Abstract The springtime sublimation process of Mars’ southern seasonal polar CO2 ice cap features dark fan-shaped deposits appearing on the top of the thawing ice sheet. The fan material likely originates from the surface below the ice sheet, brought up via CO2 jets breaking through the seasonal ice cap. Once the dust and dirt is released into the atmosphere, the material may be blown by the surface winds into the dark streaks visible from orbit. The location, size and direction of these fans record a number of parameters important to quantifying seasonal winds and sublimation activity, the most important agent of geological change extant on Mars. We present results of a systematic mapping of these south polar seasonal fans with the Planet Four online citizen science project. Planet Four enlists the general public to map the shapes, directions, and sizes of the seasonal fans visible in orbital images. Over 80,000 volunteers have contributed to the Planet Four project, reviewing 221 images, from Mars Reconnaissance Orbiter’s HiRISE (High Resolution Imaging Science Experiment) camera, taken in southern spring during Mars Years 29 and 30. We provide an overview of Planet Four and detail the processes of combining multiple volunteer assessments together to generate a high fidelity catalog of ∼ 400000 south polar seasonal fans. We present the results from analyzing the wind directions at several locations monitored by HiRISE over two Mars years, providing new insights into polar surface winds.
Quelle: Science Direct
When active, comets release dust grains of various sizes and morphologies (e.g. Schulz, Hilchenbach, Langevin, Kissel, Silen, Briois, Engrand, Hornung, Baklouti, Bardyn, Cottin, Fischer, Fray, Godard, Lehto, Roy, Merouane, Orthous-Daunay, Paquette, Rynö, Siljeström, Stenzel, Thirkell, Varmuza, Zaprudin, 2015, Hilchenbach, Kissel, Langevin, Briois, von Hoerner, Koch, Schulz, Silén, Altwegg, Colangeli, Cottin, Engrand, Fischer, Glasmachers, Grün, Haerendel, Henkel, Höfner, Hornung, Jessberger, Lehto, Lehto, Raulin, Le Roy, Rynö, Steiger, Stephan, Thirkell, Thomas, Torkar, Varmuza, Wanczek, Altobelli, Baklouti, Bardyn, Fray, Krüger, Ligier, Lin, Martin, Merouane, Orthous-Daunay, Paquette, Revillet, Siljeström, Stenzel, Zaprudin, 2016). The grains’ trajectories are influenced by solar radiation pressure to form dust tails, which lag behind the nucleus’s motion about its orbit, e.g. Fulle (2004). Analysis of dust tails’ structure can reveal key information on the dust grains’ parameters, together with the time of release from the nucleus. These parameters can in turn provide information on the activity of the nucleus.
Features commonly observed in cometary dust (type II) tails include synchronic bands, which are large-scale linear features that are coaligned with the position of the comet’s nucleus, and, much more rarely, striae. The latter are puzzling features that have only been observed in a few, generally very high production rate comets, including C/1957 P1 (Mrkos) (Sekanina, Z, Farrell, 1982), C/1962 C1 (Seki-Lines) (McClure, 1962), C/1975 V1 (West) (Koutchmy, Lamy, 1978, Lamy, Koutchmy, 1979, Nishioka, Saito, Watanabe, Ozeki, 1992), and C/1996 O1 (Hale-Bopp) (Sekenina and Pittichová, 1997).
Comet C/2006 P1 McNaught – for brevity, from hereon referred to as McNaught only – was discovered by Robert McNaught on 2006 August 7 (McNaught, 2006), and reached perihelion on 2007 January 12 at a distance of 0.17 AU from the Sun. This was well inside the perihelion distance of planet Mercury, and therefore a near-Sun comet (Jones et al., 2018). Undoubtedly a modern example of a “Great Comet”, it was easily visible from Earth through much of January 2007, but only from the southern hemisphere when at its most spectacular post-perihelion. The orbit of McNaught suggests it is was a dynamically new comet from the Oort cloud (Marsden, 2007).
Combi et al. (2011) found that the comet’s water production rate could be approximated by 5.4 × 1029r2.4 s where r is the heliocentric distance, and that it reached a peak of 5.48 × 1031 molec s on 2007 January 13. Despite the comet’s spectacular appearance around perihelion, relatively few works have been published on it based on visible wavelength observations, largely due to the difficultly in observing the object with ground-based facilities, though ground-based observations of sodium emission from the comet were possible with a solar telescope (Leblanc et al., 2008). The comet had an extensive, highly structured dust tail, displaying many striae. Observations of the comet at infrared wavelengths reveal that it had larger and more compact porous grains than many other objects (Kelley, Woodward, Harker, Wooden, Reach, Fernández, 2010, Wooden, De Buizer, Kelley, Sitko, Woodward, Harker, Reach, Russell, Kim, Yanamadra-Fisher, Lisse, de Pater, Gehrz, Kolokolova, 2014).
