Friday 28 August 2015

Observing the Giant Planets in 2016

The VLT semester goes from April to October.

Jupiter is at opposition on March 8th 2016, and available above airmass 1.8 for 6 hours on April 1st, 4 hours on June 7th, 2 hours by July 11th, 1 hour by July 26th.  Sadly Jupiter is a daytime object for much of the period when Juno is first arriving at Jupiter, but we'll hope to pick up again in the latter part of the year.

Saturn is available for 6 hours on April 1st, opposition on June 2nd 2016, and is still around for 2 hours on October 1st.

Uranus is at opposition on October 15th 2016, and is up for an hour on June 1st, 2 hours by June 14th, 4 hours by July 14th.

Neptune is at opposition on September 2nd 2016, and is up for an hour by April 17th, 4 hours by May 29th.

Tuesday 25 August 2015

ESA’s Europa Targets for JUICE

In a previous post I discussed the preliminary plans for two flybys of Europa in February 2031 by ESA's Jupiter Icy Moons Explorer (JUICE).  The flybys are constrained by a multitude of complex factors, including the illumination conditions, the geometry to allow the radar to work (we must be on the anti-jovian far side), the available data volume, the accumulated radiation dose and the need the power all the instruments simultaneously during the dense flybys.

First the basics:  Europa is a highly evolved world, with a low crater frequency implying a young age of the surface material (as low as 60 million years).  That youth is inherently linked to the ocean and gravitational tides, which trigger resurfacing, cracking and release of fresh materials from the interior. We expect a metallic core, a silicate rock mantle, and then an outer layer of water ice some 100-200 km thick, some of which is a salty liquid (evidenced from the induced magnetic response to Jupiter’s magnetic field).  The ice shell above the liquid might be 10-30 km thick (from morphological studies of landforms), although it might be as thin as 3 km in some regions.  The ice penetrating radar should be able to help us resolve that mystery.

Dark mottled terrain dominates the trailing hemisphere, with ridges and double ridges hundreds of kilometres long being the most ubiquitous landform.  The darker terrains are associated with materials like salts, sulphates, carbonates and/or sulphuric acid on the surface, whereas brighter areas are richer in water ice.  The trailing hemisphere appears brighter, possibly as a result of the more modest particle bombardment compared to the trailing hemisphere.  The main process shaping the surface appears to have been tectonism, with tidal stress (from Europa's 85-hour period for orbiting Jupiter) generating linear ridged plains with dark bands, which subsequently evolved through faulting to create the chaotic terrains at lower latitudes.  However, the mechanisms creating specific features remain highly uncertain, and surface features could be linked to the sub-surface ocean, tidal effects and possible exchange processes.

Only 10% of Europa’s surface was imaged by Galileo at a resolution of better than 100 m, and Europa remains poorly imaged at regional resolutions of 200-500 m.  The highest resolution image obtained by Galileo was at 6m/pixel, revealing the surface to be extremely rough at small scales, but this was only done once.  The JUICE team selected eight regions of interest for their geological, chemical and astrobiological significance, and then designed the flybys to give close views of these locations.  I’ll try to summarise each of those regions below - the Regio, the disrupted terrains, linear features and impact craters.
Regions of interest for JUICE, with the darker
mottled areas known as Regio.  Ground-tracks for the two
flybys are shown. Credit:  ESA Red Book.

1.  Regio:  Darker Terrains:

At low spatial resolution, Europa’s Regio are locations of darker terrain and are given names from Celtic mythology.  On the trailiing side during the approach phases JUICE will be able to observe Annwn (20N, 40E), Argadnel (15S, 151E), Dyfed (10N, 110E), Falga (30N, 150E) and Moytura (50S, 65E) Regio.  On the leading side, JUICE will observe Balgatan (50S, 330E), Powys (0N, 215E) and Tara (10S, 285E) Regio during the departure phases.

2.  Disrupted Terrain:  Chaos and Lenticulae

Conomara Chaos from Galileo.
Conamara Chaos (A1, 8N, 85E)
Fractured plates of ice that have shifted with respect to one another, possibly due to localised melting of the salty ice shell due to ice convection or oceanic plumes, form the jigsaw of the chaos terrains.  The chaos rafts show pre-existing ridged plains that have been broken up.  Some chaos units appear to stand higher than the surrounding terrain, whereas other appear to have foundered into a finer-grained material.   Reddish material is associated with the chaotic terrain, possibly hinting at a relationship with the deeper ocean.  JUICE will be able to investigate the Conamara Chaos region (A3, 8N, 85E) that has previously been explored by Galileo, but will additionally study chaos terrains in the central and northern parts of the Dyfed Regio region (B1e, B1b, B1c) associated to Conomara, where the activity has disrupted the Asterius, Glaukos, Agave and Belus Linea.

