsh, H., Khatuntsev, I., Kiselev, A., Ignatiev, N., Jurewicz, A., Lellouch, E., Lopez Moreno, J., Marten, A., Mattana, A., Maturilli, A., Mencarelli, E., Michalska, M., Moroz, V., Moshkin, B., Nespoli, F., Nikolsky, Y., Orfei, R., Orleanski, P., Orofino, V., Palomba, E., Patsaev, D., Piccioni, G., Rataj, M., Rodrigo, R., Rodriguez, J., Rossi, M., Saggin, B., Titov, D., Zasova, L., 2005. The planetary Fourier spectrometer (PFS) onboard the European Mars express mission. Planet. Space Sci. 53 (10), 963–974. Giuranna, M., Formisano, V., Biondi, D., Ekonomov, A., Fonti, S., Grassi, D., Hirsch, H., Khatuntsev, I., Ignatiev, N., Malgoska, M., Mattana, A., Maturilli, A., Mencarelli, E., Nespoli, F., Orfei, R., Orleanski, P., Piccioni, G., Rataj, M., Saggin, B., Zasova, L., 2005a. Calibration of the planetary Fourier spectrometer long-wavelength channel. Planet. Space Sci. 53 (10), 993–1007. Giuranna, M., Formisano, V., Biondi, D., Ekonomov, A., Fonti, S., Grassi, D., Hirsch, H., Khatuntsev, I., Ignatiev, N., Michalska, M., Mattana, A., Maturilli, A., Moshkin, B.E., Mencarelli, E., Nespoli, F., Orfei, R., Orleanski, P., Piccioni, G., Rataj, M., Saggin, B., Zasova, L., 2005b. Calibration of the planetary Fourier spectrometer short wavelength channel. Planet. Space Sci. 53 (10), 975–991. Grassi, D., Ignatiev, N.I., Zasova, L.V., Maturilli, A., Formisano, V., Bianchini, G.A., Giuranna, M., 2005a. Methods for the analysis of data from the planetary Fourier spectrometer on the Mars express mission. Planet. Space Sci. 53 (10), 1017–1034. Grassi, D., Fiorenza, C., Ignatiev, N., Zasova, L., Maturilli, A., Formisano, V., 2005b. Decay of a Martian dust storm as observed by the planetary Fourier spectrometer on board of Mars express, Poster at the Second General Assembly of the European Geosciences Union, Wien, Austria, 24–29 April 2005. Hansen, G.B., 2001. Empirical optical properties of martian dust, abstract no.1282, In: 32nd Annual Lunar and Planetary Science Conference, Houston, TX, March 12–16. Ignatiev, N.I., Grassi, D., Zasova, L.V., 2005. Planetary Fourier spectrometer data analysis: fast radiative transfer models. Planet. Space Sci. 53 (10), 1035–1042. Joshi, M.M., Haberle, R.M., Barnes, J.R., Murphy, J.R., Schaeffer, J., 1997. Low-level jets in the NASA Ames Mars general circulation model. J. Geophys. Res. 102 (E3), 6511–6524.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357 Leikin, G.A., Zabalueva, E.V., 1975. Hellas and dust storms on Mars. Pis’ma v Astron. Zh. 1, 36–38 In Russian. Lewis, S.R., Collins, M., Read, P.L., Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Huot, J., 1999. A climate database for Mars. J. Geophys. Res. 104 (E10), 24177–24194. Liu, J., Richardson, M.I., Wilson, R.J., 2003. An assessment of the global, seasonal, and interannual spacecraft record of Martian climate in the thermal infrared. J. Geophys. Res. 108 (E8), 8-1 CiteID 5089. Martin, T.Z., 1981. Mean thermal and albedo behavior of the Mars surface and atmosphere over a Martian year. Icarus 45, 427–446. Martin, T.Z., Richardson, M.I., 1993. New dust opacity mapping from viking infrared thermal mapper data. J. Geophys. Res. 98 (E6), 10941–10949. Montmessin, F., Forget, F., Rannou, P., Cabane, M., Haberle, R.M., 2004. Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model. J. Geophys. Res. 109 (E10) CiteID E10004. Murphy, J.R., 1997. Understanding the martian dust cycle. Adv. Space Res. 19 (8), 1289–1290. Smith, D., Neumann G., Arvidson R.E., Guinness E.A., Slavney S., 2003. Mars global surveyor laser altimeter mission experiment gridded data record, NASA Planetary Data System, MGS-M-MOLA-5-MEGDRL3-V1.0. Smith, M.D., 2002. The annual cycle of water vapor on Mars as observed by the thermal emission spectrometer. J. Geophys. Res. (Planets) 107 (E11), 25-21 CiteID 5115.