Here we report on the analysis of the striae observed in McNaught, which arguably possessed the most spectacular dust tail of recent times, and certainly the one for which is available the most comprehensive collection of observations to date. We apply a new reprojection technique, which we refer to as temporal mapping, to images of McNaught’s dust tail acquired from several sources, allowing us to track the behaviour of striae in the comet’s tail throughout the comet’s perihelion passage.
We present a sequence of images captured by the Large Angle and Spectrometric Coronagraph (LASCO) on the ESA/NASA Solar and Heliospheric Observatory (SOHO) spacecraft, that appears to record clearly for the first time the formation of several striae. Together with data from the Heliospheric Imagers (HI) that form part of the wider Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) remote sensing package on the twin NASA STEREO-A and B spacecraft, we track the formation and evolving appearance of numerous striae. We propose that the progressive realignment of many of these striae is due to interactions between electrically charged dust grains and the heliospheric magnetic field.
1.1. Striae formation theories
Most striae models describe formation through the fragmentation of larger dust particles. In addition to gravity, cometary dust grains are certainly strongly influenced by radiation pressure. As these two primary forces act in opposition to each other, the effective force acting on them is equivalent to reduced gravity. This is parameterised by the dimensionless constant βr, given as:
Sekanina and Farrell (1980) proposed that stria formation is a two-step process. They posited that the striae’s parent dust grains are released from the nucleus all with similar values. After a delay, these grains break up within a short period of time. Their fragments have a range of βr values, and radiation pressure separates the grains according to βr, forming near-linear structures. Under this scenario, along a single stria, there is expected to be a monotonic increase in βr with increasing heliocentric distance.
This model has been found to agree well with observations of striae (e.g. Pittichová et al., 1997). However, there is a serious issue with the Sekanina and Farrell two-step fragmentation scenario, in that it is difficult to explain why parent grains would all fragment after approximately the same time delay. They suggested rotational bursting generally due to uneven radiation pressure over grains’ surfaces, but this has not been widely accepted. Hill and Mendis (1980) suggested that the fragmentation of grains could occur due to electrostatic charging, with the few keV and more electrons required for this process originating in the closed current system of the induced cometary magnetotail.
Nishioka and Watanabe (1990) and Nishioka (1998) proposed a different scenario for the formation of striae. Rather than a two-step process, an almost continuous cascade of fragmentation was proposed. This cascade continues beyond where grains become too small to observe. This process would result in fragments of all values co-existing along the length of each stria. Sekenina and Pittichová (1997) pointed out that a relaxed fragmentation time should lead to wedge shaped striae, which are not observed.
Other models have also been proposed. These include those of Froehlich and Notni (1988) and Notni and Thaenert (1988), who suggested stabilising effects due to optical thickness of dust clouds. Steckloff and Jacobson (2016) considered that each stria is the result of the destruction of macroscopic sized (10–100m) boulders by rotational stresses from sublimation pressures.
Around perihelion, McNaught’s proximity to the Sun in the sky made ground-based observations particularly challenging. However, instruments aboard the SOHO, STEREO-A, and STEREO-B spacecraft provided excellent imaging data, allowing the comet’s coma and tail structures to be monitored through the perihelion passage. The activation of the SECCHI heliospheric imagers aboard the then–recently launched twin STEREO spacecraft came just in time to capture the dust tail as the comet reached perihelion, and this event was also captured by the SOHO LASCO C3 coronagraph.
The comet was well observed from Earth from the southern hemispherepost-perihelion, with some of the older dust features also seen in the northern hemisphere during the same period, despite the nucleus being below the horizon from those locations. We note that this is similar to the circumstances of Comet de Chéseaux (C/1743 X1) with its extensive, structured dust tail (Kronk, 1999). For this project we use the SECCHI HI-1 and HI-2 datasets from both STEREO spacecraft, as well as LASCO C3Clear and Blue filter data and various ground-based images. Fig. 1summarizes the images obtained from all sources. Further details of each data source are provided below.
