Study finds comet dust painted Mer­cury black

Los Angeles Times - - THE NATION - By Deb­o­rah Netburn deb­o­rah.netburn @la­times.com Twit­ter: @Deb­o­rahNet­burn

The mys­tery of Mer­cury’s ex­ces­sively dark sur­face may be solved.

A team of re­searchers at Brown Uni­ver­sity, who have pub­lished their find­ings in Na­ture Geo­science, say the planet’s inky ap­pear­ance may stem from cometary dust that “painted” the planet black over bil­lions of years.

Sci­en­tists have long won­dered why Mer­cury was so much darker than Earth’s moon, re­flect­ing just onethird the light that the moon ref lects.

The two bod­ies are of­ten com­pared, said lead au­thor Megan Bruck Syal, a post­doc­toral re­searcher at the Lawrence Liver­more Na­tional Lab­o­ra­tory. They are about the same size, and be­cause nei­ther one has much of an at­mos­phere, both are sub­ject to a con­stant bom­bard­ment from bits of space dust and other mi­crom­e­te­orites.

Many air­less bod­ies get their dark color from iron-bear­ing min­er­als on their sur­face, but ob­ser­va­tions by NASA’s Mes­sen­ger space­craft have shown that the sur­face of Mer­cury is less than 2% iron.

Sci­en­tists had to look for an­other dark­en­ing agent. “One thing that hadn’t been fully con­sid­ered be­fore was car­bon,” Bruck Syal said. “Car­bon is re­ally abun­dant in comets and could be de­liv­ered by cometary dust.”

Full-size comets could not be re­spon­si­ble for de­posit­ing enough car­bon on Mer­cury’s sur­face be­cause an im­pact would shoot ma­te­rial back into space.

Im­pacts from cometary dust, how­ever, were a dif­fer­ent story.

“The lit­tle dust par­ti­cles less than a mil­lime­ter in size are also very car­bon rich, and they come in at lower speeds,” Bruck Syal said. “We did a lot of cal­cu­la­tions about dif­fer­ent im­pact an­gles and ve­loc­i­ties and found you could re­tain most of the ma­te­rial on the planet.”

The den­sity of cometary dust in­creases as you get closer to the sun, and af­ter fur­ther cal­cu­la­tions, the re­searchers determined that Mer­cury, the clos­est planet to the sun, is prob­a­bly struck by 50 times as many bits of cometary dust as the moon.

“It’s pretty con­stant,” Bruck Syal said. “Un­like as­ter­oid or comet im­pacts that are kind of hard to pre­dict, this is more of a steady state.”

To test whether the im­pact of car­bon dust has a dark­en­ing ef­fect, re­searchers turned to the NASA Ames Ver­ti­cal Gun Range in Moun­tain View. They used a 14-foot can­non fu­eled by hy­dro­gen gas to shoot pro­jec­tiles into tar­gets at speeds of more than 3.5 miles per sec­ond, im­i­tat­ing ce­les­tial im­pacts.

By cre­at­ing tar­get ma­te­ri­als sim­i­lar to what they ex­pect to find on the sur­face of Mer­cury, the sci­en­tists were able to demon­strate that cometary dust im­pacts would in­deed pro­duce dark ma­te­ri­als on the planet.

Bruck Syal said the hy­poth­e­sis still needs to be tested fur­ther.

“I know the Mes­sen­ger mission is do­ing a low-altitude cam­paign right now try­ing to get more car­bon abun­dance data,” she said. “If we could have some out­side verification of how much car­bon there is … that would be the ul­ti­mate test.”

NASA

MER­CURY re­flects just one-third the light that the moon re­flects. Re­searchers say car­bon-rich cometary dust dark­ened the planet’s sur­face over bil­lions of years.

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