Author name code: hoekzema ADS astronomy entries on 2022-09-14 author:"Hoekzema, Nick M." ------------------------------------------------------------------------ Title: Shadow method retrievals of the atmospheric optical depth above Gale crater on Mars using HRSC images Authors: Shaheen, Farzana; Scariah, Nayama Valsa; Lala, Mili Ghosh Nee; Krishna, A. P.; Jeganathan, C.; Hoekzema, N. M. Bibcode: 2022Icar..38815229S Altcode: The 'Shadow Method' is a tool to estimate the Atmospheric Optical Depth (AOD) on Mars from the brightness of shadows. This method is derived from the equations of radiative transfer, but there are several important simplifications that together invoke errors of several tens of percent. Work by us and by others show that these errors are largely systematic and can be minimized by adding an empirical 'Correction-Factor' to the equations. To quantify this factor, we compared shadow method retrievals from orbiter images with in-situ measurements by the Curiosity rover. We analysed a set of seven images that was taken by DLR's High Resolution Stereo Camera (HRSC) on-board ESA's orbiter Mars Express. The images were taken in seven channels: NIR, red, green, blue, and in three panchromatic stereo channels S1, Nadir, and S2. All these images show Gale crater and the exploration site of the Curiosity rover therein. Comparing the rover measurement of the AOD with shadow method retrievals from 5 close-by regions yielded the following correction factors: 0.53 ± 0.03 for Nadir, 0.60 ± 0.04 for S1, 0.62 ± 0.03 for S2, 0.66 ± 0.03 for NIR, 0.64 ± 0.03 for Red, 0.55 ± 0.03 for Green, and 0.57 ± 0.03 for Blue.

We analysed 71 regions with varying altitudes between -4.6 km to +3.8 km and found that, on an average, the AOD decreases with increasing altitude. The available channels yielded the following averages of atmospheric optical depth before applying the correction factor: Nadir: 0.47; S1: 0.46; S2: 0.52; Red: 0.49; Green: 0.53; Blue: 0.57 and NIR: 0.44 while after applying the correction factor we obtain Nadir: 0.89; S1: 0.77; S2: 0.84; Red: 0.77; Green: 0.96; Blue: 1.0 and NIR: 0.67. The Curiosity rover, at an altitude of -4.87 km, measured a ground-truth AOD of 0.88.

The shadowed and sunlit comparison regions that were used for this work range in altitudes from -4.6 km to +3.8 km. The AOD decreases with altitude and the decrease yields the following scale heights: S1: 12.5 + 0.48/-0.35 km; nadir: 11.1 + 0.53/-0.33 km; S2: 14.3 + 0.48/-0.44 km; NIR: 11.1 + 0.56/-0.47 km; red: 14.3 + 0.52/-0.49 km; green: 15.6 + 0.62/-0.59 km; blue: 14.3 + 0.44/-0.42 km. For this area and around that time of day, the Mars Climate Database predicts a pressure scale height of 11.6-12.2 km, which agrees well with the scale heights that we derived for Nadir, S1 and NIR. One region (number 42) contained a cloud with an optical depth of up to 0.6-0.8; i.e., around one third of the total AOD of 1.9 ± 0.01 measured for this area. Region 29 showed the highest AOD of our sample. Title: The High Resolution Stereo Camera (HRSC) of Mars Express and its approach to science analysis and mapping for Mars and its satellites Authors: Gwinner, K.; Jaumann, R.; Hauber, E.; Hoffmann, H.; Heipke, C.; Oberst, J.; Neukum, G.; Ansan, V.; Bostelmann, J.; Dumke, A.; Elgner, S.; Erkeling, G.; Fueten, F.; Hiesinger, H.; Hoekzema, N. M.; Kersten, E.; Loizeau, D.; Matz, K. -D.; McGuire, P. C.; Mertens, V.; Michael, G.; Pasewaldt, A.; Pinet, P.; Preusker, F.; Reiss, D.; Roatsch, T.; Schmidt, R.; Scholten, F.; Spiegel, M.; Stesky, R.; Tirsch, D.; van Gasselt, S.; Walter, S.; Wählisch, M.; Willner, K. Bibcode: 2016P&SS..126...93G Altcode: The High Resolution Stereo Camera (HRSC) of ESA's Mars Express is designed to map and investigate the topography of Mars. The camera, in particular its Super Resolution Channel (SRC), also obtains images of Phobos and Deimos on a regular basis. As HRSC is a push broom scanning instrument with nine CCD line detectors mounted in parallel, its unique feature is the ability to obtain along-track stereo images and four colors during a single orbital pass. The sub-pixel accuracy of 3D points derived from stereo analysis allows producing DTMs with grid size of up to 50 m and height accuracy on the order of one image ground pixel and better, as well as corresponding orthoimages. Such data products have been produced systematically for approximately 40% of the surface of Mars so far, while global shape models and a near-global orthoimage mosaic could be produced for Phobos. HRSC is also unique because it bridges between laser altimetry and topography data derived from other stereo imaging instruments, and provides geodetic reference data and geological context to a variety of non-stereo datasets. This paper, in addition to an overview of the status and evolution of the experiment, provides a review of relevant methods applied for 3D reconstruction and mapping, and respective achievements. We will also review the methodology of specific approaches to science analysis based on joint analysis of DTM and orthoimage information, or benefitting from high accuracy of co-registration between multiple datasets, such as studies using multi-temporal or multi-angular observations, from the fields of geomorphology, structural geology, compositional mapping, and atmospheric science. Related exemplary results from analysis of HRSC data will be discussed. After 10 years of operation, HRSC covered about 70% of the surface by panchromatic images at 10-20 m/pixel, and about 97% at better than 100 m/pixel. As the areas with contiguous coverage by stereo data are increasingly abundant, we also present original data related to the analysis of image blocks and address methodology aspects of newly established procedures for the generation of multi-orbit DTMs and image mosaics. The current results suggest that multi-orbit DTMs with grid spacing of 50 m can be feasible for large parts of the surface, as well as brightness-adjusted image mosaics with co-registration accuracy of adjacent strips on the order of one pixel, and at the highest image resolution available. These characteristics are demonstrated by regional multi-orbit data products covering the MC-11 (East) quadrangle of Mars, representing the first prototype of a new HRSC data product level. Title: Scientific assessment of the quality of OSIRIS images Authors: Tubiana, C.; Güttler, C.; Kovacs, G.; Bertini, I.; Bodewits, D.; Fornasier, S.; Lara, L.; La Forgia, F.; Magrin, S.; Pajola, M.; Sierks, H.; Barbieri, C.; Lamy, P. L.; Rodrigo, R.; Koschny, D.; Rickman, H.; Keller, H. U.; Agarwal, J.; A'Hearn, M. F.; Barucci, M. A.; Bertaux, J. -L.; Besse, S.; Boudreault, S.; Cremonese, G.; Da Deppo, V.; Davidsson, B.; Debei, S.; De Cecco, M.; El-Maarry, M. R.; Fulle, M.; Groussin, O.; Gutiérrez-Marques, P.; Gutiérrez, P. J.; Hoekzema, N.; Hofmann, M.; Hviid, S. F.; Ip, W. -H.; Jorda, L.; Knollenberg, J.; Kramm, J. -R.; Kührt, E.; Küppers, M.; Lazzarin, M.; Lopez Moreno, J. J.; Marzari, F.; Massironi, M.; Michalik, H.; Moissl, R.; Naletto, G.; Oklay, N.; Scholten, F.; Shi, X.; Thomas, N.; Vincent, J. -B. Bibcode: 2015A&A...583A..46T Altcode: Context. OSIRIS, the scientific imaging system onboard the ESA Rosetta spacecraft, has been imaging the nucleus of comet 67P/Churyumov-Gerasimenko and its dust and gas environment since March 2014. The images serve different scientific goals, from morphology and composition studies of the nucleus surface, to the motion and trajectories of dust grains, the general structure of the dust coma, the morphology and intensity of jets, gas distribution, mass loss, and dust and gas production rates.
