Today we have a guest blog by JPL research scientist Laura Kerber, one of our lead researchers on Planet Four: Ridges . Laura studies physical volcanology, aeolian geomorphology, wind over complex surfaces, and the ancient Martian climate.
Hello Ridge Hunters!
Thanks to you, we have mapped ridges all over Nili Fossae and Nilosyrtis Mensae!!
As you might remember, our ultimate goal was to determine the distribution of ridges so that we could see if they were correlated with any other types of interesting features, like valley networks, clay or chlorite detections, or even just with the dichotomy boundary itself, which could have aligned with the edge of an ancient ocean. Since I have found polygonal ridge networks in other places near the dichotomy boundary, I was thinking that there might be a relationship between the hypothetical ancient ocean and the ridge networks. Indeed, there are many polygonal networks in shallow marine environments on the Earth, thought to be due to shrinking that happens when water is forced out of clay layers as they are pressed. Thanks to your efforts, we discovered that the Nili ridges are very localized along the dichotomy boundary, crowded into Nilosyrtis Mensae, Nili Fossae, and Antoniadi crater, but missing in Protonilus Mensae and further west along the dichotomy boundary. These means that something special must have been going on close to Nili Fossae. It could be that the ridges were only forming in this region, or perhaps we see a deeper exposure of the subsurface here, which allows the ridges to be exposed. One intriguing possibility is that the presence of the ridges is related to the availability of carbonate, which is a common ridge-forming substance in some terrestrial deserts (in the form of the mineral calcite). The Nili Fossae region is one of the only regions on Mars where lots of evidence for carbonate minerals has been found. Perhaps ground water circulation through fractures was happening all across Mars, but only in the area where there was CaCO3 in the water could the mineralization of these fractures take place. WE DON’T KNOW!
The next step for us to take is to study all of the great examples that you have found and to tie them to their geological context, both in terms of where they are with respect to the dichotomy boundary, but also how they relate to #darklines, glaciers, and other interesting things in the area. (I think I’ve seen enough of the area by now to say that they don’t seem to be related to glaciers).
I have been working with Meg over the last couple of weeks to get all of the data that you have collected into a usable format so that we can start to write the paper. The actual writing process will take a number of months.
Meanwhile, we decided to expand our search to a slightly different part of the Arabia region—Meridiani Planum.
Here is a map showing roughly where we have been looking (jagged gray area with a black background) in the context of the broader Arabia area. Arabia is an interesting place because it is very dusty (making it hard to see what minerals are there) and it has an unusual chemical signature (it has elevated hydrogen compared to other nearby places). The white area is where I have previously found ridges in Meridiani Planum (and the center of where we will be looking next).
If you think you spy a crater whose name sounds familiar, it could be because we’re getting closer to the territory that Mark Watney traversed in Andy Weir’s The Martian.
[Spoiler Alert]: In the book, Mark Watney has to traverse from the northern plains through Mawrth Valles (another popular landing site candidate!) to get to Schiaparelli Crater. You can chart his course here on this cool fan-made website: http://www.cannonade.net/mars.php#map
In 2004, the Opportunity Rover landed in Meridiani Planum. Its landing site was a wide, flat plain. In the 13 years since its landing, Opportunity has made some amazing discoveries, including the discovery of sedimentary rocks emplaced and modified by water, evidence that Opportunity’s landing site was close to the shoreline of an ancient, salty, shallow body of water. To the north of Opportunity’s landing site, Meridiani Planum becomes much less “plain-like”. Instead, it devolves into a tangle of arcuate, intersecting ridges. While it would be a nightmare for a small rover (or Mark Watney) to traverse, this kind of bizarre geomorphology is fascinating from a geological point of view. In particular, these ridge patterns are similar in shape and morphology to some of those shallow marine polygonal networks that I was looking for along the dichotomy boundary. On Earth, one such polygonal network can be seen an ancient shallow marine environment now exposed in Egypt’s white desert. The desert is white because of the expose chalk formations, and the entire area is criss-crossed with veins full of calcite and hematite.
