Post by Joshua E. Hedgepeth, PhD student, Centre for Planetary Science and Exploration, University of Western Ontario, Canada.
Discovered by Huygens in 1656 (Huygens, 1656), the surface of Titan was obscured by its atmosphere for centuries (Campbell, 2003; Smith et al., 1996; Muhleman et al., 1995, 1990; Kuiper, 1944). In 2004, we finally obtained high resolution images of the surface with Cassini RADAR. Cassini was equipped with a Ku-band (2.17 cm λ) radar instrument with 5 beams for collecting data (Elachi et al., 2004; Stofan et al., 2012). The long wavelength band was able to penetrate Titan’s thick haze to perform radiometry, scatterometry, altimetry and synthetic aperture radar (SAR) imaging of the surface. The SAR mode captured the highest resolution images of the surface of Titan, as high as 175 meters per pixel (Elachi et al., 2004; Lopes et al., 2010). While this may not be as high resolution as the images we have of other moons, it is high enough to obtain some spectacular images. In this blog we have a detailed look at the craters on the surface of Titan.
Image 1: Selk crater (D=84 km) located at 199.1, 6.9 latitude and longitude. The dark crater floor is representative of the smooth material, and around it is the bright rough ejecta material.
Here we see a beautiful mosaic of Selk crater (D~84 km), one of Titan’s most pristine craters (Image 1). Despite the lower resolution, RADAR provides more details than traditional images. Radar transmits radio waves that are returned as an echo to study the surfaces of different planets (Moreira et al., 2013; Neish and Carter, 2014). A perfectly smooth surface deflects the signal away from the receiver, and a rough surface scatters the signal, sending more data back to the receiver. This translates to brighter pixels where the surface is rougher and darker pixels where the surface is smoother. This is evident in the picture of Selk crater. The dark interior is composed of small sand grains that make up of the giant sand dunes that wrap around Titan’s equator (Barnes et al., 2015). You can see these in the bottom of the image in what look like scratches on the image as well as thin lines on the crater floor. Overall, the crater is still pristine (Hedgepeth et al., 2018; Neish et al., 2018), and the bright ejecta is still clearly observable, barely even obscured by the encroaching sand dunes.
Image 2: Left. Selk crater (D~84 km), one of Titan’s most pristine craters. Right. Soi crater (~85 km), one of the most degraded craters on Titan.
Selk is a rarity because Titan’s surface is undergoing significant modification (Hedgepeth et al., 2018; Neish et al., 2016, 2013). Sand dunes, dunes slowly infill Titan craters, but fluvial erosion by methane rain also acts to erode its ejecta and elevated rim. Titan’s craters constrain how much modification Titan is undergoing to age the surface and measure the level of degradation (Hedgepeth et al., 2018; Neish and Lorenz 2012). Titan has an observed 90 possible craters that gives an estimated surface age of ~200 Ma to 1.0 Ga. Comparing pristine craters on similarly sized icy moons (e.g. Ganymede) with Titan’s degraded craters reveal how much erosion and infill is occurring on the surface. Soi is an example of one of Titan’s most degraded craters (Image 2). Not only is the floor almost completely infilled by the larger material in the terrain, the rim and ejecta are far less obvious than with Selk crater. These two craters demonstrate the level of degradation that Titan’s craters experience by infill and erosion. Furthermore, the degradation of Titan’s craters speaks to the degree that erosion and infill plays in modifying Titan’s surface.
Further reading:
Barnes, J.W., Lorenz, R.D., Radebaugh, J., Hayes, A.G., Arnold, K., Chandler, C., 2015. Production and global transport of Titan’s sand particles. Planetary Science 4. https://doi.org/10.1186/s13535-015-0004-y
Campbell, B.A., 2012. High circular polarization ratios in radar scattering from geologic targets. Journal of Geophysical Research: Planets 117, n/a-n/a. https://doi.org/10.1029/2012JE004061
Elachi, C., Allison, M.D., Borgarelli, L., Encrenaz, P., Im, E., Janssen, M.A., Johnson, W.T.K., Kirk, R.L., Lorenz, R.D., Lunine, J.I., Muhleman, D.O., Ostro, S.J., Picardi, G., Posa, F., Rapley, C.G., Roth, L.E., Seu, R., Soderblom, L.A., Vetrella, S., Wall, S.D., Wood, C.A., Zebker, H.A., 2004. Radar: The Cassini Titan Radar Mapper. Space Science Reviews 115, 71–110.