Aims: We present the calibration of the raw images taken by OSIRIS and address the accuracy that we can expect in our scientific results based on the accuracy of the calibration steps that we have performed.
Methods: We describe the pipeline that has been developed to automatically calibrate the OSIRIS images. Through a series of steps, radiometrically calibrated and distortion corrected images are produced and can be used for scientific studies. Calibration campaigns were run on the ground before launch and throughout the years in flight to determine the parameters that are used to calibrate the images and to verify their evolution with time. We describe how these parameters were determined and we address their accuracy.
Results: We provide a guideline to the level of trust that can be put into the various studies performed with OSIRIS images, based on the accuracy of the image calibration. Title: Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse Authors: Vincent, Jean-Baptiste; Bodewits, Dennis; Besse, Sébastien; Sierks, Holger; Barbieri, Cesare; Lamy, Philippe; Rodrigo, Rafael; Koschny, Detlef; Rickman, Hans; Keller, Horst Uwe; Agarwal, Jessica; A'Hearn, Michael F.; Auger, Anne-Thérèse; Barucci, M. Antonella; Bertaux, Jean-Loup; Bertini, Ivano; Capanna, Claire; Cremonese, Gabriele; da Deppo, Vania; Davidsson, Björn; Debei, Stefano; de Cecco, Mariolino; El-Maarry, Mohamed Ramy; Ferri, Francesca; Fornasier, Sonia; Fulle, Marco; Gaskell, Robert; Giacomini, Lorenza; Groussin, Olivier; Guilbert-Lepoutre, Aurélie; Gutierrez-Marques, P.; Gutiérrez, Pedro J.; Güttler, Carsten; Hoekzema, Nick; Höfner, Sebastian; Hviid, Stubbe F.; Ip, Wing-Huen; Jorda, Laurent; Knollenberg, Jörg; Kovacs, Gabor; Kramm, Rainer; Kührt, Ekkehard; Küppers, Michael; La Forgia, Fiorangela; Lara, Luisa M.; Lazzarin, Monica; Lee, Vicky; Leyrat, Cédric; Lin, Zhong-Yi; Lopez Moreno, Josè J.; Lowry, Stephen; Magrin, Sara; Maquet, Lucie; Marchi, Simone; Marzari, Francesco; Massironi, Matteo; Michalik, Harald; Moissl, Richard; Mottola, Stefano; Naletto, Giampiero; Oklay, Nilda; Pajola, Maurizio; Preusker, Frank; Scholten, Frank; Thomas, Nicolas; Toth, Imre; Tubiana, Cecilia Bibcode: 2015Natur.523...63V Altcode: Pits have been observed on many cometary nuclei mapped by spacecraft. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments and models cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts. Alternative mechanisms like explosive activity have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov-Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface. Title: On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko Authors: Sierks, Holger; Barbieri, Cesare; Lamy, Philippe L.; Rodrigo, Rafael; Koschny, Detlef; Rickman, Hans; Keller, Horst Uwe; Agarwal, Jessica; A'Hearn, Michael F.; Angrilli, Francesco; Auger, Anne-Therese; Barucci, M. Antonella; Bertaux, Jean-Loup; Bertini, Ivano; Besse, Sebastien; Bodewits, Dennis; Capanna, Claire; Cremonese, Gabriele; Da Deppo, Vania; Davidsson, Björn; Debei, Stefano; De Cecco, Mariolino; Ferri, Francesca; Fornasier, Sonia; Fulle, Marco; Gaskell, Robert; Giacomini, Lorenza; Groussin, Olivier; Gutierrez-Marques, Pablo; Gutiérrez, Pedro J.; Güttler, Carsten; Hoekzema, Nick; Hviid, Stubbe F.; Ip, Wing-Huen; Jorda, Laurent; Knollenberg, Jörg; Kovacs, Gabor; Kramm, J. Rainer; Kührt, Ekkehard; Küppers, Michael; La Forgia, Fiorangela; Lara, Luisa M.; Lazzarin, Monica; Leyrat, Cédric; Lopez Moreno, Josè J.; Magrin, Sara; Marchi, Simone; Marzari, Francesco; Massironi, Matteo; Michalik, Harald; Moissl, Richard; Mottola, Stefano; Naletto, Giampiero; Oklay, Nilda; Pajola, Maurizio; Pertile, Marco; Preusker, Frank; Sabau, Lola; Scholten, Frank; Snodgrass, Colin; Thomas, Nicolas; Tubiana, Cecilia; Vincent, Jean-Baptiste; Wenzel, Klaus-Peter; Zaccariotto, Mirco; Pätzold, Martin Bibcode: 2015Sci...347a1044S Altcode: 2015Sci...347.1044S Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss. Title: Dust deflation by dust devils on Mars derived from optical depth measurements using the shadow method in HiRISE images Authors: Reiss, D.; Hoekzema, N. M.; Stenzel, O. J. Bibcode: 2014P&SS...93...54R Altcode: We measured the optical depth of three separate dust devils and their surroundings with the so called "shadow method" in HiRISE images. The calculated optical depths of the dust devils range from 0.29±0.18 to 1.20±0.38. Conservative calculations of the minimum and maximum dust loads are in the range of 4-122 mg m-3. Assuming reliable upper and lower boundary values of vertical speeds within the dust devils between 0.1 and 10 ms-1 based on terrestrial and Martian studies we derived dust fluxes in the range of 6.3-1221 mg m-2 s-1 (PSP_004285_1375), from 0.38-162 mg m-2 s-1 (ESP_013545_1110), and from 3.2-581 mg m-2 s-1 (ESP_016306_2410) for the three dust devils. Our dust load and dust flux calculations for the three dust devils are in good agreement to previous studies. Two of the analyzed dust devils left continuous dark tracks on the surface. For these dust devils we could calculate how much dust was removed by using the minimum and maximum dust fluxes in combination with measured horizontal speeds of these dust devils. Our results indicate that a dust removal of an equivalent layer of less than 2 μm (or less than one monolayer) is sufficient for the formation of dust devil tracks on Mars. This value might be used in future studies to estimate the contribution of dust devils to the global dust entrainment into the atmosphere on Mars. Title: Dust Deflation by Dust Devils on Mars Derived from Optical Depth Measurements Using the Shadow Method in HiRISE Images Authors: Reiss, D.; Hoekzema, N. M.; Stenzel, O. J. Bibcode: 2014LPI....45.1994R Altcode: Optical depth measurements of three individual dust devils and their surroundings with the shadow method in HiRISE images. Title: Mars' Atmospheric Optical Depth from Mars Express HRSC and Mars Exploration Rovers — A Comparison Authors: Stenzel, O. J.; Hoekzema, N. M. Bibcode: 2014LPI....45.1799S Altcode: Comparison of optical depth derived from Mars Express' HRSC and the Mars Exploration Rovers. A considerable correlation is found even between remote areas. Title: Optical depth of the Martian atmosphere and surface albedo from high-resolution orbiter images Authors: Petrova, E. V.; Hoekzema, N. M.; Markiewicz, W. J.; Thomas, N.; Stenzel, O. J. Bibcode: 2012P&SS...60..287P Altcode: In this paper we describe and evaluate the so-called shadow method. This method can be used to estimate the optical depth of the Martian atmosphere from the differences in brightness between shadowed and sunlit regions observed from an orbiter. We present elaborate and simplified versions of the method and analyze the capabilities and the sources of errors. It proves essential to choose shadowed and sunlit comparison regions with similar surface properties. Accurate