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Smith, M.D., 2004. Interannual variability in TES atmospheric observations of Mars during 1999–2003. Icarus 167 (1), 148–165. Smith, M.D., 2006. TES atmospheric temperature, aerosol optical depth, and water vapor observations 1999–2004. In: Second Workshop on Mars Atmosphere Modelling and Observations Abstract Book, Granada, Spain. http://www-mars.lmd.jussieu.fr/granada2006/abstracts/Smith2_Granada2006.pdf. Siili, T., Haberle, R.M., Murphy, J.R., Savijarvi, H., 1999. Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions. Planet. Space Sci. 47 (8–9), 951–970. Strausberg, M.J., Wang, H., Richardson, M.I., Ewald, S.P., Toigo, A.D., 2005. Observations of the initiation and evolution of the 2001 Mars global dust storm. J. Geophys. Res. 110 (E2) CiteID E02006. Tamppari, L.K., Zurek, R.W., Paige, D.A., 2003. Viking-era diurnal water-ice clouds. J. Geophys. Res. Planets 108 (E7), 9-1 CiteID 5073. Warren, S.G., 1984. Optical constants of ice from the ultraviolet to the microwave. Appl. Opt. 23, 1206–1225. Wolff, M.J., Smith, M.D., Clancy, R.T., Spanovich, N., Whitney, B.A., Lemmon, M.T., Bandfield, J.L., Banfield, D., Ghosh, A., Landis, G., Christensen, P.R., Bell, J.F., Squyres, S.W., 2006. Constraints on dust aerosols from the Mars Exploration Rovers using MGS overflights and Mini-TES. J. Geophys. Res. 111 (E12), CiteID E12S17.
Planetary and Space Science 55 (2007) 1346–1357 www.elsevier.com/locate/pss
The martian atmosphere in the region of Hellas basin as observed by the planetary Fourier spectrometer (PFS-MEX) D. Grassia,, V. Formisanoa, F. Forgetb, C. Fiorenzaa,c, N.I. Ignatieva,d, A. Maturillie, L.V. Zasovaa,d a
INAF-IFSI, Via del Fosso del Cavaliere, 00144 Rome, Italy b LMD-IPSL, Paris, France c Universita` degli Studi dell’Aquila, Italy d IKI-RAS, Profsojuznaja 84/32, 117997 Moscow, Russia e DLR, Berlin, Germany Accepted 2 October 2006 Available online 13 January 2007
Abstract This work presents a review of the observations acquired by the planetary Fourier spectrometer (PFS) in the region of the Hellas basin. Taking advantage of the high spectral resolution of PFS, the vertical air temperature profile can be investigated with a previously unexperienced vertical resolution. Extensive comparisons with the expectations of EMCD 4.0 database highlight moderate discrepancies, strongly dependant on season. Namely, the morning observations acquired around Ls ¼ 451 show a series of temperature deficiencies with recurrent spatial patterns in different observations, correlated with the topography profile. Trends of integrated dust loads as a function of the field of view (FOV) elevation are also described. Values are consistent with the retrieval hypothesis of a dust scale height equal to the gas one, even far from the season of main dust storms. r 2007 Elsevier Ltd. All rights reserved.
1. Introduction The Hellas basin represents a major feature in the topography of the Martian surface. Its dimensions made it the most impressive impact structure of the entire Solar system. The depth of the basin with respect to the surrounding terrains (8 km) is comparable with the scale height of the Martian atmosphere, while its diameter has a size similar to the typical length of atmospheric waves. Both factors suggest a likely role of the crater in modifying the circulation of air masses with respect to an ideal flat Mars in a variety of space scales. The location has been identified since a long time as the site of peculiar atmospheric phenomena. Visual observaCorresponding author. Current address: LESIA Observatoire de Paris, Section de Meudon 5, place Jules Janssen 92195 MEUDON Cedex, France. Tel.: +39 06 49934034; fax: +39 06 49934074. E-mail address: [email protected] (D. Grassi).
0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.12.006
tions from Earth recognized Hellas as a preferential location for the beginning of global dust storms, pointing toward slope winds driven by topography profile as a possible explanation (e.g.: Leikin and Zabalueva, 1975). Extensive records from spacecraft dataset have confirmed Hellas to be a special place for silicate aerosol phenomenology, both during the onset (Martin and Richardson, 1993; Murphy, 1997; Liu et al., 2003; Strausberg et al., 2005) as well as in the decay phases (Fenton et al., 1997) of severe dust lifting events. Tamppari et al. (2003) described the frequent occurrence of topographic water ice clouds in the region, while the contents of water vapor reported by Smith (2002) and Fedorova et al. (2004) do not present specific anomalies once the integrated amounts are normalized for the total atmospheric pressure. Several efforts have also been devoted to the numerical modeling of atmosphere dynamics in the Hellas region. Joshi et al. (1997) described a mechanism where the increase of dust loads intensifies the slope winds in the Hellas region, providing a positive feedback for the dust
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lifting. Siili et al. (1999) investigated the mesoscale interaction of slope winds with ice-edge effects, while Montmessin et al. (2004) invoked a role of regional circulation forced by impact basins in driving the transport of moisture toward the southern pole. Eventually, Colaprete et al. (2005) highlighted the role of Hellas and Argyre basins in forcing the onset of a stationary thermal wave detected in the vicinity of Southern polar region, which eventually led to the observed Southern polar cap asymmetry. This last work provides an important insight on the interpretative help offered in regional atmospheric studies by the analysis of air temperature fields. Despite the very good spatial coverage offered by the measurements of the thermal emission spectrometer (TES) on board of the Mars global surveyor satellite (Smith, 2004, and references therein), no specific analysis of the derived retrievals in the Hellas region has yet appeared in the literature. More recently, the on-going measurements campaigned by the planetary Fourier spectrometer (PFS hereon) on board of ESA Mars express (MEX) has provided further opportunities in this investigation context. This work aims to present the PFS data acquired in the region of Hellas and the observed phenomenology. Unexpected behaviors of the atmosphere are highlighted mainly by comparison against the expectations of the European Martian climate database v4.0 (EMCD hereon, Forget et al., 2006; Lewis et al., 1999).
