Lunar Reconnaissance Orbiter Diviner maps geologic context of Chang'e 3 landing site
Posted by Emily Lakdawalla
08-01-2014 13:53 CST
It's been a quiet couple of weeks on the Chang'e 3 mission, as the Moon's slow rotation takes it through a two-week-long lunar night. Sunrise will come to the landing site on Friday at about 5:30 UT, but I imagine that cold, sleeping spacecraft won't spring to life when the first photons hit the solar panels.
While fans on Earth are waiting for sunrise, a specific set of lunar exploration fans has not been idle. The science team of the Diviner instrument on the Lunar Reconnaissance Orbiter has posted a set of maps and data that provide geologic and physical context for the landing site of the Chang'e 3 mission. You don't hear about Diviner frequently, for two reasons. One: it doesn't generally produce telegenic image products. And two: it's the kind of instrument where the quality of the science slowly increases over time -- it must patiently map the whole planet, one orbital strip at a time, building up a map that doesn't just cover the entire spatial extent of the Moon but also covers all that space over time, watching how the lunar surface responds to changes in solar illumination. There's no instant science here, just patient, careful modeling work that eventually -- like now -- yields useful results.
As you'll see in the post below, Lunar Reconnaissance Orbiter has now been at the Moon long enough that Diviner can readily deliver important information about any given spot on the Moon -- in this case, one small spot in the northwestern part of the largest nearside basin, Mare Imbrium, where a Chinese spacecraft happened to land last month. Below the horizontal line is the text of the post from the Diviner website, with some added commentary in brackets by me.
The first few graphs aren't explained in great detail; they show representations of the data in the form of such things as "brightness temperature" and "bolometric temperature" that were used as inputs to generate models of the actual nighttime temperature of the lunar soil and then the abundance of rocks around the Chang'e 3 lander. That, in turn, gets them to what they call an "H-parameter map," and that's when they can start talking in detail about what kind of geology Chang'e 3 is actually sitting on -- that's when the story gets interesting.
In a perfect world, the Diviner team would have been able to communicate directly with Chinese colleagues, the two missions working together, Lunar Reconnaissance Orbiter providing orbital context to Chang'e 3, and Chang'e 3 providing ground truth to Lunar Reconnaissance Orbiter. They might even plan mutual observations, coordinated science experiments performed when Lunar Reconnaissance Orbiter happens to be passing over the Chang'e 3 landing site, just as Mars Reconnaissance Orbiter's Mars Climate Sounder (an instrument virtually identical to Diviner) has performed simultaneous observations with all Mars landers and rovers.
But it's not a perfect world, and NASA scientists are prohibited from bilateral cooperation with scientists from the Chinese space agency. That's a missed opportunity. But one thing NASA scientists can do is to post blog entries about their data on public websites for the whole world to see, and speak no words about collaboration. I hope that maybe these maps will help the Chang'e 3 science team plan scientific observations so that when China does release the science data to the public (as they have promised to do), NASA scientists will get some data that they can use for ground truthing their orbital observations. It's not perfect, and it's not scientifically irreplaceable coordinated observations, but it will have to do.
The LRO Diviner Lunar Radiometer has been mapping the entire Moon on a nearly continuous basis since July, 2009. The Diviner dataset includes excellent spatial and diurnal coverage of the Mare Imbrium region. [Because Lunar Reconnaissance Orbiter is in a polar orbit, its orbital tracks are more closely spaced at high latitudes than near the equator. The Chang'e 3 landing site is quite far north, 44 degrees, so a polar orbiter gets relatively dense coverage.] The Diviner team has produced maps of the thermal behavior and and a range of derived quantities at Chang’e 3 landing site that are described below.
Brightness temperature (TB):
Brightness temperature is the temperature that a black body (a body with emissivity = 1) in thermal equilibrium with its surroundings would have to be to produce the radiance that Diviner observes in a particular wavelength interval. [Note that this isn't exactly the same as the actual temperature of the surface; surface temperature has to be derived from brightness temperature. For surfaces emitting thermal radiation, brightness temperature underestimates the actual temperature by some amount depending on how emissive the surface is. Keeping that in mind, check out how much the temperature changes over the course of a lunar day in the graph below -- brightness temperature ranges from about 100 to about 350 kelvins, and it's coldest right before sunrise -- that is, right about now.]