knowledge of the observing geometry, including the slopes of the observed region, is important as well, since the procedure should be corrected for the non-horizontal surface. Moreover, the elaborate version of the shadow method can be sensitive to (i) the optical model of aerosols and (ii) the assumed bi-directional reflectance function of the surface. To obtain reliable estimates, the analyzed images must have a high spatial resolution, which the HiRISE camera onboard the MRO provides. We tested the shadow method on two HiRISE images of Victoria crater (TRA_0873_1780 and PSP_001414_1780) that were taken while this crater was the exploration site of the Opportunity rover. While the rover measured optical depth τ approximately in the ranges from 0.43 to 0.53 and from 0.53 to 0.59 by imaging the sun, our shadow procedure yielded τ about 0.50 and 0.575, respectively (from the HiRISE's red images). Thus, the agreement is quite good. The obtained estimates of the surface albedo are about 0.20 and 0.17, respectively. Title: Retrieving optical depth from shadows in orbiter images of Mars Authors: Hoekzema, N. M.; Garcia-Comas, M.; Stenzel, O. J.; Petrova, E. V.; Thomas, N.; Markiewicz, W. J.; Gwinner, K.; Keller, H. U.; Delamere, W. A. Bibcode: 2011Icar..214..447H Altcode: The difference in brightness between shadowed and sunlit regions in space images of Mars is a measure of the optical depth of the atmosphere. The translation of this difference into optical depth is what we name the "shadow method". Our analysis of two HRSC data-sets and a HiRISE data-set indicates that it is possible to estimate the optical depth with the shadow method. In colors between yellow and red the accuracy may be around ±15%, and in some cases ±8-10%. In other colors we found larger errors. We came to these results in two steps. First, we investigated in how far shadow method retrievals are proportional to the true optical depth. To this end we analyzed about 150 locations in Valles Marineris that were imaged by HRSC. Whereas the studied region spans about 8 km in altitude we were able to study the relation between altitude and shadow-method retrievals. Retrievals from five HRSC panchromatic (675 ± 90 nm) stereo images yielded scale-heights with an average of 12.2 ± 0.7 km, which is very close to the expected local pressure scale height. Many studies have shown that the scale-height of optical depth and pressure commonly are similar. This indicates that the shadow method retrievals are on average close to proportional to the optical depth, because otherwise these would probably not yield a correct scale-height. HRSC's red image yielded very similar results, but the blue, green, and NIR images did not. Next, we compared optical depth measurements by the two MER rovers with shadow method retrievals from orbiter images of the rover exploration sites. Retrievals with the shadow method appear systematically smaller than the rover measurements; dividing the retrievals by a "correction factor" yields an estimate of the real optical depth. Retrievals from three HRSC panchromatic stereo images of a region near the Spirit rover yielded a correction factor of 0.63 ± 0.09 when the sunlit comparison regions were at varying and more or less arbitrary distances from the shadows and 0.71 ± 0.06 when these were close together. Twenty retrievals from a HiRISE red (650 ± 100 nm) image of the Opportunity exploration site similarly yielded 0.68 ± 0.09. The results from these two case studies suggest that the shadow method has an accuracy of about ±15% or around ±8-10% in the best cases. Title: Optical Depth of the Martian Atmosphere and Surface Albedo from High Resolution Orbiter Images Authors: Petrova, E. V.; Hoekzema, N. M.; Markiewicz, W. J.; Thomas, N. Bibcode: 2011mamo.conf..110P Altcode: No abstract at ADS Title: The Scale-Height of Optical Depth in Valles Marineris as Derived from Shadows in HRSC Images Authors: Hoekzema, N. M.; Stenzel, O. J.; Markiewicz, W. J.; Petrova, E. V.; Gwinner, K. Bibcode: 2011mamo.conf..112H Altcode: No abstract at ADS Title: Limb Observations of the Martian atmosphere with Mars Express' High Resolution Stereo Camera Authors: Stenzel, O. J.; Keller, H. U.; Hoekzema, N. M.; Markiewicz, W. J.; Hoffmann, H. Bibcode: 2011mamo.conf..178S Altcode: No abstract at ADS Title: Optical depth and its scale-height in Valles Marineris from HRSC stereo images Authors: Hoekzema, N. M.; Garcia-Comas, M.; Stenzel, O. J.; Grieger, B.; Markiewicz, W. J.; Gwinner, K.; Keller, H. U. Bibcode: 2010E&PSL.294..534H Altcode: We measured the optical depth of the Martian atmosphere as a function of altitude above two opposing scree walls of the Valles Marineris, from stereo images that were taken with the High Resolution Stereo Camera (HRSC) of Mars Express on June 3, 2004, during orbit 471. The optical depths were measured from contrast differences between the stereo images with the so called "stereo method". For 7 regions in the northern wall of the Valles, we estimated the optical depth and found values between 1.0 and 1.6. These regions span more than 6 km in altitude and the results show a clear relation with altitude. A fit on these results yielded a scale-height for the optical depth of 14.0 km + 1.3/-1.1 km. The expected local pressure-scale height is smaller: 11.5-12.0 km. The difference is most likely explained by small (around 1.5%) offset errors in the intensity calibration of HRSC images. We also selected 9 regions in the opposing southern wall and from these we measured values of optical depth in the range 1.3-1.5. Our result suggests the presence of clouds above this part of the Valles because the optical depth appears almost independent of the surface altitude. Possibly these are banner clouds, forming at the edge of the canyon, that contain dust that is blown over the canyon by winds from the high plains to the South. Title: The Scale-height Of Optical-depth In Valles Marineris As Derived From Shadows In HRSC Images Authors: Hoekzema, Nick; Garcia-Comas, M.; Petrova, E. V.; Stenzel, O. J.; Markiewicz, W. J.; Gwinner, K.; Keller, H. U. Bibcode: 2009DPS....41.4904H Altcode: The optical depth of the Martian atmosphere can be estimated from the brightness of shadows with the so called "shadow method". We investigated the accuracy of this method by analyzing a set of stereo and color images observed on July 21, 2005 with the High Resolution Stereo Camera of the Mars-Express orbiter. The images show part of Valles Marineris during late afternoon and contain numerous shadows. Whereas the analyzed regions span height differences of about ten kilometers, we could study the relation between altitude and shadow method measurements. If the optical depth and gas-pressure have similar scale heights, then the accuracy with which our measurements can reproduce the pressure-scale-height tells about the accuracy of the shadow method.