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procedure is able to compute simultaneously the vertical air temperature profile (in the indicative range 5–45 km above the surface), the surface temperature and the integrated contents for silicate dust and water ice clouds. Numerical experiments on simulated observations, performed taking into account the actual NER values, allowed to estimate the random retrieval error for T(p) to be in the order of 2 K in the region of data highest sensitivity (15–20 km above the surface). A similar approach leads to fix around 0.03 the random errors for the retrievals of both dust and iceintegrated opacities. Retrievals are performed on individual LWC measurements, provided that an adequate absolute radiometric calibration is available. The algorithm takes advantage of a specific subroutine for the computation of synthetic spectra (Ignatiev et al., 2005), able to account for the effects of multiple scattering by atmospheric aerosols. For the purposes of this work, only LWC was considered. The distribution of PFS measurements in terms of latitude, local time and season at the center of the observed FOV derives from a list of operative constraints: not Sun synchronous orbit of MEX satellite, downlink needs and pointing requirements by other experiments in the payload. Namely, the first constraint results in a strong correlation between local time and season, as can be seen in Fig. 1. There, we show the distribution of PFS measurements in the Hellas basin region—as defined by the quadrant [201;701]S, [401;1001]E—currently available for analysis. Fig. 2 shows the geographic location of FOV centers for
2. Dataset 20
Local time (1/24 Sol)
The PFS instrument is a Fourier spectrometer in the payload of MEX mission; it measures the radiation emerging from the atmosphere and surface of Mars in the spectral range between 250 and 8200 cm1. The radiation field is analyzed separately in two channels. The long-wavelength channel (LWC) covers the thermal region between 250 and 1700 cm1. On the other hand, the shortwavelength channel (SWC) measures the incoming photons between 1700 and 8200 cm1, with a thermal part dominating below 2500 cm1 and the Solar-reflected radiation being the most important toward the visible region. For both channels, spectral resolution is in the order of 2 cm1 for apodized spectra, sampled with a 1 cm1 step. The instantaneous field of view (FOV) of LWC has a diameter of 12.2 km at the nominal height of the MEX pericenter (250 km), while the SWC observes a slightly smaller region 7.4 km wide. A complete description of the instrument and its radiometric performances can be found in the papers by Formisano et al. (2005) and Giuranna et al. (2005a, b). Namely, individual PFS measurements in the LWC are affected by a random noise about 0.8 ergs/ (sec cm2 ster cm1) at 500 cm1, leading to a S/N about 100 for most favorable cases. The PFS spectra can be analyzed according to the methods described by Grassi et al. (2005a). Our retrieval
15
10
5 320
0
40 Solar Longitude (LS)
80
120
Latitude
-70
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-45
-32
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Fig. 1. Distribution of PFS data in the region of Hellas basin as a function of local time, season and latitude (in color code). Representation is limited to the MEX orbit range 10-1600 (data available to the current date for atmospheric analysis).
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time and season corresponding to each individual PFS acquisition.
Location of PFS observations
3. Retrieved air temperature fields
-30
60
90
-60
Solar Longitude (LS) 0
90
180
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Fig. 2. Geographic locations of the PFS measurements given in Fig. 1. Size of the spots is not representative of the actual PFS FOV size at the time of the acquisition.
the same acquisitions. Total number exceeds 4600 individual PFS measurements. Due to the almost polar orbit of MEX satellite (i871), PFS data acquired during a pericenter pass can provide a latitude scan of the atmospheric fields along a given longitude and at a fixed local time (assuming a nadir observation geometry). For the aim of this study, we will focus our attention on the Ls periods around 3401, 451 (in the ‘morning’ data subset in Fig. 1) and 1101. The comparison of PFS retrieval outcomes against the expectations of global circulation models (GCM) of the Martian atmosphere turns out in a powerful investigation tool. GCMs encompass the dynamical factors acting on air masses and demonstrated reliable in providing accurate simulations of different atmospheric fields. In this work, we adopted as reference the EMCD 4.0 database (Forget et al., 2006), a pre-computed collection of outputs from the simulations of LMD-AOPP-IAA GCM (Forget et al., 1999). Noteworthy, EMCD 4.0 takes full advantage of an extensive assimilation of TES retrievals as well as of an accurate numerical description of water cycle (Montmessin et al., 2004). Moreover, several model runs (‘dust scenario’) allow accounting for different dust loads in the atmosphere. In our case, we adopted the reference Martian year 24 (MY24) scenario in which the GCM is forced by the dust column observations retrieved by MGS TES in 1999–2001 (before the global dust storm) in order to mimic the atmosphere fields of this particular year, which is thought to be typical. The comparison PFS retrieval—EMCD expectations was performed on individual spectrum basis. This was possible extracting from the database a set of expected atmospheric quantities for the latitude, longitude, local
