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After three and a half years in orbit around Mercury --- the tiny shriveled and pockmarked orb that boasts temperatures in excess of 650 degrees --- NASA’s MESSENGER mission to our solar system’s innermost planet is headed for a controlled late March 2015 surface impact.
“The spacecraft is getting closer to the planet than we’ve ever been which helps our [surface] composition measurements a lot,” said David Lawrence, a MESSENGER science team member at Johns Hopkins University’s Applied Physics Lab (APL). “Between now and impact the spacecraft’s elliptical orbit is going to get down to 10 km above the surface; we’re talking airplane altitudes.”
Until the $450 million MESSENGER, Mercury rarely registered as a blip on most researchers’ radars. But scientists are now in the process of sorting through a plethora of new data about the planet.
“Mercury is an oddball planet of contradictions,” said David Blewett, a MESSENGER science team member also at APL, which NASA contracted to build and operate the mission. “It has a huge iron core, but the surface rocks are almost totally bereft of iron. Mercury is the planet closest to the Sun, yet it has a "polar cap" in the form of ice(s) lurking in the eternal darkness of polar craters.”
And although Mercury has no apparent current tectonic or volcanic activity; in its distant past, after formation and cooling; the planet did shrink in a manner not unlike a shriveled orange.
Mercury’s surface also has hundreds of surface “hollows” no deeper than a km, says Lawrence, who notes that they cover the planet. “You don’t see these hollows on any other planet in the solar system,” he said. “They’re not craters and they don’t look like volcanoes.”
Blewett says they have shapes similar to certain depressions found on the South polar cap of
As the planet closest to the Sun, however, Lawrence said he would have expected such volatile elements, like sodium, chlorine and sulfur, to have already been “baked off the surface.” Yet he notes that surprisingly that’s not the case.
“But compared to the Moon, Mercury doesn’t have a lot of compositional surface variability,” said Lawrence. “In visible light, Mercury is kind of dark gray, bland and uniform.”
In fact, Mercury’s close 88-day solar orbit makes it gravitationally (or tidally-locked) into what is known as a 3:2 spin resonance. In other words, Mercury makes three complete rotations on its axis for every two orbits it makes around the Sun. And at an average solar distance of only 58 million km, to an astronaut on Mercury’s surface; the Sun would appear some two and half times larger than it does from earth.
“The 3:2 spin-orbit resonance doesn't cause one side to be much hotter than the others,” said Nancy Chabot, a MESSENGER instrument scientist at APL. “Rather, there are two colder longitudes and two hotter longitudes and the permanently-shadowed craters by Mercury's poles are cold enough to have water ice.”
MESSENGER has confirmed that Mercury does indeed have significant amounts of water ice at its poles.
“We’ve confirmed that there’s a lot of hydrogen at Mercury’s poles [indicative of water ice] where it congregates in permanently-shaded craters,” said Lawrence. “But Mercury has a lot more water at the poles than the Moon and we don’t know why. Maybe there was a recent cometary impact on Mercury.”
In principle, the act of cometary impact on Mercury, says Lawrence, would liberate the comet’s water. Although he says most of that water would be lost to space, at least some of it would find its way into the planet’s permanent “cold traps” and persist there.
Why would Mercury be more prone to attract cometary impactors than the Moon?
Mercury may simply get impacted with more cometary material than the moon, says Lawrence. It’s closer to the Sun, he says, so there are a lot of sun-grazing comets with a high probability of also impacting Mercury.
And to understand the plethora of weird and wacky extrasolar planets out there, we must first understand our own inner solar system.
“In order to really understand how solar systems and planets form and evolve, we need to understand Mercury's formation and history,” said Blewett.
The next step involves the $850 million BepiColombo mission, a joint European Space Agency and the Japan Aerospace Exploration Agency (JAXA) effort; due for launch in Summer 2016 with an arrival at Mercury in early 2024.
Two separate spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO) will collect new data on Mercury’s origin and evolution; its structure as a planet itself; its vestigial exosphere; and the origin and current state of its magnetic field and magnetosphere.
But missions to Mercury are never easy. MESSENGER took 6.5 years to get there doing orbital gravity assists around Earth, Venus and then Mercury itself in order to slow the spacecraft down enough so that it could be captured into Mercury orbit.
Partly as a result, Lawrence and colleagues are savoring the last six months of Mercury science before MESSENGER impacts the surface, likely on the side hidden from Earth --- near the end of March.
Lawrence says the plan is to take a lot of high-resolution images of the planned impact area in advance, so that when BepiColombo arrives in Mercury orbit, its two spacecraft will be able to see the effects of MESSENGER’s impact.
Thus, Mercury seems to finally be coming out of the scientific shadows.
“There’s been a resurgence of interest in Mercury; having a lot of [new] data really helps,” said Lawrence. “We’re starting to open the hood on and ask more detailed questions about [the planet].”