Various GCMs suggest a local pressure scale height of around 13 km at that moment, while the red and the five panchromatic stereo images all yielded similar scale heights with an average of 12.2 ± 0.7 km. Thus, in the color of the panchromatics (yellow to red) and in red the shadow method yielded good results. The scale height derived from the NIR image is too low: 10.6 ± 0.4 km, we speculate that this is an effect from airborne dust particles that are on average larger in the lower than in the higher atmosphere. The scale heights that were derived from blue and green images were unrealistically high: 17.0 ± 0.7 km and 14.5 ± 0.5 km respectively. This may be caused by thin white high altitude hazes, or indicate that the accuracy of the shadow method varies with optical depth in these colors. Title: Correction of HRSC Images for Atmospheric Dust Using a Simple Optical Depth Retrieval Method Authors: Stenzel, O. J.; Hoekzema, N. M.; Markiewicz, W. J.; Keller, H. U. Bibcode: 2008LPICo1447.9112S Altcode: A simple scheme is introduced to estimate the optical depth present in the martian atmosphere during a Mars Express HRSC observation. Title: Dust haze in Valles Marineris observed by HRSC and OMEGA on board Mars Express Authors: Inada, A.; Garcia-Comas, M.; Altieri, F.; Gwinner, K.; Poulet, F.; Bellucci, G.; Keller, H. U.; Markiewicz, W. J.; Richardson, M. I.; Hoekzema, N.; Neukum, G.; Bibring, J. -P. Bibcode: 2008JGRE..113.2004I Altcode: 2008JGRE..11302004I We present analysis of a bright haze observed inside Valles Marineris, which formed in mid northern spring. The data were collected by the High Resolution Stereo Camera (HRSC) and the imaging spectrometer, Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA), aboard Mars Express. This study provides a case example of the power of simultaneous multiple emission angle and hyperspectral imaging for study of aerosols and clouds in the Martian atmosphere. The haze appeared thinner after three days and disappeared in nine days. It was limited to a 2-km layer at the bottom of the canyon. The color was redder than the underlying surface. The analysis of the OMEGA spectra indicates that this haze was composed of dust particles. The dust layer appeared brighter with the HRSC stereo channels than the nadir channel due to longer scattering paths. We have estimated the optical depth of the haze by fitting both HRSC and OMEGA data with radiative transfer calculations. The retrieval of the optical depth is very sensitive to the aerosol scattering model used and the reflectance of the surface. Applying an aerosol scattering model derived from sky surveys at a constant elevation by the Imager for Mars Pathfinder, the optical depth of the haze is estimated from HRSC data to be within 1.7 to 2.3 at the wavelength (λ) of 0.675 μm. The wavelength dependence is obtained from OMEGA spectrum. It increases to 2.2-2.6 at λ = 1.35 μm and moderately decreases to 1.2-1.8 at λ = 2.4 μm. Title: Comparing the Dust Emission in the Close Vicinity of Comets 1P/Halley, 19P/Borrelly, 81P/Wild2 and 9P/Tempel1 Authors: Ho, T. M.; Knollenberg, J.; Boice, D. C.; Hoekzema, N.; Kuehrt, E.; Schulz, R.; Stuewe, J.; Thomas, N. Bibcode: 2008LPICo1405.8269H Altcode: No abstract at ADS Title: A Comparative Study of the Dust Environment near the Nuclei of Comets 1P/Halley, 19P/Borrelly, 81P/Wild 2 & 9P/Tempel1 Authors: Ho, Tra-Mi; Knollenberg, Joerg; Hoekzema, N.; Boice, Daniel; Kuehrt, Ekkehard; Schulz, Rita; Stuewe, J.; Thomas, Nicolas Bibcode: 2008cosp...37.1250H Altcode: 2008cosp.meet.1250H There have been four comets imaged by spacecrafts: 19P/Halley (HMC on Giotto); 19P/Borrelly (MICAS on DS1); 81P/Wild 2 (NavCam on Stardust); and 9P/Tempel 1 (MIR & HIR on Deep Impact). This paper presents a comparative studies of the dust emission within the first 30 - 40 km of the nuclei of these four comets. On March 14, 1986, Giotto encountered comet 1P/Halley's nucleus at a distance of 596km carrying the Halley Multicolour Camera (HMC) [1]. Five years later, Deep Space 1 obtained images of the nucleus of comet 19P/Borrelly with the Miniature Integrated Camera and Spectrometer (MICAS) at a closest distance of 2174km [2]. The next cometary flyby occurred when Stardust approached comet 81P/Wild 2 at 236km on January 2, 2004, tracking its nucleus with its optical navigation camera (NavCam) [3]. The latest close encounter occurred in July 4, 2005, when Deep Impact flew by 9P/Tempel 1 at 500km [4] carrying the Medium Resolution Instrument (MIR). Since the nuclei of 1P/Halley, 19P/Borrelly, 81P/Wild 2 and 9P/Tempel 1 have been observed under similar phase angles (108° , 88° , 73° , and 63° , respectively), we can do a comparative analysis of the inner dust environment of these data sets. The inner dust coma morphology, particularly dust jets and broader fans, of these four comets has been investigated by several authors [5]-[8]. We concentrate on the comparative study of their dust emission. The outflow of dust particles is force-free at large radial distance from the comet nucleus. Thus, integrating the intensity Ids [9] around a comet results in constant Ids. However, the integrated intensities of comets 1P/Halley and 19P/Borrelly indicate deviation from the expected behavior within the first 50 km from their nuclei [10]. 1P/Halley's Ids decreases near the nucleus surface whereas comet 19P/Borrelly's Ids increases. But at large distances, they both converge to constant values. These opposite effects in the first 50km indicate that different mechanisms dominate the intensity distribution around their nuclei. At comet 1P/Halley, dust fragmentation into optically larger particles is most likely the dominant process giving the rise of the intensity. The situation at 19P/Borrelly shows the effects of dust acceleration and fragmentation into optically smaller particles. By investigating the integrated intensity around comets 81P/Wild 2 and 9P/Tempel 1, we have obtained similar characteristics at 19P/Borrelly indicating that their inner intensity distribution is most likely dominated by the same processes. We present further comparative analyses of the dust morphology and dynamics for the four comets to constrain possible mechanisms creating the observed features. References: [1] Keller H.U. et al. (1986) Nature 321, 320-326. [2] Soderblom L.A. et al. (2002) Science 296, 1087-1091. [3] Brownlee D.E. et al. (2004) Science 304, 1764-1769. [4] A'Hearn M.F. et al. (2005) Science 310, 248-264. [5] Boice D.C. et al. (2002) EM&P 89, 301-324. [6] Soderblom L.A. et al. (2004) Icarus 167, 4-15. [7] Sekanina Z. et al. (2004) Science 304, 1769-1774. [8] Farnham T. et al. (2007) Icarus 187, 26-40. [9] Thomas N. et al. (1988) Nature 332, 51-52. [10] Ho T.M. et al. (2007) P&SS 55, 974-985. Title: Optical Depth Retrievals from Shadows in HiRISE Images Authors: Hoekzema, Nick; Thomas, N.; Markiewicz, W. J.; Keller, H. U.; Inada, A.; Delamere, W. A.; Herkenhoff, K. E.; Millazzo, M.; McEwen, A.; HiRISE Team Bibcode: 2007DPS....39.2403H Altcode: 2007BAAS...39..454H The spatial resolution of the images that the HiRISE camera is taking of the Martian surface is unprecedented and offers new ways to estimate the optical depth of the aerosols abundant in the Martian atmosphere. While resolved shadows are rare in images of Mars at spatial resolutions of tens or more meters per pixel, HiRISE's images actually often resolve the much more common smaller shadows that are cast by boulders and cliffs in the rims of craters. We used a HiRISE image of the Opportunity rover site to estimate optical depths from shadows and compared our estimates with the ground truth that the rover measured by looking at the sun. Our results suggest that retrievals of the optical depth from shadows can yield an accuracy of better than 10%. Title: Optical Depth Retrievals from Shadows in HiRISE Images Authors: Hoekzema, N. M.; Thomas, N.; Keller, H. U.; Markiewicz, W. J.; Inada, A.; Delamere, W. A.; Herkenhoff, K. E.; Milazzo, M.; McEwen, A.; HiRISE Team Bibcode: 2007LPICo1353.3226H Altcode: We use a HiRISE image of the Opportunity rover site to estimate optical depths from shadows and compared our estimates with the rover's ground truth. Our results suggest that retrievals of the optical depth from shadows can yield an accuracy of better than 10%. Title: The Scale-Height of Dust Around Pavonis Mons from HRSC Stereo Images Authors: Hoekzema, N. M.; Garcia Comas, M.; Gwinner, K.; Grieger, B.; Markiewicz, W. J.; Keller, H. U. Bibcode: 2007LPICo1353.3154H Altcode: We used HRSC stereo observations for estimating the scale-height of aerosols in the martian atmosphere and found 10.8 +0.9/-0.8 km on the flanks of Pavonis Mons. This is equal to, or very close to, the expected local gas-scale-height. Title: The Dust Scale Height Of The Martian Atmosphere Around Pavonis Mons From Hrsc Stereo Images Authors: Hoekzema, Nick; Gwinner, K.; Grieger, B.; Markiewicz, W. J.; Keller, H.; Hoffmann, H.; Meima, J. A.; Neukum, G. Bibcode: 2006DPS....38.6030H Altcode: The Martian atmosphere contains large and variable amounts of aerosols, mainly consisting of airborne dust. The High Resolution Stereo Camera (HRSC) onboard the European orbiter Mars Express is a powerful tool for studying the distribution of dust in Mars’ atmosphere. An essential parameter for such studies is the optical depth, which can often be estimated from contrast differences between HRSC stereo images with the so called ‘stereo method’. Software for this purpose has been developed at the MPS in Lindau, Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR.