2.1. Stereo A SECCHI HI
The twin NASA STEREO spacecraft follow separate orbits around the Sun, moving progressively ahead of and behind the position of the Earth (Howard et al., 2008). Each spacecraft has two Heliospheric Imagers, HI-1and HI-2. These consist of CCD cameras pointed off axis from the sun. HI-1 points closest to the sun with a field of view of 20° and solar offset of 14° (Eyles et al., 2009). The effective primary bandpass of the instrument is ∼ 630–730 nm, with additional, weaker coverage of the wavelengths ∼ 300–450 and ∼ 900–1000 nm (Bewsher et al., 2010). Images are obtained onboard measuring 2048 pixel square, but were downsampled to 1024 pixel square images for transmission during the period of interest. The pixel scale of transmitted images is70 pixel. This corresponded to a physical resolution of 43 000 km at McNaught’s head at perihelion. As the dust tail extended from the nucleus towards STEREO-A, the physical resolution for the tail was higher, reaching ∼ 33 000 km pixel.
At the time of the McNaught observations, both HI-1 instruments were being commissioned, and McNaught was serendipitously in the field of view as their observations on both spacecraft began. A few 2048 pixel-wide images were downlinked during commissioning, but given the large scale of the striae, these do not prove particularly useful for this purpose compared to the 1024 pixel-wide images. The data from this instrument were inferred to imply the presence of a neutral iron tail at McNaught (Fulle et al., 2007).
Fig. 2 shows an example image of McNaught from STEREO-A HI-1. The fine structure of the dust tail is clear, but several artefacts also appear. The HI imagers are primarily designed to image sunlight Thomson scattered by tenuous solar wind and transient structures within it, such as coronal mass ejections. The instrument is not designed to deal during standard operations with objects that are as bright as McNaught, hence much of the region around the coma is overexposed. The particularly bright coma of McNaught caused significant CCD bleeding. Furthermore, as the HI cameras do not have shutters (Eyles et al., 2009), as the pixel values are read off the detector, the camera continues to collect light, causing the bright background in parts of the image adjacent to the brightest tail regions.
The sequence of raw data from HI-1 can be viewed in Supplementary material.
The second of the two heliospheric imagers, HI-2, points further off axis with a field of view of 70° and solar offset of 53.7°, with the instrument designed to image CMEs all the way to the Earth (Eyles et al., 2009). The bandpass of HI-2 is centred on ∼ 640 nm, with a FWHM of ∼ 450 nm (Tappin et al., 2015). The pixel scale of transmitted HI-2 images is4 pixel which corresponds to a physical scale at McNaught varying between 90 000 and 200 000km, depending on the position in the dust tail.
Although the nucleus of the comet did not pass into the HI-2 field of view, the extensive tail was present in the instrument data for approximately two weeks. Fig. 3 shows an example view of McNaught from STEREO-A HI-2. Here the fine structure of the tail is much dimmer than in HI-1 images. Complicating the interpretation of some of the images is the presence of zodiacal dust, whose surface brightness is comparable to that of the striae.
2.2. STEREO B SECCHI HI
The HI instrument aboard STEREO-A has the same properties as its sister spacecraft, and both spacecraft were still very close to the Earth-Moon system at the time. The instrument door on STEREO-B opened for the first time on January 11, so the instrument was in a much earlier stage in its commissioning than its counterpart on STEREO-A. Many of its images contain artefacts, and there are fewer images taken at regular intervals. The default FITS headers for providing the celestial coordinates provided inconsistent results compared to the background starfields, so the astrometry for the files was recalculated independently using the automated service at astrometry.net (Lang et al., 2010).
We note in passing that true stereo images of Comet McNaught were taken by SECCHI HI-1A and HI-1B nineteen times between January 12 and 15. An example pair, from 00:01 on January 14, can be seen in Fig. 4. The parallax effect is clear in the two images, with a ∼ 0.3° shift in background star positions. The separation of the two spacecraft was however too small to provide useful information on the three-dimensional distribution of the cometary dust to be extracted, e.g. to confirm that the dust lay exclusively in the comet’s orbital plane. The HI-2 instrument aboard STEREO-B also captured the edge of the extended tail of McNaught. However, the instrument was unfortunately not in focus at this time and so this dataset was disregarded.
2.3. SOHO LASCO C3
The SOHO spacecraft resides ∼ 1 million km sunward of Earth at Sun-Earth Lagrange Point L1. Its LASCO coronagraph images a 15° wide circle surrounding the Sun (Brueckner et al., 1995), and is responsible for the greatest number of comet discoveries (Battams and Knight, 2017). LASCO comprises three instruments; all the McNaught data were gathered using coronagraph C3, which covers 3.7 to 30 solar radii from the Sun’s centre, recorded on 1024 by 1024 pixel images at a spatial resolution of 56 arcsec pix. Further technical information on the LASCO instrument and its calibration is covered by Morrill et al. (2006).
