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. Author manuscript; available in PMC: 2020 Nov 15.
Published in final edited form as: Icarus. 2019 Nov 15;333:273–282. doi: 10.1016/j.icarus.2019.06.009

Local time variation of water ice clouds on Mars as observed by THEMIS

Michael D Smith 1,*
PMCID: PMC6839708  NIHMSID: NIHMS1535815  PMID: 31708590

Abstract

The move of the Odyssey spacecraft during Mars Years 31 and 32 to an orbit with local time near 7:00 AM and PM has enabled the systematic retrieval of water ice cloud optical depth using THEMIS thermal infrared images at a time of day not accessible from Mars Global Surveyor, Mars Reconnaissance Orbiter, or previous Odyssey observations. Because water ice clouds form by condensation, relatively small changes in atmospheric temperature can cause clouds to form or sublimate quickly, and there can be large changes in water ice cloud optical depth over the course of a day. Retrievals of water ice cloud optical depth using THEMIS observations show significant differences in cloud locations and optical depth as a function of local time and season. Cloud optical depth generally increases from the earliest (14:30) to latest (19:30) observations. During the aphelion season the increase from afternoon to evening is primarily associated with the thickening of existing clouds, while during the equinoctial and perihelion seasons there is a proportionally greater increase associated with the formation of clouds in the evening at locations where clouds were not present during the afternoon.

1. Introduction

Water ice clouds have long been observed to be a prominent constituent in the Mars atmosphere. Beginning with their spectroscopic identification in Mariner 9 thermal-infrared observations (Curran et al., 1973), our knowledge of the general climatology of these clouds has increased markedly over the past few decades with observations from a wide range of spacecraft showing such prominent features as a low-latitude aphelion season cloud belt and polar hoods in the winter hemisphere (e.g., Viking Orbiter: Tamppari and Zurek, 2003; Mars Global Surveyor: Pearl et al., 2001; Smith, 2004; Pankine et al., 2013; Mars Odyssey: Smith, 2009; Mars Reconnaissance Orbiter: Wolff et al., 2019; Mars Express: Mateshvili et al., 2009; Madeleine et al., 2012a; Willame et al., 2017). These spacecraft observations have also enabled studies of the physical properties of the water ice aerosols that make up the clouds (e.g., Clancy et al., 2003; Wolff and Clancy, 2003; Madeleine et al., 2012a; Guzewich et al., 2014; Guzewich and Smith, 2019) and the vertical distribution of the clouds, which are found to often overlie, or “cap”, the main distribution of dust (e.g., Benson et al., 2010; Smith et al., 2013). The key role of water ice clouds in processes related to transport, radiative balance, and photochemistry in the current climate of Mars has gradually been revealed over time, and has increasingly become a focus of modeling efforts (e.g., Clancy et al., 1996; Richardson et al., 2002; Madeleine et al., 2012b). The importance of tides in controlling the relation between atmospheric temperatures and clouds has been discussed in detail by Hinson and Wilson (2004), Wilson et al. (2007), Madeleine et al. (2012b), and Wilson and Guzewich (2014). An excellent and detailed overview of Mars water ice cloud observations and modeling studies is given by Clancy et al. (2017).

Despite the advances in several areas of knowledge about Mars water ice clouds, including their climatology, general physical properties, and effect on the current climate, relatively little is known about their diurnal variation. This is primarily due to the fact that most spacecraft observations at Mars have been taken using spacecraft in Sun-synchronous orbits that only sample one daytime and one nighttime local time. Among the spacecraft that are not Sun-synchronous, the variation of clouds during daytime hours using observations from the Viking Orbiter IRTM instrument was investigated by Tamppari and Zurek (2003), while more recent observations from the Mars Express OMEGA (Szantai et al., 2017) and PFS (Giuranna et al., 2018) instruments have also been used to investigate the diurnal variations of clouds. Other investigations have examined the difference between daytime and nighttime water ice cloud using observations from Mars Global Surveyor TES (Wilson et al., 2007; Pankine et al., 2013) and Mars Reconnaissance Orbiter MCS (Wilson and Guzewich, 2014). These studies showed that nighttime clouds can have significantly higher optical depth and greater areal extent than daytime clouds. Together, these studies show important diurnal variations in the optical depth of water ice clouds with the general trends showing minimum cloudiness at midday and increasing thickness of clouds in the early morning and later afternoon hours.

The retrieval of dust and water ice aerosol optical depth using THEMIS thermal infrared observations was previously described by Smith et al. (2003) and Smith (2009). We expand on those works here by extending the timeline of observations by several Mars Years and by looking specifically at the variation of water ice cloud optical depth as a function of local time. Changes to the orbit of the Mars Odyssey spacecraft over its long lifetime have led to the orbit sampling a range in local time from the mid-afternoon to late evening hours past sunset. The new insights into the local time variation of water ice clouds gained from the THEMIS observations are the focus of this paper.

In Section 2 we describe the THEMIS instrument and the observations used for this study. Section 3 outlines the retrieval algorithm used and details the modifications to the algorithm that were made to optimize for conditions with low thermal contrast between the surface and the atmosphere. Uncertainties in the retrieved output parameters are also discussed. In Section 4 we describe the results of our analysis and compare against previous results. A summary is presented in Section 5.

2. Data set

2.1. THEMIS instrument

The THEMIS instrument is capable of imaging Mars at both thermal-IR and visible wavelengths. The thermal-IR focal plane consists of ten spectral filters covering the spectral range from about 6.5 to 15 μm. The visible wavelength focal plane consists of five spectral filters covering 450 to 850 nm (Christensen et al., 2004). In this work we use only the thermal-IR images. In the thermal-IR, Bands 1 and 2 are identical and are centered at 6.78 μm. The other eight bands (numbered 3 through 10) have central wavelengths at 7.93, 8.56, 9.35, 10.21, 11.04, 11.79, 12.57, and 14.88 μm. Except for Band 10, each thermal-IR band has a spectral width of roughly 1.0 μm allowing for a small amount of overlap between bands. Band 10 is somewhat narrower and has no spectral overlap with any other band.

A THEMIS thermal-IR image is 320 pixels across-track, with a variable length along-track depending on the observation mode. The spatial size of a THEMIS pixel on the surface is roughly 100 m producing images 32 km in width. For this study we are most interested in the local time variation of water ice cloud optical depth on a regional or global scale. Therefore, to improve signal-to-noise we spatially average the entire width of an image and 256 pixels along-track to form a “framelet” having a size roughly one-half of a degree square.