On August 17, 2005, during orbit 902 of Mars Express, HRSC imaged Pavonis Mons and regions around it in stereo; these span height differences of over 10 km. Obviously, the amount of dust in the Martian atmosphere varies with elevation. We used the images to study how optical depth depends on altitude. This yielded a scale-height of 10.8 km within an one sigma range of 10.0_11.7 km. Independent of altitude, the measured optical depths showed very strong local variations due to localized clouds. Title: The dust scale height of the Martian atmosphere around Pavonis Mons from HRSC stereo images Authors: Hoekzema, N. M.; Gwinner, K.; Markiewicz, W. J.; Grieger, B.; Portyankina, G.; Keller, H. U.; Hoffmann, H.; Meima, J. A.; Neukum, G. Bibcode: 2006epsc.conf..251H Altcode: The Martian atmosphere contains large and variable amounts of aerosols, mainly consisting of airborne dust. The High Resolution Stereo Camera (HRSC) onboard the European orbiter Mars Express is a powerful tool for studying the distribution of dust in Mars' atmosphere. An essential parameter for such studies is the optical depth, which can often be estimated from contrast differences between HRSC stereo images with the so called `stereo method'. Software for this purpose has been developed at the MPS in Lindau, Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR. On September 23, 2005, during orbit 2175 of Mars Express, HRSC imaged Pavonis Mons and regions around it in stereo; these span height differences of nearly 10 km. Obviously, the amount of dust in the Martian atmosphere varies with elevation. We use the images to study how optical depth depends on altitude. Independent of altitude, the measured optical depths show important (up to about a factor of two) local variations on horizontal scales of a few tens of kilometers that may well be related to winds and obstacles like crater walls and mountains. Title: Dust Haze in Valles Marineris Observed by HRSC and OMEGA on board Mars Express Authors: Inada, A.; Garcia-Comas, M.; Altieri, F.; Gwinner, K.; Bellucci, G.; Hoekzema, N. M.; Poulet, F.; Keller, H. U.; Neukum, G.; Bibring, J. -P.; Hrsc; OMEGA Co-Investigator Teams Bibcode: 2006cosp...36.2576I Altcode: 2006cosp.meet.2576I HRSC The High Resolution Stereo Camera and OMEGA Observatoire pour la Mineralogie l Eau les Glaces et l Activite on board Mars express MEX observed bright haze in the Valles Marineris on 25 May 2004 the orbit number 438 Overlapped images acquired later display that it appeared thinner after three days and disappeared in nine days We found that the composition is dust by analyzing OMEGA s spectrum The haze is redder than the surface below Its brightness temperature is colder than the expected temperature of the bottom of the valley without it The stereo channels of HRSC provide the information of angular dependence The brightness of the haze increases with the increase of emission angle which indicates that the optical depth of the atmosphere was thick We will show the optical depth derived from HRSC and OMEGA s data with a radiative transfer model Title: The dust scale height of the Martian atmosphere in Vallis Marineris from HRSC stereo images Authors: Hoekzema, N. M.; Markiewicz, W. J.; Gwinner, K.; Meima, J. A.; Keller, H. U.; Inada, A.; Hoffmann, H.; Neukum, G.; HRSC Team Bibcode: 2005DPS....37.3336H Altcode: 2005BAAS...37.1567H The Martian atmosphere contains large and variable amounts of dust and other aerosols. The High Resolution Stereo Camera (HRSC) onboard the European orbiter Mars Express is a powerful tool for studying the distribution of dust in Mars' atmosphere. An essential parameter for such studies is the optical depth, which can often be estimated from contrast differences between HRSC stereo images with the so called `stereo method'. Software for this purpose has been developed at the MPS in Lindau Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR.

On June 3, 2004, during orbit 471 of Mars Express, HRSC imaged regions in and around Vallis Marineris in stereo; these span height differences of more than 10 km. Obviously, the amount of dust in the Martian atmosphere varies with altitude. We used the images to study how optical depth depends on altitude. We used the Southern wall of the Vallis to estimate the atmospheric dust scale-height and found 13 km plus or minus 1 km. Independent of altitude, the measured optical depths show important (up to about a factor of two) local variations on horizontal scales of a few tens of kilometers, that may well be related to winds and obstacles like crater walls and mountains. Title: Atmospheric optical depths from HRSC stereo images of Gusev crater and elsewhere on Mars Authors: Hoekzema, N. M.; Markiewicz, W. J.; Inada, A.; Hviid, S. H.; Keller, H. U.; Gwinner, K.; Hoffmann, H.; Meima, J. A.; Neukum, G.; HRSC; MER Science Bibcode: 2004DPS....36.3708H Altcode: 2004BAAS...36.1157H Stereo images taken with the High Resolution Stereo Camera (HRSC) onboard the Mars Express orbiter offer a unique and powerful way to study Mars and its atmosphere. Since the atmosphere contains large and variable amounts of aerosols that scatter light and influence the images, image analysis requires careful consideration of atmospheric effects. An essential parameter is the optical depth, which often is retrievable from stereo observations with the 'so called' stereo method. This method uses contrast differences between stereo images to estimate optical depths. Software for this purpose has been developed at the MPS in Lindau Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR. For validation, we compared optical depths retrieved from HRSC images with measurements from the Spirit rover in Gusev. On Jan 16 2004 Spirit looked up into the Sun and measured the local optical depth at 0.87-0.89. That same day, during orbit 24 of MEX, HRSC observed Gusev. HRSC's stereo images yielded 0.91 ± 0.04, in good agreement with Spirit's ground truth.