Bolometric temperature (TBOL):
Bolometric temperature is the wavelength-integrated radiance in all seven thermal Diviner channels expressed as the temperature of an equivalent blackbody (Paige et al., 2010 [PDF]). Using data from the entire Diviner dataset to date (July 2009 - December 2013), we can calculate the maximum and minimum bolometric temperatures that have been observed at the Chang'e 3 landing site.
Regolith Temperature and Rock Abundance
The maps show Lunar surface rock abundance and nighttime rock free regolith temperatures. These data were derived from LRO Diviner channels 6-8 (wavelengths of 12-100 microns) data. [Deriving these maps from the brightness temperature data is a complex process, explained in a 2011 paper by Joshua Bandfield and coworkers (PDF).] The colorized maps are shaded using the LRO Camera derived digital terrain model. [This is a sophisticated topographic data set for the Moon developed by comparing stereo pairs of Lunar Reconnaissance Orbiter Wide-Angle Camera images to get topography from stereoclinometry, described in this paper by F. Scholten and coauthors, which is open-access.]
Taking into account the different nighttime cooling behavior of rocks and regolith, we use Diviner brightness temperatures to produce maps of rock abundance and nighttime regolith temperature (Bandfield et al., 2011 [PDF]). We then fit thermal models to the regolith temperature data to derive a subsurface profile of thermal conductivity and density (Hayne et al., 2013). The steepness of this profile is described by the “H-parameter”, where small values of H indicate more compact regolith, and large values of H are consistent with more fluffy, unconsolidated regolith. This parameter is similar to thermal inertia, a commonly used quantity in planetary science.
The H-parameter map shows that the Chang'e-3 lander (indicated by the 'X') is located in a region of typical lunar mare regolith. The red "hot spots" (low H values) reveal denser, rocky regolith surrounding many impact craters. Chang'e-3 is located adjacent to one of these craters, and therefore may find the regolith slightly more compact and rocky than average. The diffuse blue feature about 25 km to the southwest of the landing site is a "cold spot" characteristic of extremely fluffy material surrounding a very fresh impact crater. These unusual features are not well understood, but are thought to be a consequence of regolith decompression during the impact process, which gradually fades over time (Bandfield et al., 2014).
Diviner Standard Christiansen Feature (CF) Value
Diviner uses three bands near 8 microns (Ch 3, 4, & 5) to measure the Christiansen Feature (CF) and determine the bulk composition of lunar soils (Greenhagen et al., 2010, Science). The CF is related to silicate polymerization and shifts in a systematic way across lunar compositions (e.g. Conel, 1969, JGR; Logan et al., 1973, JGR; Salisbury and Walter, 1989, JGR). The median CF Value for highlands materials is 8.15 and mare materials is 8.28. Standard CF is sensitive to illumination and viewing geometry, and space weathering.
The composition of the area around the landing site is very uniform, and similar to global averages. Most of the "color" in this map is due to the combination of data observed at different local times and/or caused by small, fresh impact craters, which have CFs shifted to shorter values.
Diviner Corrected Christiansen Feature (CF) Value
Due to an observed dependence of Standard CF Value with solar illumination and instrument viewing geometry, we attempt to normalize the radiance data to equatorial noon and calculate a “corrected” CF Value (after Greenhagen et al., 2010, Science; Greenhagen et al., 2011, LPSC). Corrected CF Values are useful to comparisons across ranges of latitude and local time and for comparisons to laboratory data. Data artifacts are most noticeable in very dark terrains, which are underrepresented in the photometric database used to compile the correction.
Diviner Concavity Index (CI)
This spectral concavity index was developed to identify compositions that have CFs outside (either short- or long-ward of Diviner’s 8 micron bands (Glotch et al., 2010, Science). The CI is particularly useful for identifying highly silicic materials such as quart and alkali feldspar, which have a strong positive CI (Glotch et al., 2010, Science).
There is no evidence for highly silicic materials in or around the Chan'ge 3 landing site.
Diviner Ch 4 Brightness Temperature (TB)
The peak of the CF is most consistently near Diviner’s Ch 4 passband; therefore, Ch 4 has a TB with close to unit emissivity.