Scientific Discoveries of the
Lunar Reconnaissance Orbiter and
Opportunity Rover During Extended Phase
LUNAR RECONNAISSANCE ORBITER
The Lunar Reconnaissance Orbiter (LRO) has been orbiting the Moon for nearly 7 years. Originally in a quasi-circular 50 km orbit, after 18 months of operation LRO was moved to a ~30 km × ~180 km orbit to conserve fuel; all extended missions observations have been from the fuel-saving elliptical orbit. LRO includes seven science experiments; all remain healthy, except that the Miniature Radar Frequency (Mini-RF) transmitter ceased to function in December 2010 but still produces useful measurements as a receiver in a bi-static configuration (Earth-based assets transmit). An important legacy of the LRO mission is the vast amount of data made available to the scientific community, which is expected to be >900 TB by the end of 2018. This legacy data set will be used for decades of lunar exploration and science.
A few of the key LRO science results from the extended mission are summarized below. More than 220 new resolved impact craters were discovered as of March 2016 (Figure B.1), having diameters of 1.4 to 43 m. The number of new craters shows that the size frequency distribution is steeper than expected based on models commonly used to date surfaces. In addition to the craters themselves, >45,000 albedo marks (splotches) are observed that provide information regarding secondary cratering processes (Robinson et al., 2015).
The high-resolution LROC images also revealed numerous small-scale tectonic features with pristine morphologies, indicating that they are likely still forming, most likely due to cooling of the interior. The orientation of these scarps is not random but rather consistent with a pattern expected from stresses introduced from solid body tides with Earth (Watters et al., 2015). The Lunar Orbiter Laser Altimeter (LOLA) detected enhanced reflectivity @1064 nm in permanently shadowed regions at both the north and south poles (Lucey et al., 2014). These data, together with other data such as from the Lyman Alpha Mapping Project (LAMP) and temperatures measured by Lunar Diviner Radiometer (Hayne et al., 2015), collectively suggest that a micron-thick layer of water ice is present in these regions. The polar hydrogen distribution at both the north and south poles is asymmetric and mirrored, suggesting that true polar wander has occurred (Siegler et al., 2016). Although most volcanism on the Moon appears to have ended 2 to 3 Gyr ago, observations by LROC suggest late stage activity persisted until <100 Myr (Braden et al., 2014). The abundance of rocks in ejecta blankets is well correlated with the age of the crater from ~100 kyr to ~1.5 Gyr (Ghent et al., 2014), establishing a new “lithochronology” technique. The Mini-RF instrument is operated in concert with the Arecibo Observatory to collect bistatic radar data of the lunar nearside from 2012 to 2015; the response for the floor of the south-polar permanent shadowed region in Cabeus crater is consistent with the presence of blocky, near-surface deposits of water ice (Patterson et al., 2016).
MARS EXPLORATION ROVER OPPORTUNITY
The Mars Exploration Rover (MER) Opportunity landed on the Meridiani Planum plains of Mars in January 2004. After completing its initial 90-sol (92.5-day) mission, Opportunity entered the extended-mission phase and has remained operational for more than 12 years—more than 4,500 sols. Opportunity continues a legacy of U.S. in situ exploration of Mars that was initiated with the 1997 Mars Pathfinder mission. The rover initially traversed the Eagle Crater to look for signs of habitability, but then continued traversing tens of thousands of meters further to survey the Endurance crater, Victoria crater, Endeavour crater, and beyond (Figure B.2). Microscopic Imager (MI) glitches, flash memory data loss, and an “arthritic” robotic arm have not yet become mission-inhibiting challenges. Its instruments are all fully operational; the rover continues to survey the planet using cameras, spectrometers, and magnets, although its Rock Abrasion Tool is no longer operational. Opportunity’s ongoing observations continue to be a valuable source of insight into the ancient Mars environment. This section will summarize the key findings made by Opportunity since it began its extended-mission phase.
It is important to establish that the extended mission was vital toward characterizing past environments. The Burns Formation, named after Roger Burns, is a designation for a region-wide group of rocks exposed by impact-related crater formation or fracturing and explored by Opportunity. The observations from the Burns Formation in the Endurance crater helped support early observations of the formation in the Eagle crater, which together confirmed the past presence of water on Mars (Squyres and Knoll, 2005; Grotzinger et al., 2005).
Grotzinger et al. (2005) divided the Burns Formation into an upper, middle, and lower unit by similar depositional features and characterized eolian dune, eolian sand sheet, and damp to wet interdune environment types (called facies associations) in the Eagle and Endurance craters. All three units were composed of sandstone (Grotzinger et al., 2005). It was found that tepee-like or salt-ridge irregularities on a scoured sandstone facies suggested a regularly oscillating water table that sometimes reached the surface to create an ephemerally damp environment (Grotzinger et al., 2005). Miniature Thermal Emission Spectrometer (Mini-TES) data detected evaporite and sulfate minerals, suggesting that the grains deposited in the Burns Formation dunes were transported from an evaporite basin containing water that interacted with basalt (Grotzinger et al., 2005; McLennan et al., 2005). Bromine, which is found in very soluble minerals, was also detected in Meridiani Planum soils, suggesting acitivity by liquid water (Yen et al., 2005). Meanwhile, the unambiguous presence of jarosite—a sulfate mineral group—as an evaporite mineral suggested that Mars liquid water had a low pH because jarosite precipitates only from acidic solutions (McLennan et al., 2005; Squyres and Knoll, 2005). Additionally, hematite spherules about 4 mm in diameter, informally named “blueberries,” were theorized to be formed by a concretion process from the
breakdown of jarosite by groundwater or by oxidation of ferrous sulfates (McLennan et al., 2005). Therefore, grain formation on the Meridiani Planum of Mars was discovered to be once driven by acidic liquid water.
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