The last such framelet in any particular THEMIS image will have the most accurate calibration (an effect only noticeable in Band 10). Therefore, we use the last framelet in each thermal-IR image. The THEMIS images have a single-pixel noise equivalent delta temperature of 0.5 K at 10 μm wavelength and a typical of daytime surface temperature of 245 K (Christensen et al., 2004).

2.2. Observations used in this study

THEMIS images are generally taken to target objects of geologic interest. These targeted images typically provide adequate latitude and longitude coverage over a period of days or weeks, but a supplemental set of “atmospheric” images taken in a systematic latitude-longitude grid is also obtained approximately every 5° of Ls to ensure global-scale coverage over timescales of a couple weeks.

The top panel of Fig. 1 shows the seasonal (Ls) and latitudinal coverage of the 76,011 retrievals used in this work. The observations cover the time period from the beginning of the Odyssey mapping mission in February 2002, or Mars Year (MY) 25, Ls = 330° to May 2018, or MY 34, Ls = 180° providing nearly continuous coverage for > 8.5 Mars Years. Clearly visible is the value of the atmospheric grids for filling in gaps in coverage when data rates were low and few images were taken. The lack of retrievals in the winter hemispheres is the result both of few images taken in those regions and the lack of sufficient thermal contrast between the atmosphere and the clouds (see Section 3.2).

Fig. 1.

Fig. 1.

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(Top) The distribution of THEMIS retrievals of water ice cloud optical depth as a function of time and latitude showing the nearly continuous coverage over >8.5 Mars Years. (Bottom) The local time for the retrievals as a function of Mars Year and season. The orbit of the Odyssey spacecraft has been changed over the years to sample different local times.

An important aspect of this dataset is the variation of local time over the course of the Odyssey mission. Although the orbit of the Odyssey spacecraft has been nominally sun-synchronous, or at a fixed AM/PM local time, that local time has been changed a few times during the mission for programmatic and scientific reasons. The bottom panel of Fig. 1 shows the local time of the afternoon crossing of Odyssey’s orbit as a function of Mars Year and Ls. The quasi-sinusoidal pattern as a function of Ls is caused by the elliptic orbit of Mars. The upward and downward shift of that pattern for different Mars Years is the result of changes made to the orbit of the Odyssey spacecraft. For the first couple of Mars Years, the orbit was at roughly 5:00 AM/PM (or 17:00 h in Fig. 1). The orbit was then moved earlier, to roughly 3:30 AM/PM for MY 29 and 30. A transition to a much later local time was achieved over the course of MY 31 and 32 leading to the current orbit near 7:00 AM/PM. For any given Ls there are THEMIS observations covering a range of > 3 h of local time. The variations of water ice cloud optical depth as a function of local time are the subject of this paper.

3. Retrieval algorithm

3.1. Retrieval summary

The retrieval used here closely follows the algorithm used by Smith et al. (2003) and Smith (2009) for their earlier retrieval of aerosol optical depth from THEMIS thermal-IR images, but with modifications made to better handle the low thermal contrast between the surface and atmosphere that is typical in the most recent observations taken at later local times. In this section we give a brief overview of the Smith et al. (2003) and Smith (2009) algorithm. In the following section we describe in detail the effect of later local time on the THEMIS observations and the changes made to the retrieval algorithm to optimize for that case.

The overall aim of the retrieval algorithm is to derive a set of retrieved parameters that produce a computed spectrum that best matches the THEMIS observation. In this case, the THEMIS “spectrum” consists of the six THEMIS bands 3–8, which extend from roughly 7.5 to 12 μm wavelength and are shown in the top panel of Fig. 2. The set of three retrieved parameters, surface temperature, dust column optical depth, and water ice cloud optical depth, are fit simultaneously by linearizing the solution for radiance using the current best guess and iterating until convergence. Typically, no more than five iterations are required.

Fig. 2.

Fig. 2.

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(Top) The spectral dependence of the THEMIS thermal infrared filters. (Bottom) The relative spectral response of the THEMIS filters for surface emissivity and dust and water ice aerosols. Band 10 is dominated by CO2 gas absorption and provides a measure of atmospheric temperatures.

The forward model used to produce the computed radiance uses a plane-parallel geometry with only absorption and re-emission. Following Smith et al. (2003) and Smith (2009), we neglect scattering so that the retrieved aerosol optical depth values are absorption-only. The single-scattering albedo for dust and water ice aerosol is relatively small at thermal-IR wavelengths and numerical experiments show that a full extinction optical depth including scattering can be estimated from the absorption-only optical depths retrieved here by multiplying by either 1.3 or 1.5 for dust and water ice, respectively (Smith, 2004).

The bottom panel of Fig. 2 shows the absorption of dust and water ice aerosol as a function of THEMIS Band (or wavenumber), which was taken from the work of Bandfield and Smith (2003) and is assumed to be the same for all observations. The spectral dependence of surface emissivity is also taken from Bandfield and Smith (2003), while the amplitude of the surface emissivity as a function of latitude and longitude is taken from maps generated from TES observations (Smith, 2004). The vertical distribution of dust is assumed to be well-mixed with the background gas. Although this is clearly not the case at times (e.g., Heavens et al., 2011; Guzewich et al., 2013), it is usually a reasonable approximation when considering the column as a whole (Smith, 2004). Water ice clouds are assumed to form at the water condensation level and have a thickness of one scale height. There is assumed to be no water ice aerosol below the condensation level, which is computed using a water vapor abundance taken either from concurrent TES observations, or from TES climatology (Smith, 2004) for THEMIS observations taken after the end of systematic TES spectrometer observations (MY 27, Ls = 81°). All of the above assumptions are the same as used by Smith et al. (2003) and Smith (2009) facilitating direct comparison with those previous works.

With only THEMIS Band 10 sensitive to atmospheric temperatures, a full temperature profile cannot be retrieved from THEMIS nadir observations alone. Instead, the atmospheric temperature information given by THEMIS Band 10, which is representative of a wide range of altitudes centered at about the 0.5 mbar level, is used to modify the TES climatology temperature profile. This modification takes the form of a temperature correction that is zero at the surface and increases linearly in log-pressure (nearly linear in height) to a value ΔT at a pressure level of 0.5 mbar, where ΔT is the difference between the brightness temperature observed by THEMIS Band 10 and the TES climatology for the given Ls, latitude, and longitude. Typically, this correction is small with ΔT < 3 K, but during dust storms the correction can be much larger. Details and justification of this process is given in Smith (2009).

