NASA MESSENGER mission: a tool to study Mercury beyond its operative life

Introduction: 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.

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].

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].

Methods: 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].

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°.

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.

Results: 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.

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.

 

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 is about 20 m, and they can be as deep as about 4 m.

Discussion and Conclusion: In this work, we present the preliminary results of the systematic analysis of an aluminum cylinder, with the same mass of the MESSENGER spacecraft, impacting on one- and two-layered targets. The investigation was focused on testing the effects of the target properties on the final crater morphometry. Depending on the model setup, we found a crater diameter ranging from 5 to 20 m, and a crater depth from 1 to 4 m.

The ultimate goal of this study is to verify whether the crater formed by the crushing of the MESSENGER spacecraft might be observed by the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) [6], which is the imaging system onboard the ESA BepiColombo mission [13, 14]. SIMBIO-SYS is made up by a high resolution imaging channel (HRIC, resolution of 5 m/pix at periherm), a VIS–NIR spectrometer (VIHI), and a stereo imaging channel (STC) that will provide panchromatic 3D global mapping and colour imaging of selected regions of the Hermean surface, with a spatial resolution lower than 110 m.

Given the instrument performances and the results obtained from the numerical modelling, the MESSENGER crater might be observed by the only high-resolution channel, in the case that the near-surface stratigraphy of the Suisei Planitia is made up by a thick layer of regolith. We are going to investigate the ejecta distribution, in order to increase the MESSENGER crater observability.

Acknowledgments: We gratefully acknowledge the developers of iSALE‐2D/Dellen version (https://isale-code.github.io), including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov, and Jay Melosh. Some plots in this work were created with the pySALEPlot tool written by Tom Davison.

This activity has been realized under the BepiColombo ASI-INAF contract no 2017- 47-H.0.

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