The Surface of a Comet

This sequence of images captures the landing location of Rosetta’s Philae lander. The first image in the sequence shows the pre-landing surface, at a resolution of 1.3 m/pixel, acquired 3.5 minutes before first touchdown. The second image in the sequence shows the landing site about 1.5 minutes after first touchdown. The large circle on the left highlights the plume of dust Philae raised when it bounced off the surface on its first touchdown. The smaller circles on the right point out where the Philae lander finally settled after bouncing twice on the surface of the comet. More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/11/Philae_spotted_by_Rosetta_after_first_landing.
Image Credit:  ESA/Rosetta/NavCam

It has been a very exciting summer and fall for planetary exploration, but I have been far too busy with a new job and home renovations to write about it. Finally, I have managed to squeeze out a bit of time, just before the end of the year, to summarize the European Space Agency’s awesome landing on the surface of Comet 67P/Churyumov-Gerasimenko.

Comet Churyumov–Gerasimenko is shaped kind of like a barbell, with one smaller lobe and another larger lobe connected by a narrow “neck”. Detailed images from the OSIRIS (Optical, Spectrocopic and Infrared Remote Imaging System) camera on board the Rosetta spacecraft provided the high resolution images that were needed to understand the comet’s form and to select a target site for the Philae lander. An area on the outer edge of the comet’s smaller lobe, just outside a large, circular depression, was selected as the landing site. The selection criteria considered scientific interest, safety, and operations. This particular site was chosen because it was thought to 1) have a relatively smooth and flat surface (at least locally), providing a safe place to land, 2) receive sufficient sunlight to re-charge Philae’s solar batteries, making the location operationally viable, and 3) be close to active processes and provide access to pristine materials, making it scientifically interesting.

Comet 67P/Churyumov–Gerasimenko has a very irregular shape. Images taken by the OSIRIS camera on the Rosetta spacecraft have allowed this 3D shape model to be calculated. Philae landed on the outer edge of the comet’s smaller lobe, just outside of the large, circular depression located there.
More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/10/Shape_model_of_comet.
Image Credit:  ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Active processes on a comet can be considerably different from those on other solid bodies in the solar system, like asteroids, moons, and planets. For one, comets have a high content of volatile material that regularly sublimates every time the comet comes close to the sun (at perihelion). Thus, sublimation is the dominant process operating on comet surfaces, rather than impacts, which tend to dominate the other solar system bodies.

Cometary surface processes have been studied in the past, most notably on comet 19/P Borrelly, which was observed in 2001 when the Deep Space 1 mission flew by.  Comet Borrelly is a Jupiter-family comet, like Cheryumov-Gerasimenko, therefore both comets are expected to have similar orbits, periods, and active processes. The most notable observation on comet Borrelly was the complete absence of impact craters, down to the resolution limit of 200 m.  Small depressions of 200-300 meters were observed, but their morphology and distribution argued against an impact origin. Instead, Dr. Dan Britt and his team of researchers, who studies the comet’s surface, proposed that these pits were caused by sublimation. 

This image from Rosetta’s OSIRIS narrow-angle camera, taken from 50 km above the comet’s surface, shows the location of the Philae landing site, just outside a large circular depression on the outer edge of the comet’s smaller lobe.
More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/11/First_touchdown.
Image Credit:  ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

In this image from Rosetta’s OSIRIS narrow-angle camera, taken from 30 km above the comet’s surface, more details of Philae’s landing site start to become visible. The lander appears to have touched down on a smooth elevated surface within a rugged terrain.
More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/11/First_touchdown_close-up_1.
Image Credit:  ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

During its perihelion pass around the sun, volatile materials are sublimated from the surface of the comet. Non-volatile materials, in contrast, are left behind, building up an insulating “lag” layer that protects the surface from further sublimation and erosion. If this layer is not too thick, thermal instabilities can cause pits to form, exposing the volatile materials below. Impacts can also do the same for thicker lag deposits. In both cases, the volatiles exposed sublimate away, undermining the protective layer and causing the depressions to grow in size. This kind of growth, over may frequent perihelion passes, is thought produce a very rugged topography, with many flat surfaces and steep slopes.