The retrieval of vertical air temperature profile from PFS data relies on several preliminary assumptions. The assumed surface pressure value is provided by pres0 subroutine from the LMD team, able to take into account the MOLA topography (Smith et al., 2003) at a spatial scale comparable with PFS FOV size. The direct radiative transfer model used during retrieval includes the dust optical properties by Hansen (2001) and adopts a lognormal dust size distribution, with reff ¼ 1.65 mm and neff ¼ 0.35. Water ice clouds are modeled according the size distribution I described by Clancy et al. (2003) and the complex refractive indices given by Warren (1984). Numerical test demonstrated that other likely options for size distributions and optical constants (for dust: (n,k) from Wolff et al. (2006), size parameters from Hansen (2001) and Pollack et al. (1995), for water ice: (n,k) from Hansen ()) induce systematic variations in the retrieved T(p) with a magnitude less than 3 K even in the cases, not considered here, of very heavy aerosol loads. Absolute values of aerosol opacities are, as expected, more sensitive to these choices but relative trends between different observations remain preserved. Aerosol is considered as uniformly mixed in the atmosphere during the retrieval. Surface emissivity is taken from the maps derived from TES measurements published by Bandfield et al. (2000). Temperature profiles presented in this work were filtered preliminarily in order to accept only the cases where the solution of inverse problem was able to provide a satisfactory fit with observed data, as quantified by a w2 test. 3.1. Southern summer and early autumn Fig. 3a presents the air temperature field observed by PFS during orbit 41, at Ls ¼ 337.91, while Fig. 3b provides the differences between observations and EMCD 4.0 expectations. The initial phase of MEX activities were characterized by a substantial dust load (Grassi et al., 2005b), further confirmed by simultaneous TES-MGS observations (Smith, 2006). In this case, the observed discrepancies can be easily explained taking into account the radiative effects of excess aerosol load with respect to GCM assumption. Actually, the dust tends to raise the temperature of atmospheric layers (exception made for the very lowest altitudes) due to a more effective adsorption of Solar radiation. This behavior was already described in literature for IRIS Mariner 9 (Conrath, 1975) and IRTM Viking (Martin, 1981) data. It is interesting to observe how EMCD expectations for ‘‘warm’’ (dusty) scenario—not shown here—provide differences of the opposite sign, further confirming the role of silicate aerosols. Later during mission, dust tends to settle and the observed aerosol load becomes similar to the values assumed by the model. For
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
a
b
PFS Orbit:00041; Ls:337.9
50
PFS-EMCD Orbit:00041; Ls:337.9 10.
50 170.
8.
4.
40
6.
40
6.
8.
220.
.
210
-10 -70
10
10.
230.
220.
0
-50
-40
-30
-10 -70
-20
-60
LT:13.4
Latitude (deg)
6.
6. 4. .-2. 20.
6.
4.
0
-60
4.
20
8.
210.
.
10.
10
200.
190. 200
10.
190.
0.
18
30
12.
.
170
20
Altitude (Km)
180.
30
12.
Altitude (Km)
1349
-4-8. . -6
-50 -40 Latitude (deg)
-30 -20 LT:13.4
Fig. 3. (a) PFS-derived thermal structure of the Martian atmosphere along a latitudinal cross track acquired during MEX orbit 41. Empty squares represent the actual location of PFS acquisitions and (b) difference between PFS air temperature field and EMCD expectations.
b
PFS-EMCD Orbit:00382; Ls: 31.0
20
.
180
190.
10 0
0.
-60
20 10
-50
-40
Latitude (deg)
-30
2. 0.
-2. 0.2.
190.
-20
0.
-2.
0.
6. 4. 2.
0
180
.
-10 -70
30
200.
17
-2.
4.
170.
2.
-2.
0.
150.
Altitude (Km)
160.
30
0.
12.
40
160.
Altitude (Km)
40
6. 4. 16. 14.
150.
2.
50
160.
4.
PFS Orbit:00382; Ls: 31.0 50
10. 8.
a
-10 -70
0. 4.2 .
-60
LT: 9.5
2.
-50 -40 Latitude (deg)
4. 0.
-30 -20 LT: 9.5
Fig. 4. Same as Fig. 3, for orbit 382.
Ls around 301 some systematic differences between observations and modeling still exist (Fig. 4a and 4b), but they are often in the same order of magnitude of temperature retrieval error.
3. A temperature excess south of the southern rim of the crater, at the same altitude of previous structures. 4. A third weak temperature deficiency close to the surface, at the same longitude range of the previous structure.
3.2. Southern late autumn
Other possible details include a further temperature minima at the northern rim (below 10 km), while the strong gradients observed below 5 km from the surface at 55S fall in an altitude range not effectively sampled by PFS-derived retrievals. The most striking characteristics of these patterns are given by their constant aspect in a relatively broad seasonal range. Possibly, already present since Ls ¼ 301, they are evident at least until Ls ¼ 511. More specifically, it was possible to identify a subset of orbital passages in a relatively narrow range of Ls (36.1–56.7) and local time (9.3–7.7) and to reconstruct the tri-dimensional difference field, with the assumption that differences arise from spatial locations of acquisitions instead of temporal variations. Spatial coverage proceeded systematically from E to W, by means of orbits 422, 444, 466, 488, 510, 521, 532, 554 and 587. Results are presented in Fig. 8. Here, we can appreciate correlations between topography and temperature differences also on the horizontal dimension. In the altitude range 10–20 km, a moderately warm region defines the north, west and south rims of the Hellas depression. Above 30 km, the northern
A more complex behavior becomes evident beyond Ls ¼ 401. Figs. 5–7 show some examples of PFS air temperature fields measured in this period, along with the corresponding differences with respect to EMCD expectations. The general trends observed in the data panels (a) are in good agreement with the global circulation characteristics foreseen by the model. Namely, the temperature inversion observed around 30 km, 60S is consistent with the descending branch of the weak equinox Hadley cell generated by the Solar heating at the equator. The difference panels (b) highlight, however, systematic patterns, well outside the retrieval error bars. These characteristics can be listed as follows: 1. A first temperature deficiency, of moderate amplitude, centered just north of the north rim of the basin, at 40 km altitude. 2. A second, stronger, temperature deficiency, located almost at the center of the basin at the same altitude of the previous one, more extended toward the surface.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
PFS-EMCD Orbit:00466; Ls: 41.6 50
10 150.
