Year in review (Part II) – Through the solar system and beyond
For NASA’s unmanned explorers, 2012 was an astonishing year filled with discoveries and observations inside our solar system, at the outer reaches of our solar system, and in the star systems beyond our own. Leading these discoveries were the NASA probes MESSENGER, GRAIL, Cassini, Voyager I, and the Kepler Space Telescope.
MESSENGER at Mercury – Water on the inner-most planet:
Beginning 2012 with a mission extension to March 2013, NASA’s MESSENGER spacecraft in orbit of Mercury offered one of the most exciting missions of the 2012 year.
Beginning the year with a series of orbital adjustments, the MESSENGER spacecraft successfully completed its first orbit correction maneuver on 2 March to bring its periapsis (time of closest approach to Mercury’s surface) from 400 kilometers to just 200 km.
Inserted into a highly eccentric orbit around Mercury, MESSENGER’s original orbit took it from 400 km above Mercury’s surface to 15,200 kilometers altitude every 12 hours.
The maneuver on 2 March lasted 171 seconds and used all four of the medium-sized monopropellant thrusters on the deck opposite most of the science instruments. The maneuver was completed while MESSENGER was 148 million km from Earth.
The lowering of MESSENGER’s orbit was conducted over a series of three burn operations on 2 March, 16 April, and 20 April to reduce the spacecraft’s orbital rate from 12 hours to just eight hours – providing 50 percent more low altitude observation opportunities of Mercury’s northern polar regions, including the region’s permanently shadowed craters.
Also occurring in March was the release of the first full Mercury solar day data that was gathered by MESSENGER during the spacecraft’s third through sixth month in orbit.
Data gathered by the spacecraft during this time revealed many aspects of Mercury’s unique characteristics, including its global magnetic field, the dynamics of its exosphere, its surface composition, its geological evolution, and its interior structure.
Data of this magnitude and precision had never before been recorded about the inner-most planet in our solar system.
As mission Principal Investigator Sean Solomon stated, “Mercury has presented us with many mysteries to date, and solving those mysteries will take new ideas and new analyses from throughout the scientific community.”
And the unlocking of Mercury’s secrets kept coming. After compiling the first year’s-worth of data from MESSENGER, many interesting observations regarding Mercury’s landscape, its inner core, and polar shadowed regions were announced on 21 March.
A surprising find from these initial observations was the discovery that the interior of the Caloris impact basin, a basin that is 1,500 km wide, has a surface floor that stands higher than the impact basin’s rim.
As related by project scientist Maria Zerber, “The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.”
But perhaps even more surprising was the information gathered from the first precise model of Mercury’s gravity field, which, when combined with the topographic data and earlier information regarding the planet’s spin state, shed light on the planet’s internal structure, the thickness of its crust, the size and state of its core, and its tectonic and thermal history.
From this data came the discovery that Mercury’s core is 85 percent of its planetary radius, which was even larger than previous estimates and is unusual for a planet of Mercury’s small size.
Moreover, it was discovered that parts of Mercury’s core are at least still partially liquid, defying previously held beliefs that Mercury’s sufficiently small size would have caused its interior to cool to the point where the core would be completely solid.
Discovery of the partially molten core state of Mercury’s interior was gained through subtle dynamical motions measured from Earth-based radar and combined with parameters of the gravity field as observed by MESSENGER.
Based on these new observations, it is now believed that Mercury has a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deep liquid core layer, and possibly a solid inner core – all of which will have implications for our understanding of how Mercury’s magnetic field is generated and how the planet evolved from a thermal perspective.
Also revealed in March was information regarding previously observed radar-bright features – thought to consist predominantly of frozen water ice – in Mercury’s polar regions.
Information from MESSENGER’s first full Earth-year in orbit of Mercury confirmed that these radar bright features were located in areas of permanent shadow in Mercury’s southern polar region and that the deposits were also present in shadowed regions in the northern polar regions of the planet.
However, MESSENGER, in its initial year, was not able to confirm whether or not these deposits were in fact water ice.
By mid-year, MESSENGER scientists were able to observe waves at the boundary of Mercury’s magnetosphere, leading to the conclusion that waves driven by the Kelvin-Helmholtz (KH) instability play a key role in driving Mercury’s magnetosphere.
KH waves can develop at boundaries between two media that are in relative motion to one another. In space plasmas, such waves can transfer mass and energy across the boundary between two otherwise separated regions.