The surface features that Dr. Britt and his team observed at comet Borrelly support this hypothesis. They saw smooth areas, which were interpreted to represent accumulations of non-volatile materials. They saw pits, which they think are indicative of relatively thin lag deposits. And they saw rugged topography, with many steep slopes interspersed with relatively flat regions, suggesting prolonged erosion of a lag deposit through undermining at steep slopes.

This image, taken by Philae's down-looking descent ROLIS imager from 40 m above the comet’s surface, shows that the landing surface is quite rough at smaller scales, in comparison to its smooth appearance in earlier images. The area is covered by debris, ranging in size from mm to meters, with the big bolder at the top right corner being 5 m in diameter. The black bar in the same corner is a section of Philae’s landing gear.
More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/11/Comet_from_40_metres.
Image Credit:  ESA/Rosetta/Philae/ROLIS/DLR

The topography of comet Cheryumov-Gerasimenko appears to have many similar elements. Philae’s landing area in particular seems to have an abundance of steep slopes with flat surfaces at their tops and bottoms. Upon closer approach, these “flat” areas are shown to be covered by rugged debris, containing many different particle sizes, from fine dust to meter-sized boulders. Although the resolution of the Deep Space 1 images was not sufficient to observe roughness of this size on comet Borelly, photometric analysis of that data did suggest that smooth units were rougher at smaller scales. Thus, even in this respect, comet Borelly and comet Cheryumov-Gerasimenko appear to be similar.

This is the first image ever taken from the surface of a comet! Acquired by Philae's CIVA camera, this two-image mosaic shows what appears to be a very rough and rugged cliff, illuminated by the sun. One of the lander’s three feet can be seen in the centre left.
More details about this image can be found at: http://www.esa.int/spaceinimages/Images/2014/11/Welcome_to_a_comet.
Image Credit:  ESA/Rosetta/Philae/CIVA

All of this implies that the surface of Cheryumov-Gerasimenko was probably formed by similar processes as comet Borelly. Thus, the surface of Cheryumov-Gerasimenko most likely represents erosive sublimation processes, where a relatively thin lag deposit was breached, presumably by thermal instabilities from below, creating pits that grew and coalesced to form the rugged terrain we see.
However, it is still not clear how the large circular feature near the Philae landing site was formed. It is possible that this feature represents a pit that grew to a particularly large size. It is also possible that this feature is an old impact crater that was too big to be completely eroded away by sublimation. More work is clearly required to answer this question.

Unfortunately, we may never get any more data from Philae on the surface of the comet. Because of its two bounces, Philae did not stay where it was originally targeted to land, but instead came to rest in the shadow of a cliff.  As a result, the solar panels could not keep the lander operational and at half past midnight (GMT) on Nov 15, 2014 Philae stopped transmitting to the Rosetta spacecraft. There is a slight possibility that when the comet makes its closest approach to the sun on August 13, 2015, Philae’s solar panels may receive enough energy to wake up the lander and re-establish communications. Until then, we will have to be satisfied with orbiter data from the Rosetta spacecraft, which continues to collect data, currently from 20 km above the comet’s surface, but with future flybys planned to approach closer than 8 km. Exciting times, indeed!
Sources:

Britt, D.T. et al. The morphology and surface processes of Comet 19/P Borrelly. Icarus  167, 2004, DOI: 10.1016/j.icarus.2003.09.004.

“J” Marks the Spot for Rosetta’s Lander, ESA’s Rosetta Blog, Nov. 15, 2014.

Pioneering Philae Completes Main Mission before Hibernation, ESA’s Rosetta Blog, Sept. 15, 2014.

Is Venus Active?

