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Author name code: hoekzema
ADS astronomy entries on 2022-09-14
author:"Hoekzema, Nick M."
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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.
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. <P />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. <P />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.
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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.
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.
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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.
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. <BR /> 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. <BR /> 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. <BR /> 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.
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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
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.
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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
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.
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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.
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<SUP>-3</SUP>. Assuming reliable
upper and lower boundary values of vertical speeds within the dust
devils between 0.1 and 10 ms<SUP>-1</SUP> based on terrestrial and
Martian studies we derived dust fluxes in the range of 6.3-1221 mg
m<SUP>-2</SUP> s<SUP>-1</SUP> (PSP_004285_1375), from 0.38-162 mg
m<SUP>-2</SUP> s<SUP>-1</SUP> (ESP_013545_1110), and from 3.2-581 mg
m<SUP>-2</SUP> s<SUP>-1</SUP> (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.
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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.
2014LPI....45.1994R Altcode:
Optical depth measurements of three individual dust devils and their
surroundings with the shadow method in HiRISE images.
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Title: Mars' Atmospheric Optical Depth from Mars Express HRSC and
Mars Exploration Rovers — A Comparison
Authors: Stenzel, O. J.; Hoekzema, N. M.
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.
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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.
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.
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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.
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.
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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.
2011mamo.conf..110P Altcode:
No abstract at ADS
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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.
2011mamo.conf..112H Altcode:
No abstract at ADS
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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.
2011mamo.conf..178S Altcode:
No abstract at ADS
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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.
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.
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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.
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. <P />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.
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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.
2008LPICo1447.9112S Altcode:
A simple scheme is introduced to estimate the optical depth present
in the martian atmosphere during a Mars Express HRSC observation.
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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.
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.
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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.
2008LPICo1405.8269H Altcode:
No abstract at ADS
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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
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
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
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.
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.
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. <P />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.
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
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
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. <P />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
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. <P />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.
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 Bi<SUB>2</SUB>S<SUB>3</SUB> (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
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
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
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.
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. <P />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. <P />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. <P />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.
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.
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. <P />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.
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.
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.
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.
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.
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.
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.
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.
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.
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 K<SUB>2V</SUB>
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 K<SUB>2V</SUB> 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.
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$<SUB>2V</SUB>$ 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.
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 K<SUB>2V</SUB> 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.
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 K<SUB>2v</SUB> 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.
1994chdy.conf..111H Altcode:
No abstract at ADS
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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.
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.
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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.
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.
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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.
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.
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.
1989ASSL..157..271L Altcode: 1989IAUCo.113..271L; 1989plbv.coll..271L
No abstract at ADS