Spirit landed in a contrast-rich region. For accurate retrievals, sufficient contrast is essential. Low contrast regions, say <5% in RMS, generally yield large errors. In addition, careful consideration of topography proves crucial. The stereo-method works best on flat terrain since then the surface looks almost the same in all images, even though these were observed from different viewing angles. Retrievals from regions with a lot of sharp topography, such as cliffs and crater edges, generally are unreliable since such features can look very different from different perspectives. We use HRSC stereo images, and the Digital Terrain Models derived from these, to study effects from topography and contrast. Title: Indication of a near surface cloud layer on Venus from reanalysis of Venera 13/14 spectrophotometer data Authors: Grieger, B.; Ignatiev, N. I.; Hoekzema, N. M.; Keller, H. U. Bibcode: 2004ESASP.544...63G Altcode: 2004ppae.work...63G Radiance measurements by an entry probe during its descent through the atmosphere allow to retrieve a vertical profile of the optical properties. The retrieval problem is in principle similar for the Venera probes, the last of which landed on Venus in 1982, and the Huygens probe, which will land on Titan in January 2005. However, for the optically very thick atmosphere of Venus, an approximation of the angular dependence of the radiance allows an analytical retrieval of the optical properties, while this is not possible for the optically less thick atmosphere of Titan. Therefore the Titan Inverse Radiation Model (TIRM) has been developed, which numerically computes the radiative transfer and estimates optical properties by assimilating measurements from the Descent Imager/Spectral Radiometer of the Huygens probe. Both methods - the analytical approximation and a modified version of TIRM - are used to estimate the extinction profile throughout Venus' atmosphere from Venera spectrophotometer measurements. We find a pronouced layer of increased extinction at an altitude of 1-2 km above the surface indicated by the data of Venera 13 as well as by the data of Venera 14. This can be interpreted as a cloud deck. It may be related to surface areas of high radar reflectivity and low radio emissivity which can be noticed at higher elevations in the Magellan Venus orbiter data. The material forming the cloud deck and accumulating onto the highlands of Venus could be small solid particles of PbS (galena) or Bi2S3 (bismuthite). Title: Atmospheric and Variable Features Phenomena with HRSC on Mars Express Authors: Markiewicz, W. J.; Hoekzema, N. M.; Keller, H. U.; Neukum, G.; Hoffmann, H.; Gwinner, K.; HRSC Co-Investigator Team Bibcode: 2004cosp...35.3742M Altcode: 2004cosp.meet.3742M In January 2004 the High Resolution Stereo Camera (HRSC) onboard the Mars Express orbiter began its primary mission of mapping Mars. In this talk we will report on the HRSC science results related to the atmospheric and surface variable features phenomena. Data available at the time of writing this abstract show a rather dusty atmosphere with optical depth equal to approximately one. Limb images show rich layer structure which will be analyzed for the vertical distribution of the aerosols. Some of the images show details of thin clouds. It is expected that data to be obtained between now and the COSPAR meeting will include phenomena related to the Martian atmosphere and surface variable features. Title: Observations of Martian Clouds by High Resolution Stereo Camera (HRSC) on board Mars Express for the First Six Months Authors: Inada, A.; Hoekzema, N.; Markiewicz, W. J.; Keller, H. U.; Gwinner, K.; Muller, J. P.; Hoffmann, H.; Neukum, G.; HRSC Co-Investigator Team Bibcode: 2004cosp...35.3497I Altcode: 2004cosp.meet.3497I High Resolution Stereo Camera (HRSC) onboard Mars Express has provided images of water ice clouds / haze since January 2004. The wavelength dependence on the reflectance of ice particles will be shown by analyzing the color images. The center of wavelength for blue, green, panchromatic, red, and infrared bands is 440, 530, 675, 750, and 970 nm respectively. The panchromatic bands are for a nadir, two photometric, and two stereo channels. Using the shadow and stereo methods the optical depth of clouds / haze are estimated. Furthermore, we will present the height of ice condensed level by analyzing stereo images. Title: Atmospheric Optical Depths from HRSC Stereo Images of Mars Authors: Markiewicz, W. J.; Hoekzema, N. M.; Keller, H. U.; Inada, A.; Petrova, E.; Gwinner, K.; Hoffmann, H.; Neukum, G.; HRSC Co-Investigator Team Bibcode: 2004cosp...35.3752M Altcode: 2004cosp.meet.3752M In January 2004 DLR's High Resolution Stereo Camera (HRSC) onboard the Mars Express orbiter began its primary mission of mapping Mars in high resolution and in stereo. Studies of the Martian surface that need atmospheric correction need the optical depth of the atmosphere as a primary input, as do most studies of the atmosphere itself. We present first results of estimates of Martian atmospheric optical depths as retrieved from HRSC images. The data analyzed to date of submission of this abstract shows in most places a very dusty atmosphere with typical values of optical depth of about unity. There are however large global differences with optical depth being only about 0.1 for example north of Olympus Mons. We will present latitude-longitude as well as spectral dependance of the optical depth as retrieved from the images. Two methods were used for these retrievals. The first one uses the intensity differences between shadowed and sunlit regions. The second analyzes contrast differences between the images within stereo triplets of the HRSC. Title: Indication of a Near Surface Cloud Layer on Venus from Reanalysis of Venera 13/14 Spectrophotometer Data Authors: Grieger, B.; Ignatiev, N.; Hoekzema, N.; Keller, H. U. Bibcode: 2003DPS....35.3709G Altcode: 2003BAAS...35.1487G In 1982, Venera 13 and 14 reached the surface of Venus as the last, most developed probes of the Venera lander series. During the descent, the spec\-tro\-pho\-to\-me\-ter measured the radiance inside the atmosphere at different directions and wavelengths. In January 2005, the Descent Imager/Spectral Radiometer (DISR) on board the Huygens probe will make similar observations --- albeit at higher spatial and spectral resolution --- during its descent through Titan's atmosphere. To retrieve the optical properties from DISR data, the Titan Inverse Radiation Model ({\sf T!RM}) has been developed.

Deep in an optically thick atmosphere like Venus', the volume extinction coefficient can approximately be estimated from zenith and nadir radiances. The respective analysis of Venera spectrophotometer data at high vertical resolution yields a pronounced peak of extinction 1--2 km above the surface of Venus.

To check wether this peak could be an artefact due to the neglection of the near surface in the analytical approximation applied, we alternatively reconstruct the extinction profile with {\sf T!RM}. The radiative transfer computations imply assimilation of Venera 14 measurements at 705 nm and take into account the surface considering different albedo values. The modeled extinction profiles differ only marginally from the analytical approximation (cf. figure), supporting the indication of a pronounced layer of increased extinction, possibly due to clouds. The peak 1--2 km above the surface shows also up for Venera 13 data and over the complete observed wavelength range of 480--1140 nm.

This work was supported by the German Aerospace Center (DLR), contract number 50 OH 98044. Title: Optical Depth Retrievals from HRSC Stereo Images Authors: Hoekzema, N. M.; Gwinner, K. Bibcode: 2003mars.conf.3153H Altcode: The HRSC camera of the Mars Express orbiter will map Mars in stereo. Software has been developed to retrieve optical depths of the martian atmosphere from these stereo images. We present examples of retrievals from airborne HRSC images of the Alps. Title: Martian Atmospheric Optical Depth and Surface Albedo from MEX Orbiter Stereo Images Authors: Hoekzema, N. M.; Markiewicz, W. J.; Keller, H. U.; Gwinner, K.; Neukum, G. Bibcode: 2003DPS....35.1410H Altcode: 2003BAAS...35Q.936H A primary task for the Mars Express (MEX) orbiter is to map Mars in stereo with the High Resolution Stereo Camera (HRSC). The Martian atmosphere contains large amounts of dust and other aerosols that scatter light and influence the images. Therefore, image analysis requires careful consideration of atmospheric effects. An essential parameter in this context is the optical depth. During periods that the spacecraft circles within a few tens of degrees from the terminator it will usually be possible to map optical depth from normal HRSC stereo images by analyzing contrast differences. The expected precision is better than about 0.1. We present the theory behind such retrievals and discuss its applications and limitations. The method is validated with airborne HRSCa images of the French Alps. We discuss two alternative ways to analyze images. The first is using large images; the second is dividing larger regions into smaller sub-regions and analyze those separately.

