Volatile-rich impact ejecta on Mercury

Post contributed by Dr Jack Wright, School of Physical Sciences, The Open University, UK.

The Caloris basin is the largest (~1,500 km across), well-preserved impact structure on Mercury (Image 1a; Fassett et al., 2009). Hummocky plains around Caloris host numerous, steep-looking, conical knobs (Image 1b). The obvious explanation for the hummocky plains is that they formed from material ejected by the Caloris impact ~3.8 billion years ago. It follows that the knobs probably formed from discrete ejecta blocks. What isn’t obvious is why many of these blocks, which hypothetically could have formed with a variety of shapes, exist as steep cones in the present day. If these knobs really did form as Caloris ejecta, then they offer a rare opportunity to study materials ejected from Mercury’s interior with remote sensing techniques.

Image 1: Mercury and the circum-Caloris knobs. (a) Enhanced colour limb view of Mercury from the MESSENGER spacecraft. The Caloris basin’s interior is made of volcanic plains that appear orange in this data product. The arrow indicates the location of (b). (b) Examples of circum-Caloris knobs just outside the Caloris rim. Mosaic of MESSENGER MDIS WAC frames EW0220807059G, EW0220807071G, and EW0220763870G. ~86 m/pixel.


Impact Crater Degradation on Mercury

Post by Mallory Kinczyk, PhD candidate, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University

The formation of impact craters may be the most ubiquitous exogenic surface process in the Solar System. These craters take on many shapes and sizes and can hint at underlying rock types, tell us about the nature of the impactor, and can shed light on the body’s geological history. Even on bodies without atmospheres, erosive forces are at play, changing the crater shape through time via processes such as seismic shaking and disruption from debris thrown outward by subsequent, nearby impacts. Because Mercury is the only terrestrial planet without an atmosphere, it maintains a unique snapshot of the inner Solar System’s impactor population (Image 1) and, in turn, can shed light onto Earth’s own geological history.

converted PNM file

Image 1: View of Mercury from the MESSENGER spacecraft, which orbited Mercury between 2011 and 2015 (Image PIA17280). A variety of impact crater sizes and shapes are evident from very fresh craters to subdued to almost completely obliterated crater forms. Bach crater (arrow) hosts a well-defined central peak ring, but its subdued form indicates that it has been disrupted by subsequent craters. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.


It is Mercury’s fault(s)…

Post contributed by Valentina Galluzzi, INAF, Istituto di Astrofisica e Planetologia Spaziali (IAPS), Rome, Italy

Any celestial body that possesses a rigid crust, be it made of rock (e.g. terrestrial planets, asteroids) or ice (e.g. icy satellites), is subject to both endogenic and exogenic forces that cause the deformation of crustal materials. As a result of the mass movement, the brittle layers often break and slide along “planes” commonly known as faults. In particular, tensional, compressional and shear forces form normal, reverse and strike-slip faults, respectively. On Earth, plate tectonics is the main source of these stresses, being a balanced process that causes the lithospheric plates to diverge, converge and slide with respect to each other. On Mercury, there are no plates and therefore the tectonics work differently. Instead its surface is dominated by widespread lobate scarps, which are the surface expression of contractional thrust faults (i.e. reverse faults whose dip angle is less than 45°) and this small planet is in a state of global contraction.


Image 1. Endeavour Rupes area on Mercury, image is centred at 37.5°N, 31.7°W. Top: MESSENGER MDIS High-Incidence angle basemap illuminated from the West (HIW) at 166 m/pixel. Bottom: MESSENGER global DEM v2 with a 665m grid [USGS Astrogeology Science Center] on HIW basemap, the purple to brown colour ramp represents low to high elevations, respectively. Endeavour Rupes scarp is high ~500 m. For scale, Holbein crater diameter is approximately 110 km.


Recent vents and channels on the Cerberus plains of Mars: lava or water?

Posted by Rebecca Thomas, Department of Physical Sciences, The Open University, UK.

Recent channelized flows from vents in the Cerberus plains of Mars demonstrate the difficulties of uniquely ascribing process to landforms on other planets.  The image below shows two fissures emanating from a wrinkle ridge. Both fissures appear to be sources of approximately contemporaneous channels running down onto the surrounding plains (Thomas, 2013). The channel in the west is constructive and differs from that in the east which is clearly shows several phases of incision (Image 1).

Image 1: a. Vents and channels in the Cerberus plains, Mars (156.9° E, 7.1° N); b. incised channel; c. constructed, leveed channel. (HiRISE ESP_016361_1870)

Image 1: a. Vents and channels in the Cerberus plains, Mars (156.9° E, 7.1° N); b. incised channel; c. constructed, leveed channel. (HiRISE ESP_016361_1870)


Young volcanism on Mercury

Post by Carolyn Ernst, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.

Prior to 2008, less than half of Mercury’s surface had been imaged at close range, during the flybys of Mariner 10 in the mid-1970s. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of the planet in 2008 and 2009 on its way to insertion into orbit about Mercury on 18 March 2010 and viewed most of the planet’s surface that had never before been seen by a spacecraft. These MESSENGER images have helped to confirm some Mariner-10-based hypotheses and have elicited new science questions to be investigated.


Image 1: Narrow-angle camera mosaic of Rachmaninoff basin, 290 km in diameter, as seen during MESSENGER’s third Mercury flyby on 29 September 2009. Orthographic projection, ~ 440 m/pixel, centered at ~28ºN, 58ºE. MESSENGER images 0162744128 and 0162744150, credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.


Mercury’s Mighty Valles

Post by Dr. Paul K. Byrne, Carnegie Institution of Washington, USA

 Channel-like landforms termed “valles” (sing. “vallis”) have been observed on the Moon, Mars, and Venus, and recent results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission show that the innermost planet hosts its own brand of valles, too. Resembling the broad outflow channels on Mars and Venus, five shallow, linear depressions form a channelized network at high latitudes in Mercury’s northern hemisphere. These valles are situated adjacent to expansive northern volcanic plains that cover some 6% of the planet’s surface, and likely conducted voluminous, low-viscosity lavas from these plains southward.

Image 1: This vallis has the steep sides, smooth floors, and erosional residuals characteristic of Mercury’s broad valles, and likely channeled lavas from left to right across the image. The image has a field of view of ca. 150 km. The 57°N parallel and 115°E meridian are shown, and Kofi basin is labeled. The image is a portion of MESSENGER’s Mercury Dual Imaging System (MDIS) global monochrome basemap, which has a resolution of 250 meters per pixel.


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