Pluto and its Satellites from the New Horizons Spacecraft

underworld and its Satellites from the New Horizons SpacecraftCAROLINE MOOREAbstractThe New Horizons spacecraft has provided the first close-up subject field of infernal vicinity and its satellites. Much more analysis is required of the data scarce the early findings have revolutionised our understanding of the infernal region system. Discoveries such as the complexity of underworlds rally, the up-to-the-minute geological activity, the atmospheric hazes, lower-than-predicted es patee rate and the erectst k in a flashn glacier in the solar system were completely unexpected. Plutos moon Charon has surprised with its dark red polar cap and architectonic belt and data from the smaller moons supports the hypothesis that they were formed out of the remnants from the collision that formed the Pluto-Charon binary.IntroductionSince its discovery in 1930, with a semi-major(ip) axis of 39.5AU, Pluto has been considered an north-polar oddity. Beyond the realm of the gas giants, Pluto did not fit into any known solar system architecture until 1992 when the existence of the Kuiper knock (30-50AU from the Sun) was support by the discovery of the first Kuiper Belt object (KBO). Now more than 1,000 KBOs have been identified, including five dwarf planets, and it is estimated that more than 100,000 objects larger than 100km orbit the sun wi polished the belt. Its believed that the Kuiper Belt contains go forthover remnants from the beginning of the solar system and that sending the New Horizons mission to explore Pluto, its moons and another(prenominal) KBOs would provide valuable insights into the formation of the solar system.The fastest spacecraft ever launched, New Horizons started its mission on 19 January 2006 and flew past Jupiter in February 2007 for a gravity boost which reduced journey time to Pluto by four old age. It conducted a six-month-long reconnaissance flyby study of Pluto and its moons in summer 2015, culminating with the closest approach to Pluto on 14 July 2015. As substantially as the first mission to an ice dwarf planet, New Horizons is also the first mission since Voyager in the 1970s to an unexplored planet.The official NASA science goals for the Pluto-system geographic expedition element of the New Horizons mission were prioritised into three categories required, important and desired. A key goal was mapping the climbs of Pluto and Charon with an average stoppage of one kilometre (the outdo the Hubble Space Telescope can do is a 500km resolution) and mapping the surface patch of the various geological regions of the two bodies. Another key goal was find out the composition, structure and leave out rate of Plutos atmosphere. The lower priority goals include measurement of surface temperature and a search for additional satellites or rings around Pluto. The all-embracing list of science goals appears in Appendix 1.The seven instruments on New Horizons were selected to meet these science goals. They argon t he most capable suite of instruments ever launched on a first reconnaissance mission to an unexplored planet (now dwarf-planet). They include an resourcefulness spectrometer to probe atmospheric composition and planet structure a in sight and infr argond camera to obtain steep-resolution colour maps and surface composition maps a long-range telescopic camera for high up-resolution surface images member spectrometers to measure charged particles in and around Plutos atmosphere a detector to measure masses of space-dust particles and two copies of a radio science experiment to rise atmospheric structure, surface thermal properties and planet mass. The seven instruments are listed in Appendix 2.Although considerably more work needs to be done to die the data received from New Horizons it is now clear that all NASAs science objectives have been met. On 14 July 2016, the anniversary of the fly-by in 2015, NASA published Principal Investigator, Alan Sterns, lead ten discoveries so far from the Pluto element of the mission. They range from the unexpected complexity of Pluto and its moons to the lower than predicted escape velocity, and the ten have been used as a focus for this dissertation.The geology of Plutoprevious to New Horizons, the best images of Plutos surface were obtained from the Hubble Space Telescope. A colour map released in 2010 isnt sharp enough to see any features, such as craters or mountains, but does show a degree of complexity and variegation with white, dark-orange and charcoal-black terrain. However, the evidence revealed by New Horizons of current geological activity was completely unexpected and the variety of landscapes on Pluto is also much greater than expected. Hummocky cratered uplands, washboard terrain (expanses of parallel ridges and troughs), chaotic blocky mountains, cellular and non-cellular newton ice plains, pitted hummocky nitrogen ice plains and rugged dark highlands all feature.A prominent feature of the encounter hemisphere (EH) is Sputnik Planum (SP), an 870,000km oval-shaped plain on the left side of the heart-shaped Tombaugh Regio. SP is most likely composed of volatile ices N, CO and CH and is the largest known glacier in the solar system. Computer simulations have been produced to show that the surface of SP is cover with icy, churning, convective cells which recycle their surfaces every 500,000 years. The modest internal heat of Pluto causes great blobs of solid nitrogen to rise up, then cool and sink game down. This helps explain why no impact craters have been observed on SP which has a crater retention age of no greater than 10 trillion years.Pluto and its largest satellite Charon are both tidally locked which means that one hemisphere of Pluto is locked facing one hemisphere of Charon. They both spin and orbit in 6.4 days. Data from New Horizons shows that SP is roughly exactly opposite Charon the chance of this happening randomly is 5%. It is proposed that a subsurface ocean exists under SP and that over millions of years the planet has spun around, aligning the subsurface ocean and SP above it, almost exactly opposite the line connecting Pluto and Charon.Along the west margin of SP there extends for hundreds of kilometres a dis sustained chain of mountains consisting of discrete angular blocks with apparently random orientations and sizes up to 40km across and 5km high (calculated by shadow length). Prior to New Horizons it was known that N, CO and CH ices existed on the surface of Pluto, but once the images of these mountains were viewed it became clear that these ices could not support such high elevations and therefore water ices must be present. This has now been confirmed spectroscopically by New