Hi there! You know Mercury, the innermost planet of the Solar System. It has recently been explored during 4 years by the American spacecraft MESSENGER, which gave us invaluable data on its surface, its magnetic field, its interior…
Today I present you a study on the ice on Mercury. It is entitled Constraining the thickness of polar ice deposits on Mercury using the Mercury Laser Altimeter and small craters in permanently shadowed regions, by Ariel N. Deutsch, James W. Head, Nancy L. Chabot & Gregory A. Neumann, and has recently been accepted for publication in Icarus.
We know that there is some ice at the surface of Mercury, and the study wonders how much. Since Mercury is close to the Sun, its surface is usually hot enough to sublimate the ice… except in permanently shadowed regions, i.e. in craters. For that, the authors compared the measured depth of small craters, and compared it with the expected depth from the excavation of material by an impactor. The difference is supposed to be ice deposit.

Mercury and MESSENGER

The planet Mercury is known at least since the 14th century BC. It was named after the Roman messenger god Mercurius, or Hermes in Greek, since the messengers saw it at dawn when they left, and at dusk when they arrived. The reason is that Mercury is in fact pretty close to the Sun, i.e. three times closer than our Earth. So, usually the Sun is so bright that it prevents us from observing it. Unless it is below the horizon, which happens at dawn and at dusk.

Mercury makes a full revolution around the Sun in 88 days, and a full rotation in 58 days. This 2/3 ratio is a dynamical equilibrium, named 3:2 spin-orbit resonance, which has been reached after slow despinning over the ages. This despinning is indeed a loss of energy, which has been favored by the tidal (gravitational) action of the Sun. This resulting spin-orbit resonant configuration is a unique case in the Solar System. A consequence is that the Solar day on Mercury lasts 176 days, i.e. if you live on Mercury, the apparent course of the Sun in the sky lasts 176 days.

The proximity of the Sun makes Mercury a challenge for exploration. Mariner 10 made 3 fly-bys of it in 1974-1975, mapping 45% of its surface, and measuring a tiny magnetic field. We had to wait until 2011 for the US spacecraft MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) to be the first human-made object inserted into orbit around Mercury. The orbital phase lasted 4 years, and gave us a full map of the planet, gravity data, accurate measurements of its rotation, a list of craters, measurements of the magnetic field,…

The instrument of interest today is named MLA, for Mercury Laser Altimeter. This instrument used an infrared laser (wavelength: 1,064 nanometers) to estimate the height of the surface from the reflection of the laser: you send a laser signal, you get it back some time later, and from the time you have the distance, since you know the velocity, which is the velocity of the light. And in applying this technique all along the orbit, you produce a map of the whole planet. This permits for instance to estimate the size and depth of the craters.

Ice on Mercury

The discovery of ice at the poles of Mercury was announced in 1992. It was permitted by Earth-based radar imagery made at Goldstone Deep Space Communications Complex in the Mojave desert, in California (USA). Ice is pretty easy to uncover, because of its high reflectivity. But this raises some questions:

1. How can ice survive on Mercury?
2. How much ice is there?
3. How did it arrive?

The first question is not really a mystery. Because of its long Solar day and its absence of atmosphere (actually Mercury has a very tenuous exosphere, but we can forget it), Mercury experiences huge variations of temperature between day and night, i.e. from 100K to 700K, or -173°C to 427°C, or -279°F to 801°F (it is in fact not accurate at the 1°F level…). So, when a region is illuminated, the water ice is definitely not stable. However, there are regions, especially at the poles, which are never illuminated. There ice can survive.

The last two questions are answered by this study.

Ice is still present in craters

For not being illuminated, it helps to be close to a pole, but the topography can be helpful as well. The surface of Mercury is heavily cratered, and the bottoms of some of these craters are always hidden from the Sun. This is where the authors looked for ice. More precisely, they investigated 10 small craters within 10 degrees of the north pole. And for each of them, they estimated the expected depth from the diameter, and compared it with the measured depth. If it does not match, then you have water ice at the bottom. Easy, isn’t it?

Well, it is not actually that easy. The question is: did the water ice arrive after or before the excavation of the crater? If it arrived before, then the impactor just excavated some ice, and the measurements do not tell you anything.

Another challenge is to deal with the uncertainties. MLA was a wonderful instrument, with an accuracy smaller than the meter. Very well. But you are not that accurate if you want to predict the depth of a crater from its diameter. The authors used an empirical formula proposed by another study: d=(0.17±0.04)D^0.96±0.11, where d is the depth, and D the diameter. The problem is the ±, i.e. that formula is not exact. This uncertainty is physically relevant, since the depth of the crater might depend on the incidence angle of the impact, which you don’t know, or on the material at the exact location of the impact… and this is a problem, since you cannot be that accurate on the theoretical depth of the crater. The authors provide a numerical example: a 400-m diameter crater has an expected depth between 21.2 and 127.7 m… So, there is a risk that the thickness of ice that you would measure would be so uncertain that actual detection would be unsure. And this is what happens in almost of all the craters. But the detection is secured by the fact that several craters are involved: the more data you have, the lower the uncertainties. And the ice thickness derived from several craters is more accurate than the one derived from a single crater.

Results: how much ice?

And the result is: the ice thickness is 41⁺³⁰_-14m. The uncertainty is large, but the number remains positive anyway, which means that the detection is positive! Moreover, it is consistent with previous studies, from the detection of polar ice with Goldstone facilities, to similar studies on other regions of Mercury. So, there is ice on Mercury.
An extrapolation of this result suggests that the total mass of water ice on the surface of Mercury is “10¹⁴-10¹⁵ kg, which is equivalent to ~100-1,000 km³ ice in volume, assuming pure water ice with no porosity” (quoted from the study).

The origin of ice

Mercury is a dense planet, i.e. too dense for such a small planet. It is widely accepted that Mercury as we see it constituted a core of a proto-Mercury, which has been stripped from its mantle of lighter elements. Anyway, Mercury is too dense for the water ice to originate from it. It should come from outside, i.e. it has been brought by impactors. The authors cite studies stating that such a quantity could have been brought by micrometeorites, by Jupiter-family comets, and even by a single impactor.

Another spacecraft soon

Such a study does not only exploit the MESSENGER data, but is also a way to anticipate the future measurements by Bepi-Colombo. This mission will be constituted of two orbiters, one supervised by the European Space Agency (ESA), and the other one by the Japanese agency JAXA. Bepi-Colombo should be launched in October 2018 from Kourou (French Guiana), and inserted into orbit around Mercury in April 2026. Its accuracy is expected to be 10 times better than the one of MESSENGER, and the studies inferring results from MESSENGER data can be seen as predictions for Bepi-Colombo.

The study and its authors

• The study. You can find a preliminary version here,
• The LinkedIn profile of Ariel Nolan Deutsch,
• The web page of James W. Head, III,
• the one of Nancy L. Chabot,
• and the one of Gregory A. Neumann.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and (NEW) Pinterest.