A Piece of Mars: Sometimes in the floors of small craters, the wind blows in from several directions to produce odd polygon-shaped dunes that look like crochet (maybe Mars is making sweaters for its craters – it is, after all, a cold place). This “sweater” segment is 480×270 m (0.3×0.17 mi) in size (the “stitches” are ~20 m, or 66 ft, across). The smaller interior lines are younger windblown features, that are superposed on the larger structures – their alignment is strongly controlled by the topography of the larger polygonal “stitches”. (HiRISE ESP_017833_1975, NASA/JPL/Univ. of Arizona)
Using a combination of space telescope data, as well as recent data acquired with the SOFIA Airborne telescope and lab experiments, a team of astronomers including researchers from the SETI Institute and Jet Propulsion Laboratory have revealed the presence of dust of exogenic origin at the surface of dwarf planet Ceres. This contamination likely stems from a dust cloud formed in the outer part of the main belt of asteroids following a collision in recent times. That study challenges the relationship proposed between Ceres and asteroids in the C spectral class and instead suggests an origin of this dwarf planet in the transneptunian region. This study was published on January 19 2017 in Astronomical Journal.
A Piece of Mars: Get out your 3D blue/red glasses (or look here for a 2D version if you can’t find them). This is a 3.2×1.8 km (2×1.13 mi) scene showing dark dunes carving lanes 50-70 m (165-230 ft) deep into a stack of brighter sedimentary layers. Over time, the sand wears down the rock into yardangs, the elongated remnants of rock the sand didn’t manage to reach. Here we see the process ongoing; perhaps in a few million years there will be nothing left but a few streamlined peaks. Those murdering basterds [sic]. (HiRISE ESP_034419_2015, NASA/JPL/Univ. of Arizona)
The tortoise: The rippled surface at the top is high ground: the top of a dune. Wind pushes the ripples toward a steep sunlit slope, creating long thin, dark avalanches that slowly inch the slipface forward. At the bottom of the slope, which is shielded from winds blowing from the top, ripples have been formed by wind blowing from the left.
The hare: Oblivious to both the slow progression of ripples and dunes, 5-25 m wide dust devils have blazed on by, leaving behind erratic trails.
(HiRISE ESP_048592_2070, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Mars rarely does anything without drama. Long ago in this 0.96×0.54 km (0.6×0.34 mi) scene, large ripples formed and then, presumably, lithified (turned into rock). Some time after that, an impact formed the crater in the center, throwing debris into an ejecta blanket that covered the lithified ripples. That ejecta blanket sat around long enough to acquire some smaller impact craters of its own. Since then, most of that ejecta blanket has eroded away, exposing the ripples to view once again. (HiRISE ESP_011699_1910, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Nicholas Steno was a 19th century geologist, who came up with some principles that are still used today to guide interpretation of exposed sedimentary rocks. The principles seem a bit obvious, but then some of the most profound principles can be like that. Emily Lakdawalla of the Planetary Society describes them in more detail here, with really good examples. You can use these principles to do forensics on a landscape, to see what happened and when.
You can see all three principles at work in this image.
#1: Stuff makes horizontal layers. (This isn’t always true, e.g., dunes and deltas make tilted layers, but most sediments pile up into flat, horizontal layers.) You can see that at work here: A thick layer of dark gray stuff once piled up on a flat surface of brighter stuff. Some of the dark gray stuff has since eroded away, but you can see that both the gray and the brighter stuff originally piled up in flat-lying layers.
#2: Older stuff is at the bottom. (Because newer stuff buries the older stuff, like the papers on my desk and the veggies in my fridge.) In this image, the brighter stuff must be older than the darker gray stuff, because the bright stuff is on the bottom.
#3: You can’t see the layers until they’re exposed by erosion or tectonics. (Because they’re buried. So if you see layers, you know something has happened so you can see them.) You can see the edges of the dark gray stuff, so you know it’s been partially eroded away – otherwise you’d never know the underlying bright stuff was ever there. Some of the material from the dark gray layer has been reformed into dark bedforms on the brighter layer, and those bedforms are probably the youngest features in this scene.
What I like most about this image has to do with yet another principle of layered stuff: Things that cut across other things are younger. Things that have been cut across are older (Like if you chop down a tree, then the axe cuts on the tree trunk must have been made after the tree itself grew. Duh, right?). You can see that in this image: on top of the dark gray layer are some old bedforms. They must be quite old, even cemented or lithified (turned into rock that the wind can’t easily move), because they’ve been cut by erosion at the edge of the gray layer. So not only was the gray layer once more extensive, but it had ripples on it, and those ripples formed and became immobile before that erosion ever happened.
(HiRISE ESP_030460_1525, NASA/JPL/Univ. of Arizona)
A Piece of Mars: A low, broad dune occupies the center of this 800×450 m (0.5×0.28 mi) scene, blown by a dominant wind towards the lower left. The slip face on the lee side has several small avalanches, formed as the slope oversteepens (this is how dunes crawl along the surface). Upwind, among other fainter lines, is a prominent bright line: it is a former slip face of this dune, possibly formed from a thick accumulation of bright dust (maybe there was a big dust storm that year). Farther upwind, another dune slowly approaches. (HiRISE ESP_033955_2065, NASA/JPL/Univ. of Arizona)
I co-organized a session for the AGU 2016 meeting entitled “P42A: Solar System Small Bodies: Asteroids, Satellites, Comets, Pluto, and Charon“. Below the info on the session and the schedule.
We have three invited talks that will describe the New Horizons data of Charon, color of Kuiper Belt Object from a ground-based survey and a theoretical study of the formation of the asteroid belt.
Abstract: The composition and physical properties of Small Solar System Bodies
(SSSBs), asteroids and dwarf planets, remnants of the formation of planets, are key to better understand our solar system. Increased knowledge of their surface properties and their potential as resources are also necessary to prepare for robotic and human
exploration. Hints about the internal structure and composition of SSSBs
have been acquired recently thanks to flyby/rendezvous data from space
missions, study of complex multiple asteroid systems, or close encounter
between asteroids. In this session we will discuss results bringing
information on the internal structure and composition of SSSBs based on
space and ground-based data, numerical models, as well as instrument/mission
concepts in the prospect of future exploration. (more…)
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A Piece of Mars: Higher ground is to the left. You’re seeing a tan layer sandwiched between two gray layers in this 0.96×0.54 km (0.6×0.34 mi) scene. Large ripples have accumulated in the lowest area to the right, which is the floor of an old river channel. Ripples have also formed on the gray upper layer. But not the middle tan layer – maybe it’s too fine-grained to erode into sand grains, or maybe it erodes too slowly to allow any eroded sand grains to pile into ripples before they’re blown away. (HiRISE ESP_048196_1995, NASA/JPL/Univ. of Arizona)
AGU Fall meeting is starting tomorrow. I co-organized a session entitled “Detection and Direct Imaging of Habitable Exoplanets: Progress and Future” to discuss the potential of new and future facilities and modeling efforts designed to detect, image and characterize habitable exoplanets, studying their formation, evolution and also the existence of possible biospheres. Topics that are covered in this session include signs of exoplanet habitability and global biosignatures that can be sought with upcoming instrumentation; instrument requirements and technologies to detect these markers; strategies for target selection and prioritization; and impacts of planetary system properties, ground-based and space telescope architectures.
We have two invited talks, one by George Ricker on TESS and a second one by Shawn D Domagal-Goldman on HabEx, two NASA missions that could play a major role on identification and characterization of Earth-Like exoplanets.
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