Most McNaught images were gathered using a broadband “clear” filter covering wavelengths of 400 to 850 nm. These images, whilst capturing a great degree of detail in the dust tail, were overexposed in the comet’s near-nucleus and coma regions. They suffer issues with overexposure of McNaught’s bright nucleus, although the effect is less severe than with the STEREO heliospheric imagers. The instrument also includes 5 other narrower wavelength range filters (Morrill et al., 2006). As has also occurred with other anticipated bright comets, LASCO was commanded to obtain numerous images using the colour filters when McNaught was within the C3 field of view. In order to maximize data return, these images only included subframes of the entire C3 field of view, and were compressed onboard using a lossy algorithm prior to transmission to Earth. Despite the lower quality of these images compared to the clear images, they are very valuable for the study of the striae, as they record structures much closer to the comet’s nucleus than in images obtained using the clear filter, which were generally overexposed around the comet’s head. Although the clear filter images were least affected by data compression artifacts, much of the analysis of LASCO data here concentrates on images obtained using the instrument’s blue filter.
The LASCO C3 Clear data was used with several calibration algorithms already applied, referred to as Level 1 calibration. These calibration steps include corrections for the flat field response of the detector, radiometric sensitivity, stray light, geometric distortion, and vignetting (Morrill et al., 2006). Unfortunately for the Blue filter dataset, Level 1 calibration is not available, and the raw level 0 data had to be used instead. In this analysis, as we concentrate on the morphology and dynamics of tail features and do not rely on any absolute calibration of the images, we deemed the unprocessed Level 0 images to be of sufficient quality for this study. This does however mean that the Sun’s dust (F) corona and stray light are present in these images, but these largely structureless background features were largely removed by the enhancement techniques described below
Fig. 5 shows a LASCO clear filter image of the comet, enhanced with multiscale gaussian normalisation (Section 2.4.1). McNaught’s nucleus was within the LASCO C3 field of view from 2007 January 12 01:42 UT to January 15 21:54 UT. The sunward edge of the dust tail remained in the field of view for several hours after the nucleus left, but with no clearly discernible structures visible within it.
Fig. 6 shows a combination of imaging data from SOHO LASCO and STEREO SECCHI HI-1 and HI-2, demonstrating the relative scales of the images returned by the different instruments.
2.4. Enhancement techniques
The dust tail striae are features present over a wide dynamic range in the images. Enhancement techniques were found to be required to trace the positions of the striae, and to also address the presence of zodiacal light in the images, i.e. the extension of the F corona. Two complementary enhancement techniques were employed. In presenting the enhanced images in this paper, as the brightness scale is largely arbitrary, we do not provide quantitative brightness values.
2.4.1. Multiscale Gaussian Normalisation
The large range in brightness over the tail region makes it difficult to extract fine structure information over the whole range of intensities and physical scales, even with the use of logarithmic brightness scaling. Morgan and Druckmüller (2014) developed the Multiscale Gaussian Normalisation technique, MGN, for application to such situations. At several length scales, convolution of the image with Gaussian kernels enables the calculation of local means and variance. These are used to offset and normalise pixel values locally to a mean of zero and unity standard deviation. The set of normalised images are then recombined into a global image, where fine structure across all parts of the image are visible. Fig. 2 shows the success of this method for revealing fine structure in cometary dust tails. The process is very efficient, working in a matter of seconds. We use this technique here for all three datasets.
2.4.2. Difference images
In certain cases MGN enhancement fails to enhance fine detail, probably due to noise. After the comet left the field of view of the SECCHIHeliospheric imagers, striae persisted in the tail and gradually faded from view. The features could eventually no longer be resolved by eye, even with MGN enhancement. However, difference images created by subtracting the previous image frame from each image reveal faint fine structure. This allows us to extend the useful range of data by several days.
2.5. Ground based data
The comet reached a peak magnitude of −5.5 (Marsden, 2007), when it was visible in the daylight sky near the Sun. Post-perihelion, it was easily visible to the naked eye in the southern hemisphere, when the nucleus was not observable from north of the equator. Some older striae were however visible from the northern hemisphere during that period. Many professional and amateur astronomers photographed the object. Here we have used wide angle photographs taken by author Sebastian Deiries at the European Southern Observatory in Chile, and wide angle photographs and composites taken by author Miroslav Drückmuller in Chile and Argentina.