The Meridiani ridges are similar to the ridges that we have been finding so far in that they are intersecting, but those of you who have been with the project for a while will see immediately how different they look.
While the Nili ridges were rather narrow, discrete, angular, and polygonal, the Merdiani ridges are feathered, arcuate, varying in width, and flat-topped. They also seem to merge together in multiple areas, like in this image, where there seems to discrete ridges but also amalgamations of ridges that form a kind of mesa.
The other strange thing about the Meridiani ridges is that they are not always the same color. For the most part, the ridge-forming unit is white and the background plain is dark, but sometimes it looks like the opposite, as in the above image.
We will keep you updated as work progresses on the first paper. Meanwhile, we will work on getting you some images of Meridiani Planum to map!
If you are bored while you are waiting, try looking along the dichotomy boundary the other direction… around the Isidis Basin and into Nepenthes Mensae. Maybe the ridges appear on either side of the Isidis Basin, and represent circulation of groundwater caused by the remnant heat of the Isidis impact……
Michael produced these great plots below showing the fan and blotches identified in each subject image showing 6 overlaping subjects. We have overlap to ensure that we don’t miss marking features at the edges of subject images. We had to cut up the HiRISE images into smaller chunks in order to get the resolution needed and make the Planet Four website as easy to use as possible.
Each color in the plot below represents a Planet Four subject image. The dashed blue lines are to show the overlap region boundaries and the solid blue lines are the boundaries between the subject images.
These are all from Ithaca where the fans tend to be very wide. It’s a very flat region on the Martian South Polar region, which might have something to do with it. So it’s one of our best cases to look at what we should do about combining the sources from different subjects in the overlap region.
Looking at these plots, we see fan directions aren’t impacted. That although in some cases we have to fans or two blotches on top of each other with different widths and extents, the direction of the source is well represented. By using shapely we’ll be able to deal with this. For the project’s first paper, we’re focused on wind directions so we’re calling the catalog done for now and will do the Shapely stage next for fan and blotch areas and counts.
We can now confidently turn your clicks into wind directions. This is a big milestone for the project. It means we get on to writing the second half of the Planet Four paper, talking about the catalog and what we see for wind directions. Onwards and upwards!
I thought I’d share a figure from last week’s science team call that the science team discussed. Michael was looking at combining clustered features with Shapely, a Python package for manipulation and analysis of planar geometric objects. Partly this is to investigate whether this could be used to deal with differing clusters in the overlap regions between neighboring subject images and also test out if we can use the software package to easily calculate the total area covered by the seasonal fans and blotches. Shapely does a good job of merging the blotches together as you can see from the figure below. This definitely looks like a way forward for calculating the total surface area per time of year covered in dark fan material.
Thanks to your help, we’ve finished search area two for Planet Four: Ridges. We’re working on analyzing the results and hopefully starting work on a paper based on those results. Laura has come up with a new region and slightly different type of polygonal ridge to search for. We’re working on getting that dataset processed and uploaded to the site. We hope to have this completed by the end of September with updated tutorials. We’ll keep you posted. In the meantime, Planet Four and Planet Four: Terrains could use some help if you can spare the time to classify an image or two.
We’re now working on dealing with the last major component of the Planet Four data processing pipeline, the overlap regions of neighboring subject images. We divide each HiRISE images into many smaller 840×648 pixel subimages or subjects that we show on Planet Four. To make sure we capture fans and blotches that are the edges of our subject images, we have a 100 pixel overlap between the neighboring left and bottom subject images. This means that we have duplicate markings that cover the same source which we need to identify as being the same source to allow for counting the number of seasonal sources and to also accurately measure the shape of very large fans or blotches.
We spent part of the last science call looking at some examples of overlap regions and the outputting fan and blotch shapes after clustering to decide what to do.
If you focus on the center sources in the two plots above, you see there are lots of markings identifying the same shape from the different subject images that contained varying parts of the central blotch or fan. Based on what we see, we think we if in the overlap region we only keep the largest source and anything that extends beyond that we will accurate identify the fan or blotch being marked. We’re going to test that this week and review the output from the catalog for a small portion of the overlap regions to confirm.