Standard optical depth retrievals, via methods common in Earth remote sensing, are currently hampered by the absence of regions on Mars for which the surface albedo is well known. We suggest a way to improve this situation. It should be possible to map the surface albedo from stereo images that are obtained while MEX is rotated from its normal nadir pointing position towards the direction of flight by e.g., 60'. We offer the basic theory behind such albedo mapping, and discuss some of its applications and limitations. We hope for an accuracy in the retrieved albedos of up to 1--2 Title: Small-scale topology of solar atmospheric dynamics. V. Acoustic events and internetwork grains Authors: Hoekzema, N. M.; Rimmele, T. R.; Rutten, R. J. Bibcode: 2002A&A...390..681H Altcode: We use high-quality observations from the Dunn Solar Telescope at NSO/Sacramento Peak to study spatio-temporal co-location of acoustic flux events in the photosphere and internetwork grains in the chromosphere. The events are diagnosed as sites with excess upward-propagating five-minute waves measured from Dopplergrams. The grains are repetitive bright internetwork features in simultaneous \CaII \KtwoV filtergrams. We find that the largest-flux sites in the granulation have appreciably larger than random probability to co-locate with exceptionally bright chromospheric internetwork grains, at an average delay of about two minutes which is likely to represent sound travel time to the chromosphere. This finding strengthens the case for acoustic grain excitation. Title: Optical Depth Of The Martian Atmosphere From Hrsc Stereo Images Authors: Hoekzema, N. M.; Markiewicz, W. J.; Keller, H. U. Bibcode: 2002EGSGA..27.4731H Altcode: The Mars Express Orbiter will arrive at Mars in the beginning of 2004. One of the primary tasks of this mission will be to map in stereo at least half of the Martian surface with the High Resolution Stereo Camera (HRSC). HRSC will image the sur- face in five angle stereo as well as in five colours between 440 nm and 970 nm. The Martian atmosphere contains large amounts of dust as well as other aerosols which scatter and absorb the incident solar light. Interpretation of the images with respect to the surface requires careful consideration of this atmospheric effect. An essential pa- rameter of such analysis is the total optical depth. The brightness differences between illuminated and shadowed regions offer one way to estimate optical depths. Analyzing the differences between stereo images offers another. We present preliminary results of the optical depth retrieved from HRSC images taken of Stromboli Island from an aeroplane. We discuss the accuracy and limitation of these retrievals in the context of the coming flight of HRSC to Mars. Title: Optical Depth of the Martian Atmosphere from Orbiter Stereo Images Authors: Keller, H. U.; Hoekzema, N. M.; Markiewicz, W. J. Bibcode: 2001DPS....33.1918K Altcode: 2001BAAS...33.1071K The Mars Express orbiter will arrive at Mars in the beginning of 2004. One of the primary tasks of this mission will be to map in stereo at least half of the Martian surface with the High Resolution Stereo Camera (HRSC). HRSC will image the surface in five angle stereo as well as in five colours. The Martian atmosphere contains large amount of dust as well as other aerosols which scatter light and influence the obtained images. Interpretation of the images requires careful consideration of this atmospheric effect. An essential parameter of such analysis is the total optical depth. Stereo images offer a reliable way to retrieve the optical depth as demonstrated in Earth remote sensing with for example, the MISR instrument onboard the TERRA satellite. Retrieval is possible since a nadir view has a shorter atmospheric path than the forward and backward ones. We present preliminary results of the optical depth retrieved from HRSC images of Stromboli Island taken from an aeroplane. We discuss the accuracy and limitation of these stereo retrievals in the context of the coming flight of HRSC to Mars. Title: Methods to Estimate Optical Depth of the Martian Atmosphere from Orbiter Images. Authors: Hoekzema, N. M.; Markiewicz, W. J.; Keller, H. U. Bibcode: 2001DPS....33.3407H Altcode: 2001BAAS...33.1098H It is now well known that the solar photons scattered in the Martian atmosphere produce a significant diffuse illumination of the surface. The spectral characteristics of this illumination reflect the optical properties of the aerosols and are different from that of the direct solar illumination. Interpretation of the orbiter images requires careful separation of this atmospheric effect. The required analysis yields information on the compositions and properties of the aerosols. Probably the most significant parameter of the problem is the total optical depth of the atmosphere. Here, we present a method to estimate the optical depth directly from orbiter images. The method compares shadowed and illuminated areas of the surface. Since only the diffuse light illuminates shadowed regions an estimate of the optical depth with minimal number of assumptions can be made. Effects of shadows on scales beyond the resolution (micro-shadowing) can also be included. We tested our method by comparing the results from Viking orbiter images of the Viking Lander sites with direst data from the landers. The accuracy of the method is shown to be a few percent. In one set of the images of the Viking I lander we are able to identify ground fogs and/or hazes by direct inspection of the images and the variations of the optical depth with wavelength in different parts of the image (different local solar time). These variations correlate with variations measured by the lander. Title: Small-scale topology of solar atmosphere dynamics. IV. On the relation of photospheric oscillations to meso-scale flows Authors: Hoekzema, Nick M.; Brandt, Peter N. Bibcode: 2000A&A...353..389H Altcode: We use a high quality five-hour sequence of images of the solar photosphere taken at the Swedish Solar Vacuum Telescope, La Palma, to study the relation of meso-scale divergence with the amplitudes of photospheric intensity oscillations. Meso-scale flows are determined by local correlation tracking of the granulation. Statistical evaluation of the co-location probability of regions with more than twice the average oscillation amplitude and regions with high or low meso-scale divergence shows that high amplitude regions are preferentially located in regions of negative divergence (convergence) and avoid meso-divergent regions. Additionally we confirm the increase of granule brightness in regions of positive divergence with the opposite effect in regions of convergence. Title: On the Relation of Photospheric Oscillations to Meso-Scale Flows Authors: Hoekzema, N.; Brandt, P. Bibcode: 1999ASPC..183..473H Altcode: 1999hrsp.conf..473H No abstract at ADS Title: Small-scale topology of solar atmosphere dynamics. III. Granular persistence and photospheric wave amplitudes Authors: Hoekzema, N. M.; Brandt, P. N.; Rutten, R. J. Bibcode: 1998A&A...333..322H Altcode: We use a superb five-hour sequence of 900 solar images taken at La Palma to study long-duration persistence in the solar granulation, in the context of the long-lived ``intergranular holes'' discovered by \cite*{Roudier+others1997} %T AA: intergranular plumes + BP formation and the contention that these mark sites of convective downflow plumes. We develop a procedure to locate ``persistency regions'' that contain granular brightness maxima or minima over extended periods (up to 45 min), while allowing for lateral drifts due to horizontal flows. Statistical evaluation of the co-location probability for different pixel classes is first used to quantify the likelihood of long-term stationarity for different granular brightness classes and for the persistency regions, and then to evaluate the amount of preferential alignment, at different frequencies and time delays, between excessive Fourier modulation and granular brightness and persistence. The results support the existence of long-lived intergranular holes. There is large persistency difference between the brightest and the darkest features; some of the latter have location memories as long as two hours. In addition, the darkest intergranular features are found to be sites of enhanced Fourier modulation in the 3-min acoustic regime, improving earlier results through much higher statistical significance. However, the persistency