3.2. Effect of local time variation on the retrieval

The later local times sampled by the more recent THEMIS observations (bottom panel of Fig. 1) cross the evening and morning terminators, when both surface and atmospheric temperatures are changing rapidly. These changes greatly affect the observed spectrum and necessitate careful analysis of the retrievals.

During the daytime, surface temperatures are generally warmer than the temperature of the atmosphere where the bulk of dust and water ice clouds reside, and so the spectral features caused by atmospheric dust and water ice aerosols are seen as absorption features in the nadir geometry spectra taken by THEMIS. The opposite is usually true during the night, with dust and water ice seen as emission features because they are warmer than the cold surface. At some time during the morning and evening the surface and atmospheric temperatures are equal, and when that happens no spectral features from aerosols can be seen.

Fig. 3 illustrates this effect. The left-hand panel shows a set of averaged low-latitude THEMIS observations taken near perihelion (Ls = 255°) when dust is prominent. In these spectra, Band 3 is mostly transparent and so is close to the surface temperature, while Band 10 gives atmospheric temperatures at about 0.5 mbar (~25 km) level. In the afternoon (16:00 h) the dust absorption is clearly seen as the “V” shaped feature centered at Band 5. At 17:00 h the dust absorption is not as prominent, but still clearly visible, but it is basically gone at 18:00. By 19:00 the dust is warmer than the surface and presents an emission feature. All of the nighttime observations also show dust in emission.

Fig. 3.

Fig. 3.

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A series of averaged low-latitude THEMIS spectra as a function of local time shown in terms of brightness temperature. In the left panel showing perihelion (Ls= 255°) conditions, the dust feature centered at Band 5 changes from being observed in absorption during the afternoon to emission during evening and overnight hours. In the right panel showing aphelion (Ls= 105°) conditions, the water ice feature in Bands 7–9 remains in absorption longer into the evening even while the dust appears in emission.

The right-hand panel of Fig. 3 shows a set of averaged low-latitude THEMIS observations taken during the aphelion season (Ls = 105°) when water ice clouds are prominent. As with dust, at 16:00 h water ice shows a clear absorption feature as the “bowl” in THEMIS Bands 7, 8, and 9. For later local times in the evening, water ice clouds remain in absorption even at 19:00 when the dust can be seen in emission as a small inverted-V in Band 5. At night (4:00–6:00) the water ice feature appears to be in emission along with dust, although these spectra are somewhat difficult to interpret just by looking at them.

The spectral features from dust or water ice will disappear at the local time when the surface temperature is the same as the atmospheric temperature where the aerosol resides, which in turn depends on the vertical distribution of the aerosol. In fact, this property provides a high-level validation of our assumption of the aerosol vertical distribution stated in the previous section in that the observed dust and water ice features transition from absorption to emission at about the same local time as expected by our forward model using the THEMIS Band 10 atmospheric temperature correction and our assumption for the aerosol vertical distribution. In particular, in the evening hours, dust changes from absorption to emission much earlier than water ice clouds, implying that overall the dust is warmer than the clouds. Temperatures in the evening decrease with altitude, which is consistent with both our assumption and the fact that in limb-geometry observations, water ice is often seen to “cap” or lie above the bulk of the dust (e.g., Heavens et al., 2011; Smith et al., 2013).

To first order, the uncertainty in the retrieval of aerosol optical depth (dust or water ice) is inversely proportional to the temperature difference (or “thermal contrast”) between the surface and the atmosphere at the level of the aerosol. When the thermal contrast goes to zero, the spectral features caused by the aerosol disappear and a retrieval is no longer possible. In previous works (Smith et al., 2003; Smith, 2009) sufficient thermal contrast was ensured by only performing the retrieval on observations with a surface temperature warmer than 220 K. However, in reality the thermal contrast varies as a function of season, local time, latitude, longitude, and even the aerosol (dust vs. water ice) being considered, and there are many observations with surface temperatures colder than 220 K that have sufficient thermal contrast to perform a reliable retrieval (at least for water ice).

We wish to maximize the range of local times for the retrievals in this work so we require a more precise determination of the thermal contrast. Given our assumptions, the effect of thermal contrast on the observed spectrum can be computed numerically for both dust and water ice aerosols as the change in radiance for a given change in aerosol column optical depth. Focusing on water ice clouds, Fig. 4 shows this quantity, ∂I/∂τice as a function of local time for two different seasons. A latitude-longitude grid of points taken between 30° N and 30° S was used for the computation. Surface and atmospheric temperatures were taken from the Mars Climate Database (Forget et al., 1999; Millour et al., 2018) with the water condensation level computed using water vapor abundance from TES climatology. Radiance at 805 cm−1 (near the peak for water ice absorption, see Fig. 2) was computed for two different water ice column abundances to compute the derivative. In this representation, a negative value for ∂I/∂τice indicates that the water ice cloud spectral feature is seen in absorption (radiance decreases as cloud optical depth increases), while a positive value indicates an emission feature.

Fig. 4.

Fig. 4.

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The change in expected observed radiance [W cm−2 ster−1 wavenumber−1] for a given change in water ice optical depth as a function of local time for aphelion (top) and perihelion (bottom) conditions. A latitude-longitude grid of points taken between 30° N and 30° S was used for the computation. This quantity is directly related to the thermal contrast between the surface and the atmosphere where clouds reside. A larger absolute value of this quantity indicates that the observations are more sensitive to aerosol and a reliable retrieval can be performed. The red line at ∂I/∂τice = −1 ×10−6 W cm−2 ster−1 wavenumber−1 is our threshold for sufficient sensitivity for an acceptable retrieval. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

As expected from the THEMIS observations shown in Fig. 3, the water ice cloud absorption features that are clearly visible during midday become less and less prominent for later local times, eventually becoming small in the late evening and nighttime hours. We have selected a value of ∂I/∂τice = −1 × 10−6 W cm−2 ster−1 wavenumber−1 as the threshold for thermal contrast being great enough to accept a retrieval of column water ice optical depth. Retrievals with ∂I/∂τice < −1 × 10−6 W cm−2 ster−1 wavenumber−1 are accepted for further analysis. This threshold is somewhat arbitrary, but is based on the observed variation (or “noise”) in the retrieved cloud optical depth as a function of the thermal contrast parameter. Different threshold values could potentially be used depending on how many retrievals were averaged together. It is also true that retrievals with ∂I/∂τice > +1 × 10−6 W cm−2 ster−1 wavenumber−1 could also be accepted, although there are relatively few THEMIS observations meeting that threshold and so for simplicity they have not been included in this work.

