Friday, 26 April 2013

Blowing in the Martian Wind

Dune field near Northern Polar Cap
This image of a vast dune field near the northern pole of Mars was taken in August 2010, by the Thermal Emission Imaging System instrument on NASA's Mars Odyssey orbiter.
Image Credit: NASA/JPL-Caltech/ASUption
Over the past two weeks, I have gotten involved in the Hawai'i Space Exploration Analog and Simulation (HI-SEAS) mission as a First Tier Support person. What this means is that every Tuesday night, from 10 pm to 2 am my time (4-8 pm Hawai'i time), I am on call to help the crew in whatever they may need. The crew is simulating a mission to Mars, so they are on a 20 minute time delay, each way, and cannot access information quickly. This is where I come in.

Last Tuesday, I was asked to provide information on martian winds.  There was a strong wind advisory at the HI-SEAS mission site and crew member Kate Green wanted to compare what she was experiencing with potential conditions on Mars.  What I found and passed on was so interesting, I thought I should write about it.

The atmosphere of Mars is very thin, about 100 times less dense than the Earth's. In principal, one would expect that such a thin atmosphere could NOT lift sand and dust particles and move them around. And in fact, this is exactly what models that estimate the global circulation of the martian atmosphere predict, that winds there are too weak to lift sand and dust grains. The model predictions are confirmed by various martian landers; wind speeds that are strong enough to pick up sand grains have been measured only very rarely.
Dunes in Herschel Crater
Dunes on the floor of Herschel crater on Mars. The wind here blows from the top of the image to the bottom, forming these crescent-shaped dunes, with long tails that merge into each other. The tiny ripples on the dune surfaces are also formed by the wind.
Image Credit: NASA/JPL-Caltech/University of Arizona

But, as far back as the 1970's we have known about the existence of vast dune fields on Mars from images sent back by the Viking missions. Dunes are formed by winds moving large amounts of sand and dust around. So, if current conditions aren't right for getting sand grains moving, these dunes must be fossils from Mars' ancient past, and reflect a time when the atmosphere was much thicker.

However, several recent studies have found evidence of current activity in Mars' sand dunes. In March of 2010, NASA released an image set from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter ,which shows the appearance of new streaks on a dune slope in Nili Patera. These streaks indicate that winds have moved sand up the gentle side of the dune, over the dune crest, and down the steep side of the dune in a span of about 15 weeks.
Nathan Bridges of the Applied Physics Laboratory at Johns Hopkins University, working with a team of researchers from various other institutions, has measured the active dune motion in Nili Patera. He and his team looked both at the full dunes and also the smaller scale sand-ripples that can form on dune surfaces. They found that ripples moved a distance of up to 4.5 meters over a period equal to about 100 Earth days. Dune crests moved about 0.3 meters over a period of about 3 Earth years. These results show that dunes on Mars are currently active, and not just at the surface, the entire sand volume of the dune must be mobile.
Changes on Dune Slip Face, Nili Patera, Mars
Two images of the same dune face in Nili Patera, Mars (June 30, 2007 on the left and Oct. 13, 2007 on the right) show creation of new streaks.  Winds here blow from right to left.
Image Credit: NASA/JPL-Caltech/U of Arizona/International Research School of Planetary Sciences

Granted, these movement rates are slow, about 10 to 100 times slower that what we see for similar-sized dunes on Earth. However, this is only slightly slower than sand migration in the Victoria Valley dune fields of Antarctica on Earth, where the dunes have sand volumes 1000 times smaller than the martian Nili Patera dunes.  Such comparisons allow us to conclude that martian dunes form about 1000 times slower than dunes on Earth, which means the Nili Patera dune field probably formed in about 10,000 years. Now, this may seem like a long time, but the climate changes that Mars experiences due to fluctuations in its orbit take much longer than this to occur. Thus, these dunes probably formed in the current climate and are not just left over relics from an earlier time being moved around a little bit by the modern weak atmosphere.

So, how can the current thin atmosphere move sand and dust around when models and surface measurement say this should be very difficult to do? Well, the answer has two parts.

Part One: Models of the entire martian climate are, out of necessity, very course. The computing power and time needed to perform such calculations for the whole globe are rather large, so modellers look only at the big features. This means that variations on small scales aren't considered. But, very strong and localized winds can be created by abrupt changes in topography, where sudden differences in elevation produce changes in temperature and initiate convection.  These kinds of localized winds, which are believed to be responsible for various dune fields all over Mars, would not show up in global circulation models and would not be measured by landers, unless they happened to land in just the right place.