Thera Macula, from Paul Schenk's Atlas of the
Galilean satellites. 
Thera (47S, 179E) and Thrace (46S, 188E) Macula (A3)
These two features, enriched in dark materials potentially emplaced in a liquid state, were considered the highest priority for the flyby.  Thrace is the largest of the Maculae at 180 km diameter, Thera is only 95 km wide.  Here we find chaos material with a matrix of pre-existing structure associated with dark plains, possibly with emplacement of liquid via some sort of cryovolcanism.  Three other Maculae (Boeotia, Castalia and Cyclades) are found in the southern hemisphere (54, 2 and 64S, respectively) around the antijovian point. The inset region of Thrace was imaged by Galileo at 40m per pixel, the rest of the image has a scale of 250m.

Lenticulae (A5, 45N, 145E):
Pits, spots and domes all suggest ice shell convection and are found throughout Europa’s mottled terrain.  These relatively recent circular/elliptical domes and pits (10-15 km across) are associated with prominent intersecting ridges Minos, Udeaus and Cadmus Linea.  Within the domes there is a texture referred to as ‘microchaos’.  Their possible origin is from upwelling of warm, ductile ice, with evidence of melting and exchange with the subsurface.
Lenticulae (Latin for 'freckles') on Europa.  NASA / JPL / University of Arizona / University of Colorado

3.  Linear Ridges, Bands and Fractures    

Linear features cross Europa’s surface for hundreds of kilometres, possibly due to fracturing of the icy crust.  Ridges have widths as wide as 2 km and can be several hundred metres high.  Some are double structures separated by a central trough; some are cycloidal and form chains of arcs.

 Three common morphologies of linear features on Europa (a) trough (b) ridge (c) band.  NASA / JPL / Marshall and Kattenhorn

Double ridge on Europa, Feb 1997, NASA / JPL / ASU

Ridged Plains (8S, 140E)
Complex network of ridges, bands and chaos.  Double ridges could form from extrusion or intrusion of water or warm ice, with frictional shear heating from motions (strike-slip faults) along fractures causing warming and melting, creating mobile ice to squeeze through fractures to form the ridge.  Bands could be formed by the pulling apart of the crust by separation and spreading.  

Band Wedges (A6, 5S, 160E):  
These appear to be lineaments that were opened, separated and then filled by a darker (low albedo) non-ice material, much like sea-floor spreading on the Earth.   These wedge-shaped pull-apart bands provide evidence for the original configuration of the ice before the surface began to move.  The youngest bands tend to be the darkest, whereas older bands are bright.

Fractures are narrower than the ridges and bands, and are seen down to the 10-m resolution limit of the best images to date.  They can exceed 1000 km in length, cutting across nearly all other features to suggest that deformation of the ice shell occurs over short timescales.  The youngest fractures could even be active today, in response to tidal flexing.

Pwyll crater with bright rays, NASA/JPL/Arizona

4.  Impact features:

Only 24 impact craters larger than 10km in diameter have been identified on Europa, providing strong evidence for a youthful surface.  Taliesin (22S, 222E) is the largest at 50 km diameter, followed by bright Pwyll at 45km diameter.  Multi-ring structures from an impact, can provide information about the physical properties of the sub-surface.  There are very few large craters on the surface, hinting at an age of around 60 million years. Pwyll on the trailing hemisphere (25S, 89E), named for the Celtic god of the underworld, is the most striking of the impact features, and the bright rays suggests it formed less than 5 million years ago.  Pwyll is shallow and relaxed. Multi-ring structures like Tyre (34N, 214E) suggest that the impactor punched through 20-km thick ice, with a central peak from a rebound and surrounded by faulting.

More details of these features can be found in the Gazetteer of Planetary Nomenclature maintained by the USGS:

ESA's Europa Flyby Plans

While excitement builds for NASA’s flyby mission for Europa, ESA’s plans for the Jupiter Icy Moons Explorer (JUICE) are now in the implementation phase (known as B2), following successful mission adoption in November 2014.  JUICE will conduct two close flybys of Europa over a couple of weeks in October 2030, before moving onto a wider jovian orbit to complete a reconnaissance of the rest of the jovian system, ending up in orbit around Ganymede in 2032-2033.  JUICE’s sophisticated instrument suite of cameras, spectrometers (UV, near-IR and sub-mm), field and plasma instruments, laser altimeter and sub-surface radar will study Europa’s surface and sub-surface with a clarity, resolution and sensitivity far in excess of any previous exploration, including the ill-fated Galileo mission.  JUICE aims to understand the composition of non-ice material on the surface (particularly those related to habitability of the sub-surface ocean); search for liquid water below the surface; and to study any active processes.