0
16
0.
-10 -70
-6.
-8.
30 0.
0.
-10. -8.-6.
-40
-30
-10 -70
-20
-60
4.
-50
LT: 8.9
Latitude (deg)
-2.
2.
-4-2. .0.
-50
2.
0.
10
170.
-60
-6. -4.
20
0
190.
-4
.
170.
-2. .
0.
-4
Altitude (Km)
160.
20
18 0.
Altitude (Km)
150.
30
-6.
2.
40
40
-4.
0.-2.
.
150
140.
-2 .
b
PFS Orbit:00466; Ls: 41.6 50
-2.
a
6. 8. 4.
1350
-40
-30
-20
LT: 8.9
Latitude (deg)
Fig. 5. Same as Fig. 3, for orbit 466.
b 50
0.
.
-10 -70
-60
180
170.
-50
-40
0. -2 -6. .
10 0
.
.
0.
. -4. -8-6.
0.
160
-30
0.
-2. -4.
-10 -70
-20
-4.
0.
-60
-50 -40 Latitude (deg)
LT: 8.4
Latitude (deg)
-4. .
20
19
0
2.
-2
Altitude (Km)
140.
0. 15
10
2.
4.
180.
30
-2.
170.
.
4.
.
0.
20
160
0. -2. -4.
-6
6.
30
16
Altitude (Km)
40
2. 0.
4. 8.
40
PFS-EMCD Orbit:00510; Ls: 47.1 -6.
15
140
2.
PFS Orbit:00510; Ls: 47.1 50
0.
a
-30 -20 LT: 8.4
Fig. 6. Same as Fig. 3, for orbit 510.
40
15
0.
160.
-60
-50
1
-40
-30
2. 0.
-4 0.
-2.
0. -4.-2. -6.
2.
-8.
0.
170.
Latitude (deg)
.
2.
0
.
-10 -70
80.
10
-4
0
190.
-6.
10
20 .
180.
30
-10
140
.
0.
170.
-2.
2.
.
160
-20
-2. -6.-4. -8.
0.
-10 -70
-60
LT: 8.1
-50
-2.
-40
Latitude (deg)
-4.
.
16
30 20
4. 4.
150
150.
PFS-EMCD Orbit:00543; Ls: 51.2 50 6.
.
2.
140
40 Altitude (Km)
b
PFS Orbit:00543; Ls: 51.2 50
Altitude (Km)
a
-30
-20
LT: 8.1
Fig. 7. Same as Fig. 3, for orbit 543.
rim becomes colder, to eventually lead to the situation observed at 40 km, with the northern rim of crater quite well defined by the temperature deficiency. The central part of basis remains colder than model expectations in the entire range of altitudes. Very peculiar behavior is observed later in the eastern part of the basin. Orbits 583, 594 and 605 present trends generally consistent with the ones described above for the western side of crater (Fig. 9a). Since orbit 616, the deep temperature deficiency suddenly disappears (Fig. 9b), and is absent also in following orbits 627 and 638. Panels c–f of Fig. 9 demonstrate how this change should be ascribed to a sudden change in the PFS derived fields. The wide spatial scale involved, the consistently homogeneous trends of the two orbit groups and the rejection of poor-fit
cases make us confident on the real occurrence of this phenomenon. 3.3. Southern winter Another spatial contiguous group of orbits encompass the 988, 1032, 1043, 1065, 1076 and 1109th passes. In this case, the Ls and local time ranges covered are, respectively, (106.1–121.7) and (16.0–14.8). Despite a limited latitude coverage due to MEX orbital evolution, a clear temperature excess could be observed on the northern rim of the basin around 40 km altitude (Fig. 10). Magnitude of differences tends to vanish at the center of the crater in the entire range of altitudes effectively probed by PFS measurements, suggesting, once again, a relation with topography.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
1351
Fig. 8. Tri-dimensional air temperature difference field derived from PFS observations for Ls ¼ 451 and LT ¼ 8.5.