Dust tail structures have traditionally been investigated using Monte-Carlo models of dust populations, and comparing the results directly to images of comets. Many such studies have been successful, but these approaches do have limitations. Kharchuk and Korsun (2010) reported on their efforts to model the striae visible in McNaught’s tail. The results were somewhat satisfactory, but failed to reproduce the correct orientation of the striae, which in their simulations were aligned with the position of the comet’s nucleus, and were therefore synchronic bands caused by variations in dust release rates at the nucleus.
Traditionally synchrone and syndyne lines are plotted alongside on overlaid onto images, which does allow for the model to be intuitively matched to the tail by eye. However, this method suffers due to the compression of lines near the nucleus, and rarefaction in the furthest and oldest regions of the tail. It is therefore difficult to relate dust features in these regions to one another, particularly across long time sequences where near nucleus features may have travelled out and reached the far tail, and especially when the viewing geometry or instrument has changed.
We introduce here a new dust tail analysis technique which presents dust features in a consistent way, by morphing images of comets onto a matrix of assumed dust age and sensitivity to solar radiation pressure. This temporal mapping technique has several advantages:
A single dust tail feature can be analysed over an extended period of time, despite changes in its physical scale, transient motion and viewing orientation
Changes in the dust tail during periods when the appearance of a tail changes rapidly can be easily resolved
Issues concerning observing geometry are removed, e.g. when the observer is close to the orbital plane of the comet and dust tail features are foreshortened
Images of an individual comet tail obtained by different observers can be easily compared
Important additional parameters, such as heliocentric distance, orbital plane coordinates and dust phase angle can be considered and evaluated for the observations
We have implemented the temporal mapping technique in Python, as detailed below. As McNaught was a very bright, high activity comet for which we have high quality dust tail imaging data, it is a perfect subject for the application of the technique.
3.2. The Finson Probstein model
The Finson–Probstein model provides a good basis for cometary dust tail modelling (Finson and Probstein, 1968). This assumes that only gravity and radiation pressure act on the dust grains after their release from the nucleus. Using the parameter βr introduced in Section 1.1, an equation of motion for a dust particle can be based on the acceleration a, defined by:
Where r is the heliocentric distance, a the acceleration of the dust outwards in the heliocentric radial direction, G the gravitational constantand Ms the solar mass. We make several assumptions in our use of the model:
The dust is ejected with zero velocity relative to the comet and hence the physics only evolves in two dimensions; the dust tail structure remains entirely in the orbital plane of the comet. Whilst the sublimating gas in reality imparts a velocity to the dust grains, the initial motion is smaller than the velocity imparted by radiation pressure and gravity. Studies of comets at orbital plane crossings show that the bulk of the dust tail remains close to the orbital plane to a reasonable approximation. However, there is some initial sorting of grain size due to smaller particles being accelerated to higher speeds before decoupling from the outflowing gas. As we assume the dust is sorted by the time it reaches the far tail, this is not significant.
The value of βr is constant for a particular dust particle. As dust fragments or sublimates, the value of βr will be affected. Striae models that include fragmentation and its effects are discussed in Section 1.1.
The optical thickness of the dust cloud does not affect the βr value over time.
Note that our technique makes no assumptions about the mass or the physical cross-section of the individual dust grains. We only define a βrvalue for each grain.
These assumptions reduce the problem to two parameters: βr, and the time of dust particle emission, denoted by te. Each image is therefore deterministic, where every combination of βr and te values maps to a unique position in the tail.
3.3. Creating a temporal map
Creating a temporal map requires the following:
Astrometry for the image; effectively right ascension, RA, and declination, dec, for every pixel. For spacecraft data in FITS format this is often accessible in the FITS metadata. For ground based images it can usually be determined using the astrometry.net website (Lang et al., 2010).
Orbital ephemerides for the comet and its observer location relative to the sun. We use ephemerides from NASA’s JPL Horizons system (Giorgini et al., 1997). A one minute temporal resolution is used to guarantee positional accuracy.
The time of the image. For spacecraft data this is known, and many ground based observers provide this with their images. Otherwise it is necessary to manually calculate this by using the celestial coordinates of the comet. If the astrometry for the background image is correct, then the exact position of the comet in the image and thus its location along its orbit can be used together with the orbit ephemeris to calculate the time at which an image was taken.
For a desired ma