By Simon Buckley, VOG Group
It has been absolutely fascinating to follow the landing of the Mars 2020 Perseverance rover and its mission startup over the last couple of weeks. What spectacular footage – the culmination of years of meticulous science and engineering to safely land the rover on the surface of another planet.
Vista across the Jezero crater landing site, composed and rendered in LIME.
This feat has inspired us to look at the back catalogue of Mars missions and spatial data that has been collected and made available over the years. In honour of the month of Mars, we’ve prepared an exploration of the Martian landscape – in LIME – which we hope does justice to the amazing datasets and surface topography. All data is credit NASA/JPL/University of Arizona. Composed and rendered in LIME: no Photoshopping applied.
Incredibly, Mars has been mapped in sometimes greater detail than our own planet Earth. Thanks to open data policies a vast amount is available to us to use in science projects or even just to browse and wonder at the richness of topography, geomorphology and geology exposed on the vegetation-free surface of Mars. The main data sources we’ve compiled here come from the Mars Reconnaissance Orbiter (MRO), launched in 2005, which hosts the Context Camera (CTX) and High Resolution Imaging Experiment (HiRISE) sensors. In addition (in case any more acronyms are needed😀), we pulled in the Mars Orbiter Laser Altimeter (MOLA) global terrain dataset acquired on the earlier Mars Global Surveyor mission. All are available from the USGS Astrogeology Science Center and the University of Arizona’s Lunar & Planetary Laboratory websites.
Let’s first take a look at the Jezero crater landing site for the Perseverance rover. This has been imaged in stereo with both CTX and HiRISE cameras and processed to generate digital terrain model and orthoimage mosaics. The CTX imagery gives the wider area context (6m ground sample distance in the orthoimage and 20m DEM spacing) for the science teams to assess, before they target the HiRISE sensor on areas of high interest to be captured at a stunning 0.25m ground sample distance (making for 1m DEM spacing). See Fergason et al. (2020) for more details on the imaging sensors and product generation.
Since LIME v2.0, it is straightforward to import digital elevation models into your project. Remember to change the file filter on the Import 3D Model dialog to show elevation data types, then crop the DEM and change the sampling as desired (we used 1/4 or 1/16 resolution for the HiRISE DEMs). The DEMs are loaded as any other 3D model in LIME, and they can be inspected using the model Display Settings. Here we visualise the Elevation of the Jezero landing site (1.3x vertical exaggeration):
Just look at those craters, dry valleys and the deltaic fan feature suggesting the earlier presence of water on Mars (Goudge et al., 2017) and focus of the Mars 2020 mission. The Perseverance landing site is close to the bottom right “finger” of the fan. Now looking at the fan from the west:
The same view but visualising the surface slope to better enhance the surface topography (including sand dunes in the large crater and stacked against cliffs and gullies):
Using the default LIME terrain colour ramps just felt wrong – far too Earth-like to represent the tans and rust colours of the Red Planet! As a result, the team has added a selection of colour ramps that really evoke the Martian terrain, inspired by this great overview of hypsometric tints for Mars [and after reading the cited paper by Crameri et al. (2020), it is looking like the rainbow ramp will get kicked further down the list in future LIME releases…].
Next, we drape the orthoimages onto the respective DEMs using the Add Overlay Texture tool (with type georeferenced image). One current limitation is that LIME expects a georeferenced image to have an accompanying world file, while the original data stores the georeferencing information in the image header itself. It’s therefore necessary to resave the image with a world file in a GIS or using GDAL (i.e. gdal_translate with option -co “TFW=YES” for tif files) – something we aim to optimise in the future (Update Oct 2021: this is no longer necessary in LIME v2.3).
HiRISE orthoimage overlayed on DEM: Jezero crater, Mars.
The greyscale orthoimages draped on the terrain models give a good impression of the Jezero crater. But how to make it look more like Mars? This is where we push LIME to the limits and take advantage of the highly customisable 3D model and texture visualisation settings to make renderings inspired by artists such as Kevin Gill!
Looking over the deltaic fan feature from the north.
To get to the above image, we take the base CTX/HiRISE terrain and orthoimage overlay data as the starting point, spending a bit of time to fly around to find an interesting view. Here I was after a vantage point close to the surface to give an impression of a wide Martian vista. Vertical exaggeration is tweaked slightly to some add some extra relief.
The orthoimages have already been radiometrically corrected, and natural lighting is captured. However, I found the shadows quite harsh, so dynamically adjusted the overlay brightness and contrast to give a softer effect. I also wanted to enhance the panchromatic imagery with colours more appropriate to Mars. To achieve this a few different effects were applied:
The last thing to do was to create a more realistic horizon than the standard LIME 3D scene background. Again, some creative use of Panels was in order. Here we create a large vertical Panel aligned roughly with the southern edge of the 3D model. The material was set to a darker sky colour picked from a surface panorama captured by Perseverance. Next, the 3D scene background colour was set to a lighter sky colour picked from the same panorama. Finally, the weighting sliders (found under the Display Settings – 3D Model – Advanced tab) of the Panel were used to create soft transparency gradients to give a realistic horizon.
The result can be saved to a Custom View in the project, so it is easy to go back to at any point with the correct viewport and display settings configuration. And of course, we can make a high-resolution screen capture of our Martian rendering.
Changing the 3D view to look down on the fan.
Another scene of the Perseverance landing site, this time changing the single mesh material colour to terrain elevation.
Orthographic (map) view of blended elevation data/orthoimagery, overlayed with transparent contours and 2500m grid lines.
HiRISE data from Deuteronilus Mensae, showing some cool geomorphology - grooved lobate flow features potentially indicative of subsurface ice flow.
Finally, the global MOLA DEM rendered in LIME (with 2000km grid overlay) and taking advantage of the new colour ramps to highlight the north/south divide to give a “flooded Mars” look.
Ready to find out more about the growing capabilities of LIME? Your starting point is here.
Crameri, F., Shephard, G.E. & Heron, P.J., 2020. The misuse of colour in science communication. Nature Communications, 11, 5444. https://doi.org/10.1038/s41467-020-19160-7
Fergason, R.L., Hare, T.M., Mayer, D.P., Galuszka, D.M., Redding, B.L., Smith, E.D., Shinaman, J.R., Cheng, Y., Otero, R.E., 2020, Mars 2020 Terrain Relative Navigation Flight Product Generation: Digital Terrain Model and Orthorectified Image Mosaics, 51st Lunar and Planetary Science Conference.
Goudge, T.A., Milliken, R.E., Head, J.W., Mustard, J.F. and Fassett, C.I., 2017. Sedimentological evidence for a deltaic origin of the western fan deposit in Jezero crater, Mars and implications for future exploration, Earth and Planetary Science Letters, 458: 357-365. https://doi.org/10.1016/j.epsl.2016.10.056
LIME 2.3.2 is now available for download!
LIME 2.3.1 is available for download now.
Prof John Howell (VOG Group and U. Aberdeen) will hold a half-day workshop on virtual field trips (VFTs) at BSRG 2021...