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Steno’s principles, or “how to make sense of pretty landscapes”

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)

The trail of a dune

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)

An AGU 2016 Session on Solar System Small Bodies

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…)

Them that make ripples and them that don’t

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)

Fall AGU meeting: Detection and Direct Imaging of Habitable Exoplanets

screen-shot-2016-12-11-at-10-51-19-pmAGU 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.


Martian spiders

A Piece of Mars: Martian spiders, or araneiforms, are geological structures found at high latitudes on Mars. The dark splotch with branching arms in this 0.48×0.27 km (0.3×0.17 mi) scene is a good example. They form in the springtime, when bright frost still covers a darker sandy soil, but some sunlight filters through the frost to warm the underlying surface. Sublimation of gas (under the frost but just above the soil) creates enough pressure that little explosions occur like dry geysers, punching through the frost and blowing up sand that then falls back to the surface as a dark splotch. If the wind is blowing when this happens, then the dark splotch is carried a ways downwind, but that hasn’t happened in this case. (HiRISE ESP_048189_0985, NASA/JPL/Univ. of Arizona)


A Piece of Mars: The dunes climbing over a rocky surface in this 0.96×0.54 km (0.6×0.34 mi) scene are mostly yellow because they’re covered (and therefore kept immobile) by dust. The crest of one dune, though, shows recent activity: dark sand has been pushed by the wind up the lower right side, and then shot (cannonball-style) over the brink, where it slowly piles up on the upper left side. This pileup is called grainfall, because that’s what the sand grains have done here (rather than sliding downhill, avalanche-style, which is called grainflow). There’s a dune on the left side of the image that hasn’t experienced this activity, maybe because it’s a little more sheltered from the wind. (HiRISE ESP_047779_1655, NASA/JPL/Univ. of Arizona)

Bedforms on crater rims

A Piece of Mars: Dunes and ripples most commonly form in topographic lows. But not in this 0.96×0.54 km (0.6×0.34 mi) scene. Here, and in other places on Mars, these bedforms (called TARs) form on plains, and sometimes appear to cling to the rims of craters – which are topographic highs, not lows. It’s not clear how this happens: Does the topography of the crater rim provide a wind shadow that allows windblown sediment to accumulate there? Or was there simply more loose material on the crater rims to begin with, allowing these things to form in place? I’m open to suggestions. (HiRISE ESP_047787_1910 NASA/JPL/Univ. of Arizona)

Shadows behind boulders

A Piece of Mars: Bright material (either dust or sand) has accumulated in the lee of wagon- to car-sized boulders in this 0.96×0.54 km (0.6×0.34 mi) scene. It’s perhaps something like the Rocknest sand shadow that Curiosity visited a few years back. The wind blows from lower right to upper left, carrying along sediment that occasionally gets trapped in the protected areas behind the boulders. These sand shadows aren’t very thick, as the underlying texture (polygonal terrain!) is visible through them. (HiRISE ESP_047798_1150, NASA/JPL/Univ. of Arizona)

Let’s end this political hangover. It is time to fight


It was a tough night … one in which any dreams we may have had of “American exceptionalism” were crushed.

Instead of serving, as it has so nobly, for more than two centuries as a beacon of hope and light to people everywhere, this nation will instead see our first African-American president hand the keys to the White House to a low-grade reality TV star who is endorsed by David Duke and the KKK, “alt-right” crazies, and American Nazis. To the horror of many of us, our next president will be someone who explicitly rejects science, reason, and the values we all hold dear—values that are our only hope for moving this country and the world forward. (more…)