This hot spot on Idunn Mons, a volcano on Venus, was discovered by Suzanne Smredar and her coworkers in 2010. Here, Magellan radar imagery (in brown) is draped over Magellan topographic data, showing off the landscape of the region. The elevation has been exaggerated 30 times in order to highlight the topography. On top of all this, data from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on board ESA’s Venus Express orbiter, is overlain to show temperature variations. Red colours indicate the warmest places and purple the coolest. This hot spot is thought to indicate the presence of geologically young lava flows, less than 2.5 million years old, but it does not prove the existence of current active volcanism.  
More details and the original image can be found at http://sci.esa.int/venus-express/46816-surface-warmth-on-a-volcano-on-venus/
Image Credit: ESA/NASA/JPL

One of the most interesting things to come out of this year’s Lunar and Planetary Science Conference is the identification of potentially active volcanoes on Venus. Venus has long been known to have lots of volcanoes, but evidence for active volcanism on the planet has been elusive until now.

Venus, despite being similar to the Earth in mass and size, is a very different kind of planet. It is shrouded in a thick atmosphere of carbon dioxide and has clouds of sulfuric acid. As a result of this thick atmosphere, pressure on the surface of Venus is 93 times the atmospheric pressure on the surface of the Earth and temperatures are a sizzling 900 degrees Fahrenheit.

The thick atmosphere also makes it very difficult to see the surface of Venus. Regular cameras that capture visible light can’t see beneath the thick atmosphere. However, radar and infra-red instruments can penetrate the dense atmosphere to let us study the planet’s surface. From 1990-1994, NASA’s Magellan mission mapped the entire surface of Venus using radar. More recently, the Venus Monitoring Camera (VMS) on board the European Space Agency’s Venus Express mission has been gathering data from both the planet’s surface and atmosphere.

NASA’s Magellan mission to Venus mapped the entire surface of the planet, using radar instruments to peer beneath the thick atmosphere that otherwise obscures the surface of Venus from view. This image is centred more-or-less on Diana Chasma.  Maat Mons is the purple/pink volcano north east of centre with Ganiki Chasma extending northward nearby. 
This image can be found at http://photojournal.jpl.nasa.gov/catalog/PIA00159
Image Credit: NASA/JPL/USGS

The VMS collects data at four different wavelengths of light. The atmosphere of Venus is transparent to one of these wavelengths, namely the near-infrared wavelength of 1.01 microns (1 micron = 1000 nanometers). The radiation reflected from the surface of Venus at this wavelength is highly dependent on the surface temperature. A team of scientists at the Max Planck Institute for Solar System Research, led by Eugene Shalygin, have been using this fact to study temperature variations on the planet’s surface.

Such data can only be collected at night, when heat from the sun is not a factor, and only when cloud variability is low, to ensure that differences are in fact due to changes at the surface. With these constraints, the team was able to collect data from a total of 36 different orbits of the Venus Express spacecraft, providing a series of observations over time.

From the VMS time series data, Eugene Shalygin and his team observed a number of bright spots, representing an estimated 980-1520 degrees Fahrenheit, well above planet's typical temperatures. Most spectacularly, these bright spots appeared suddenly (after having been absent in preceding orbits), persisted through several orbits, and then disappeared in subsequent orbits. “We were looking for these spots for several years (and) didn't find," anything, said Alexander Bazilevskiy, a senior scientist on the team. The scientists conclude that these transient bright spots must represent some kind of localized process that released hot matter to the surface in a short period of time. In other words, these spots suggest that an active volcanic event occurred right before our eyes!

All of the identified bright spots are located at the edge of Ganiki Chasma, a young rift zone in the vicinity of Maat Mons. Maat Mons is a tall shield volcano that erupted between 10 -20 million years ago, so this region is known to have been volcanically active in the past. The bright spots could, therefore, represent either long lava flows, which stretch for about 25 kilometers (16 miles), or a chain of small cinder cones or volcanic hot spots.