Part Two: The wind speed required to pick up sand grains and get them moving is not the same as that required to keep them going once they are started. Wind moves sand by picking up and throwing individual sand grains, and doing this continuously with millions of grains at a time. But, these thrown grains eventually drop back down to the ground and when they do, they can dislodge other grains, causing them to move as well. This added energy from the falling grains means that slower winds can keep already moving sand going. On Earth, only 80% of the wind strength needed to pick up a grain is enough to keep grains moving. On Mars, the lower gravity means that once a sand grain is thrown, it travels higher, gathering more momentum, and the thinner atmosphere means that air resistance is lower, so a sand grain on Mars has more energy when it comes back down. As a result, only 10% of the wind energy needed to pick up sand grains on Mars will be enough to keep the whole process going once it gets started.

So, despite a relatively thin atmosphere, winds on Mars can and do move sand around. Local strong winds get things started and then even weak winds can keep things going. The results are spectacular dunes forms on the surface, which are active even today.

Kok, 2012, Martian sand blowing in the wind, Nature, V 485, May 17, 2012, 312-313.

Bridges et al., 2012, Earth-like sand fluxes on Mars, Nature, V 485, May 17, 2012, 339-342.

Sunday, 14 April 2013

Astounding Tiger Stripes of Enceladus

Last Thursday, I joined the Planetary Society's Hangout: Ice Giants with Heidi Hammel. The hangout dealt with the ice giants Uranus and Neptune, but during the discussion Saturn's moon Enceladus was mentioned, and it made me want to delve more into this intriguing planet.

Enceladus and its Tiger Stripes
False colour image of Saturn's moon Endceladus, showing four pronounced "tiger stripes", which appear blue, in the southern hemisphere.
Image Credit: NASA/JPL/Space Science Institute
Enceladus is a fascinating planetary body. Its surface is an interesting mix of cratered terrain and relatively smooth regions with large fractures. There are four very prominent fractures in the southern hemisphere, which show up as blue stripes in false-colour images. These features have been dubbed "tiger stripes" and each stripe given a name: Alexandria, Cairo, Baghdad, and Damascus.

The distinctive colour of these tiger stripes shows that their composition is different from the surrounding terrain. It is believed that the stripes are "blue" because they have not been covered by fine-grained water ice particles from Saturn's E ring, in which Enceladus orbits. This would suggest that the tiger stripes must be very young, possibly less than 1000 years old!

Furthermore, data from the Cassini mission to Saturn indicates that these stripes are very hot (minus 135 degrees Fahrenheit) relative to the surrounding regions (minus 330 degrees Fahrenheit). Temperature maps from the Composite Infrared Spectrometer (acquired in March 2008) show that the hottest regions line up with the tiger stripes.  Heat, must therefore be escaping from Enceladus' interior along these fractures.
Heat map of Enceladus' "tiger stripes"
The heat map of Enceladus' "tiger stripes", overlain on visible image data, shows that heat is escaping the planet from these fractures.

Most spectacularly, plumes of vapour and ice particles have been observed jetting from the hottest parts of the tiger stripes. These plumes consist predominantly of water, with small amounts of methane, carbon monoxide, carbon dioxide and other simple and complex organics, making them similar to comets in composition.  Furthermore, it is thought that the whole of Saturn's E ring has been made from these particles that were spewed from Enceladus. The discovery of these jets in November of 2005 added Enceladus to the very short list of places in the Solar System where active volcanoes have been observed.
Individual jets emanating from the surface of Saturn's moon Enceladus
False-colour image of individual jets emanating from the surface of Saturn's moon Enceladus. These plumes consist mostly of water vapour and ice particles, with traces of other organic compounds.  
Image Credit: NASA/JPL/Space Science Institute

However, not all the plumes are neatly associated with the 4 named tiger stripes. A poster presentation by Patthoff and Kattenhorn at the recent Lunar and Planetary Science Conference suggests that current plume activity could be coming from older fracture sets.

The currently named tiger stripes are just the most prominent and youngest set of such fractures.  Older sets of fractures, which look very similar to the current tiger stripes, have also been identified. These are termed ancient tiger stripes. It is suspected that, in their day, ancient tiger stripes probably behaved similarly to the current tiger stripes, but then became less active as new stripes formed. This suggests that the south polar terrain of Enceladus has had a long history of tiger stripe activity, with a rotating series of fractures that continuously maintain Saturn's E ring.

What Patthoff and Kattenhorn did was calculate the diurnal stresses on Enceladus as it orbits Saturn, to see which stripes were under tension (being pulled apart) and which were under compression (being squeezed) at the time the plumes were observed. It is expected that stripes that are being squeezed together should not produce plumes, while those that are being pulled apart would allow materials to jet from the interior. The Patthoff and Kattenhorn model shows that for some plume events, the nearby current tiger stripes were under compression (being squeezed), while certain ancient tiger stripes were under tension (being pulled apart). They therefore conclude that the tidal stresses caused by Enceladus orbiting around Saturn may allow ancient tiger stripes to be re-activated and produce plumes even after younger stripes have developed.

