<p><strong>Introduction:</strong> The morphology of impact craters depends on the target properties, including changes in density, strength, water content, porosity, and composition (e.g., [1,2,3]), allowing in turn to study the interior of planetary bodies.</p> <p>Along with naturally formed impact structures, there are a number of artificial craters, produced by a metallic module while the main spacecraft observes the process and the impact products [4]. One benefit of this method is that the initial impact energy is well known, and therefore any variation with respect to the expected shape depends on the target properties only. Spacecrafts crushing on planetary surfaces at the end of their operative life can also provide valuable information on the target material properties and near-surface stratigraphy, when they are imaged by subsequent space missions [5].</p> <p>In this work, we analyze the formation of the crater formed by the NASA MESSENGER spacecraft, which impacted on the Hermean surface on April 2015. We investigate the artificial crater via numerical modelling by testing several targets scenarios, and discussing the outcomes in view of a possible future observations [6].</p> <p><strong>Methods:</strong> Numerical models have been carried out with the iSALE shock physics code [7, 8, 9, 10], which is well tested against laboratory experiments and other hydrocodes [11].</p> <p>We approximate the MESSENGER spacecraft as an aluminum cylinder, which is described by the aluminum Tillotson Equation of State and the Johnson-Cook strength model), and has impact velocity of ~4 km/s, and impact angles of 30°, 45°, and 90°.</p> <p>The impact location is within the Suisei Planitia, a poorly cratered smooth plains located north-east of Caloris basin [12], and it is assumed of basaltic composition. The surface is approximated as a two-layer target, composed by a lower crust layer described by a pressure and damage-dependent strength model [9], overlaid by a regolith layer, described by the Drucker-Prager strenth model. The thickness of the upper regolith layer was varied from 0 (one layer) up to 20 m. Several strength and inner friction values have been tested. The porosity was set to 40% and 10% for the regolith and the crust, respectively. We tested surface temperature of 440 K and 700 K, while no thermal gradient was implemented within the target.</p> <p><strong>Results: </strong>In Fig. 1, we compare the formation of the MESSENGER crater, when considering (on the right) or not (on the left) the presence of an upper regolith layer.</p> <p><img src="" alt="" /></p> <p><strong>Fig. 1. Comparison of time series models, with a regolith thickness of 0 m (left) and 2 m (right). The colour map refers to the three different materials used: fuchsia for the projectile, green for the basaltic crust, and pale yellow for the regolith.</strong></p> <p> </p> <p>Generally, we found that craters forming in competent rock surfaces are <10 m wide, and about 1 m deep. In two-layer targets, with a thin upper regolith layer (1 to 2 m), the final craters display a flat floor morphology, where the flat bottom occurs in correspondence of the transition between the rheologies governing the two layers. Craters can be as much as about 20 m in diameter, and 2 m deep. In the case of thick regolith layers (>5 m), the crater develops entirely in the upper regolith layer, and thus their final shape depends on the regolith properties. The diameter

MESSENGER MESSENGER, MErcury Surface, Space ENvironment, GEochemistry and Ranging, was an unmanned NASA and APL spacecraft.[7] It was orbiting and studying the planet Mercury.[7] Its mission lasted 10 years, 8 months and 28 days. It was launched on August 3, 2004[7][8] at Cape Canaveral Air Force Station. It was aboard a Boeing Delta II rocket.[8] After launch, the probe did several fly-bys and deep space manoeuvres to gain the right trajectory and speed.[7] It completed 30% mapping of Mercury on January 14, 2008. MESSENGER made one more pass by Mercury in 2009, and on March 18, 2011 began to orbit Mercury.[7][9] 100% mapping was completed in March 2013 and the probe continued its studies. On April 30, 2015, it crashed into Mercury. It crashed near the crater Janáček. References - ↑ "NASA extends spacecraft's Mercury mission". UPI. November 15, 2011. Retrieved December 20, 2012. - ↑ Wu, Brian (April 3, 2015). "NASA Set to Extend Mercury Mission for Another Month". Johns Hopkins University APL. The Science Times. Retrieved April 4, 2015. - ↑ "MESSENGER's Operations at Mercury Extended". Johns Hopkins University APL. SpaceRef.com. April 3, 2015. Retrieved April 4, 2015.