Mercury has water-ice!!!

Mercury

The Mariner 10 space probe made three passes by the planet Mercury in 1974 and 1975. But it could only explore 45 percent of the planet surface. Therefore Scientists continued ground observations of planet Mercury.

In 1991, planetary scientists Duane Muhleman and Bryan Butler from Caltech and martin Slade from the Jet Propulsion Laboratory decided to study Mercury by radar. What they did was that they sent a beam of 8.5 GHz, 460 kW microwaves  from a half-million watt power transmitter radar system in a 70-meter dish antenna at NASA Deep Space Network , Goldstone, California targeting Mercury.  The radar waves reflected from Mercury were received by the National Radio Observatories 26 VLA (Very Large Array) antennas. Then scientists filtered and processed the returned waves and produced a map of Mercury according to the radar reflectivity of its surface .This the map they got.

Radar image in 1991

In this radar image red indicates strong reflection and yellow, green and blue progressively weaker reflection. You can see a bright red spot at the Mercury’s north pole. In 1994, the same observing team discovered similar reflection’s from the Mercury’s south pole.

Then in 1999, astronomer John Harmon at the Arecibo Observatory in Puerto Rico repeated the 1991 study. This time he used a more powerful beam of microwaves emitted by the Arecibo Radio telescope and the reflected waves were detected by the VLA radio telescope array in New Mexico. This is one of the high resolution images of Mercury’s North Pole that was made by using the data of reflected waves.

Radar image made in 1999

Scientists were surprised to see these reflections from Mercury. Their deduction was that such kind of strong reflections can never be made from a momentary “glint” off a crater wall. When studied further these characteristics of reflections were similar to the ones from the water ice on Mars and icy moons of Jupiter. Although water ice found on earth absorbs radio waves, ice at very low temperatures is a very effective reflector of radio waves. So you know what I mean. Can there be ice on the surface of such a scorching hell like Mercury? Well, scientists say why not?

We all know that Mercury is the smallest planet in the solar system and also the closest to the sun. Being so small to retain an atmosphere its surface temperature varies horribly. Therefore its day side is hot as frying 427 degrees Celsius where as its night is frigid minus 173 degrees Celsius. As the axial tilt of Mercury is nearly two degrees, some crater floors in the poles of the Mercury are in constant shade and never get sunlight, resulting a temperature below minus 170 degrees Celsius. With no atmosphere to trap heat these areas can trap cold and called as “cold traps”.   Debris or vapor drifting from interplanetary space could be permanently captured in cold traps as ever deepening rimes of frost.  

Scientists propose that meteorites in the past potentially carried large amounts of water to Mercury’s surface. Also outgassing of water from the planet’s interior could also provide a non-negligible flux of water to the surface. If any water found its way to the cold traps it would freeze and remain. Also it has been revealed that there should be several meters or more thick  at least 90 percent pure ice to give such strong reflections, indicating that each deposit was laid down in one or a small number of rapid events, such as a large comet impact.   Scientists say that to remain the ice stable there should be a thin (10-20 cm) layer of regolith (layer of dust, soil and broken rock)on them.

Considering the bright radar spots to be frozen ice deposits scientists have calculated that there could be 10¹⁴ to 10¹⁵ kg of ice. I would give a data to compare this. The Antarctic ice sheet here in the earth has a mass of about 4 x 10¹⁸ kg. That really is lot of water. Isn’t it?

   

The MESSENGER space probe, only one to visit after Mariner 10 reached the Mercury in March 2011. It created a global map of Mercury by sending back 87,000 images within its first year. Scientists examined the topographical data and created a 3D model of Mercury’s poles. Here, they could discover cold traps with permanent shade. Also by processing images in the poles they could produce the illumination maps which are given below.

An illumination map of Mercury's south pole(left), black areas are cold traps.The respective visual image(right)
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institute of Washington  

The researchers superimposed observations of radar bright patches by the Arecibo Observatory on the latest photos of Mercury's poles taken by the MDIS imaging instrument aboard Messenger.Stunningly, the areas with constant shade or cold traps perfectly matched the radar bright spots, supporting the water-ice hypothesis. But this evidence does not confirm that there is water ice. Because, sulfur has also been proposed as the cause of radar-bright spots. Therefore in order to affirm the composition of cold traps, looking for hydrogen on Mercury by examining neutrons was done by MESSENGER’s neutron spectrometer.  

Top:Mercury's north pole in visible light, Credit: NASA/JHUAPL/CIW
Bottom: Same place in Radar image, Credit: NAIC/Arecibo

Two images superimposed, yellow patches are bright radar spots,Credit:NASA

Neutrons are created when cosmic rays hit Mercury. They break atoms into their component particles. Scientists can determine the abundance of hydrogen on Mercury by mapping the relative flux of neutrons at different energy levels. Hydrogen in Mercury’s soil would slow down the particles, making them less energetic. As the most likely source of hydrogen in the solar system is water this is an interesting experiment. Currently, Scientists are still crunching the numbers of received data.

Finally we can hope that discovery and confirmation of water-ice on Mercury’s poles would be a great potential for human colonization on planet Mercury in future.