However, the harsh radiation environment of Europa, coupled with the desire to study Jupiter and Ganymede over a 3-year mission, means that JUICE’s brief foray close to Europa will only occur once, with two quick flybys to limit the radiation dose.  Indeed, particle fluxes are 20 times higher at Europa than they are at Ganymede.  We chose to focus these two flybys on regions of high interest for geology, chemistry and astrobiology, including those where Galileo image suggested potential recent activity - chaotic terrains where exchange of materials between the surface and subsurface might have been possible, providing potential insights into the nature of Europa’s subsurface ocean.  Two flybys will no-doubt leave us wanting many more, but it should be remembered that Ganymede is the primary target of this mission.

Regions of interest and trajectory for the two JUICE
flybys of Europa, from the JUICE Definition Study Report.
JUICE will fly on the anti-jovian side of Europa (the far side, from Jupiter’s perspective), with a closest approach of 400 km.  We need to be on the far side for the radar sounding to work.  The far side must also be sunlit for the remote sensing experiments.  This will provide regional (500-1000 m resolution) and local (50 m resolution) imaging for the study of geological processes; the potential for radar sounding down to a maximum penetration depth of 9 km (depending on the surface properties) with a vertical resolution of 50 m or larger; and laser altimetry with a vertical resolution of less than 5 m.

Plans for the Flybys

The spacecraft will approach Europa from the trailing hemisphere (longitude of 90 degrees) that receives the largest radiation dose from the co-rotating jovian magnetosphere; closest approach will be over the anti-jovian point (longitude 180 degrees); and departure is then over the leading hemisphere (longitude 270 degrees).  One flyby will occur over the northern hemisphere (up to latitudes of 45 degrees), the second will be over the southern hemisphere.    JUICE will zip past at 3.6-3.9 km/s.  

The Europa flybys represent the most highly packed observational sequence of the JUICE mission.  Observations will start many hours before the closest approach, using regional imaging and spectroscopic mapping to compare the geology and composition of the leading and trailing hemispheres. JANUS will lead 3 spacecraft slews during the inbound and outbound phases, MAJIS will lead 2, with other instruments riding along to view both the nadir and limb of the moon.   All instruments will be operating simultaneously when the spacecraft is within a distance of 150,000 km of Europa, placing one of the toughest requirements on the design of the JUICE spacecraft.  

During the 2 hours surrounding the closest approach, JUICE will switch from the power-optimised yaw-steering mode to the inertial pointing mode.  After more spacecraft slews, the instruments become purely nadir-pointing for the ±30 minutes around closest approach, and then active instruments (laser altimetry and sub-surface radar) operate for the ±7 minutes surrounding closest approach.  Depending on data volume constraints, distant Europa observations for plume searches will be performed two days either side of the closest approach, and distant views of Europa will continue to be used to study materials ejected from Europa’s surface, such as plume activity from the southern pole.  

Eight regions of interest have been identified, seven of which are on the trailing hemisphere to be viewed during approach.  I’ll try to describe each of these regions in a future blog post, and why we think they’ll be exciting to explore.  The plans continue to be formulated as JUICE moves through the implementation phase, under the guidance of the working groups for the SWT (science working team), but a comprehensive summary of the plans can be found in the JUICE Definition Study Report (Red Book) here:

NASA's Europa Swiss Army Knife

This week at the Applied Physics Laboratory (APL) in Maryland, the Outer Planet Assessment Group (OPAG) is meeting and sharing ideas and progress, and I'm indebted to all those tweeters who contributed to the #OPAG hashtag during the meeting to allow me to write this summary. It also provides an opportunity to discuss the newest of NASA’s missions, the as-yet-unnamed Europa mission.  The mission is expected to perform 45 flybys at altitudes ranging from 25 to 2700 km.  All of the PIs of the selected instruments were present to describe the experiments that will be onboard, described by Bob Pappalardo (mission project scientist) as sending a versatile Swiss Army knife to the outer solar system.  Although this isn’t the orbital mission that we’d originally planned when this was still part of EJSM (the Europa Jupiter System Mission prior to 2010), NASA expects that the flyby mission will still recover 90% of the intended science return.