The range of ∂I/∂τice for a given local time in Fig. 4 is indicative of the variation of that quantity with latitude and longitude. This is shown explicitly in Fig. 5 where the red squares indicate those positions in the latitude/longitude grid where the threshold condition ∂I/∂τice < −1 × 10−6 W cm−2 ster−1 wavenumber−1 is met for a local time of 19:00 h. The spatial pattern here is caused by the local value of thermal inertia (e.g., Mellon et al., 2000). For this particular local time, the regions with high thermal inertia are desirable since for that case the surface temperature cools more slowly and thus during evening hours a sufficient temperature difference is maintained between the surface and the atmosphere where clouds form.

Fig. 5.

Fig. 5.

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For an evening local time of 19:00 h representative of current THEMIS observations, the red boxes show those regions where there the condition ∂I/ ∂τice < −1 × 10−6 W cm−2 ster−1 wavenumber−1 is met allowing a reliable retrieval of water ice cloud optical depth. The green boxes indicate those areas where consistently good retrievals are available for all seasons. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The spatial pattern of coverage shown in Fig. 5 can potentially introduce serious biases into comparisons of retrieved cloud optical depth between different local times. At the earlier local time observed by THEMIS during MY 30 and 31 (~16:00) essentially complete spatial coverage is possible in the latitude range between 30° N and 30° S for all seasons, while this is clearly not the case for the THEMIS observations taken during MY 33 and 34 (~19:00). Water ice clouds are known to exhibit strong spatial variations (e.g., Smith, 2004), with much higher column optical depth over the Tharsis region than at many other longitudes, for example. Since the ∂I/∂τice < −1 × 10−6 W cm−2 ster−1 wavenumber−1 threshold excludes the Tharsis region (among others) this would introduce a bias. Therefore, for interannual comparisons we have chosen two regions shown by the green boxes in Fig. 5 (latitude of −10° to +20°, longitude of 15–60 W and 230–300 W) where there are consistently good retrievals available for all seasons and evening local times available in the THEMIS observations.

3.3. Estimation of uncertainties

Uncertainty in the retrieved values of water ice column optical depth arise from a number of different sources including instrument noise and calibration, and the various assumptions and simplifying approximations we have made in the retrieval algorithm. As previously described by Smith et al. (2003) and Smith (2009), the formal propagation of random instrument noise is small since the framelets we use for the retrievals are the average over many (320 × 256) pixels. Systematic errors are not reduced by averaging pixels and it is these errors that dominate the overall uncertainty.

We have described above how thermal contrast becomes small at later local times and so it is important to compute it as accurately as possible. The surface temperature is directly retrieved from the THEMIS observations, but it is not immediately obvious that using TES climatology derived from observations taken at ~14:00 h can be used for the THEMIS retrievals at much later local times, even with the temperature correction using THEMIS Band 10. We have explored this approximation in detail using temperature profiles taken from the Mars Climate Database (Forget et al., 1999; Millour et al., 2018). In this experiment we first compute ∂I/∂τice using the correct surface and atmospheric temperature profile for a given local time as given by the Mars Climate Database. We then simulate the approximation we use for THEMIS by computing ∂I/∂τice using the correct surface temperature but the atmospheric temperature profile from 14:30 local time. In this experiment there is no attempt to correct the temperature profile at all, so this is a worst-case scenario. Fig. 6 shows the results. For each local time shown, a grid of points covering all seasons, latitudes (−10° to 20°), and longitudes were computed with each result shown as a plus sign. The red bar is the average value. The bias in our assumption is small (only a couple percent), even as late as 21:00. At the latest local times observed by THEMIS (~19:30) the error in thermal contrast (and thus in the retrieved cloud optical depth) is rarely greater than about 5% and never greater than about 10%.

Fig. 6.

Fig. 6.

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The results of a numerical experiment where we compute ∂I/∂τice using the correct surface temperature but the atmospheric temperature profile from 14:30 local time using data from the Mars Climate Database, which simulates the effect of our assumptions in estimating a temperature profile for the retrievals. Each plus sign shows the computed fractional error in ∂I/∂τice while the red bar is the average error for each local time. At the latest local time observed by THEMIS (19:30) the error in thermal contrast (and thus in the retrieved cloud optical depth) is rarely greater than about 5% and never greater than about 10%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Since uncertainty in the retrieved optical depth is closely related to the inverse of thermal contrast, and thermal contrast systematically decreases with later local time, the uncertainties in the cloud optical depths presented here are also a function of local time. For local times earlier than roughly 17:00 the uncertainty estimate for an individual retrieval given by Smith (2009) as the greater of ± 0.04 or 10% is appropriate. For the latest local times we estimate that the uncertainty can be up to 2–3 times greater. Although there is significant noise in the individual retrievals at 19:00 h, spatial and seasonal averaging can be used to more clearly identify trends.

4. Results

4.1. Long-term record of THEMIS retrievals

Although the focus of this paper is the variation of water ice cloud optical depth as a function of local time, in Fig. 7 we present a summary of the > 8.5 Mars Year record of THEMIS atmospheric retrievals. Shown are the dust column optical depth, the atmospheric temperature derived from THEMIS Band 10 (indicative of ~0.5 mbar level), and water ice column optical depth. Some moderate amount of smoothing in Ls and latitude has been performed to better emphasize climatological trends. Since we assume that the dust is well mixed, it is natural to present the dust optical depth normalized to a standard reference pressure level, which is 6.1 mbar in this case. This has the effect of showing the dust column as a mixing ratio with the background gas. There is no normalization performed for the water ice optical depth since clouds are not well mixed. These data are available in datafile archived with this article.

Fig. 7.

Fig. 7.