For Mercury, this boundary is between the relatively dense and fast streaming solar wind and the more rarefied magnetosphere of the planet.
Long-term observation of these waves at Mercury mean that the KH waves develop more readily and are much more important for mass and energy transfer from the solar wind into the magnetosphere than had previously been believed.
And the discoveries were not over yet. By 21 September, information from MESSENGER’s X-ray Spectrometer revealed a chemical diversity in Mercury’s surface that was previously unknown.
Based on findings from MESSENGER it is now understood that Mercury’s volcanic, smooth plains differ in composition from the older surrounding terrain. The older terrain contains higher ratios of magnesium to silicon, sulfur to silicon, and calcium to silicon, while containing lower ratios of aluminum to silicon.
These different compositions suggests that the smooth plains material erupted from a magma source on the planet that was chemically different from the source of the material in the older regions, shedding new light on the complex and unique geological history of Mercury.
Furthering the discoveries about Mercury’s surface, by 15 November, MESSENGER scientists had discovered assemblages of tectonic landforms unlike any other previously found on Mercury or anywhere else in the solar system.
These land formations are believed to have formed by faulting in response to horizontal contraction or shortening as the planet’s interior cooled and the surface area shrank, causing blocks of crustal material to be pushed together and form unique land surface features.
But the biggest discovery of all came in late-November of this year when NASA’s scientists reported that MESSENGER had, via its neutron spectrometer, confirmed evidence for the presence of both water ice and organic compounds in the permanently shadowed craters of Mercury’s north pole.
There is water ice on one of the most inhospitable hunks of rock in the solar system – a rock located a mere 57.9 million kilometers (average distance) from the sun.
Ebb and Flow – GRAIL categorizes the lunar gravity field:
Beginning 2012 with one of the spacecraft in lunar orbit (orbit achieved 31 December 2011) and the second one entering orbit of the moon on 1 January 2012, NASA’s GRAIL spacecraft began the science and data collection phase of their multi-month mission on 27 March 2012.
An ambitious and unique mission, GRAIL utilized two identical spacecraft, named Ebb and Flow, to create the most precise map of the lunar gravity field to date.
Mapping of the lunar gravity field was accomplished by flying the two spacecraft in precise formation using radio signals bouncing back and forth between the two spacecraft to accurately measure and define the distance between them.
As the two spacecraft flew over areas of greater or lesser gravity, the distance between the two spacecraft changed slightly, changes that were recorded by the radio signals between the two spacecraft.
After eight months of data collection, the most accurate lunar gravity map was compiled by NASA and released to the general public.
Just shy of one year after entering lunar orbit, on 17 December 2012, the GRAIL spacecraft were sent into a collision course with the moon’s surface, thus ending their mission.
The impact location for Ebb and Flow was named in honor of American space icon and former shuttle astronaut Sally K. Ride who passed away earlier in the year after a long battle with cancer.
Cassini at Saturn – an incredible legacy continues:
To say that the Cassini mission to Saturn has been nothing short of marvelous would be an understatement. Marking its eighth year of operations at Saturn in July 2012, the Cassini orbiter spent the year cataloging new discoveries and new dynamics of the Saturnian system.
From early in the year, Cassini’s focus on Titan revealed new information about the weather patterns of the moon.
Stemming from a new analysis of radar data, information about the Titan’s climactic and geological history was gained via observation of dune fields, the second-most dominant land formation on Titan.
The dune fields in question are significantly larger than those found on Earth, averaging 1.2 miles in width, hundreds of miles in length, and 300 feet or more in height.
Radar data from Cassini allowed scientists to discover that the sizes of the dunes on Titan are controlled by at least two factors: altitude and latitude.
In terms of altitude, it was discovered that the more elevated dunes tended to be thinner and more widely separated than dunes at lower elevations.
This discovery allowed scientists to postulate that the sand on the surface of Titan is not made of silicates as it is on Earth but of solid hydrocarbons precipitated out of the atmosphere.
For the latitudinal effect on the sand dunes, it was observed the Titan’s dunes were confined to the moon’s equatorial region.
As stated by project scientist Alice Le Gall, “Understanding how the dunes form as well as explaining their shape, size, and distribution on Titan’s surface is of great importance to understanding Titan’s climate and geology because the dunes are a significant atmosphere-to-surface exchange interface.”
