Icarus 202 (2009) 444-452 t . mow, atttt;cits is vhtl bl t Mac r s„ In THEMIS observations of Mars aerosol optical depth from 2002-2008 Michael D. Smith NASA Gaddat d Space Fligin CeaW' GrecN. fc 440207: ii. UM A R T I C L E I N F 0 A B S T R A C T .Ankle hi.'tcTY: We use infrared images obtained by the Thermal Emission Imaging System (THEMIS) instrument on-- Received 23 January 2309 board Mars Odyssey to retrieve the optical depth of dust and water ice aerosols over more than 3.5 Revised IS 'Oarch 2609 martian years between February 2002 (MY 25, 330'-jC; = and Detember 2008 (MY 29. Ls ^ I33°). Ace=pfed 21 €March 200-91 These data provide an important bridge between earlier :ES observations and recent abservadons from Available anTme 25 M3 r€li 2009 Mars Express and Gars Reconnaissance Orbiter. An improvement to our earlier retrieval ISmith, MR. Keywm)Fd5: Randfiield, J.L.. Christensen, P.R., Richardson, M.L. 2€701 J. Geophys, Res. 108, doi:10.1029")WIJE0021141 Mars. armasphere to include atmospheric temperature information from THEMIS hand 10 observations leads to much Mars. climate improved retrievals during the largest dust storms. 1ehe new retrievals show moderate dust storm activity Atmospheres, structure during Mars Years 26 and 27, although details of the strength and timing of dust storms is different from year to year. A planer en[irciing dust storm event was observed during Mars Year 28 near Southern Hemisphere Summer. Solstice. A beet of tow-latitude water ice clouds was observed during the aphelion season during each year, ;bars Years 26 through 29,i 'rhe optical depth of water ice clouds is somewhat higher in the'rHEMIS retrievals at - 5:00 PM local t me than in the TES retrievals at ­2:00 PM, suggestive of possible local time variation of clouds. Pubiished by Elsevier Inc. 1. introduction and ItTRO{MEx observations provides an opportunity for the vali- dation and cross-catibration between all the different instruments. The continued successful operation of the Thermal Fmission Togethcr, these datasets nor., cover a period of more than five mar- Imaging Systems (THEMIS) instrument on-board the Mars Odyssey tian years, enabling the initial exploration of interannual variations spacecraft has allowed for the Iong-term monitoring of the Mars in atmospheric conditions. atmosphere for more than 3.5 martian years (Christensen et al., The retrieval of dust and water ice aerosol optical depth using 2003 1. The atmosphere has been studied using both the visible and TIIEMB infrared observations was previously described by Smith the infrared portions of the THEMIS instrument. Thermal infrared et al. ;20113;, and more information on the THEMIS instrument observations made by THEMIS in nine spectral bands enable the can be found in Christensen et al. ;2003). A recent review of all retrieval of the spatial and seasonal variation of dust and water ice previous spacecraft observations of aerosols in the martian atmo- column-integrated aerosol optical depth, as well as surface tem- sphere is also given by Smith (2008). In this paper, we describe perature and a vertically-averaged atmospheric temperature (Smith improvements to the Smith et al. (20031 retrieval as well as re- et al., 20013). THEMIS visible images have been used to charac- sults updated to the current time. In Section 2 we describe the terize the morphology and seasonal dependence of dust and ice set of'rHEMiS observations used in the retrieval of aerosol optical clouds (lnada et al., 2007), and to characterize rnesoscale clouds depth. In Section 3 we describe the retrieval algorithm, focusing on (McConnochie et al., 21306). the improvements that have been made to the Smith et at. (2003) The retrievals of aerosol optical depth from THEIyl7S observa- algorithm. In Section 4 we present the results of the retrieval along tions provide a crucial link between the observations of aerosols with a discussion of tesuirs, and we summarize our findings in taken by the Mars Global Surveyor TES (e.g, Smith, 2004) and Mars Section 5. Orbiter Camera (MOC) instruments (e.g. Cantor et al., 2001; Cantor, 2007) and those taken by instruments ern-board the currently- 2. data set operabrig ]Mars Reconnaissance Orbiter (MRO) (e.g. Wolff et al„ 2009) and Mars Express (MEx) orbiter (e.g. Zasova et al., 20175: 2.1. THEYldS instrument Rannou et al„ 2006). The long overlap iii time (more than one martian year) between the THEMIS observations and both MGS The THEMIS instrument obtains images of Mars using one of two separate focal planes. One focal plane contains ten spectral fil- E-r.^ai: sG`dresc: ittiCtia€.3.ds;!!irhi^^ nzsa.