Zhiyong Xiao

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations of Mercury’s high northern latitudes reveal a contiguous area of volcanic smooth plains covering more than ~6% of the surface that were emplaced in a flood lava mode, consistent with average crustal compositions broadly similar to terrestrial komatiites. MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity

Rimless shallow depressions on Mercury may still be forming by outgassing, volcanism, sublimation, or space weathering. High-resolution images of Mercury’s surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury’s interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system’s innermost planet.

Mercury's Hollows: Constraints on Formation and Composition from Analysis of Geological Setting and Spectral Reflectance

[1] Landforms unique to Mercury, hollows are shallow, flat-floored irregular depressions notable for their relatively high reflectance and characteristic color. Here we document the range of geological settings in which hollows occur. Most are associated with impact structures (simple bowl-shaped craters to multiring basins, and ranging from Kuiperian to Calorian in age). Hollows are found in the low-reflectance material global color unit and in low-reflectance blue plains, but they appear to be absent from high-reflectance red plains. Hollows may occur preferentially on equator- or hot-pole-facing slopes, implying that their formation is linked to solar heating. Evidence suggests that hollows form because of loss of volatile material. We describe hypotheses for the origin of the volatiles and for how such loss proceeds. Intense space weathering and solar heating are likely contributors to the loss of volatiles; contact heating by melts could promote the formation of hollows in some locations. Lunar Ina-type depressions differ from hollows on Mercury in a number of characteristics, so it is unclear if they represent a good analog. We also use MESSENGER multispectral images to characterize a variety of surfaces on Mercury, including hollows, within a framework defined by laboratory spectra for analog minerals and lunar samples. Data from MESSENGERs X-Ray Spectrometer indicate that the planets surface contains up to 4% sulfur. We conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low-reflectance material relative to average surface material. Hollows may owe their relatively high reflectance to destruction of the darkening agent (sulfides), the presence of alteration minerals, and/or physical differences in particle size, texture, or scattering behavior.

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400u2009nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations

Received 12 April 2012; revised 31 August 2012; accepted 1 November 2012; published 31 March 2013. [1] Volcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have formed by lava erosion. We used analytical models to estimate eruption flux, erosion rate, and eruption duration to characterize the formation of candidate erosional features, and we compared results with analyses of similar features observed on Earth, the Moon, and Mars. Results suggest that lava erupting at high effusion rates similar to those required to form the Teepee Butte Member of the Columbia River flood basalts (0.1–1.2 � 10 6 m 3 s –1 ) would have been necessary to form wide valleys (>15km wide) observed in Mercury’s northern hemisphere, first by mechanical erosion to remove an upper regolith layer, then by thermal erosion once a lower rigid layer was encountered. Alternatively, results suggest that lava erupting at lower effusion rates similar to those predicted to have formed Rima Prinz on the Moon (4400 m 3 s –1 ) would have been required to form, via thermal erosion, narrower channels (<7km wide) observed on Mercury. Although these results indicate how erosion might have occurred on Mercury, the observed features may have formed by other processes, including lava flooding terrain sculpted during the formation of the Caloris basin in the case of the wide valleys, or impact melt carving channels into impact ejecta in the case of the narrower channels. Citation: Hurwitz, D. M., J. W. Head, P. K. Byrne, Z. Xiao, S. C. Solomon, M. T. Zuber, D. E. Smith, and G. A. Neumann (2013), Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations, J. Geophys. Res. Planets, 118, 471–486, doi:10.1029/2012JE004103.

Duration of activity on lobate-scarp thrust faults on Mercury: Thrust Fault Activity on Mercury

Lobate scarps, landforms interpreted as the surface manifestation of thrust faults, are widely distributed across Mercury and preserve a record of its history of crustal deformation. Their formation is primarily attributed to the accommodation of horizontal shortening of Mercurys lithosphere in response to cooling and contraction of the planets interior. Analyses of images acquired by the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during flybys of Mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains. However, the full temporal extent of thrust fault activity on Mercury, particularly the duration of this activity following smooth plains emplacement, remained poorly constrained. Orbital images from the MESSENGER spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states. Analysis of these stratigraphic relations indicates that contraction has been a widespread and long-lived process on the surface of Mercury. Thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of Mercurys subsequent history. Such deformation likely resulted from the continuing secular cooling of Mercurys interior.