Maat Mons is a very large shield volcano on the surface of Venus. It is thought to have erupted between 10 -20 million years ago. Potential evidence of current volcanic activity has been found in the vicinity of this large volcano.This image shows a computer-generated perspective view of the volcano, looking from the north.
This image can be found at http://photojournal.jpl.nasa.gov/catalog/PIA00106
Image Credit: NASA/JPL

But, before getting too excited, we should be cautioned that the volcanic nature of the observed bright spots has not yet been confirmed. Shalygin and his team plan to look for more bright spots in the VMS data and also to sift through the historical Magellan radar data to see if they can find additional evidence of volcanic activity.

For more details on the location of the spots and to see an example of the VMS discovery data, check out Lunar and Planetary Science Conference Abstract #2556, where Shalygin and his team first presented their findings.

Sources:

Hall, S. Active Volcanoes on Venus? Sky and Telescope, March 24, 2014.
http://www.skyandtelescope.com/astronomy-news/active-volcanoes-on-venus/

Klotz, I. Active Volcanoes Revealed on Venus, Discovery News, March 18, 2014, Space.com http://www.space.com/25106-venus-volcanoes-active.html

Shalygin, E.V. et al. Bright transient spots in Ganiki Chasma, Venus. 45th Lunar and Planetary Science Conference, Abstract #2556, 2014. http://www.hou.usra.edu/meetings/lpsc2014/pdf/2556.pdf

The Curious Layers of Mars

It has been an extremely long and hard winter this year in the north-eastern portions of North America and I find myself in need of some cheering up. Nothing is so cheery as pretty pictures of planetary surfaces. So today I am going to talk about some of the beautiful imagery that's been coming from the Curiosity rover on Mars, specifically the lovely layers of sedimentary rocks.

Many layers of soft and hard sandstone rocks make these step-like structures on Mars. The Curiosity rover collected this image in Gale Crater on Feb. 25, 2014.
 To learn more about this image, go to the JPL Space Images website.
Image Credit: NASA/JPL-Caltech/MSSS

Curiosity landed in the Gale impact crater on Mars in August of 2012 and has been exploring the floor of that crater ever since. The going is slow though. The rover's average travelling speed is about 30 meters per hour. In comparison, most people can easily walk 3 kilometers (3000 m) per hour. But the rover is actually even slower than its average travelling speed. First, it's goal is to explore Mars, so it makes many stops to gather science data. Second, it needs to pick its path carefully to avoid obstacles. So it's not always following the straightest route. In the early days of the mission, sharp rocks were puncturing the rover's aluminum wheels, but now careful route planning has helped to minimize such damage. But that care takes time. In the end, Curiosity has moved only about 4 kilometers from its landing site in the first 561 martian days of operations (a martian day is about 40 minutes longer than an Earth day).

The Curiosity rover landed on the floor of Gale impact crater on Mars, just north of Mount Sharp in the centre of the crater. The landing location is highlighted here by the black oval.
To learn more about this image, go to the NASA mission page: http://www.nasa.gov/mission_pages/msl/multimedia/pia15093b.html#.UzejcYXiiCo.
Image Credit: NASA/JPL-Caltech

The type of terrain Curiosity has been passing through has changed in those 4 kilometers, though. The rover's original landing site was relatively flat with only a scattering of small pebbles on the ground. This is actually a good thing for a landing site, since you wouldn't want your rover to land on a cliff or large boulder.

The Bradbury Landing site (named after Martian Chronicles author Ray Bradbury) is very flat and shows no evidence of layering. The mountain in the distance at the top centre of this image is Mount Sharp.
To learn more about this image, go to the JPL Space Images website.
Image Credit: NASA/JPL-Caltech/MSSS

Ever since it landed, Curiosity has been steadily making its way to the large mountain in the centre of Gale crater, called Mount Sharp. The base of Mount Sharp, which is about 20 kilometers south from the landing site, is the rover's ultimate destination. This region is of great interest to scientists because it contains a very thick exposure of layered rocks, which may reveal several billion years worth of clues about this region's history. 