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An overview of THEMIS retrieved aerosol optical depth and Band 10 atmospheric temperature over >8.5 Mars Years. Shown is the zonal average of each quantity as a function of time and latitude. (Top) Dust optical depth at 1075 cm−1 scaled to an equivalent 6.1-mbar pressure surface to remove the effect of topography. (Middle) THEMIS Band 10 atmospheric temperature, representative of a wide range of heights centered at about 0.5mbar. (Bottom) Water ice optical depth at 825 cm−1.

A close examination of the results from MY 33 and 34 reveals a greater number of accepted retrievals for water ice than for dust. Unlike in previous work where a simple criterion based on surface temperature was used as quality control to accept a retrieval, here we use a more sophisticated set of criteria based on the sensitivity of the observed spectrum to each aerosol. For water ice, we require that the sensitivity ∂I/∂τice < −1 × 10−6 W cm−2 ster−1 wavenumber−1 as explained in detail in Section 3.2. For dust, we require that the sensitivity to dust ∂I/∂τdust < −3 × 10−7 W cm−2 ster−1 wavenumber−1 or ∂I/∂τdust > 3 × 10−7 W cm−2 ster−1 wavenumber−1 (at 1025 cm−1). Because the “V”-shaped dust spectral feature is much more distinctive than the broad bowl-like water ice spectral feature, we have found that the retrieval is able to provide reliable retrievals at lower sensitivities for dust, and we have also allowed the relatively small number of observations showing dust in emission. This appears at first to be a more relaxed requirement than that for water ice, but in practice it turns out to be more restrictive because the lower altitudes where the bulk of dust is located are more often very close to the same temperature at the surface during MY 33 and 34.

Fig. 7 shows the familiar seasonal patterns of a relatively warm and dusty perihelion season (Ls = 180°–360°) and a relatively cool and cloudy aphelion season (Ls = 0°–180°) that have been observed before by THEMIS and several other spacecraft (e.g., the reviews given by Smith, 2008; Smith et al., 2017; Clancy et al., 2017; Kahre et al., 2017). The timing, duration, and strength of large-scale dust storms vary significantly from one Mars Year to the next. In particular, the dust storm season during MY 33 appears to have been relatively muted compared to previous years. Atmospheric temperatures respond to a combination of orbital (perihelion vs. aphelion) and seasonal (summer vs. winter) variations, as well as to direct heating by the absorption of sunlight by dust and to general circulation patterns. Water ice clouds show a generally repeatable pattern (for MY 26–31) with significant low-latitude cloudiness beginning soon after Ls = 0°, reaching peak intensity between Ls = 60° and 120°, and then gradually diminishing after that.

4.2. Local time variation of clouds

Besides the well-known interannual variation in dust column optical depth and atmospheric temperature (e.g., Smith, 2004), the other apparent interannual variation in Fig. 7 is a notable increase in water ice cloud optical depth beginning in MY 32. However, we interpret this increase not as a real secular trend in cloud optical depth, but rather as an artifact of how the local time observed by THEMIS has varied over the years (recall the bottom panel of Fig. 1). The small amount of interannual variation in cloud optical depth at a fixed daytime local time has been documented by spacecraft observations including Mars Global Surveyor TES (Smith, 2004), Mars Reconnaissance Orbiter MCS (Wilson and Guzewich, 2014), and MARCI (Wolff et al., 2019), and the THEMIS retrievals for MY 28–31 shown in Fig. 7. The MARCI retrievals possibly show a slight secular increase in cloud optical depth from MY 29 to 33, but this has magnitude ~0.01 over that period, which is an order of magnitude smaller than the increase observed in THEMIS retrievals between MY 31 and 34. Furthermore, the increase in retrieved THEMIS cloud optical depth aligns precisely with the move of the Odyssey spacecraft’s orbit to a later local time. Therefore, we are confident that the dominant source of the variations in retrieved cloud optical depth from one Mars Year to the next are caused by changes in the observed local time and not by a secular trend.

Fig. 8 shows the THEMIS retrievals of water ice cloud column optical depth as a function of Ls and local time. To make a consistent comparison across all Mars Years, only observations contained within the two regions outlined by green boxes in Fig. 5 are used (latitude of −10° to +20°, longitude of 15–60 W and 230–300 W). Smoothing has also been performed with a box 15° wide in Ls and 0.5 h wide in local time to better show the trends. Apparent in both Figs. 7 and 8 is a significant increase in water ice cloud optical depth at later local time. During the late afternoon hours (15:00–18:00) there is a systematic increase in clouds for later local times, but the increase intensifies after sunset at 18:00.

Fig. 8.

Fig. 8.

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The variation of water ice cloud optical depth retrieved from THEMIS observations as a function of season (Ls) and local time. To make a consistent comparison across all Mars Years, only those retrievals located within the two regions outlined by green boxes in Fig. 5 are used. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

During the afternoon, the increase in cloud optical depth appears to be mostly restricted to the aphelion season, with few clouds observed for any local time between 14:30 and 17:00 between Ls = 210° and 330°. The aphelion season cloud belt is also present for an extended period of Ls at 17:00 h compared to earlier local times. Clouds appear at a somewhat earlier Ls but the larger change is that the cloud belt persists to a significantly later Ls date after 17:00 h with clouds remaining beyond Ls = 180°.

In the evening after sunset (18:00) clouds are present in these regions year-round although they are still most prominent during the aphelion season. The increase in cloud optical depth between the earliest and latest local times is about 30% at Ls = 105°, but closer to 100% at Ls = 30° and 180°. Therefore, especially later in the season after Ls = 150°, the increase in clouds after 18:00 is not simply a scaling of optical depth observed earlier in the afternoon, but represents the formation of clouds at seasons and locations where they were not present at 15:00.

Figs. 9 and 10 illustrate the above points. Each figure shows maps of the retrieved water ice cloud column optical depth for different local times. Fig. 9 shows maps for the period Ls = 60°–150°, during the season of peak cloudiness, and Fig. 10 shows maps for the period Ls = 150°–210°, during the season where clouds generally only form after sunset. In these maps there is no smoothing performed, so they also give an indication of the quality (or noise level) of the retrievals at the different seasons and local times.

Fig. 9.

Fig. 9.

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Maps of water ice cloud optical depth retrieved from THEMIS during the aphelion season (Ls=60°–150°) for three different local time periods.

Fig. 10.

Fig. 10.

