NASA’s Messenger mission is expected to end on April 30th at 3:30 PM EDT with a crash landing on Mercury. Messenger has run out of fuel and NASA is using this phase of the mission to utilize the spacecraft’s low orbit to obtain very high-resolution images of Mercury’s surface. One of Messenger’s many remarkable discoveries was the confirmation that ice exists in the polar regions of Mercury, the planet closest to the Sun.
Messenger’s voyage to Mercury began on August 3, 2004. The trip to Mercury took almost seven years and Messenger finally achieved orbit on March 18, 2011. Why did the trip take so long? Even though Mercury is only, on average, 48 million miles from Earth, Messenger’s voyage to Mercury was 4.9 billion miles. The trajectory to Mercury involved one flyby of Earth, two flybys of Venus, and finally, three flybys of Mercury itself to insert Messenger into orbit. A video the Messenger’s journey to Mercury is below:
Messenger was the first spacecraft to reach Mercury since Mariner 10 did so in 1975. During this gap, tantalizing evidence emerged that ice might exist on the polar regions of Mercury. The Arecibo Radio Telescope (the same one Jodie Foster used in Contact and James Bond destroyed in Goldeneye) detected strong evidence that ice exists in the shadowed regions of polar craters on Mercury. That made the confirmation of ice deposits on Mercury one of the prime objectives of the Messenger mission. So how does ice exist on the closest planet to the Sun? The answer lies in two facts, the small axial tilt of Mercury and the lack of an atmosphere on Mercury.
The axial tilt of Mercury is 2.11 degrees compared to Earth’s axial tilt of 23.5 degrees. At the poles, this represents the highest angle above the horizon the Sun obtains at the Summer Solstice. The image below is Noon on June 21st at the North Pole on Earth complete with faux landscape courtesy of Starry Night.
Now lets take a look at the North Pole of Mercury when the Sun is at its highest.
Quite a difference, this is the same altitude the Sun has about 15 minuets after sunrise on Earth in the mid-latitudes. Think about how long the shadows are at that time. On Mercury, you have craters as deep as 1 km. When the Sun stays low in the sky, and craters are that deep, there will be areas in those craters that will never see sunlight.
One might ask, with Mercury being so close to the Sun it must be hot enough to melt ice even if there is no sunlight in the craters. However, Mercury has practically no atmosphere, and with an atmosphere lacking, there is not any wind to transport heat from sunlight areas to dark areas on Mercury. The gravity on Mercury is only 38% that of Earth. As a consequence, the escape velocity of Mercury is 4.3 km/s compared to Earth’s 11.2 km/s. Here, Mercury’s closeness to the Sun plays a key role. The hotter the temperature, the faster atoms and molecules are accelerated. On Mercury, atmospheric gases are accelerated faster than the escape velocity causing atmospheric loss. On Earth, cooler temperatures and a higher escape velocity enable Earth to retain its atmosphere. Hence, over the course of time, Mercury will lose any atmospheric gases it may have.
Mercury’s surface bears a resemblance to the Moon. The Moon also lacks an atmosphere and is unable to transfer heat from the daylight to the dark side. On Earth, the atmosphere distributes heat across the surface and as a result, there is only an order of difference of a few degrees between day and night. On the Moon, the same distance to the Sun as Earth, the temperature can range from 2500 F on the dayside to -2400 F on the dark side. On Mercury, the difference is more extreme. The range can be from 8000 F on the dayside to -2800 F on the dark side. A region in a crater that is permanently in shadow will never approach the sublimation point (meaning changing from ice to water vapor, the atmospheric pressure on Mercury is too low for liquid water to exist) of ice.
Messenger’s detection of ice on Mercury is a classic case of matching a predicted result with observation. Messenger used a neutron spectrometer to measure the amount of neutrons emitted from the surface of Mercury. These neutrons are radiated as a result of high-energy cosmic rays striking the surface and ejecting them into space. Areas with ice absorb these neutrons, leading to a lower count in those regions. Scientists built a model predicting the neutron count, matched it up with observation, and voila, as the video below illustrates, a match was found.
This is a map of Mercury’s North Pole released by the Messenger mission in 2012. The red regions are shadowed regions and the yellow indicate areas of ice.
In 2014, Messenger obtained visual confirmation of the existence of ice in the Kandinsky crater located near the North Pole of Mercury. The image was taken by Messenger’s wide angle camera (WAC) with a 600 nanometer (orange) broadband filter. Normally, this camera’s function was to image stars for calibration purposes, but it had the capability to capture ice in the craters of Mercury lying beneath the shadows using scattered sunlight. This capped off a 25 year hunt for proof of existence of ice on the closest planet to the Sun.
This image compares a view of the crater without the filter (left) and with the filter (right) that can detect detail in the dark recesses of the crater.
Even though Messenger is in its final days, it is still productive. As its orbit decays and the spacecraft gets closer and closer to the surface, this will enable Messenger to take some of the highest resolution images ever of a planetary surface. Messenger was selected by NASA for its mission in 1999. The total cost over those 16 years has been about $450 million. That is about the same as it cost to launch one shuttle mission. Or on more Earthly terms, the same amount Texas A&M will spend to renovate its football stadium. Messenger has sent back over 10 terabytes (or 10,000 gigabytes) of data and over 200,000 images that will keep planetary scientists busy for years to come. At least until 2024, when ESA’s BepiColumbo mission will reach Mercury.