Ever since arriving at Bradbury Landing, Curiosity has been making its way south to Mount Sharp, stopping at a number of planned waypoints along the way. This image shows exactly where Curiosity has traversed, up to day 561 (March 5, 2014) of the mission. Murray Buttes is thought to be a good entry point to the base of Mount Sharp.
To learn more about this annotated image, go to JPL Curiosity Rover Multimedia website.
Image Credit: NASA/JPL-Caltech/Univ. of Arizona

But, Curiosity did not have to go all the way to Mount Sharp to find layered rocks. Those it found relatively soon after landing. More recently, on Feb. 25, 2014, Curiosity arrived at a location where many different types of layered rocks can be seen in one place (see the image at the top of this post).

The layered rocks here are thought to be sandstones. This is a type of rock that is literally made up of sand grains glued together by some kind of cement. If the cement is made up of clay materials, the sandstone is relatively soft and can be easily eroded by wind and water. If the cement is made up of hard quartz minerals, the sandstone is very durable and hard to erode. When layers of hard and soft sandstone occur together, they make step-like structures, where the hard sandstone forms caps protruding over the soft sandstone that is eroding away.

Scientists are not clear on why there would be so many different layers of soft and hard sandstone in this one place, or how they formed. But, they are hoping for some answers soon. About 400 meters away is the planned Kimberly waypoint. This area was identified as a point of interest from satellite images, because four different-looking rock types seem to intersect there. And it is expected that layered sandstones will be found at the Kimberly waypoint. The Curiosity rover will stop there for a time to conduct scientific investigations.

Antarctica Beneath the Ice

This winter has been a particularly brutal one in my part of the world (south-eastern Canada), with snow coming earlier than usual, extremely cold temperatures persisting for prolonged periods of time, and brutal ice storms causing massive power outages. With all this cold and snow, I thought it would be appropriate to talk about another cold place on Earth, Antarctica.

Sitting at the southern pole, Antarctica is almost completely (98%) covered by an ice sheet. Reaching thicknesses of up to 3km in places, this glacier flows under its own weight and is estimated to hold more than 50% of the world's fresh water. It is anticipated that melting of this great ice sheet, due to global climate change, will contribute significantly to sea level rise. But it is still not clear exactly how the glacier will react to climate change, because the glacier and the bedrock it sits upon is poorly understood.

Antarctica is almost completely covered by a large ice sheet. A new data set, called Basemap2, uses over 26 million data points to determine the surface elevation of the ice (right image), the thickness of the ice, and the topography of the underlying bedrock (left  image). The vertical scale in these images has been exaggerated 17 times, to make the mountains and valleys easier to see.
To learn more about these data sets, go to the NASA Feature website. There you will also find an interactive map of the two datasets, which lets you switch between them, making them easier to compare.
Image Credit: NASA's Goddard Space Flight Center

 Luckily, scientists have recently produced a new data set that, in addition to revealing the stunning topography hidden beneath the glacier, will help modellers resolve these questions. Led by Dr. Fretwell at the British Antarctic Survey, an international consortium of scientists has released the Bedmap2 dataset. Bedmap2 builds on a previous data set called Bedmap (produced in 2001), providing surface elevation, ice thickness, and bedrock topography for all of Antarctica, south of 60° S.

To produce the new data set, the researchers incorporated an additional 25 million measurements and processed these using modern GIS techniques and hardware, which made manipulation of such large data sets possible. For the earlier Bedmap data set, which used only 1.4 million data points, this number had to be reduced for processing to be manageable. In addition, the more recent data points were collected using modern Global Positioning Satellite (GPS) technology, which helped to pin-point the data more precisely. The original Bedmap data didn't always have this level of precision.