Once we sort what to do in the overlap regions, the focus should be writing all of the steps in the processing pipeline into the paper draft.
I’m pleased to announce that our first scientific paper for Planet Four: Terrains was accepted to the journal Icarus. Below is a snapshot from the top of the paper manuscript, and the paper is publicly available via the free preprint we’ve put online here.
A big thank you to all the volunteers who contributed to the publication. We acknowledge everyone who contributed to the project on the results page of the Planet Four at: http://p4tauthors.planetfour.org
The paper presents the first spider and swiss cheese terrain catalog derived from your classifications. 90 CTX images comprising ~11% of the Martian South Polar region southward of -75 N latitude were searched by Planet Four: Terrains volunteers. This comprised approximately 20,000 subject images reviewed on the Planet Four: Terrains website with 20 independent reviews. The P4: Terrains search coverage is shown below:
Applying a weighting scheme, we combine classifications together to identify spiders and swiss cheese terrain. The weighting scheme isn’t testing anyone, but it helps us find more spiders by allowing us to pay slightly more attention to those that are better at identifying spiders and help increase the overall detection efficiency of the project. Details can be found in the paper.
Using the weighting scheme each Planet Four: Terrains subject has a spider score which is the sum of the weights of the volunteers who identified spiders in the image divided by the sum of the weights of the volunteers who reviewed the subject image. Using classifications from Anya and I for a very small subset of the subject images, we found a spider scores above which we’re highly confident the identifications have few false positives.
To our surprise when we compared to the map of the secure spider locations to the geologic map of the South Polar region, we found araneiforms or spiders where we didn’t expect them to be. In previous surveys of the South Polar region, araneiforms were found to be located only on the South Polar Layered deposits (SPLD). The SPLD has been measured to have a height of ~4 km and covering a surface area of ~90,000 square kilometers mainly comprised of varying dust and water ice layers as well as some buried carbon dioxide and water ice deposits. Previous works have theorized that something about the unconglomerated nature of the SPLD, might make it easier for spiders to form there than other areas of the South Polar region
To confirm these identifications were real, we needed HiRISE imaging. CTX has a resolution of 6-8 m/pixel. HiRISE can resolve up to a coffee table on Mars with a resolving power of 30 cm/pixel. With HiRISE we could see the wiggly dendritic nature of the channels and as well see seasonal fans to confirm that these form via the carbon dioxide jet process. 8 areas outside of the SPLD were targeted last Summer and Fall by HiRISE.
Below are just a few examples of the HiRISE subframes of these regions off the SPLD:
There be spiders! Araneiform channels can clearly be seen in the images above. The HiRiSE images confirm the spider/araneiform identification. We also see seasonal fan activity as well. For the first time we have found spiders/araneiforms outside of the SPLD!
This result is exciting. For some of these areas we have sequences with HiRISE taken over time which we hope we can put it into Planet Four to measure how the fans sizes and appearance are different from their counterparts on the SPLD. Now we get a chance to study how these locales off the SPLD are similar or differ from the SPLD and try to learn why these areas and not others have spider channels.
We’ve only searched a small fraction of the Martian South Polar region. We have more images on the site to expand the search area to see where else spiders/araneiforms may be. Help us today by classifying an image or two at http://terrains.planetfour.org
Planet Four volunteer Peter Jalowiczor got asked a great set of questions after his public talk to his local Astronomical society. Below you’ll find the replies from Anya:
What evidence is there for cracking (of the ice) in the Martian surface?
We have actually observed the cracks in the seasonal ice layer appearing and evolving. We have seen them in multiple locations and several years. There is a paper on this:
Portyankina, G., Pommerol, A., Aye, K.-M., Hansen, C. J., & Thomas, N. (2012). Polygonal cracks in the seasonal semi‐translucent CO2 ice layer in Martian polar areas. Journal of Geophysical Research: Planets, 117(E2), DOI: http://doi.org/10.1029/2011JE003917. It has examples of observed cracks in spring from southern and northern hemispheres.