Enceladus continues to astound us!

Planetary Society Hangout: Ice Giants with Heidi Hammel

Patthoff and Kattenhorn, 2013, The contribution of ancient tiger stripes to plume activitiy and energy flux on Enceladus, LPSC 44, Abstract #1675.

Wednesday, 3 April 2013

Impossible Lunar Magma Chambers

One of my very first science microblogs from the Lunar and Planetary Science Conference  last month was about a talk given by Katelyn Lehman, an undergraduate student, suggesting the existence of lunar magma chambers. At the time, I was skeptical, because current thinking is that these can't occur on the Moon. But, new work is suggesting that they can, and do.  So, as promised, here is a full discussion on the topic.

On the Earth, hot magma often stalls in shallow rocks rather than erupting at the surface. There it forms a magma chamber, where it sits and cools.

But such magma chambers are thought to be unlikely on the Moon. The density of the lunar crust is very low, so hot magma is not buoyantly stable within this crust. Just like water under ice, the neutral buoyancy zone of magma is just below the lunar crust (which is about 40 kilometers thick). So, that is where the magma stays and collects, deep in the lunar interior.

Neutral Buoyancy of Magma on the Moon block diagram
The magma that makes basaltic rocks is less dense than the lunar crust. On the Moon, therefore, magma stalls just below the crust. As the Moon cools, the mantle below the crust becomes more solid, and magma stalls even further below the crust.
Image credit: Head and Wilson, 1992

If a lot of magma collects in one place below the crust, it can build up enough pressure to fracture the overlying rocks and rise up through the crust in thin vertical sheets, called dykes.  If the pressure of the magma is high enough, it will make it to the surface and extrude as lava, forming dark, smooth mare flows. But, if the pressure is too low, the magma will not make it to the surface. It will freeze in the dykes, or if the dyke is very thick, the dense magma could sink back down below the less dense crust. This is why magma chambers are not expected to occur in the Moon's crust. The physical properties of the magma and crust don't support them.

However, recent research is suggesting that some magma chambers may be found on the Moon.  Walter Kiefer, a colleague of Katelyn Lehman, has used high resolution gravity data to show that very dense material must be located not too far below the surface in the Marius Hills region of the Moon.  This material is too dense to represent the crust of the Moon, so it must be something else.

Gravity data has been used since the Apollo era to show that large concentrations of massive, dense material (called mascons) are located at depth on the Moon. But mascons are very large features, associated with big basins, and so assumed to represent thinning of the crust due to huge impacts. The new gravity features are much smaller and shallower, and so cannot be related to crustal-scale processes. They must, therefore, represent shallow magma chambers, where basalt magma intruded into the Moon's crust.

Gravity anomaly of the Marius Hills region of the Moon
Very high resolution gravity data of the Marius Hills region of the Moon shows that there are two small positive gravity anomalies located in this region.  These are thought to represent relatively shallow magma chambers,  which are quite rare on the Moon.
Image credit: Kiefer, 2012

Backing up this interpretation, Katelyn Lehman's research has shown that the Marius Hills region of the Moon contains a number of regions that are rich in the mineral plagioclase. Plagioclase-rich materials are not expected in a mare area like the Marius Hills, where thick basalt lavas mean that the crust is not likely to be exposed, even by impact events.  Katelyn and her team, therefore, interpret these plagioclase areas to represent a different type of volcanism, one that would have resulted when magmas evolved after sitting in a shallow magma chamber for some time.

So, while the physics of magma and lunar crust means that magma chambers are not likely to occur on the Moon, it appears that it is not impossible. And, at least in the Marius Hills region, one of these rare lunar magma chambers may have in fact been formed.

Head and Wilson, 1992. Lunar mare volcanism: Stratigraphy, eruption conditions, and the evolution of secondary crusts, Geochim. et Cosmochim. Acta, V56, 2155-2175. DOI: 10.1016/0016-7037(92)90183-J.

Kiefer, 2012. Gravity constraints on the subsurface structure of the Marius Hills: The magmatic plumbing of the largest lunar volcanic dome complex, J. Geophys. Research - Planets, DOI: 10.1029/2012JE004111.

Lehman, et al., 2013, Composition analysis of the Marius Hills volcanic complex using Diviner Lunar Radiometer Experiment and the Moon Mineralogy Mapper, LPSC 44, Abstract #1225.