Sinus Iridum - China's Next Stomping Grounds

Clementine mosaic of the near side of the Moon, with arrow pointing to Sinus Iridum.
Image Credit: NASA/JPL/USGS, Annotation by Irene Antonenko

Earlier this week, China's National Space Administration (CNSA) held a press briefing to provide more details about their Chang'e 3 lunar lander, which is scheduled to launch early Monday morning on Dec 2, 2013 around 1:30 am Beijing Standard Time.  That should put the lander on the Moon some time by the middle of December, making it the first spacecraft to execute a soft landing on the Moon in over 37 years. The last man-made lunar visitor to do so was the Soviet Luna 24, which landed in August of 1976.

The Chang'e 3 lander will also deliver a rover, named Yutu, the legendary "Jade Rabbit" companion of the goddess Chang'e.  The last man-made vehicle to roam the Moon's surface was the Soviet Lunakhod 2, which landed in January of 1973 and operated on the surface for 4 months. China's Yutu is expected to roam around the Moon for 90 days and cover about 5 square kilometers of terrain.

The all important landing spot for the Chang'e 3 lander and Yutu rover is to be in Sinus Iridum, or the Bay of Rainbows, on the near side of the Moon.  No spacecraft has ever landed there before. The Soviet Luna 17, which delivered the rover Lunakhod 1, came close, landing over 200 km to the south, on the edge of Mare Imbrium (the Sea of Rains). But even that was a long time ago and over 40 years have passed since then. It is expected that technological advances since that time will make it possible to obtain far better data, allowing us to study the lunar surface in unprecedented detail.

Artist's conception of the Chang'e 3 lander and its accompanying rover Yutu.
Image Credit: Beijing Institute of Spacecraft System Engineering

But, it is also important to study the landing site as much as possible before arrival. Most obviously, this is necessary to ensure a safe site, both for landing and operating on the surface. Secondly, it is important to select a site that is scientifically useful. We have been to the Moon before, so to get the most bang for our buck, we want to make sure we are seeing something different from what we have seen before.  And finally, we want know the landing area as well as possible, so that we can quickly understand and interpret the information we see when we get there. This allows scientists to make critical decisions about where to send the rover next without wasting precious surface time on elementary data analysis.  With that in mind, here is a short primer on the Chang'e 3 landing site.  

Topography of the Sinus Iridium region. Lunar Reconnaissance Orbiter Camera (LROC) data has been processed to determine the topography of the surface. Colours represent elevation, with yellows and oranges representing high topography and blues indicating low lying surfaces. The topography data has been overlain on top of LROM wide angle camera mosaic imagery, helping to highlight the difference between high, rough terrain, and smooth low-lying areas.
     Feel free to explore this region in more detail at the LROC Act-React Quick Map on-line web tool.
Image Credit: NASA/GSFC/Arizona State University

Sinus Iridum is a small lava filled impact crater about 250 km in diameter, which sits beside, and opens into, the much bigger Mare Imbrium, which is also a lava-filled impact structure (up to 1800 km in diameter). The Iridum crater sits on top of an uplift structure (called an inner ring) inside the Imbrium basin and, therefore, must have formed after the Imbrium impact event. The topography of the pre-existing Imbrium impact basin seems to have controlled how the later Iridum crater formed, so that the parts of the Iridum crater that face the centre of Imbrium (to the southeast) are overall lower than the parts that face away. This fact played an important role when both the structures were flooded with basaltic lava, as the topographically lower portions of the Iridum crater were completely covered over, while the higher portions in the northwest remained untouched.

Not all this lava flooded the area at once. It came in relative dribs and drabs. Geologists have used things like surface albedo (or brightness) and the crater counting to show that different parts of the sinus and mare were emplaced at different times, with younger flows covering over older flows in some places, but not in others. These resulting variations can be seen on geologic maps, where the different basalt units are represented by varying colours. In addition, remote sensing studies of the iron content of these basalts also shows that there is great variation between the different flows.

The flooded mare regions of both Imbrium and Iridum are crossed by mare ridges. These are long linear hills that extend for tens to hundreds of kilometers across the mare surface, often running parallell to impact rim and ring structures buried under the mare. It is believed they form when thick piles of basaltic lava cool and contract. This leaves an  upper-most "chill crust", which cooled much earlier and is now too large and loose for the underlying contracted pile, forcing it to bunch up and wrinkle like a table cloth. 

Geologic map of Sinus Iridium and the northwest parts of Mare Imbrium. Purple colours represent materials from the Iridum crater rim. Greens, yellows, and browns represent materials from various younger craters. And pink and grey areas show the different units of mare basalts.
   You can download this geologic map at Lunar and Planetary Institutes Resources portal.
Image Credit: USGS

Map showing the iron content of surface materials in the Sinus Iridium region. Reds to yellows indicate relatively high iron content, while greens and blues show a lower iron content. The high iron regions (red) appear to correspond to the grey mare unit in the geologic map at left.
   You can explore the iron content of the Moon at the USGS Map-a-Planet Explorer web page.
Image Credit: USGS

The exact location where Chang'e 3 and Yutu are going to land is still not known. However, it is likely that these wrinkle ridges will play a role in the landing site. Wrinkle ridges may be able to provide some shade during parts of the lunar day, which can help to stabilize temperatures for the operating spacecraft. Temperatures on the lunar surface reach 390K (117 degrees Celcius) in the day time, which is higher than the temperature of boiling water, and drop  down to 110K (-163 degrees Celcius) at night. The spacecraft cannot function at either of these temperature extremes for prolonged periods of time, so landing in an area where shade is available, at least some of the time during the day, can be very useful for improving spacecraft life.

Personally, I can't wait to see what Chang'e 3 and rover Yutu will discover on the surface of Sinus Iridum. Here's hoping for a successful launch, uneventful journey, and a very happy landing!

Sources:
Lakdawalla, 2013, Chang'e 3 may launch December 1 with Yutu rover, will not harm LADEE mission, The Planetary Society Blogs, Nov 27, 2013.

Schaber, 1969, Geologic Map of the Sinus Iridum Quadrangle of the Moon, I-602 (LAC-24), USGS.

Wang et al., 2013, The Chang E-3 Landing and Working Area Selecting: Based on the Lunar Digital Terrain Model, 21st International Conference on Geoinformatics, DOI: 10.1109/Geoinformatics.2013.6626076

Coming soon: China launches Chang’e-3 lunar probe, China.Org.Cn, Nov 29, 2013.