Following this discovery came the announcement on 2 March that Cassini had detected molecular oxygen ions around the icy moon Dione, confirming the presence of a very tenuous atmosphere on the Saturnian moon.
At Dione’s surface, its atmosphere is as dense as Earth’s atmosphere is 300 miles above the surface.
Thus, Dione’s atmosphere is technically an exosphere.
The oxygen in Dione’s atmosphere is believed to derive from either solar photons or energetic particles from space that bombard the moon’s water ice surface and liberate oxygen molecules.
Later in March, Cassini returned evidence that stress fractures on the moon Enceladus are caused by the moon’s interaction with Saturn.
This information allowed scientists to correlate the icy jets of water vapor from fissures on Enceladus’s surface with the way Saturn’s gravity stresses and stretches the fissures.
The observations of the stress fissures provided more evidence for a large, subterranean body of liquid water that would be necessary to allow Enceladus to flex enough to generate stress great enough to deform the surface of the moon.
By late April, Cassini had returned evidence that revealed a striking similarity between a lake on Titan and a mudflat in Namibia.
The new evidence from Cassini revealed that the lake on Titan could in fact be a depression that drains and refills from below its surface and not be completely filled with liquid hydrocarbons as originally thought.
This would correspond to the way that the Etosha salt pan on Earth fills from a shallow layer of groundwater that rises during the rainy season and then drains and leaves sediment-like tidemarks showing the previous extent of the water level.
According to project scientist Nicolas Altobelli from the European Space Agency, “These results emphasize the importance of comparative planetology and modern planetary sciences: finding familiar geological features on alien worlds like Titan allows us to test the theories explaining their formation” as we see them on Earth.
Also in April, Cassini aided scientists in their understanding of how the F ring of Saturn behaves.
Previously, abnormalities had been seen in the behavior of the F ring. This, thanks to new information from Cassini, is now understood to be caused in part by strange, half-mile sized objects punching through part of the F ring, leaving glittering trails behind them.
The small objects appear to collide with the F ring at gentle speeds in collisions that drag glittering ice particles out of the ring with them, leaving a trail 20 to 100 miles long.
The information from Cassini revealed that these objects are in part responsible for the eddies that ripple around the ring – thus aiding the understanding of how these eddies are formed.
Furthermore, Cassini returned even more evidence in April regarding another of Saturn’s prominent features, its moon Phoebe.
During recent observations, Phoebe was found to be more planet-like than originally thought. Based on Cassini observations, it is now believed that Phoebe is a planetesimal whose development was arrested billions of years ago and that the moon is actually a Kuiper Belt Object, the same region where Pluto currently resides.
By late June, attention turned back to the moon Titan when information from the Cassini spacecraft revealed that the moon likely harbors a layer of liquid water under its icy surface.
During several observational periods, Cassini witnessed a large amount of squeezing and stretching as Titan orbited Saturn. These observations led scientists to determine that if Titan were composed entirely of stiff rock, the gravitational attraction of Saturn would cause bulges, or solid tides, on the moon of only 3 feet in height.
However, Cassini witnessed tides of approximately 30 feet in height, suggesting that Titan is not made entirely of solid rocky material.
Using this information, scientists concluded that a liquid layer must exists below the moon’s surface and act as the main cause for the tidal bulging observed by Cassini. Furthermore, this subterranean liquid layer would most likely be composed of water because Titan’s surface is composed mainly of water ice.
While the discovery is important from a potential life perspective, scientists were quick to point out that a subterranean water layer does not have to be huge or deep to create the amount of tidal bulging observed on Titan.
Currently, Cassini’s mission is slated to last well into the year 2017 based on current funding levels – ensuring at least five more years of exploration of the Saturnian system.
Voyager 1 – the outskirts of the solar system:
For over a year now, NASA has stated that Voyager 1 could exit the solar system at “any moment.”
This year, hopes were high that Voyager 1 would actually do just that – exit the solar system and become the first human object to escape the sun’s direct sphere of influence.
But it was not meant to be in 2012.
On 14 June, NASA issued a press release stating that Voyager 1 had encountered a region in space where the intensity of charged particles from beyond our solar system had markedly increased.
This marked increase is one of three points of data which must make significant swings in order to indicate that a new era of space exploration has begun.
In particular, the second measurement that would indicate Voyager 1’s exit from the solar system and entry into interstellar space would be the rapid decline of energetic particles generated by the sun.
