A Piece of Mars: Gray dunes have migrated over reddish rock, moving toward a narrowing cleft surrounded by tall tan cliffs. Bright lines on the dunes are exposed internal layers (bones of the dunes, really) that show you where the lee-side slopes once were (so you can tell they’ve moved to the left). The cliffs are made of layered rocks (extra points if you can find the fault), suggesting these are sedimentary layers, laid down long ago in Mars’ geologic past. The whole HiRISE image is worth a long look, it’s really amazing. (HiRISE ESP_049009_1520, NASA/JPL/Univ. of Arizona)
In May 2016, Michael Gillon and his team announced the discovery of three Earth-sized exoplanets around TRAPPIST-1, an ultra cool M-dwarf star, using the small TRAPPIST telescope at ESO-La Silla, Chile. It was an exciting discovery—yet on that day no one could possibly have imagined that less than a year later they would make another significant discovery involving the same system. But here we are: today, they announced in Nature the discovery of seven potentially habitable Earth-like worlds.
The star, named TRAPPIST-1, is a fairly inconspicuous star in our Milky Way. Small (8% the mass of the sun) and cold (half the temperature of the sun), it is a member of an ultra-cool dwarf population that represents 15% of the star population of our galaxy. In 2016, Gillon and his team detected the transit (i.e., the shadow of a planet passing between its host star and us) of three exoplanets at the inner edge of the habitable zone of their star.
A Piece of Mars: This is the crest of one of the largest dunes on Mars (0.5×0.5 km or 0.31×0.31 mi). The wind mostly blows from the right, slowly pushing sand up the windward slope. But frost accumulates on (and probably in) the sand during winter, and sometimes it gets too heavy and slides down the steepest slope (toward the left), carving out big gullies in the sand. And then the wind blows some more, trying to erase the gullies by 1) making ripples, 2) burying the gullies (the featureless blue patches are grainfall, which is a fancy term for sand that fell as airfall), and 3) forming dust devils that leave faint but wide tracks. Who wins this fight, wind or ice? Neither: gravity wins (it usually does). (HiRISE ESP_020876_1330, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Is this 480×270 m (0.3×0.17 mi) scene showing a 150 m (492 ft) wide yin-yang symbol on Mars? Sort of, maybe, if you blur your eyes and lend me artistic license, but it’s not doing so intentionally. One side of the crater is dark and the other is light. Both have their tone because of windblown material blown from the same direction, but the different materials collected where they did for different reasons. The dark material is probably mafic sand (iron and magnesium-rich, like what’s found near many volcanoes), which was bounced along the ground from the lower right, and collected in the lee of the crater rim. The bright material is much finer-grained, dust carried aloft, and it probably settled down on the far side of the crater, and outside as well, as the crater rim poked into the wind and provided enough shelter to let some of the bright material settle out as airfall. (HiRISE ESP_016496_2000, NASA/JPL/Univ. of Arizona)
A Piece of Mars: The focus of this 0.96×0.96 km (0.6×0.6 mi) scene is one of many two-faced dunes on Mars. The bright sunlit slope is one face, formed recently by wind blowing from the upper right. The dark shaded slope is the other face – it’s a little older, formed by wind blowing from the left. Together these two winds alternate, probably in different seasons, forcing the sand into a needle-shaped point that carries sand in a direction that is, give or take, the sum of those two winds. Two-faced dunes like this are rare on Earth, as winds here typically quickly erase older crestlines. (HiRISE ESP_021716_1685, NASA/JPL/Univ. of Arizona)
A Piece of Mars: How do you tell when a planetary landscape shows Mars, instead of Mercury or the Moon or Europa? The easiest way to tell is to look for both craters and dunes, like what’s shown here in this 640×360 m (0.4×0.22 mi) scene. Not all martian landscapes have either feature, and there are some other worlds that do have both (Earth, Titan, maybe Pluto, and probably Venus but we need better data…), but it’s a pretty good bet that if you see both features together, you’re looking at Mars. Anyway, in this lovely view, the dark gray terrain (you’ll see boulders if you look closely enough!) is being eroded away slowly, revealing a much older, brighter surface beneath it. Unfortunately for those who would study ancient terrains on Mars, much of that older, lower surface is covered in dunes. But I like the dunes – they give us information about surface erosion rates and wind patterns. One person’s signal is another person’s noise. (HiRISE ESP_047762_1585, NASA/JPL/Univ. of Arizona)
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)