The new data comes from a wide variety of sources, representing 83 different survey campaigns, run by various nationalities, and collected using ground, air, and space platforms. A large part of the data comes from the Operation IceBridge campaign. This airborne mission was flown from Punta Arenas, Chile, collecting laser altimeter and ice-penetrating radar data between 2009 and 2011. The radar instrument, called the Multichannel Coherent Radar Depth Sounder (MCoRDS), was operated by the Center for Remote Sensing of Ice Sheets at the University of Kansas. MCoRDS sent radar signals down through the ice and recorded the returning signals, which gave information on the ice surface, the internal layering within the ice, and the bedrock below. This data was processed to determine the surface elevation of the ice and the ice thickness, which were used to calculate the bedrock topography. However, ice-penetrating radar instruments, which tend to work best in flat areas, don't do so well in steep mountainous regions.  In such areas, surface elevation data from NASA's Ice, Cloud, and Land Elevation Satellite (ICESat) proved useful. The laser data from Operation IceBridge was used to verify the accuracy of the surface elevation data from these sources. In some cases, ground-based data was available, including over-snow radar, seismic sounding, surface elevation, bathymetry, rock-outcrop, grounding line, and ice-extent datasets. Also, when the density of the data was particularly low, satellite gravity data was used to determine ice thicknesses.

The result of all this data is that Bedmap2 provides higher resolution, greater coverage, and improved precision over the original Bedmap product. The large amount of data points allows the data to be interpolated over a 1 km grid, but the uncertainties can be high; up to 130 m in ice surface elevation and up to 1000 m in ice thickness.
  
Even so, Bedmap2 highlights the beauty of the bedrock under the ice. The better resolution shows off smaller features that could never be seen before, revealing the full scale of the mountain ranges, valleys, basins, and troughs. The new data has also found that the deepest bedrock elevation is actually deeper (by 15%) than previously believed. In addition, several other deep points (about 2.5 km below sea level) have now been identified on the Antarctic continent. It is not clear how accurate the numbers for these deep points are, but it is certain that the deepest point for any continent on the Earth is located somewhere in Antarctica. No other continental areas even come close to such depths.

The Bedmap2 data set also tells us a lot about the Antarctic ice sheet. Volumes calculated from the data indicated that 27 million km3 of water are stored in the ice sheet, which can potentially contribute 58 m to sea level rise if the Antarctic glacier should melt. These estimates are very close to the values that were determined from the original Bedmap product, but now our confidence in them is much greater. Also, we now know that the ice sheet is on average 4.6% thicker than was previously believed and that a much greater volume of ice exists below sea level.

The Basemap2 data set (right image) builds upon a previous version, called Basemap (left image).  The higher resolution and greater coverage of Basemap2, gives us better precision, making it easier to see the spectacular mountain ranges, valleys, and rugged terrain.
To learn more about these data sets, go to the NASA Feature website. There you will also find an interactive map of the two datasets, which lets you switch between them, making them easier to compare.
Image Credit: NASA's Goddard Space Flight Center

Mapping the thickness, volume, and bedrock of the Antarctic glacier helps us to understand how ice sheets respond to changes in ocean and air temperatures. Specifically, the shape and structure of the bedrock below the ice controls how the ice sheet moves, affecting its shape and thickness. For example, ice will flow faster downhill, be thinner at the top of the hill, and thicker at the bottom. Conversely, uphill slopes and bumpy terrain in the bedrock can slow down an ice sheet, or even hold it in place temporarily. Bedmap2 provides the level of detail that is necessary to understand these effects, allowing researchers to build more realistic and accurate models that simulate ice motion.

But the task in still not quite finished. There are still many places in the Bedmap2 data set where the amount of data is very poor or even completely non-existent. Dr. Fretwell and his colleagues have identified what they call 2 "poles of ignorance", regions where no data exists for several hundred kilometers. Clearly, the need for more data gathering exists. Which means we can all look forward to a Bedmap3 sometime in the future.

Sources:
NASA's IceBridge Mission Contributes to New Map of Antarctica, NASA News, July 4, 2013.

Fretwell, et al. 2013, Bedmap2: Improved ice bed, surface, and thickness datasets for Antarctica, The Cryosphere, 7, 375-393, doi:10.5194/tc-7-375-2013.