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Maps of water ice cloud optical depth retrieved from THEMIS during the equinoctial season (Ls = 150°–210°) for three different local time periods.

The top panel of Fig. 9 shows the spatial distribution of clouds observed during the daytime aphelion cloud belt season. The same pattern has been documented before by TES (Smith, 2004), THEMIS (Smith et al., 2003), MARCI (Wolff et al., 2019) and other spacecraft (see the review by Clancy et al., 2017). At later local times (middle and bottom panels) an enhancement of the clouds is observed. While the latitudinal extent of the cloud belt does appear to expand somewhat at later local times, for the most part it still maintains the same general spatial pattern with the highest cloud optical depths at the same locations. The spatial patterns of thermal inertia are evident here at the later local times. While there are still retrievals available in regions with high thermal inertia, there are no valid retrievals in regions with low thermal inertia since the surface quickly cools enough that the requirement for thermal contrast is no longer met.

In Fig. 10 we view the equinoctial period Ls = 150°–210° after the main part of the daytime aphelion cloud belt has dissipated. In afternoon observations (top panel) there are still significant clouds over the Tharsis volcanoes, the Valles Marineris, and Elysium Mons, but only a trace of the broad equatorial cloud belt remains. At later local times, and especially after the sun has set at 18:00, the THEMIS observations show not just an enhancement of the clouds observed during the afternoon, but that clouds have formed at locations where there were no significant clouds earlier in the day. These clouds persist in a similar geographical pattern beyond Ls = 210°, although with reduced optical depth. THEMIS observations taken at ~19:00 during both MY 33 and 34 follow the same general seasonal and spatial patterns.

4.3. Comparison with other observations and datasets

There has been relatively little previous work done characterizing the local time variation of water ice clouds because recent NASA Mars orbiters have had Sun-synchronous orbits. Still, the tendency for water ice cloud optical depth to increase from early to late afternoon hours has been noted in earlier observations from THEMIS (Smith, 2009), telescopic observations (e.g., Wolff et al., 1999; Glenar et al., 2003), Mars Global Surveyor (Wilson et al., 2008), and Mars Reconnaissance Orbiter (Malin et al., 2008; Wolff et al., 2019). Analysis of the thermal-IR observations made by the Viking Orbiters by Tamppari and Zurek (2003) also showed minimum cloud optical depth at midday with higher values earlier in the morning and later into the afternoon.

More recently, observations from the Mars Express orbiter, which is not in a Sun-synchronous orbit, have provided some additional information about the diurnal variation of clouds. Retrievals using thermal-IR spectra taken the Planetary Fourier Spectrometer by Giuranna et al. (2018) show that during the season and latitude band of the aphelion season cloud belt (Ls = 50°–140°; latitude = 0°–30°), water ice cloud opacity was minimum at 12:00, rising to peak values roughly 3 times higher at sunrise (6:00) and sunset (18:00). Overnight cloud opacity was roughly twice that at noon. Although the magnitude of this local time variation is greater than that observed in the THEMIS observations, the overall character is similar. Retrievals from near-IR spectra taken by the OMEGA instrument by Szantai et al. (2017) are restricted to daylight hours since near-IR observations require reflected sunlight, but also show a similar trend with lowest daytime cloud optical depth around noon and increasing cloudiness both in the early morning and late afternoon hours.

Finally, we can also compare the THEMIS retrievals against model output from the Mars Climate Database (MCD). The MCD (Forget et al., 1999; Millour et al., 2018) predicts a similar local time variation during the aphelion season (say, Ls=60°–150°) with minimum water ice cloud optical depth during midday and larger values in the evening and early morning. Between 15:00 and 19:00 h local time, modeled cloud optical depth from the MCD increases by about a factor of two, a significantly larger increase than that observed in the THEMIS retrievals. A more serious difference between THEMIS observations and the MCD is the seasonal behavior after sunset. Although the cloud optical depth retrieved from THEMIS at 19:00 is definitely greatest during the aphelion season, there is still significant cloudiness throughout the year with averaged cloud optical depth never falling below about 25% of its peak annual value. On the other hand, outside of the aphelion season the modeled cloud column optical depth from the MCD drops to values an order of magnitude less than annual peak values. Clearly, further observations of the complete diurnal cycle of water vapor and water ice clouds are needed to better constrain models.

5. Summary

Thermal infrared observations taken by THEMIS have been used to retrieve the column-integrated optical depth of dust and water ice aerosols for period of time covering > 8.5 Mars Years. The change in the orbit of the Mars Odyssey spacecraft over its long lifetime has enabled a range of different local times to be sampled and the variation of water ice cloud optical depth with local time to be explored. In particular, the recent move of the Odyssey spacecraft to an orbit with local time near 7:00 PM enables systematic retrieval of water ice clouds at a time of day not accessible from Mars Global Surveyor, Mars Reconnaissance Orbiter, or previous Odyssey observations.

At the evening local times sampled by the current THEMIS observations, the thermal contrast, or temperature difference, between the surface and the atmosphere becomes small reducing the sensitivity of the observations to the presence of aerosols. Special care has been taken for the retrievals presented here to compute this thermal contrast for each observation to ensure that only those observations with sufficient sensitivity to atmospheric aerosols are retained for analysis. We find that regions with relatively high thermal inertia (and thus retaining high surface temperatures after sunset) provide the largest thermal contrast and most reliable retrievals.

During the afternoon hours between 14:30 and 17:00 h local time there is a moderate and systematic increase in the retrieved optical depth of water ice clouds during the aphelion season. The seasonal period when clouds are observed becomes gradually longer later in the afternoon, but there remain very few clouds in the perihelion season. After sunset (18:00 h), clouds are observed during all seasons to rapidly expand to be present at locations where clouds were essentially nonexistent during the afternoon.

During the aphelion season the moderate (~30%) increase in cloud optical depth in the evening hours is primarily the result of thicker clouds at the same locations where clouds existed earlier in the afternoon. At the equinoctial seasons (Ls = 0° and 180°) there is a much larger increase in cloud optical depth from afternoon to evening and the observations show clouds forming in the evening at locations where they were not present in the afternoon.

The trend of increasing cloud optical depth for later local times in the afternoon and evening is generally consistent with modeling results and previous retrievals using other observations from other spacecraft, but the results presented here are different in the details of that variation. For example, the MCD model results show much less of an increase in evening cloud optical depth during the perihelion season than observed with THEMIS, while retrievals from Mars Express PFS spectra show a larger local time variation during the aphelion season than presented here. More observations from current and future spacecraft are needed to better characterize this important aspect of the current Mars climate.

