Messenger confirms theories, sees surprises at Mercury

click to enlarge This spectacular view of the crater Degas was obtained as a high-resolution targeted observation (90 m/pixel). Impact melt coats its floor, and as the melt cooled and shrank, it formed the cracks observed across the crater. For context, Mariner 10’s view of Degas is shown at left. Degas is 52 km in diameter and is centered at 37.1° N, 232.8° E. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The
spacecraft entered orbit around Mercury on March 18, 2011 UTC, becoming
the first spacecraft ever to do so. Tens of thousands of images of
major features on the planet — previously seen only at comparatively low
resolution — are now available in sharp focus. Measurements of the
chemical composition of Mercury’s surface are providing important clues
to the origin of the planet and its geological history. Maps of the
planet’s topography and magnetic field are revealing new clues to
Mercury’s interior dynamical processes. And scientists now know that
bursts of energetic particles in Mercury’s magnetosphere are a
continuing product of the interaction of Mercury’s magnetic field with
the solar wind.

This
week, Messenger completed is first perihelion passage from orbit, its
first superior solar conjunction from orbit, and its first
orbit-correction maneuver. “Those milestones provide important context
to the continuing feast of new observations that Messenger has been
sending home on nearly a daily basis,” offers Messenger Principal
investigator Sean Solomon of the Carnegie Institution of Washington.

A surface revealed in unprecedented detail

Among
the fascinating features seen in Messenger flyby images of Mercury were
bright, patchy deposits on some crater floors. Without high-resolution
images to obtain a closer look, these features remained a curiosity. New
targeted Mercury Dual Imaging System images at up to 10 meters per
pixel reveal these patchy deposits to be clusters of rimless, irregular
pits varying in size from hundreds of meters to several kilometers.
These pits are often surrounded by diffuse halos of higher-reflectance
material, and they are found associated with central peaks, peak rings,
and rims of craters.

click to enlarge Major-element composition of Mercury’s surface materials, depicted on the same graph, as measured by the Messenger XRS. Mercury has lower Al/Si and higher Mg/Si than typical lunar surface materials and terrestrial basalts, indicating a lower fraction of the common mineral plagioclase feldspar. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

“The
etched appearance of these landforms is unlike anything we’ve seen
before on Mercury or the Moon,” says Brett Denevi, a staff scientist at
the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel,
Md., and a member of the Messenger imaging team. “We are still debating
their origin, but they appear to have a relatively young age and may
suggest a more abundant than expected volatile component in Mercury’s
crust.”

Mercury’s surface composition

The
X-ray Spectrometer (XRS) — one of two instruments on Messenger designed
to measure the abundances of many key elements on Mercury — has made
several important discoveries since the orbital mission began. The
magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged
over large areas of the planet’s surface show that, unlike the surface
of the Moon, Mercury’s surface is not dominated by feldspar-rich rocks.

click to enlarge As a result of the north-south asymmetry in Mercury’s internal magnetic field, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. In particular, the magnetic “polar cap” where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury’s surface contributes both to the generation of the planet’s tenuous atmosphere and to the “space weathering” of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

XRS
observations have also revealed substantial amounts of sulfur at
Mercury’s surface, lending support to prior suggestions from
ground-based telescopic spectral observations that sulfide minerals are
present. This discovery suggests that the original building blocks from
which Mercury was assembled may have been less oxidized than those that
formed the other terrestrial planets, and it has potentially important
implications for understanding the nature of volcanism on Mercury.

Mapping of Mercury’s topography and magnetic field

Messenger’s
Mercury Laser Altimeter has been systematically mapping the topography
of Mercury’s northern hemisphere. After more than two million
laser-ranging observations, the planet’s large-scale shape and profiles
of geological features are both being revealed in high detail. The north
polar region of Mercury, for instance, is a broad area of low
elevations. The overall range in topographic heights seen to date
exceeds 9 kilometers.

Two
decades ago, Earth-based radar images showed that around both Mercury’s
north and south poles are deposits characterized by high radar
backscatter. These polar deposits are thought to consist of water ice
and perhaps other ices preserved on the cold, permanently shadowed
floors of high-latitude impact craters. Messenger’s altimeter is testing
this idea by measuring the floor depths of craters near Mercury’s north
pole. To date, the depths of craters hosting polar deposits are
consistent with the idea that those deposits occupy areas in permanent
shadow.

Energetic particle events at Mercury

click to enlarge A cross-section of Mercury’s magnetosphere (in the noon-midnight plane, i.e., the plane containing the planet-Sun line and Mercury’s spin axis) provides context for the energetic electron events observed to date with the Messenger XRS and GRS high-purity germanium (HpGe) detectors. The Sun is toward the right; dark yellow lines indicate representative magnetic field lines. Blue and green lines trace the regions along Messenger’s orbit from April 2 to April 10 during which energetic electrons were detected and Messenger’s orbit was within ± 5° of the noon-midnight plane. The presence of events on the dayside, their lack in the southern hemisphere, and their frequency of occurrence at middle northern latitudes over all longitudes point to a more complex picture of magnetospheric activity than found at Earth. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

One
of the major discoveries made by Mariner 10 during the first of its
three flybys of Mercury in 1974 were bursts of energetic particles in
Mercury’s Earth-like magnetosphere. Four bursts of particles were
observed on that flyby, so it was puzzling that no such strong events
were detected by Messenger during any of its three flybys of the planet
in 2008 and 2009. With Messenger now in near-polar orbit about Mercury,
energetic events are being seen almost like clockwork.

“We
are assembling a global overview of the nature and workings of Mercury
for the first time,” adds Solomon, “and many of our earlier ideas are
being cast aside as new observations lead to new insights. Our primary
mission has another three Mercury years to run, and we can expect more
surprises as our solar system’s innermost planet reveals its long-held
secrets.”

More findings from Messenger

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