The universe is full of surprises. More than once I have read an article about a new discovery and told myself something like this: “That can’t be right! They must have messed up somewhere, and we’ll read about how in a few weeks.” Sometimes I’ve been right. Far more often, I’ve been wrong.
Such was the case with the reports of ice in the polar regions of the planet Mercury in the early 1990s. Mercury is the planet nearest our sun, with an average distance less than 40% that of Earth; this means it gets sunlight that is almost seven times as intense as Earth’s. High noon at the equator on Mercury sees temperatures hot enough to melt lead. How could ice exist in such a place? This twenty-year-long story of increasingly sophisticated investigations is a great illustration of how confirming what we thought we knew—and finding out things we never suspected—how both of these are part of the process of scientific discovery.
Let’s go back to that original 1991 report. What made scientists think there was ice present? Using Earth-based radio telescopes, powerful radar beams were bounced off Mercury, and the returning signals used to create an image.
Several things can make such a radar image look bright, but for our purposes only two are important:
• Surface roughness—If the surface is rough on the same scale as the radar wavelength (a few centimeters) it will scatter the beam and the image will look dark. If the surface is smooth, it will reflect the beam and the image will look bright. This is what you are seeing in the areas marked as craters. Their floors are smoother than the surrounding terrain.
• Water ice—Water’s physical properties make it highly reflective of radar beams. The presence of ice would also create a bright image.
If this image looks pretty rough and low-resolution, consider that it was created by sending a radar beam from Earth and using whatever bounced back! It should come as no surprise that it could only show the crudest features of the planet. But instrumental advances, and the use of the world’s largest radio telescope at Arecibo, Puerto Rico
allowed a far more detailed radar image of the north pole of Mercury in 1999.
Now we can see that permanently shaded crater floors—areas of Mercury that never see the sun—are the areas showing the highest reflectivity and presumably contain the most ice.
Fast-forward to the present, where we have a spacecraft in orbit around the planet. MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging)–someone at NASA really stretched with this acronym—slipped into orbit in March 2011 and has been using an array of instruments to map and analyze the planet. What has it found?
Here is the cover of a recent issue of Science Magazine.
What it shows is a combination of the highest surface temperature in this region (Mercury’s north pole is at the center of the image) superimposed on the topography. Red areas reach temperatures above 450 K (350° F); blue and purple areas inside the craters never exceed 100 K (-280° F). These areas are cold enough to shelter water ice and frozen organic compounds.
So far none of this is real news—it is just confirming what we first suspected in 1991. But here is the surprise. Areas that are bright in radar are NOT bright in visible light. In other words, those areas that are reflecting radar in ways that indicate the presence of water ice? They look dark.
Here is what the authors of the Science papers think is going on.
• The dark deposits are thought to be organic materials that can survive higher temperatures without being “boiled off”.
• The organic material is assumed to have been directly deposited with the ice as part of the same process.
• The ice deposits show evidence of having been larger in the past. Dark areas that show no particular radar reflectivity are relic deposits of this earlier era.
• The presence of these organic materials strongly indicates that comets are the source of these deposits. (See this earlier post for more about comets.)
The series of images and captions that follow are taken from a press conference last November, and show how the scientists believe the deposits (both organic and icy) were formed. The full suite of media materials can be found here. It brings to mind the famous quote of the British geneticist J.B.S. Haldane: “I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.” Indeed!
A high-latitude impact crater illuminated by the angled rays of the Sun creates a region of very warm temperatures on the illuminated rim, lower temperatures on the illuminated floor of the crater, and extremely cold temperatures in regions of permanent shadow.
A comet or water-rich asteroid that also contains organic compounds impacts Mercury.
The water and organic compounds are spread over a wide geographic region, and a small fraction of both compounds migrate to the poles where they can become cold-trapped as ices.
Over time, the water ice in the warmer regions vaporizes, leaving behind the more stable organic impurities at the surface.
The ice retreats further to a stable long-term configuration. In the coldest areas, water ice remains on the surface. In the warmer areas, the ice is covered by an ice-free surface layer that is rich in organic impurities that have been darkened by exposure to Mercury’s space environment.