Cooling fractures in impact melt deposits on the Moon and Mercury: Implications for cooling solely by thermal radiation

We study the distribution, morphology, and geometrical properties of fractures in several young impact melt deposits on the Moon and Mercury, and the ways that these fractures may form from cooling by thermal radiation. In each impact melt complex, the topography of the underlying terrain determines the orientation of cooling fractures, such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the units margins. Solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures, but too much solid debris prevents cooling fractures from growing to macroscopic sizes. The appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification. Tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the Moon and Mercury, which may represent the initial stage of columnar joints formation, but the cooling rate caused solely by thermal radiation is not large enough to form well-organized columnar joints that feature polygonal colonnades. We therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies.

Effect of Topography Degradation on Crater Size‐Frequency Distributions: Implications for Populations of Small Craters and Age Dating

Whether or not background secondary craters dominate populations of small impact craters on terrestrial bodies is a half-century controversy. It has been suggested that small craters on some planetary bodies are dominated by background secondary craters based partly on the steepened slope of crater size-frequency distribution (CSFD) towards small diameters, such as the less than ~1 km diameter crater population on the lunar mare. Here we show that topography degradation enlarges craters and increases CSFD slopes with time. When topography degradation is taken into account, for various-aged crater populations, the observed steep CSFD at small diameters is uniformly consistent with an originally shallower CSFD, whose slope is undifferentiated from the CSFD slope estimated from near-Earth objects and terrestrial bolides. The results show that the effect of topography degradation on CSFD is important in dating planetary surfaces, and the steepening of CSFD slopes is not necessarily caused by secondary cratering, but rather a natural consequence of topography degradation.

Subsurface Structures at the Chang'e-3 Landing Site: Interpretations from Orbital and In-Situ Imagery Data

The Chang’e-3 (CE-3) spacecraft successfully landed on one of the youngest mare surfaces on the Moon in December 2013. The Yutu rover carried by CE-3 was equipped with a radar system that could reveal subsurface structures in unprecedented details, which would facilitate understanding regional and global evolutionary history of the Moon. Based on regional geology, cratering scaling, and morphological study, here we quantify the subsurface structures of the landing site using high-resolution orbital and in-situ imagery data. Three layers of lunar regolith, two layers of basalt units, and one layer of ejecta deposits are recognized at the subsurface of the landing site, and their thicknesses are deduced based on the imagery data. These results could serve as essential references for the on-going interpretation of the CE-3 radar data. The ability to validate our theoretical subsurface structure using CE-3 in-situ radar observations will improve the methods for quantifying lunar subsurface structure using crater morphologies and scaling.

Geological Characteristics of Von Kármán Crater, Northwestern South Pole‐Aitken Basin: Chang'E‐4 Landing Site Region

Von Karman crater (diameterxa0=xa0~186xa0km), lying in the northwestern South Pole-Aitken (SPA) basin, was formed in the pre-Nectarian. The Von Karman crater floor was subsequently flooded with one or several generations of mare basalts during the Imbrian period. Numerous subsequent impact craters in the surrounding region delivered ejecta to the floor, together forming a rich sample of the SPA basin and farside geologic history. We studied in details the targeted landing region (45.0–46.0°S, 176.4–178.8°E) of the 2018 Chinese lunar mission ChangE-4, within the Von Karman crater. The topography of the landing region is generally flat at a baseline of ~60xa0m. Secondary craters and ejecta materials have covered most of the mare unit and can be traced back to at least four source craters (Finsen, Von Karman L, Von Karman L, and Antoniadi) based on preferential spatial orientations and crosscutting relationships. Extensive sinuous ridges and troughs are identified spatially related to Ba Jie crater (diameterxa0=xa0~4xa0km). Reflectance spectral variations due to difference in both composition and physical properties are observed among the ejecta from various-sized craters on the mare unit. The composition trends were used together with crater scaling relationships and estimates of regolith thickness to reconstruct the subsurface stratigraphy. The results reveal a complex geological history of the landing region and set the framework for the in situ measurements of the CE-4 mission, which will provide unique insights into the compositions of farside mare basalt, SPA compositional zone including SPA compositional anomaly and Mg-pyroxene annulus, regolith evolution, and the lunar space environment.