Starship Asterisk*

Paul Geissler wrote:

---

Dunes and Ripples in Valles Marineris (ESP_025297_1730)

Two types of wind-blown sedimentary deposits are pictured in this scene of the floor of Ius Chasma in the Valles Marineris.

Smaller light toned ridge-like ripples draped over the bright bedrock are visible in this subimage. Long dark sand dunes superpose the small ridges, indicating that the sand dunes formed more recently than the ridges. Earlier observations have shown that the small ridges, known as "transverse aeolian ridges" or TARs, appear to be fixed in place. They are typically found near the equator of Mars, in places where the winds alternate in direction over daily or seasonal cycles.

Detailed investigations by the MER rovers revealed that the TARs were armored by a surface layer of coarse granules left behind after the smaller particles were winnowed away by the wind. These granules are too large to be lifted by the wind and preserve the TARs from further erosion. In contrast, recent HiRISE observations show that many of the dark sand dunes on Mars are actively on the move.

From these facts, we can hypothesize the recent history of the aeolian deposits here in Ius Chasma. Unconsolidated sediments were formed in the canyon by impact fragmentation and erosion of the steep canyon walls. These sediments were eroded and shaped into dunes by the winds that blow up and down the canyon, alternating direction between day and night. Eventually the tireless winds won the battle over sediment supply, and the remaining sediments were sifted into the TARs visible in the image, oriented perpendicular to the length of the canyon (transverse to the winds).

More recently, a fresh supply of sand was introduced into the canyon that the winds have not yet had time to tame. Instead of forming simple crescent dunes, the bidirectional winds have shaped the sand into long linear dunes punctuated by short slip faces. At high resolution, we see that the linear sand dunes are ruffled along their lengths by ripples transverse to the wind directions. Ultimately, most of this sand will be swept away, leaving the TARs trapped behind.

Serina Diniega wrote:

---

Looking for an Impact Crater on a Dune (ESP_025481_1385)

This dune field image was requested since a prior lower-resolution (THEMIS VIS) image of this area had suggested the possible presence of an impact crater on the dunes. Finding an impact crater on a dune field would be quite important: as of 2011, no craters have been found on Martian dunes, which strongly supports the hypothesis that the dune fields are very young features and either formed or have actively evolved during the last tens of thousands of years.

No crater is visible in this dune field image, which is consistent with the "active" and "fresh" appearance of these dunes: (1) they have sharp crestlines and (2) slipfaces (the downwind slope; generally pointing towards the south-southwest in this field, although there are some signs of a reversing slipface, implying that some past winds have pushed sand towards the northeast-north northeast) are generally very smooth in appearance, except for some small avalanche features (which form as sand accumulating at the upper-portion of the slipface).

The search for a dune-field crater will need to continue ...

Alfred McEwen wrote:

---

Phoenix Lander Almost 2 Mars Years after Landing (ESP_025786_2485)

This is one of a series of images to monitor frost patterns at the Phoenix landing site.

The lander and backshell are visible, but not the parachute or the dark halo around the lander which is covered by dust.

This is the same appearance to the hardware as 1 Mars years ago, in 2010. For views of the lander during the active mission in 2008, see our special releases page.

Alfred Mcewen wrote:

---

Spirit Lander and Bonneville Crater in Color (ESP_025815_1655)

HiRISE has never before imaged the actual lander for the Spirit rover in color, on the west side of Bonneville Crater. The lander is still bright, but with a reddish color, probably due to a dust cover (lower left in the subimage).

A bright spot from a remnant of the heat shield is still visible on the north rim of Bonneville Crater. The backshell and parachute are still bright, but were not captured in the narrow color swath.

The rover itself can still be seen near "home plate" in the Columbia Hills, but there is no obvious sign of rover tracks--erased by the wind.