Mars lander "Curiosity" is on the surface

It will be impossible to outdo the NASA press releases on the exciting mission now underway on the surface of Mars. So, for my international readers who may not be receiving so much information on the mission, here is the JPL link to Curiosity, as well as to the two previous landers, Spirit and Opportunity:

http://marsrovers.jpl.nasa.gov/

Enjoy!

More evidence of geologic activity on Mars: sand dune changes

In this photo sand and ice are cascading down the slipface of a dune,
covering seasonal ice.  The arrow points to a small cloud of dust kicked
up by this cascading.

Extraterrestrial sand dunes exist on Mars and Titan.  There has been some question about how active the dunes that cover a large region of the northern latitudes of Mars are.  Although there have been some indications of small changes, most data suggested that the dunes are currently stable, perhaps held together by ice. However recent images from the HiRISE experiment show substantial changes that are triggered by the sublimation of CO2 as the seasons change. This work is reported by Hansen et al. in Science this week (vol. 331, no. 6017, pp. 575-578, 2011).

Sand dunes have a windward side, where the sand is pushed up onto the dune, and a slip face on the lee side. Sand moves on the dunes by saltation (bouncing) and avalanching (called "grainfall" in this Science article). On earth, they have a variety of shapes and, in spite of the differences in climate, have similar morphologies on Mars.

Kelso dunes in the Mojave desert, California,
Photo by Mark A. Wilson, Wiki

Every year, in Mars winter, the polar regions are covered by seasonal caps of CO2 frost. In the spring, when the CO2 sublimates sand grains can move forming the avalanches and dust cloud shown in the figure above. Hansen et al. found that the dunes showed new alcoves, gullies and extension of their aprons.  These changes were also remobilized by the wind, forming ripples and erasing the gullies.  Even if the dunes contain a large amount of ice internally, their surface layers, perhaps to a depth of 25 cm, is dessicated and able to move.

Saidmarch, Blackhawk, and Heart Mountain landslides

The Saidmareh landslide in Iran.

Geology.com is a great source of information for this blogger, and the image from the right, originally from NASA is from that site; it was also featured on Dave Petley's landslide site in 2009.  This landslide, which occurred about 10,000 years ago, is believed to be the largest yet identified on the surface of the earth.  About 20 cubic kilometers of limestone slid 1600 meters (a mile) vertically, spread across the Karkheh River and its valley. Some material traveled 14 kilometers. The slide dammed the Karkheh River, causing a landslide lake to form behind the earthen dam.  This lake eventually breached the dam.

Massive landslides are often triggered by earthquakes.  In the U.S., one of the most catastrophic occurred in 1959 in southwestern Montana.  An earthquake, M 7.3-7.5, caused a huge landslide that killed 28 people and cost $11 million 1959 USD in damage.  This slide blocked the Madison River, resulting in the creation of Quake Lake.  The earthquake is known as the Hebgen Lake earthquake.  Fearing that the lake would burst through the dam in a catastrophic flood, the Army Corps of Engineers almost immediately began to cut a channel into the slide, and within a month, water was flowing through this cut.  In contrast, the landslide dam blocking the Karkheh River in Iran lasted long enough that 150 meters of sediment accumulated at the bottom of the lake before the dam failed.

Landslides that travel long distances occur not only on Earth, but also on Venus, Mars, and Io. The conditions that permit such large, heavy masses to travel long distances have been, and are still, subjects of controversy.  The runouts exceed distances calculated from simple models in which friction is a retarding force.  One hypothesis, based on field observations of the base of the Blackhawk Landslide in California, is that there is a cushion of air that lubricates the base of the landslide.  Another suggestion is that internal vibrations could "fluidize" the rock debris, making the effective coefficient of friction much lower than would be characteristic of a sliding solid mass.

Within the U.S., the Heart Mountain landslide in northwestern Wyoming has a runout distance of about 50 km.  How it traveled so far has been a source of scientific controversy for decades.  In a recent paper, Goren et al. have proposed that a feedback between "shear heating, thermal pressurization, and thermal decomposition of carbonates" at the sliding interface accounts for the large runout distance. The model suggests that the sliding velocity was a few tens of meters per second to more than 100 m/s, and that it took only a few tens of minutes for the whole sliding event.

Katabatic winds on Mars

Figure: This view of the north polar region of Mars shows the icy polar cap, about 1,000 km across.  The large canyon (arc arc)  in the lower right is Chasma Boreale which is about as long as the Grand Canyon, and up to 2 km deep.  The dark spiraling bands are troughs. Credit: NASA

The north polar region of Mars contains spiraling troughs up to 10 km in width and 1 km depth.  Winds spiral out from the north pole and in many places cross the troughs at nearly right angles. (In other places, such as the large Chasma Boreale) they flow down the canyons.  By comparison with winds on earth that flow down off high terrain, the winds on Mars have been called katabatic winds. Simulations suggest that horizontal wind velocities in some places on Mars may reach 30 m/s.  In the second figure here, streaks descending the slopes of one of the spiraling canyons are taken to indicate winds pouring over the rim of the canyon. They are eroding grooves into the slope and entraining material, presumed to be a mixture of ice and dirt.  The grooves are being carved by longitudinal vortices in the boundary layer of the winds.  The spacing of the grooves--hundreds of meters--suggests that the boundary layer is hundreds of meters thick (approximately two times the spacing of the grooves). Near the base of the canyon, the winds decelerate--possibly through a hydraulic jump--and the organized structure of the vortices is disturbed.  The entrained material is being dumped out of suspension as indicated by the turbulent clouds.

Added on December 18: Here's a New York Times article about katabatic winds in the Antarctic.