The mission is now in Phase A, which means that it’s still in flux and being formulated, with many ideas on the table - Hubble is still searching for further evidence of plume activity from Europa’s south pole; potential contributions from ESA are being explored (such as landers, life detection, small free flyers), and about 250 kg of launch mass is being retained from the Phase A studies for more consideration in 2016.  Out of 33 submitted proposals, nine instruments have been selected to explore habitable conditions on Jupiter’s enigmatic moon:

1.  UVS - an ultraviolet spectrograph from Kurt Retherford and colleagues at Southwest Research Institute that’s a clone of the one being flown on ESA’s JUICE spacecraft, which will continue to hunt for plumes from Europa’s subsurface, determine their composition and chemistry, sources and sinks, structure and variability.  It uses a combination of UV emissions, surface reflections and transmissions (e.g., stellar and solar occultations) to detect and characterise Europa’s surface and tenuous atmosphere.

2.  EIS - the Europa dual imaging camera system led by Zibi Turtle of APL, combining both a narrow (NAC) and wide angle camera (WAC) with colour and stereo capabilities to understand the formation of landforms, the potential for current activity on the surface, and characterise the ice shell and ice-ocean interface.  The NAC has a 2-axis gimbal that allows the FOV to be moved both within and beyond the field of the WAC, achieving resolutions of 0.5 m from an altitude of 50 km above Europa’s surface.  The WAC will have a resolution of 4m from 50 km altitude, over a wider surface area.  The whole surface of the moon could potentially be mapped at a resolution of 50m.  The stereo capabilities will allow the creation of a digital terrain model (DTM) with a 4m vertical precision from the 50-km flyby altitude.  

3.  MISE - the mapping infrared spectrometer for Europa led by Diana Blaney (JPL) will be used to map the history of geologic activity on the surface, including a search for currently active areas.  Covering 0.8-5.0 µm with a 10 nm spectral resolution, this near-infrared spectrometer is a common feature of missions - Cassini, Juno and JUICE all have similar experiments (VIMS, JIRAM and MAJIS).  MISE will get resolutions of 25 m on a local scale, 300 m on a regional scale, and 10 km on a global scale, assuming 100-km altitude flybys.  Near-IR reflectance spectra will allow distinguishing between hydrate regions, sulphate regions, and even search for trace organics.

4.  E-THEMIS is the thermal instrument on board (so glad NASA chose to take a mid-infrared instrument, and I wish ESA/JUICE had one too!), provided by Phil Christensen of Arizonal State University.  The instrument will search for thermal anomalies on the surface (particular associated with any active venting), with a resolution of 5x22 m from 25 km altitude and a precision of 0.2 K for 90-K surfaces and 0.1 K for 220-K surfaces.  It has three filters, 7-14, 14-28 and 28-70 µm.

5.  REASON - the Radar for Europe Assessment and Sounding: Ocean to Near-Surface provided by Don Blankenship and team at Texas (best acronym ever), and a sister instrument for the RIME instrument on ESA/JUICE.  It’s a dual frequency radar at 60 MHz, with 15-m vertical resolution for shallow sounding to 4.5 km depth, or 150-m vertical resolution for ocean sounding below 4.5-km depth.  It will study the surface, sub-surface, using reflectometry to study near-surface roughness, porosity and composition, and search for an ice-ocean interface and evidence of exchange processes beneath Europa’s surface.  

6. PIMS - the plasma instrument for magnetic sounding provided by Joe Westlake from Johns Hopkins APL, which will work to characterise the salinity and depth of Europa’s oceans by measuring the plasma environment surrounding Europa and the magnetic induction response as conductive Europa moves through Jupiter’s magnetosphere.

7.  ICEMAG - the mission magnetometer provided by Carol Raymond of JPL to characterise Europa’s interior, thermal evolution, atmospheric sources and sinks, and the coupling between Europa and Jupiter’s ionosphere.  ICEMAG can also look at Europa’s exosphere by looking at dynamic species coming off Europa during each flyby.  

8.  MASPEX - a mass spectrometer provided by Hunter Waite and colleagues at the Southwest Research Institute (SwRI) to sniff out the exospheric (and ejected surface) composition.  Particles can be sputtered from Europa’s surface due to bombardment by energetic particles, or can simply sublimate from the surface, creating a density enhancement over the sunlit region.  Plume material would also contribute, being transported equatorward and deposited at lower latitudes to join other materials to be sputtered.

9.  SUDA - a dust experiment to characterise the surface using lofted dust detected with low-altitude flyby, provided by Sascha Kempf of University of Colorado, Boulder.  The Galileo dust detector had previously found that each of the Galilean satellites were wrapped in dust clouds of surface ejecta.  euro

More information on each of these instruments can be found here:
…and I’ll try to compare the capabilities of the Europa mission to those of JUICE (which will be performing two Europa flybys) in future blog posts.