Hedgepeth, J.E., Neish, C.D., Turtle, E.P., Stiles, B.W., 2018. Impact Craters on Titan: Finalizing Titan’s Crater Population, in: Titan Is Terrific. Presented at the 49th Lunar and Planetary Science Conference, Lunar and Planetary Institute, Woodlands, Texas.
Huygens, C., 1656. De Saturni luna observatio nova. In Opera varia.
Kuiper, G.P., 1944. Titan: a Satellite with an Atmosphere. Astrophysical Journal 100, 378.
Lopes, R.M.C., Malaska, M.J., Solomonidou, A., Le Gall, A., Janssen, M.A., Neish, C.D., Turtle, E.P., Birch, S.P.D., Hayes, A.G., Radebaugh, J., Coustenis, A., Schoenfeld, A., Stiles, B.W., Kirk, R.L., Mitchell, K.L., Stofan, E.R., Lawrence, K.J., 2016. Nature, distribution, and origin of Titan’s Undifferentiated Plains. Icarus 270, 162–182. https://doi.org/10.1016/j.icarus.2015.11.034
Moreira, A., Prats-Iraola, P., Younis, M., Krieger, G., Hajnsek, I., Papathanassiou, K.P., 2013. A tutorial on synthetic aperture radar. IEEE Geoscience and Remote Sensing Magazine 1, 6–43. https://doi.org/10.1109/MGRS.2013.2248301
Muhleman, D.O., Grossman, A.W., Butler, B.J., 1995. Radar Investigation of Mars, Mercury, and Titan. Annual Review of Earth and Planetary Sciences 23, 337–374. https://doi.org/10.1146/annurev.ea.23.050195.002005
Muhleman, D.O., Grossman, A.W., Butler, B.J., Slade, M.A., 1990. Radar Reflectivity of Titan. Science, New Series 248, 975–980.
Neish, C.D., Carter, L.M., 2014. Planetary Radar, in: Spohn, T., Breuer, D., Johnson, T.V. (Eds.), Encyclopedia of the Solar System. Elsevier, Amsterdam ; Boston, pp. 759–777.
Neish, C.D., Lorenz, R.D., 2012. Titan’s global crater population: A new assessment. Planetary and Space Science 60, 26–33. https://doi.org/10.1016/j.pss.2011.02.016
Neish, C.D., Lorenz, R.D., Turtle, E.P., Barnes, J.W., Trainer, M.G., Stiles, B., Kirk, R., Hibbitts, C.A., Malaska, M.J., 2018. Strategies for Detecting Biological Molecules on Titan. Astrobiology 18, 571–585. https://doi.org/10.1089/ast.2017.1758
Neish, C.D., Molaro, J.L., Lora, J.M., Howard, A.D., Kirk, R.L., Schenk, P., Bray, V.J., Lorenz, R.D., 2016. Fluvial erosion as a mechanism for crater modification on Titan. Icarus 270, 114–129. https://doi.org/10.1016/j.icarus.2015.07.022
Smith, P.H., Lemmon, M.T., Lorenz, R.D., Sromovsky, L.A., Caldwell, J.J., Allison, M.D., 1996. Titan’s Surface, Revealed by HST Imaging. Icarus 119, 336–349. https://doi.org/10.1006/icar.1996.0023
Stofan, E.R., Wall, S.D., Stiles, B.W., Kirk, R.L., West, R.D., Callahan, P.S., 2012. Cassini RADAR Users Guide. NASA and Jet Propulsion Lab.
Planetary Geomorphology Image of the Month 