4. Retrieved dust load fields PFS data can provide an estimate of the dust opacity integrated along the entire atmospheric column on single spectrum basis. Further insights on the vertical distribution of these aerosols can possibly be gained from nadir measurements once we assume that: 1. the number of dust particles per unit volume ndust obeys to an exponential decay dustðzÞ ¼ ndust ð0Þ ez=hdust
(1)
being z the altitude above the reference geoid 2. the parameters ndust(0) and hdust can be considered as constants in a limited spatial range. For the former hypothesis, Z 1 tdust / ndust ðzÞ dz ¼ hdust ndust ð0Þ ezsurf =hdust (2) zsurf
therefore, for the latter hypothesis, hdust can be estimated by a best-fit procedure from a series of (tdust, z) measurements over a region of rapidly varying topography. Latitudinal cross sections derived from PFS data over the Hellas rims represent a suitable dataset for this study. Fig. 11 provides the outcomes from two different orbital passes during Southern late autumn. Measurements are classified as north or south according to the crater rim on which they were acquired. For orbit 466, northern data follows quite strictly Eq. (2). The values of hdust are very close to the effective scale heights of the atmosphere1 (8165 vs. 8170 m), suggesting an effective mixing of the dust in the lowest 1 A figure of merit for atmospheric scale height is given by H ¼ RT/g, being R the gas constant, g the Mars gravity acceleration and T the ‘‘mean’’ atmospheric temperature. From Figs. 5 and 6, this mean value can be set to 160 and 180 K, respectively, on the Southern and Northern rims of the crater.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
-40
-30
PFS Orbit:00605; Ls: 58.9
d
140.
15 0. .
Altitude (Km)
160
180.
150.
10 0
190. 180.
160.
-60
-50
-40
Latitude (deg)
6.
20
-2.
10
.
170
0.
-10 -70
.
0.
0.
16
-60
-50
190
0.
16
-40
-30
180.
0.
-60
170.
-50
-40
-20
Latitude (deg)
-30
15
160.
150.
10
-10 -70
140.
0.
30 20
-20
LT: 7.4
EMCD Orbit:00616; Ls: 60.2 50
0
18
0
Altitude (Km)
160.
17
Altitude (Km)
f
0 15
10
0.
19
40
.
18
0.
15
Latitude (deg)
140.
30 20
0.
160.
30
LT: 7.5
15
40
2.
4.
0.
-10 -70
-20
-20
LT: 7.4
140. 16
0
EMCD Orbit:00605; Ls: 58.9 50
0.
e
-30
-30
PFS Orbit:00616; Ls: 60.2
170.
-10 -70
-40
50 40
160. 170.
-50
0. 15
0.
150.
20
-60
Latitude (deg)
14
50
30
--84-6.-.2 ..
-10 -70
-20
. -2 -4.
-8.-6.
-8.
LT: 7.5
Latitude (deg)
40
10 0
-8.
-50
20
0.
16
-60
170.
-60
30
-50
180.
-1-80 .
2.
4.
2.
0. 2.
-6.
-4. 0. -6.
0 -10 -70
Altitude (Km)
Altitude (Km)
-4.
0. -2.
0. -2. -4.
4.
2. . 22.
2.
-4.
Altitude (Km)
-2.
-2 -8.-6. .
4.
40
8.
20 10
-2.
0.
6. 4.
-6. -8.
30
PFS-EMCD Orbit:00616; Ls: 60.2 50 8.
2.
2.
-4. -2.
4. 6.
40
c
b
PFS-EMCD Orbit:00605; Ls: 58.9 50 8.
a
140 .
1352
-40
190.
-30
-20
Latitude (deg)
Fig. 9. Air temperature fields from orbit 605 and 615. Panels a and b PFS measurements—EMCD expectations, panels c and d. PFS measurements, panels e and f EMCD expectations.
levels. Southern data are much more sparse, but still provides hdust values similar to atmospheric scale height (9010 vs. 9190 m). Agreement is not so good for orbit 511 (9482 vs. 8170 m on south, 12049 vs. 9190 m on north), but still compatible once the not negligible errors related to data spread (2–3 km for the cases presented here) and limited population is taken into account. PFS-derived measurements of tdust can be compared against the EMCD assumptions (current version of parent GCM does not provide forecast for dust load; this parameter is initialized to the values observed by TES instrument and given by Smith, 2004). For an effective comparison, dust loads are scaled to the reference pressure level of 610 Pa (Fig. 12) assuming uniformly mixed aerosols. The period of southern late autumn is characterized by a general low dust content in the atmosphere.
Nevertheless, PFS data remain usually smaller than model assumptions in almost the entire range of latitudes covering the Hellas basin. Also relative trends differ. While EMCD assumes a local dust maxima over the crater (even after scaling to reference level), PFS data point toward a decrease toward the South pole, with a discontinuity between 40 and 50S. In any case, no correlation appears evident between the discrepancies in the dust fields and those in air temperatures, even once a bidimensional map is taken into account. More specifically, the warm zone on the southern rim of the carter observed in this season (Section 3.2, point 2), if related to dust radiative effects, would require an higher aerosol content with respect to model expectations, while the opposite is observed here. Low amounts of observed (and expected) dust rule out aerosols effects in the explanation of the observed air temperature phenomenology.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
5. Retrieved water ice load fields
6. Possible ambiguities in data interpretation
Thermal IR spectra measured by PFS can also effectively constraint the column-integrated content of water ice in the atmosphere of Mars. During the retrieval, we assume the ice as well mixed in the atmosphere, even if the vertical trends of air temperature as well as the likely vertical variations of water vapor mixing ratio allow the existence of stable water ice only in a limited range of altitudes in the atmosphere. This fact leads us to avoid any normalization to a reference pressure level while producing Fig. 12b. This map refers to the southern late autumn, and a correlation could be observed with air temperature deficiency on the Northern rim of the crater and local ice enrichment. Noteworthy, EMCD parent GCM is able to foresee this enhancement (Fig. 12c), but not to take into account the radiative effects of clouds in the energy budget of the atmosphere.