regions containing intergranular holes do not seem to produce the excess acoustic emission that would be expected above downflow plumes. Title: Small-scale topology of solar atmosphere dynamics. I. Wave sources and wave diffraction Authors: Hoekzema, N. M.; Rutten, R. J.; Brandt, P. N.; Shine, R. A. Bibcode: 1998A&A...329..276H Altcode: We study the small-scale topology of dynamical phenomena in the quiet-sun internetwork atmosphere, using short-duration Fourier analysis of high-resolution filtergram sequences to obtain statistical estimates for the co-location probability of different fine-structure elements and wave modes. In this initial paper we concentrate on the topology of short-duration Fourier amplitude maps for the photosphere and the simultaneously observed overlying chromosphere. We find that these maps portray a complex mix of global modes and locally excited waves which necessitates a statistical approach. Various aspects including mesoscale patterning indicate the presence of subsurface wave sources and of subsurface wave diffraction by convective inhomogeneities. Title: Small-scale topology of solar atmosphere dynamics. II. Granulation, K2v grains and waves Authors: Hoekzema, N. M.; Rutten, R. J. Bibcode: 1998A&A...329..725H Altcode: We continue studying the small-scale topology of dynamical phenomena in the quiet-sun internetwork atmosphere through statistical estimation of the co-location probability of different fine-structure elements and wave modes. In this paper we chart spatial alignments between the granular brightness structuring of the photosphere, Ca ii K2V brightness patterns in the chromosphere, and wave amplitude patterns in both regimes as a function of time delay between the occurrences of the various features. These charts confirm the presence of excess 2--4 min waves above dark intergranular lanes, the absence of excess 5 min waves above bright granules, the absence of expected alignments between photospheric and chromospheric wave patterning, and the broad-band nature of Ca ii K2V grain formation. In addition, they show significant alignments at large time delays that seem to be regulated by mesoscale patterning and pattern = migration. Title: Dynamical Relations between Photosphere and Chromosphere Authors: Hoekzema, N. M. Bibcode: 1997ASSL..225..281H Altcode: 1997scor.proc..281H This report presents statistical relationships between dynamical features in the photosphere and chromosphere in quiet-Sun internetwork regions. Short-duration Fourier analysis is combined with statistical correlation to establish correspondence between granulation features and oscillation amplitudes in the photosphere and the presence of Ca II K$2V$ bright points in the overlying chromosphere. The results agree with the time delays between photospheric cause and chromospheric effect in the numerical simulations of acoustic shock propagation by Carlsson \& Stein (1994) Title: Ultraviolet Jets and Bright Points in the Solar Chromosphere. II. Statistical Correlations Authors: Hoekzema, N. M.; Rutten, R. J.; Cook, J. W. Bibcode: 1997ApJ...474..518H Altcode: We use HRTS-VI rocket observations of the solar chromosphere to search for relationships between high-Dopplershift ``jets'' observed in the C I lines near λ = 156 nm and internetwork ``bright points'' observed in the λ = 160 nm continuum, in sequel to the analysis by Cook et al. which failed to find a direct connection between these phenomena. We now use the same data to establish statistical correlations between C I Dopplershift and 160 nm brightness modulation in internetwork areas. These mean relations emerge only after extensive spatial averaging and have small amplitude, but are definitely significant. They show that both C I Dopplershift and 160 nm brightness participate in oscillatory behavior with 3 minute periodicity and mesoscale (8 Mm wavelength) as well as small-scale (1.4 Mm wavelength) spatial patterning. We find spatial and temporal phase relations between Dopplershift and brightness that confirm that jets and bright points should not be interpreted as isolated entities. Rather, they are chromospheric manifestations, with much pattern interference, of the oscillatory acoustic shock dynamics in the internetwork which also cause Ca II K2V grains. Additional small-scale modulation is present which we attribute to waves with f-mode character. Title: Ultraviolet Jets and Bright Points in the Solar Chromosphere. I. Search for One-to-One Relationships Authors: Cook, J. W.; Rutten, R. J.; Hoekzema, N. M. Bibcode: 1996ApJ...470..647C Altcode: Ultraviolet spectrograms and spectroheliograms of the solar chromosphere are used to test the suggestion of Dere, Bartoe, & Brueckner and Rutten & Uitenbroek that bright points in quiet Sun cell interiors observed at = 1600 A, chromospheric jets observed in C I lines near λ = 1560 Å, and Ca II K2v bright points are associated with each other and that they are all manifestations of the same wave interaction in the nonmagnetic chromosphere. We search for spatio-temporal connections between 1600 Å bright points and C I blue jets using data from the High Resolution Telescope and Spectrograph VI rocket flight, comparing 1600 A spectrohellograms and a cospatial C I Doppler shift map on a pixel-by-pixel basis. We find no direct evidence for spatial colocation of bright points and jets, not for instantaneous correspondence and also not when allowing for phase delays as long as 3 minutes. Also, the average brightness evolution and its rms fluctuation are not obviously different between sites of large C I blueshift and the remaining surface. Title: On CI jets and 160 nm internetwork bright points Authors: Hoekzema, N. M. Bibcode: 1994chdy.conf..111H Altcode: No abstract at ADS Title: Walraven photometry of stars near the luminous blue variable AG Carinae. Authors: Hoekzema, N. M.; Lamers, H. J. G. L. M.; van Genderen, A. M. Bibcode: 1993A&AS...98..505H Altcode: We present Walraven-photometric data of 114 stars of V in the 5.9 to 14.7 range, within 17 arcmin of the Luminous Blue Variable AG Car (HD 94910). The results are expressed in terms of V-magnitude in the Johnson system and in observed and extinction-free colors in the Walraven system. Title: The distance and reddening of stars near the luminous blue variable AG Carinae. Authors: Hoekzema, N. M.; Lamers, H. J. G. L. M.; van Genderen, A. M. Bibcode: 1992A&A...257..118H Altcode: Stars of V less than about 13.5 in the region close to the Luminous Blue Variable AG Car (within 17 arcmin) have been studied in the Walraven photometric system. The observed colors are used to derive the values of Teff and log g. The intrinsic colors, predicted by model atmospheres, and the absolute visual magnitudes, predicted by stellar evolutionary tracks, are used to derive the extinction and the distances of 43 stars. We find a concentration of early type stars in the direction of AG Car with distances between 1 and 10 kpc and E(B - V) in the range of 0.13 to 0.68. The extinction versus distance relation suggests a distance of 4 to 10 kpc for AG Car. Combining all distance criteria for AG Car, we find a distance of 6 +/- 1 kpc, and Mbol = -10.8 +/- 0.4 mag with variable M(v). AG Car does not belong to the Car OB1 or OB2 associations at 2.5 kpc. We did not find a cluster of luminous stars at the distance of AG Car, but we identified a few stars which may belong to the same cluster as AG Car. Title: The distance and evolutionary phase of the luminous blue variable AG Car. Authors: Humphreys, R. M.; Lamers, H. J. G. L. M.; Hoekzema, N.; Cassatella, A. Bibcode: 1989A&A...218L..17H Altcode: The paper presents evidence that the luminous blue variable AG Car is at a much greater distance than has been previously assumed in the literature. The variation of interstellar extinction with distance for stars in its direction shows that AG Car's distance is much greater than 2 to 2.5 kpc and consistent with a distance of 5 kpc or more. Its kinematic distance from its radial velocity and galactic rotation is 6.4 to 6.9 kpc. The derived parameters of AG Car are presented. Title: The Algol Type Binary Qs-Aquilae Authors: Heintze, J. R. W.; Spronk, W.; Hoekzema, N. Bibcode: 1989SSRv...50..344H Altcode: No abstract at ADS Title: AG Carinae : Variability, Extinction, Distance and Luminosity Authors: Lamers, H. J. G. L. M.; Hoekzema, N.; Trams, N. R.; Cassatella, A.; Barylak, M. Bibcode: 1989ASSL..157..271L Altcode: 1989IAUCo.113..271L; 1989plbv.coll..271L No abstract at ADS