If you keep up with science-related news at all, you may have heard that the MESSENGER spacecraft orbiting Mercury crashed into the surface of the innermost planet on April 30th, 2015, after running out of propellant. The spacecraft was travelling at 4 kilometers a second (over 8700 miles an hour) and should have made a crater 16 meters (52 feet) in diameter. MESSENGER (MErcury Surface, Space ENvironment, and Ranging) was launched on August 3rd, 2004. Due to the complications of orbital mechanics, considerations of costs, and other factors, its path through the solar system did not place the spacecraft in orbit around Mercury until March 2011.
With the proverbial death of MESSENGER, it seems fitting to write about the spacecraft in tribute. Therefore, today’s paper is “Thermal Evolution of Mercury as Constrained by MESSENGER Observations“, written by Nathalie C. Michel, Steven A. Hauck II, Sean C. Solomon, Roger J. Phillips, James H. Roberts, and Maria T. Zuber. The work was published in the Journal of Geophysical Research: Planets in May 2013.
The history of a planet in terms of heat–its origin, its ‘loss’ to space–is important in understanding how a planet came to be what we see today. Today’s paper is looking at that for Mercury, asking: what’s the history of convection in the mantle? What does this mean for volcanic activity?
Among models that have been run in other literature, there has been disagreement about whether or not there is ongoing convection in Mercury’s mantle at present. The mantle is the section between the surface crust and the inner core, which is semisolid and can move; convection refers to the movement of heat using a medium like a hot fluid–or in this case, the mantle.
The authors of this paper used an asymmetric, 2-dimensional model with varying parameters to try to solve the mystery of present-day mantle convection within Mercury. Their answer?
They’re not sure, either.
The way to look at convection in an equation-filled modeling system is to look at the Rayleigh number. The Rayleigh number is a parameter that depends on several factors. Below a certain Rayleigh number, there is no convection. Above a certain Rayleigh number, known as the ‘critical’ number, convection can occur. Since the Rayleigh number involves a lot of different things, convection would or would not occur within a model, depending on the variables used. In general, if the modeled Mercury had a mantle thicker than about 400 kilometers (about 250 miles), there would be convection from the beginning of the planet to the present day. As the thickness of the mantle decreased, the results of the model suggested convection only happened in the earlier parts of Mercury’s lifespan.
There are several other factors that could influence convection. For one, the initial start of the convection itself, whatever it was that first got it going. In the long-term, the models found this did not change the behavior. The extreme variations in surface temperature (from proximity to the sun, an eccentric orbit, and the length of a year compared to the length of a day for Mercury) did little to affect the outcome of the model.
There appears to have definitely been mantle convection in the past. Mantle convection is tied to volcanism–it is, in many ways, the movement of lava. The model results suggest that there was active magma in the past. This fits with the observed smooth plains on the surface of Mercury. In some models, this activity in the magma continued until about one billion years ago (geologically speaking, a fairly short amount of time).
In the end, we’re still not sure if there is ongoing mantle convection on Mercury. We need more data, more precise knowledge to put constraints on our models. MESSENGER was orbiting Mercury for two years after this article came out, and may have shed further light. Additionally, the European Space Agency and Japan Aerospace Exploration Agency are combining forces for the next planned mission to Mercury, the BepiColumbo project with a planned 2016 launch. MESSENGER did good work, and the spacecraft will be missed–but not forgotten, as we continue to sift through the invaluable data it sent back after 4104 orbits around Mercury.
Sources used to write this article.
“Artist’s Impressions: MESSENGER” N.p., n.d. Web.
The image of MESSENGER at the beginning of the article.
Astronomy Picture of the Day. “APOD: 2015 May 1 – MESSENGER’s Last Day on Mercury.” N.p., 2015 May 1. Web.
Image of the impact zone for MESSENGER.
“FAQ: MESSENGER” N.p., n.d. Web.
FAQ page for the MESSENGER mission. Used for timeline and a few other bits of information.
“MESSENGER Web Site.” N.p., n.d. Web.
A website devoted to the MESSENGER mission.
Michel, Nathalie C., et al. “Thermal evolution of Mercury as constrained by MESSENGER observations.” Journal of Geophysical Research: Planets 118.5 (2013): 1033-1044.
The main paper for today.
National Aeronautics and Space Administration. “Unmasking the Secrets of Mercury.” N.p., 2015 May 1. Web.
NASA page from which I took the final image in the post.
Turcotte, Donald Lawson, and Gerald Schubert. Geodynamics. 2nd ed. Cambridge ; New York: Cambridge University Press, 2002. Print.
A textbook from my undergraduate years. Used to remind myself about convection and the other two types of heat transfer (conduction, radiation), and also for information about the Rayleigh number.