6.1. Air temperatures
40. km
20
-20
13 -30
Latitude
6 0 -6
-40
-50
-13 -60
-20 60
65
70
75
80
Longitude (E) Fig. 10. Tri-dimensional air temperature difference field derived from PFS observations for Ls ¼ 1131 and LT ¼ 15.5.
a
1353
EMCD 4.0 derives from GCM runs performed on a moderate resolution topography grid (3.751latitude by 5.6751 longitude), adopted to achieve acceptable computational times and dataset distribution size. The comparison of model expectations against the PFS-derived data on an absolute altitude scale (i.e. referred to the standard MOLA geoid) becomes therefore ambiguous. For a given observation, strong surface elevation differences between the 51 map and the average actual surface elevation in the area encompassed by PFS FOV cannot be ruled out. Three different approaches can be adopted to overcome the problem:
1. Relative altitude: Data are compared considering instead of the absolute altitude above the geoid, the relative altitude above the corresponding adopted surface, where expectations and observations have possibly two different absolute surface elevations. Eventually, the differences can be referred back to the surface elevation of PFS data. This approach was used for the analysis presented above. 2. Comparison on pressure grid: PFS-derived profile are computed assuming the surface pressure returned by the pres0 subroutine on the basis of an high-resolution topography map, not the surface pressure expected by EMCD 4.0. The difference in surface pressures in PFS data and EMCD expectation makes, once again, a direct comparison ambiguous. 3. Comparison on s coordinate: The quantity s ¼ log10 (p/psurface) allows a comparison of PFS data and EMCD expectations, as long as each dataset maintains its own value of psurface. Direct tests demonstrated that the structures described in Section 3 for air temperature
b
Fig 11. Observed integrated dust opacities as a function of surface elevation for two PFS orbits inside the Hellas basin. Straight lines represent best-fit curves according Eq. (1). The measurements are classified in two populations according to latitudes.
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
1354
-5 0.1
-10 -60
-50 -40 Latitude (deg.)
-30
-20
Norm. dust opacity @ 1100 cm -1
0
0.2
0.0 -70
b
Orbit: 0510 5
Altitude (km)
Norm. dust opacity @ 1100 cm -1
Orbit: 0466 0.3
0.3 5
0.2
0
-5 0.1
Altitude (km)
a
-10 0.0 -70
-60
-50 -40 Latitude (deg.)
-30
-20
c
Fig. 12. (a) Comparison between the dust loads observed by PFS (solid line) and those assumed by EMCD (dot-dashed line) for two orbits during southern late autumn. Red line provides the topography profile. Values are referred to the standard 610 Pa level. (b) Water ice load field derived from PFS observations for Ls ¼ 451 and LT ¼ 8.5. Unit is column integrated optical depth at 830 cm1. (c) Water ice volume mixing ratio expected by EMCD 4.0 at around 8am LT for Ls ¼ 30–60 at Longitude ¼ 61.9E.
differences are still present at the same locations even adopting the comparison on s grid. Therefore, they cannot be explained as artifacts due to ambiguity in the definition of reference vertical grid. The horizontal trends shown in Fig. 8 are made ambiguous by the joint contribution of spatial and temporal variations. This source of ambiguity can be investigated considering three orbits closely spaced in time, covering the regions respectively east, in the middle and west of main basin. Fig. 13 presents data from three orbital passages with these characteristics. Panel a shows the air difference field derived from orbit 422, at the center of the basin. Panel b provides the same quantity for orbit 429, on the far east side of the basin. In the latter case, magnitudes of differences northward of 50S are negligible. Noteworthy, just a weak trace can be found of the (45S,
40 km) deficiency observed in panel a at the center of the basin. This suggests that the feature shall be ascribed mainly to topography effects instead of latitudinal variations. The same holds true for the northern rim deficiency (at 25S), virtually absent in Fig. 13b. On the other hand, the temperature excess southward of 60S at 40 km is also evident outside the basin, pointing toward an origin not immediately related to the basin presence. Panel c presents the difference field derived from orbit 426. This panel is more similar to a with respect to b, but the magnitude of the deficiency at 40S remains relatively small. Noteworthy, the southern temperature excess has almost disappeared, highlighting possibly a longitudinal dependence. Note also the generally different trends observed in the eastern and western parts of the basin. These comparison allows us to state that, despite the unresolved ambiguities related to observational biases,
ARTICLE IN PRESS D. Grassi et al. / Planetary and Space Science 55 (2007) 1346–1357
6.2. Dust loads
PFS-EMCD Orbit:00422; Ls: 36.1 -2.
2.
-2.
. -4-6. . -8
-6 .
-8-1. 0.
4.
-10.
30
0. 2.
-4 .
-2 .
-6.
-8.
-8-6 . .
20
-1
2. . 68.
40
10
0.
Altitude (Km)
-2. 4. -
-6 . -8.
0.
50
-1 ---14. 1 12 6.. 0.
0
. -4
-8--26. .0 ..
-10 -70
-60
2.
0.
-50 -40 Latitude (deg)
-2.
0.
-30 -20 LT: 9.3
12.
