Mercury was once expected to be too hot for ice, because it lies so close to the sun, but radar observations revealed bright spots near its poles, and NASA’s MESSENGER spacecraft later confirmed that many of them were water ice, preserved inside craters where sunlight never reaches.
Mercury is the planet closest to the Sun, and for a long time this was reason enough to assume that it couldn’t hold ice anywhere. The surface temperature under direct sunlight can reach more than 400°C. However, there is water ice on Mercury, trapped in craters near its poles where the Sun has never reached, a NASA spacecraft has confirmed.
This claim seems like a contradiction. The explanation is geometry.
Radar tip
The first signal came in 1991, when the Arecibo Radio Telescope in Puerto Rico bounced radar off Mercury and picked up unusually bright spots near the poles. Bright radar returns of this sort are the kind of things water ice produces, and many of the spots line up with large craters mapped by the Mariner 10 spacecraft in the 1970s.
It was a strong hint, not proof. Radar brightness can be caused by other substances as well, so for two decades polar spots have been a promising suspect awaiting a closer look.
Why can ice survive near the sun?
The reason why ice can exist on the hottest planet is because of how Mercury is located. Its axis of rotation is almost perfectly upright with respect to its orbit, and is tilted only about a hundredth as far as the Earth rotates. With no tilt, the floors of deep craters near the poles don’t catch a single ray of sunlight.
These permanently shaded floors are among the coldest places in the solar system, much colder than anywhere else on Pluto, reaching temperatures of around minus 170 degrees Celsius and remaining there for billions of years. The planet could bake on average and retain some pockets that have never been touched by the Sun. Ice trapped in one of them has nowhere to go.
What MESSENGER has already shown
A closer look came from MESSENGER, NASA’s spacecraft that orbited Mercury from 2011 to 2015. In 2012, its team announced not one, but three measurements pointing in the same direction.
A neutron spectrometer detected its signature Hydrogen is concentrated in the ArcticIn the amounts you would expect from buried water ice. The laser altimeter found that the polar deposits were either unusually bright or unusually dark, which is where temperature models said surface ice should be located, or ice under a dark cover. Those thermal models tied the whole picture together. Combined, as well The mission team concludedWater ice is the main component of Mercury’s north pole deposits.
Convergence is the point. It is possible to argue with any one of these readings. It’s hard to dismiss an independent three-way agreement, which is why 2012 is when radar doubts became a certainty.
The ice is mostly hidden
There is one detail that complicates the elegant image of shiny polar ice. In most deposits, ice is not exposed at the surface at all. It is buried beneath a dark layer, thought to be rich in complex organic compounds, in spots cold enough to keep the ice buried but too warm to keep it stable on the surface.
Only in very cold craters is the ice exposed and bright. So The image is layered: Dark organic matter at the top, and water ice at the bottom, with the proportions determined by how cold each floor of the crater is.
What is still open
Where the water came from has not been completely settled. Impacts from water-bearing comets and asteroids are the usual explanation, with some possible contribution from chemistry driven by the solar wind. No matter what it reaches, the cold traps near the poles catch it.
The same phenomenon appears at the moon’s poles, which is important for anyone planning to use lunar ice as a resource. Mercury itself will soon receive another visitor: the European and Japanese BepiColombo mission is on its way and set to settle into orbit in the coming years, carrying tools that can sharpen what MESSENGER started. The address is already secure. There is ice on the planet closest to the sun, and it has been in the dark for a very long time.