While Voyager 1’s instruments have shown a steady decline in the amount of energetic particles streaming away from the sun, the amount of particles still affecting Voyager 1 had not, as of 14 June, dropped off in any significant amount.
Lastly, the third dataset relates to the direction of the magnetic field lines surrounding Voyager 1.
As long as Voyager 1 is contained within the heliosphere, these magnetic field lines will run east to west in relation to the spacecraft. As soon as Voyager 1 crosses the boundary into interstellar space, scientists believe these magnetic field lines will orient into a more north-south direction relative to the spacecraft’s axis.
And by August, the second of these key indicators changed!
With an ever-increasing rate of change, Voyager 1’s cosmic ray instrument showed that on 28 July the level of high-energy cosmic rays originating from outside of the solar system jumped by an impressive 5 percent at the same time that the low-energy particles originating from inside the solar system drop by half.
While fluctuations followed, and several conditions returned to previous levels in the following days, the turbulent nature of the outer solar system and the dramatic jump in high-energy cosmic rays from outside the solar system coupled with the drop of low-energy particles from inside the solar system firmly placed Voyager 1 closer to its inevitable moment of crossing out the solar system.
Then, in late November, came word that NASA was to hold a press conference about the Voyager 1 spacecraft, fueling speculation that the craft had indeed finally passed out of the solar system.
But as with all of the expectations before, a different surprise came on 3 December – the announcement of the discovery of a new region at the outer reaches of the solar system.
This new region, called the magnetic highway, was discovered by Voyager 1 and was found to contain charged particles where the sun’s magnetic field lines are connected to the interstellar magnetic field lines.
The connection allows lower-energy charged particles that originate from inside our heliosphere – or bubble of charged particles the sun emits around itself – to escape the solar system while at the same time allowing higher-energy particles from outside the solar system to stream in.
In essence, Voyager 1 will spend the remaining days of 2012 as it has its entire life – safely contained within the solar system.
But that historic day is coming when Voyager 1 will exit the solar system.
As related by project scientist Edward Stone, “We believe this is the last leg of our journey to interstellar space. Our best guess is it’s likely just a few months to a couple years away. The new region isn’t what we expected, but we’ve come to expect the unexpected from Voyager.”
Beyond the solar system – the quest to find exoplanets:
In terms of unmanned exploration, 2012 saw the continued pursuit to find planets outside of our own solar system.
In 2012, an impressive 95 planets were confirmed to be orbiting stars other than the sun, including one in our galactic next-door-neighbor’s house.
The discovery of a planet in the Alpha Centauri system, a system located just 4.3 light years from Earth, was announced on 16 October.
The planet, called Alpha Centauri B b, is the smallest-mass terrestrial planet yet discovered via the radial velocity detection technique.
At approximately 1.13 times the mass of Earth, it is the lightest exoplanet ever discovered around a star similar to that of the sun and is also the closest exoplanet discovered to our own solar system.
Using data compiled over a four-year period, European astronomers discovered a tiny but very real radial velocity signal coming from the Alpha Centauri B star every 3.2 days.
Radial velocity is an exoplanet detection method that measures very small wobbles in the motion of the star created by the gravitational pull of an orbiting planet. The effect is extremely minute, and in the case of Alpha Centauri B, only caused the star to move back and forth by no more than 51 centimeters per seconds.
Astronomers were able to see a 51 centimeter per second wobble in a star located 25,666,079,113,090 miles away.
The newly discovered planet orbits its host star every 3.236 days at a distance of approximately 6 million kilometers, ten times closer to its star than Mercury’s average distance is to the sun.
Given that Alpha Centauri B is a sun-like star, the new planet’s proximity to the star rules out any chance of it being habitable to human life.
The surface temperature of the planet is estimated to be approximately 1200 degrees C, far too hot for liquid water and above the melting temperatures of many silicate magmas. For comparison, the surface temperature of the planet Venus, the hottest planet in our solar system, is 462 degrees C.
Nevertheless, the detection of an Earth-sized planet in the closest star system to our own is a huge advancement in the search for exoplanets.
At the end of 2012, combining information from NASA’s Kepler Space Telescope, 817 planets have been confirmed around 642 stars, and there are 2,320 Kepler planetary candidates awaiting confirmation.
(Images via NASA, JPL, PHL and ESA).