Was there certainty that the channels were not ridges? (Yes, this is an optical illusion!)
Yes, there is certainty. We know the direction of sun illumination on every image that HiRISE (or any other camera) takes. We can compare it to the locations of shadowed and illuminated sides of the channels. We have done it multiple times and it always fits to the assumption that those are channels not ridges.
Where does the blue colour come from?
This question has 2 parts:
1) Camera technology: the blue color is detected by the CCD that has a filter in front of it and thus only sensitive to blue part of visible spectra. In HiRISE we call it blue-green channel and highest sensitivity centered at 536 nm.
2) Light scattering by different materials: when light hits the surface of Mars, it is scattered differently by different material. Martian “soil” most efficiently scatters red light and makes Mars look brown-red. Fresh frost scatters rather efficiently most of sunlight spectral range, but particularly well in the blue part. This is true for both, CO2 or water frost. Thus, the blue patches in HiRISE images in polar regions are where CO2 or water ice lies at the top of the martian soil.
Is the North Polar region of Mars going to be investigated in the same way as the South polar region?
We would like to do that, given we get support and funding.
What height are the geysers?
We have not observed them in action, which means only theoretical estimates exist. For a jet that is constantly outgassing early in spring from underneath 1-m thick ice layer with a vent that is <1m2, the maximum estimated height is 70 m. If the pressure under the ice first builds up and then releases in eruption-style event, the height estimate is several times higher but highly uncertain.
Is there an imaging dataset, perhaps an experiment on a satellite, which could enable these heights to be measured more accurately?
Not currently. We are proposing for a small mission to be able to do just that.
How often do we return to each of the imaged areas? Surely there must be some follow-up to see how the features have developed.
We image every location several times per spring. Our main locations (Inca City, Ithaca, Giza, etc.) get up to 10 images per season. We also image them every summer when they are free of ice. Repeated imaging in summer is targeted to detect the changes in the araneiform structures, but it is very tricky goal, as the atmospheric and illumination conditions should be very similar in order to definitely detect any topography changes. Right now we have 5 martian years of observations but no certain detected topography changes.
Today we have a guest post by Planet Four volunteer Peter Jalowiczor.
As a Planet Four contributor for a number of years this blog is a description of a presentation I recently gave at my local Astronomical society. It was based, of course, on my P4 work and included a preliminary discussion on Mars before starting on the focus of the talk – this project. About two months earlier I had given a talk based on material provided by Meg so everyone was already familiar with the project as well as everyone involved. Here is how the second talk was structured.
Introduction to P4
P4 is a citizen science project designed to help planetary scientists identify and measure features on the surface of Mars, the images on the site are around the Southern polar region, an area of Mars that we know little about and the majority of which have never before been seen by human eyes!
Mars throughout the Ages
Following the introduction came a brief discussion of the history of observation from the first observations of planetary motion by Kepler and the first telescopic observations of Mars by Galileo, to maps (of Mars) by Christiaan Huygens, Giovanni Cassini on to Beer & Madler, Richard Proctor, Schiaparelli, Camille Flammarion and of course Percival Lowell and his famous Martian Canals. But, how was this all relevant to P4? The answer was that the HiRISE experiment on the MRO spacecraft was mapping areas around the South Pole…
Mars in the Solar System
How does Mars compare as a planet within the terrestrial group? Following a comparative overview of the five terrestrial bodies (including the Moon) in the inner Solar System. In this group, Mars is intermediate in its properties. For example, compared to the largest body (the Earth) and the smallest (the Moon) it displays features on its surface, which are Earth-like: evidence of river valleys, lakes and the likelihood of an ancient Ocean in the Northern hemisphere. Whereas in the Southern hemisphere, the landscape is more primitive and cratered, more Moon-like. A discussion into why this was so and the importance of the surface area to volume ratio and its effect on planetary evolution across the terrestrial group. Mars’ properties in its own right was briefed.