30
The retrieval of dust scale height is made ambiguous by the validity of the hypothesis listed in Section 4. Namely, ndust(0) has to vary with latitude along the basin, as demonstrated by Fig. 12. Also Fig. 11 shows that, at a fixed altitude, retrieved southern dust loads tend to be much smaller than northern counterparts (i.e. best-fit lines in Fig. 11 are parallel, not overlapped). However, our computations for the retrieval of hdust were performed including just the steeper parts of orbital passages (slope 4300 m/deg latitude). This resulted in individual subset of measurements (north and south) extended for 5–81 in latitude, to be compared with the 351 diameter of the impact basin. The ratio between these two figures makes us confident that retrieved hdust are really representative of vertical distribution of dust and not of horizontal variations. Even more important, PFS tdust reported in Fig. 12 shows negligible variations on the steeper parts of orbital passages, where values remain consistent with those observed outside the main basin.
2. 0.
2.
7. Possible interpretations
20
4.
Altitude (Km)
8.
40
4. 2.
2. 0. -2. 6.4.
0.
PFS-EMCD Orbit:00429; Ls: 37.0 50
10
2. 0.
-2.
-6.
0
-4
.
-10 -70
-60
-50 -40 Latitude (deg)
-30 -20 LT: 8.5
A complete and self-consistent mesoscale modeling of the regional circulation in the Hellas basis is beyond the purposes of this paper. Nevertheless, some qualitative interpretations can be attempted:
PFS-EMCD Orbit:00426; Ls: 36.6 50
-4.
0.
-6.
-8.
0 -10 -70
-4.
-2.
-8. -6. -4.
30
10
0.
-2.
0.
Altitude (Km)
2.
-4.
-2.
40
20
1355
-60
-6. -4.
-2.
20..
-50 -40 Latitude (deg)
-2.
-30 -20 LT: 9.3
Fig. 13. Air temperature difference fields derived from three orbits closely spaced in time, covering respectively the middle (panel a, orbit 422), eastern (panel b, orbit 429) and western (panel c, orbit 426) regions of Hellas region.
some features in the air temperature difference fields seems to be related to topography of the considered region. Among them, temperature deficiencies at the northern rim and at the center of the basin are well evident in the morning hours around LsE451.
the main reason of disagreement between GCM expectations and observations is usually represented by the dust load assumed during model computations. A possible explanation of late southern autumn observations could consists in a less dusty atmosphere around 40 km high over Hellas, because of the low altitude of the surface. This simplistic picture, however, does not consider that stretching of vortex tubes as air passes over the Hellas topographic depression tends to create a ‘‘low-pressure zone’’ over and an ascendance of air. Thus, we would expect dust to be lifted higher there, and this dust to be transported eastward in the zonal flow. Since we observe the opposite (i.e. cooler temperature on the east side), the dust distribution does not represent a likely cause. Moreover, the simultaneous PFS dust estimation does not highlight any meaningful correlation between the dust and temperature discrepancies in EMCD and actual Martian conditions. Conversely, the previous scheme could work for water and water ice. Unlike dust, water ice induces a cooling. However, the model foresees an ice-rich zone near the surface (Fig. 13 ter), which possibly would represent an explanation for the doubtful low-altitude temperature deficiency on the northern rim but does not explain the 40 km discrepancy. A second ice layer at that altitude could not be ruled out a priori, but appears as an ad hoc model. The PFS-derived cloud coverage and its correlation with temperature deficiencies points toward a role of water ice but, unfortunately, PFS dataset alone does
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not allow to constraint directly the vertical location of the observed cloud coverage on the eastern part of the basin, precluding firmer interpretation. Dynamic of the atmosphere represents a further hypothesis. Observed structure could be related to the strength of the adiabatic cooling and warming resulting from the Meridian circulation. Unfortunately, this scheme is highly model sensitive and possible consistencies between computations and data could be suspected to be due to ad hoc initial constraints. Nevertheless, this hypothesis is consistent with the fact that PFS observes warmer temperature at 40 km near the pole and colder temperature around 40S–50S.
8. Conclusions and guidelines for further investigations PFS data allowed a first direct detection of anomalies in the air temperature fields in the region of Hellas basin. These features are likely related to the dynamical effects of the topography over the global air circulation. The dust remains well mixed in the atmosphere despite the sedimentation expected away from main dust rise events, suggesting a persistent effective vertical mixing of the atmosphere. Future work will be firstly devoted to the analysis of later PFS measurements. The joint study of temperature profiles derived from TES spectra and MaRS radio occultations could possibly complement our experimental scenario. Nevertheless, an effective understanding of the described phenomenology will require an intensive modeling work by means of mesoscale circulation codes, with possible inclusion of radiative effects of water ice clouds and a dust content directly initialized on the basis of PFS retrievals. Acknowledgments For funding: ASI, ESA. For code development: R. Haus and D. V. Titov, G.A. Bianchini and CISAS. For calibration and SC operations: M. Giuranna, A. Mattana and F. Nespoli. Preliminary results of this study were presented at the 37th annual meeting of the Division for Planetary Sciences of the American Astronomical Society, Cambridge, UK, September 4–9 2005. A sponsorship kindly agreed by Dr. A. L. Graps allowed D.G. to attend to the meeting. The members of PFS team wish to thank the LMDAOPP-IAA teams for their excellent work in developing, testing, distributing and supporting the usage of the results of their global circulation model, for the benefit of the entire scientific community. References Bandfield, J.L., Hamilton, V.E., Christensen, P.R., 2000. A global view of Martian surface compositions from MGS-TES. Science 287 (5458), 1626–1630.
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