At the time of this writing, the THEMIS instrument remains fully operational and continues to take visible and thermal-infrared images of Mars further extending what is currently the longest continuous record of Mars atmospheric observations from a single instrument. Future observations from THEMIS will continue to provide additional new important information about the local time and interannual variations of atmospheric conditions.

Acknowledgements

The author acknowledges financial support from the NASA Mars Odyssey project and is grateful for the work done by the THEMIS operations team at Arizona State University who performed all the sequencing and calibration needed to obtain the THEMIS data set.

References

  1. Bandfield JL, Smith MD, 2003. Multiple emission angle surface-atmosphere separations of Thermal Emission Spectrometer data. Icarus 161, 47–65. [Google Scholar]
  2. Benson JL, Kass DM, Kleinböhl A, McCleese DJ, Schofield JT, Taylor FW, 2010. Mars’ south polar hood as observed by the Mars Climate Sounder. J. Geophys. Res 115, 12015 10.1029/2009JE003554. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Christensen PR, Jakosky BM, Kieffer HH, Malin MC, McSween HY Jr., Nealson K, Mehall GL, Silverman SH, Ferry S, Caplinger M, Ravine M, 2004. The Thermal Emission Imaging System (THEMIS) for the Mars 2001 Odyssey mission. Space Sci. Rev 110, 85–130. 10.1023/B:SPAC.0000021008.16305.94. [DOI] [Google Scholar]
  4. Clancy RT, Grossman AW, Wolff MJ, James PB, Rudy DJ, Billawala YN, Sandor BJ, Lee SW, Muhleman DO, 1996. Water vapor saturation at low altitudes around Mars aphelion: a key to Mars climate? Icarus 122, 36–62. 10.1006/icar.1996.0108. [DOI] [Google Scholar]
  5. Clancy RT, Wolff MJ, Christensen PR, 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, 5098 10.1029/2003JE002058. [DOI] [Google Scholar]
  6. Clancy RT, Montmessin F, Benson J, Daerden F, Colaprete A, Wolff MJ, 2017. Mars Clouds, Chapter 5 in “The Atmosphere and Climate of Mars”. Cambridge University Press, 10.1017/9781139060172.005. [DOI] [Google Scholar]
  7. Curran RJ, Conrath BJ, Hanel RA, Kunde VG, Pearl JC, 1973. Mars: mariner 9 spectroscopic evidence for H2O ice clouds. Science 182, 381–383. 10.1126/science.182.4110.381. [DOI] [PubMed] [Google Scholar]
  8. Forget F, Hourdin F, Fournier R, Hourdin C, Talagrand O, Collins M, Lewis SR, Read PL, Huot JP, 1999. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res 104, 24155–24176. 10.1029/1999JE001025. [DOI] [Google Scholar]
  9. Giuranna M, Wolkenberg P, Carraro F, Grassi D, Aronica A, Aoki S, Scaccabarozzi D, Saggin B, Formisano V, the PFS team, 2018. New dataset of atmospheric parameters retrieved by PFS-MEx. In: Poster Presented at the Mars Atmosphere Data Assimilation Workshop, 29–31 August 2018, Le Bourget du Lac, France,. http://www-mars.lmd.jussieu.fr/mada2018/Abstracts/Giuranna_MADA2018.pdf. [Google Scholar]
  10. Glenar DA, Samuelson RE, Pearl JC, Bjoraker GL, Blaney D, 2003. Spectral imaging of Martian water ice clouds and their diurnal behavior during the 1999 aphelion season. Icarus 161, 297–318. [Google Scholar]
  11. Guzewich SD, Smith MD, 2019. Seasonal variation in Martian water ice cloud particle size. J. Geophys. Res 124, 636–643. 10.1029/2018JE005843. [DOI] [Google Scholar]
  12. Guzewich SD, Talaat ER, Toigo AD, Waugh DW, McConnochie TH, 2013. Highaltitude dust layers on Mars: observations with the Thermal Emission Spectrometer. J. Geophys. Res 118, 1177–1194. 10.1002/jgre.20076. [DOI] [Google Scholar]
  13. Guzewich SD, Smith MD, Wolff MJ, 2014. The vertical distribution of Martian aerosol particle size. J. Geophys. Res 119, 2694–2708. 10.1002/2014JE004704. [DOI] [Google Scholar]
  14. Heavens NG, McCleese DJ, Richardson MI, Kass DM, Kleinböhl A, Schofield JT, 2011. Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: 2. Implications of the thermal structure and aerosol distributions for the mean meridional circulation. J. Geophys. Res 116, E01010 10.1029/2010JE003713. [DOI] [Google Scholar]
  15. Hinson D, Wilson RJ, 2004. Temperature inversions, thermal tides, and water ice clouds in the martian tropics. J. Geophys. Res 109, E01002 10.1029/JE002129. [DOI] [Google Scholar]
  16. Kahre MA, Murphy JR, Newman CE, Wilson RJ, Cantor BA, Lemmon MT, Wolff MJ, 2017. The Mars Dust Cycle, Chapter 10 in “The Atmosphere and Climate of Mars”. Cambridge University Press, 10.1017/9781139060172.010. [DOI] [Google Scholar]
  17. Madeleine JB, Forget F, Spiga A, Wolff MJ, Montmessin F, Vincendon M, Jouglet D, Gondet B, Bibring JP, Langevin Y, Schmitt B, 2012a. Aphelion water-ice cloud mapping and property retrieval using the OMEGA imaging spectrometer onboard Mars Express. J. Geophys. Res 117, E00J07 10.1029/2011JE003940. [DOI] [Google Scholar]
  18. Madeleine JB, Forget F, Millour E, Navarro T, Spiga A, 2012b. The influence of radiatively active water ice clouds on the Martian climate. Geophys. Res. Lett 39, 23202 10.1029/2012GL053564. [DOI] [Google Scholar]