The Planet Four Project
A detailed description of the project starting with the HiRISE experiment. From an altitude of 200 to 400 kilometers above Mars, surface images are acquired containing individual, basketball-size (30 to 60 centimeters) pixels, allowing features 1.2m to 2.4m across to be resolved. Large swaths of the surface are imaged: 20,000 x 126,000 pixels and the image is broken up into individual P4 tiles.
Citizen scientists mark dark fans and dark blotches that appear and disappear during Spring/ Summer around the South Pole of Mars. The dark fans and blotches appear in the Southern spring when the ice cap begins to thaw and sublimate back into the atmosphere. The fans and blotches then disappear at the end of the summer when there is no more ice left. How do these fans form, how they repeat from Spring to Spring? What does this tell us about the surface winds on the South Pole? How would these features enable scientists to build-up a global map of wind-directions on Mars.
30 images (tiles) were shown from categories such as: #spiders, #fans, #blotches, #yardangs, #dendritic features, #Mars-has-the-blues…
Results of work carried out by myself. Occasionally, during classification, the #measurement tag was used by myself to measure various features within the tile. This started out as no more than an intention to log the size of features. After a few years a few hundred measurements had accumulated.
I found that:
- This was biased towards the measuring of blotches.
- 31-40m was the modal, or most commonly occurring size of blotch within the tile.
- 10m-90m – fan sizes were grouped in this region.
- Where the blotch, spider or fan went outside the range, an attempt was made to estimate its size.
- The images of the tiles here ranged from ~150m x 200m to 366m x 475m.
However, the constraints of the tile parameters mean that the sizes may not be a true reflection of reality; and this was a simple experiment in science, The results could be improved with a larger sample size (more measurements) particularly if measurements taken outside the tiles.
This is based on questions/ discussions, which came up both during and after the lecture. Some I answered such as the linear venting (to the question below) as it was asked before the relevant slide came up! For some of the other questions I gave partial answers and we agreed that this should be forwarded on to Meg, and the Planet Four Science Team is going to answer these in a following blog post.
- What evidence is there for cracking (of the ice) in the Martian surface?
- Was there certainty that the channels were not ridges? (Yes, this is an optical illusion!)
- Where does the blue colour come from?
- Is the North Polar region of Mars going to be investigated in the same way as the South polar region?
- What height are the geysers?
- Is there an imaging dataset, perhaps an experiment on a satellite, which could enable these heights to be measured more accurately?
- How often do we return to each of the imaged areas? Surely there must be some follow-up to see how the features have developed.
The talk started with the good news of a certain Sagan Medal recipient and ended with the news that there was now an alternative to Mars bars with Martian cake, however this had to be refrigerated at Martian polar temperatures…
Now that we have frozen development of the clustering algorithms for both blotches and fans and reviewed the stage of combining the different types of clusters together, we are working on the next issue. This is dealing with markings from the overlap regions of Planet Four subject images. For most Planet Four subject images, there is 100 pixel overlap with a neighboring subject image. So in our current catalog we have duplicates possibly of seasonal fans and blotches that appeared in more than one Planet Four subject image.
We’ve started to look into what’s the best wave to deal with this. The main reason we have the overlapping regions between adjacent Planet Four subject images is to make sure we identify seasonal fans that would get cut off between the two subject images if there was no overlap. The overlap should ensure at least one of the subject images has full view of the source, so we don’t miss anything.
Here’s an example of four adjacent subject images combined and the clustered markings drawn on.
You can see that in the current catalog we get two overlapping blotches with slightly different orientations and centers generated from combining the volunteer classifications. If you look at what is in our catalog for each subject image that makes up the ensemble, you see that for one of the subject images the blotch is only partially on the image. The resulting blotch marker from the combined classifications is also partially on the image, it extends beyond. We might be able to use this fact that people extended the drawing markings to fit the shape if the source went over the edge, to identify those seasonal fans and sources that extend beyond the extend of the subject image and when that happens use the catalog entry from the overlapping subject images where the source was fully in the subject image. We’re testing this hypothesis by performing a manual review in the next week or so of the catalog output of a sample of overlap regions.