^c!E_ ters covering the thermal infrared between 6.5 and l5 film, while 0019-1055 - see Mort matter Publishet3 by Elsevier lnc. dci: i0.1`-}iUiJ.t^ 4rL`S,ZCF^0.Cr3.07.7 Th'Eh3lS abse; vanor ; of Uan, aerasni optiml depiir fl, vm 2002-20,08 445 90 60 ara ^ Vag... •^^ ^, „ .:J^ 'n; ;..i. 30 0 -30 -60 MY 26 yi MY 27' : MY 28 • "^ MY 29 -90 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 L$ Fig,]. The diiv t 'ron in season (r,) and lannade or the THEMIS retrievals of aerosol optical dcpth. 'there is neaay-costinueaas coverage for more than 1.5 Martian years. SmA gaps in coverage are caused by so€ar tali€uiwoan and 3aacecraft anamaiies, the other contains Five spectral filters at visible wavelen;ths be- 25, L, ^ 330', to the end of December 2008 'MY 29, L, = 183'). tween 0,45 and 0.85 pin (Christensen et al., 2003;. In this work Although the number of available images per day varies some- we use only the images taken in the thermal infrared. The first what with time, the coverage is nearly continuous over these 3.5 two THEMIS spectral bands are identical and are centered at martian years with only short interruptions caused by solar con- 6.78 lam, Sands 3 through 10 are centered at 7.93, 8.56, 935, 10-21, junction or spacecraft anomalies. 17.04, 11.79, 12.57, and 14.88 pm, respectively, with spectral widths Fig. 1 shows the seasonal ( ks) and latitudinal coverage of the that allow for slight overlaps between adjacent bands (except for THEMIS infrared images used in this work. Although the THEMIS Band 10, which is somewhat more narrow and has no spectral instrument cannot provide a continuous pole-to-pale mapping, be- overlap with any other band?. cause of data rate constraints, the coverage is still more than suf- The THEWS images are 320 pixels across-track and are built up ficient to provide a valuable picture of the seasonal and spatial in the along-track direction over time by spacecraft motion. Images trends of atmospheric aerosol optical depth. in particular, the value are of variable length depending on the purpose of the observation of the atmospheric grids can easily be seen during the aphelion and the available resources available for downlink of the data. The season (LS =W-180') when data rates have typically been low. size of a THEMIS pixel at the surface is 100 in. The local time of the THEMIS observations varies between roughly 4:0(1 and 6:00 PM, 3. Revised retrieval algorithm which is somewhat later than either the 2:00 Plot mean local time of Mars Global Surveyor or the 3:00 HVI mean local time of the 'the retrieval used here is the same as Smith et al. (2003) algo- ;Mars Reconnaissance Arbiter. rithm, except modified to include atmospheric temperature infor- We are most interested in the variations of atmospheric con- mation from the THEMIS observations themselves. Here we give a ditions on scales larger than . the THEMIS pixel size, so for each brief outline of Smith et al. '2003; algorithm, and in the next sec- THEMIS image we use as our observation the average of a block of tion we describe the revision to include temperature information data 320 pixels wide (the entire width of the image) by 256 pixels from the THWIS observations. long. The last 256 pixels of each image are used for this average The basic idea of the THEMIS retrieval is to find the values for because they have the most accurate calibration (an effect only no- surface temperature and dust and wirer ice aerosol optical depth ticeabie for Band 10). The resulting average covers an area 32 by that provide the best fit between the computed and observed ra- 26 km in size, or roughly one-half degree square. diance spectra in THEMIS Bands 3-8 (roughly 8-12 tiro). The three quantities, surface temperature, dust optical depth, and water ice 2.2. Qttservatigtts used in this study optical depth are fit simultaneously by linearizing the solution for radiance about the current best guess and iterating until solution. THEMIS images are divided between those targeted at objects Convergence typically occurs in less than five iterations, of geologic interest, Unerat mapping of the entire surface, and a Radiance is computed using a purely absorbing plane-parallel latitude--longitude grid of observations taken every couple weeks atmosphere, Thus, the optical depth values reported here should intended to survey atmospheric conditions. Although the targeted be considered as effective absorptive optical depth only, not full observations generally provide a reasonable globai-scale sampling extinction optical depth that includes scattering. Numerical exper- over titre, the atmospheric grid has proven to be very useful as iments show that extinction optical depth is roughly 1.3 times a means to "fill in" any spatial gaps in the coverage of targeted absorption optical depth for dust, and 1.5 times absorption opti- observations.