  19. Malin MC, Calvin WM, Cantor BA, Clancy RT, Haberle RM, James PB, Thomas PC, Wolff MJ, Bell JF, Lee SW, 2008. Climate, weather, and north polar observations from the Mars Reconnaissance Orbiter Mars Color Imager. Icarus 194, 501–512. 10.1016/j.icarus.2007.10.016. [DOI] [Google Scholar]
  20. Mateshvili N, Fussen D, Vanhellemont F, Bingen C, Dekemper E, Loodts N, Tetard C, 2009. Water ice clouds in the Martian atmosphere: two Martian years of SPICAM nadir UV measurements. Planet. Space Sci 57, 1022–1031. 10.1016/j.pss.2008.10.007. [DOI] [Google Scholar]
  21. Mellon MT, Jakosky BM, Kieffer HH, Christensen PR, 2000. High-resolution thermal inertia mapping from the Mars Global Surveyor Thermal Emission Spectrometer. Icarus 148, 437–455. 10.1006/icar.2000.6503. [DOI] [Google Scholar]
  22. Millour E, Forget F, Spiga A, Vals M, Zakharov V, Montabone L, Lefèvre F, Montmessin F, Chaufray JY, López-Valverde MA, González-Galindo F, Lewis SR, Read PL, Desjean MC, Cipriani F, the MCD development team, 2018. The Mars Climate Database (Version 5.3). Scientific Workshop “From Mars Express to ExoMars”, 27–28 February 2018, ESAC Madrid, Spain https://www.cosmos.esa.int/documents/1499429/1583871/Millour_E.pdf. [Google Scholar]
  23. Pankine AA, Tamppari LK, Bandfield JL, McConnochie TH, Smith MD, 2013. Retrievals of Martian atmospheric opacities from MGS TES nighttime data. Icarus 226, 708–722. 10.1016/j.icarus.2013.06.024. [DOI] [Google Scholar]
  24. Pearl JC, Smith MD, Conrath BJ, Bandfield JL, Christensen PR, 2001. Observations of Martian ice clouds by the Mars Global Surveyor Thermal Emission Spectrometer: the first Martian year. J. Geophys. Res 106, 12325–12338. 10.1029/1999JE01233. [DOI] [Google Scholar]
  25. Richardson MI, Wilson RJ, Rodin AV, 2002. Water ice clouds in the Martian atmosphere: general circulation model experiments with a simple cloud scheme. J. Geophys. Res 107, 5064 10.1029/2001JE001804. [DOI] [Google Scholar]
  26. Smith MD, 2004. Interannual variability in TES atmospheric observations of Mars during 1999–2003. Icarus 167, 148–165. 10.1016/j.icarus.2003.09.010. [DOI] [Google Scholar]
  27. Smith MD, 2008. Spacecraft observations of the Martian atmosphere. Annu. Rev. Earth Planet. Sci 36, 191–219. 10.1146/annurev.earth.36.031207.124334. [DOI] [Google Scholar]
  28. Smith MD, 2009. THEMIS observations of Mars aerosol optical depth from 2002–2008. Icarus 202, 444–452. 10.1016/j.icarus.2009.03.027. [DOI] [Google Scholar]
  29. Smith MD, Bandfield JL, Christensen PR, Richardson MI, 2003. Thermal Emission Imaging System (THEMIS) infrared observations of atmospheric dust and water ice cloud optical depth. J. Geophys. Res 108, 5115 10.1029/2003JE002115. [DOI] [Google Scholar]
  30. Smith MD, Wolff MJ, Clancy RT, Kleinböhl A, Murchie SL, 2013. Vertical distribution of dust and water ice aerosols from CRISM limb-geometry observations. J. Geophys. Res 118, 321–334. 10.1002/jgre.20047. [DOI] [Google Scholar]
  31. Smith MD, Bougher SW, Encrenaz T, Forget F, Kleinböhl A, 2017. Thermal Structure and Composition, Chapter 4 in “The Atmosphere and Climate of Mars”. Cambridge University Press, 10.1017/9781139060172.004. [DOI] [Google Scholar]
  32. Szantai A, Audouard J, Forget F, Madeleine J-B, Pottier A, Millour E, Gondet B, Langevin Y, Bibring J-P, 2017. Construction of a 4D water ice cloud database from Mars Express/OMEGA Observations – derivation of the diurnal Martian cloud life cycle. In: 6th Mars Atmosphere Modeling and Observation Workshop Granada, Spain, January 17–20, 2017. [Google Scholar]
  33. Tamppari LK, Zurek RW, 2003. Viking-era diurnal water-ice clouds. J. Geophys. Res 108, 5073 10.1029/2002JE001911. [DOI] [Google Scholar]
  34. Willame Y, Vandaele AC, Depiesse C, Lefèvre F, Letocart V, Gillotay D, Montmessin F, 2017. Retrieving cloud, dust and ozone abundances in the Martian atmosphere using SPICAM/UV nadir spectra. Planet. Space Sci 142, 9–25. 10.1016/j.pss.2017.04.011. [DOI] [Google Scholar]
  35. Wilson RJ, Guzewich SD, 2014. Influence of water ice clouds on nighttime tropical temperature structure as seen by the Mars Climate Sounder. Geophys. Res. Lett 41, 3375–3381. 10.1002/2014GL060086. [DOI] [Google Scholar]
  36. Wilson RJ, Neumann GA, Smith MD, 2007. Diurnal variation and radiative influence of Martian water ice clouds. Geophys. Res. Lett 34, L02710 10.1029/2006GL027976. [DOI] [Google Scholar]
  37. Wilson RJ, Lewis SR, Montabone L, Smith MD, 2008. Influence of water ice clouds on Martian tropical atmospheric temperatures. Geophys. Res. Lett 35, 7202 10.1029/2007GL032405. [DOI] [Google Scholar]
  38. Wolff MJ, Clancy RT, 2003. Constraints on the size of Martian aerosols from Thermal Emission Spectrometer observations. J. Geophys. Res 108, 5097 10.1029/2003JE002057. [DOI] [Google Scholar]
  39. Wolff MJ, Bell JF III, James PB, Clancy RT, Lee SL, 1999. Hubble space telescope observations of the Martian aphelion cloud belt prior to the Pathfinder mission: seasonal and interannual variations. J. Geophys. Res 104, 9027–9041. [Google Scholar]
  40. Wolff MJ, Clancy RT, Kahre MA, Haberle RM, Forget F, Cantor BA, Malin MC, 2019. Mapping Water Ice Clouds on Mars With MRO/MARCI. (Submitted to Icarus).

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