- Title: Darkening of Mercury’s surface by cometary carbon
- Authors: Megan Bruck Syal, Peter H. Schultz, Miriam A. Riner
- First author’s institution: Lawrence Livermore National Laboratory
- Status of the Paper: Published in Nature Geoscience
What’s the issue?
Today’s Astrobite presents an explanation of a feature of the Solar System‘s smallest and innermost planet: the darkening of Mercury. So far astronomers could get no satisfaction in explaining the fact that Mercury’s surface is darker than the Moon’s. In principle, iron is the most common cause for darkening of bodies without an air atmosphere. The problem for Mercury is: Mercury’s surface consists of less iron than the Moon. Thus, there has to be a material different from iron that “paints” Mercury black.
Shooting on a Range and Gambling in Monte Carlo for Science
The authors of the article propose that carbon, instead of iron, can darken the surface of Mercury sufficiently. Indeed they do experiments at NASA’s Ames Vertical Gun Range, where they shoot projectiles onto Moon-like material mixed with/without organics. When the projectile is shot on the material with organics, the heat induced by the impact causes the formation of carbon. As you can see in Figure 1 (Figure 3 in the paper) the reflectance of light from the surface of the two materials is significantly lower, when carbon is present and the values are consistent with the darkest areas on Mercury.
At this point you may ask: Fine, but why should there be more carbon on Mercury than on the Moon? The explanation of the authors is based on two observations.
- Comets consist on average about 18% of carbon.
- The number of cometary impacts per unit area decreases roughly inversely with decreasing radial distance from the Sun.
Considering that enough of the impacting material retains on Mercury, the authors suggest that the larger amount of meteoritic impacts enrich the surface of Mercury more in carbon than the surface of the Moon. To be precise, they consider only small meteorites – so called micrometeorites – and they assume a constant spherical size of 0.25 cm and constant speed of 20 km/s. You may argue now that this is a drastic simplification since meteorites have varying sizes and corresponding higher speeds. The authors are aware of that, but they argue that larger objects have higher speeds such that they will not be captured by Mercury’s gravitational field. Thus the impact of larger objects is negligible and micrometeorites are the dominating objects for impact.
The authors test their idea with a Monte Carlo code, where they compute the percentage of retaining meteorites on the Moon and Mercury for different impact angles. In Figure 2 of this Astrobite (Figure 1 in the paper) you can see the probability for a micrometeorite of certain impact angle and the mass fraction of the objects that retain on Mercury. The impactors (micrometeorites) are resolved by several grid cells in the code. The drop in the retainment fraction at 30° reflects the fact that the micrometeorite has relatively seen more energy compared to the target than at different angles. The results are also similar for tracer particles that follow the movement of impactors during the simulation. The authors explain the differences at 15° with asymmetric shock conditions such that some mass of the impactors does not stay on the surface. However, this process is – in contrast to the first method – not resolved by tracing the micrometeorites as single objects by tracer particles.
Altogether, you can see that a sufficient amount of mass from the impactors stays on the surface (on average 83 % for Mercury and 63 % for the Moon) and the authors conclude that approximately 50 times more carbon-rich micrometeorites are delivered to Mercury than to the Moon. Together with the result from the shooting experiment that carbon darkens the surface efficiently, micrometeorite impact may cause the darkening of Mercury. In other words: Falling stones paint it black.
Attribution for the image of Mercury: By NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Edited version of Image:Mercury in color – Prockter07.jpg by Papa Lima Whiskey. (NASA/JPL ) [Public domain], via Wikimedia Commons