Horizons. Because water ice is buoyant with respect to N2 and CO ice, some small blocks can be carried along by convective or advective motions and larger blocks can be undermined, shifted and rolld. Because of this its possible, if the solid N/CO ice is sufficiently deep, that some of the smaller mountains observed may be drifting within the plains, although the elevation of the largest mountains on the western margin of SP suggests that they are most likely grounded on the basement. It is not known why there are no mountainous terrains at the eastern edge of SP.At a few locations at the eastern boundary of SP and the pitted uplands, smooth materials connect with SP along the floors of troughs 1.5 to 6km wide. High-phase imaging of the southernmost of these systems has shown clearly visible medial flow lines within the troughs, with the ice in the troughs sloping at an angle of 2-3 degrees over 50km. This implies glacial-like flow of the plains material into SP. At present it is unknown if the flowing ice carved the troughs.Cthulhu Regio (CR) is a large dark area ranging from 15N to 20 S and bordering TR at clxE and stretching almost halfway around the planet to 20E . The region, comprised of a variety of geographical terrains, is covered by a thin dark mantle likely to be deposits of atmospheric Tholin. Tholin is a hydrocarbon formed by the action of sunlight on the methane in Plutos atmosphere. The methane molecules link together in progressively longer chains and as they get heavier they form a haze which eventually settles to the surface.Two broad quasicircular mounds, south of SP, might have an logical argument involving cryovolcanism. The smaller, Wright Mons, is 3-4km high and 150km across, with a central depression at its summit at least 5km deep with a rim showing concentric fabric. The larger, Piccard Mons, is similar but reaches 6 km high and 225km across. If their origin is cryovolcanic it would entail materials much stronger than N ice.There are features on the EH which suggest prolonged tectonic activity. Numerous belts of line up troughs and scarps, that can reach several hundred kilometres in length and several kilometres high, are often observed to cut across pre-existing landforms as e asily as branch into each other and these have been interpreted as extensional fractures in going stages of degradation. The differing fault trends and states of degradation suggest several distortion episodes and prolonged tectonic activity. The great length of individual faults on Pluto, their scarp steepness and spectral evidence strongly suggest a thick water-ice lithosphere rather than a thin one or one made of any of Plutos volatile ices.Pluto displays a wide variety of crater morphologies and sizes vary from 0.5 to 250km, not including any possible ancient basin underlying SP. Crater densities vary widely, from heavily cratered portions of CR to the apparently un-cratered SP. From the total cumulative crater size-frequency dispersal its been concluded that Plutos surface, as a whole, dates back nearly to the time of the end of Late Heavy Bom interdictdment (LHB)- perhaps 4 billion years ago. On the EH only the eastern portion of CR appears to approach the saturation crate r densities expected of a terrain that has survived from the LHB itself. In contrast the water-ice mountains and the mounds mentioned previously are very untested and no craters, down to a diameter of 2km, have been notice on SP. This implies a model crater retention age of no greater than 10 million years for SP and perhaps much less.The atmosphere of PlutoA major goal of the new Horizons mission was to explore and characterise the structure and composition of Plutos atmosphere. Much more work is required to fully analyse the data obtained, but already understanding of Plutos atmosphere has been revolutionised. Ground based major eclipse had shown an atmosphere around Pluto composed primarily of N with trace amount of moneys of CH, CO and HCN, with complex surface interaction and an uncertain surface pressure of 3-60 bar and a warm stratosphere at 100K above a much c ripened surface (38-55K).The New Horizons trajectory allowed near simultaneous radio (using REX) and solar (us ing ALICE) occultations. The spacecraft passed almost diametrically behind Pluto, as viewed from Earth, with ingress near the centre of the anti-Charon hemisphere and egress near the centre of the Charon facing hemisphere. The atmospheric structure at pinnacles 0 to 50km was retrieved from REX. A strong temperature inversion at both ingress and egress was found for altitudes below 20km, accordant with measurements taken from Earth. However new evidence of horizontal variations in temperature was discover from two notable differences between the REX profiles at entry and exit. First, the temperature inversion at entry is greater than that at exit the derived mean upright gradient in the lowest 10km of the inversion is 6.4 0.9 Kkm at entry but only 3.4 0.9 Kkm at exit. Second, the temperature inversion at entry ends abruptly at an altitude of 4km, marking the top of a distinctive boundary stage. The temperature inversion at exit, however, appears to extend all the way to the sur face, with no evidence for a boundary layer at this location. These differences in temperature structure cannot be accounted for by night-time radiative cooling or daytime solar heating within the atmosphere because the radiative constant of Plutos atmosphere is round 700 Pluto days.From REX data, surface pressure has been estimated at 11 1 bar at entry and 10 1 bar at exit. Analysis of stellar occultation data from 2012 and 2013 yielded essentially the same result indicating that the mass of Plutos atmosphere has not changed significantly in recent years.REX data shows that at occultation exit, temperature side by side(predicate) to the surface is 45 3K this may be indicative of a surface material less volatile that N ice because a surface covered in N ice would have a temperature of 37.0K to remain in vapour pressure equilibrium with the measured surface pressure of Pluto. At occultation entry, close to the region SP, the mean temperature in the lowest 4km above the surface is 37 3K close to the saturation temperature of N. It is suggested that this layer of cold air could maturate directly from sublimation of the N ices in SP. Calculations have shown that it would take approximately two years for downward heat conduction in the overlying temperature inversion to designate and an inversion that extends to the ground. So the observed boundary layer would have vanished on this timescale without the resupply of cold N further confirmation of SP as a sublimation source.Models indicate that photochemistry in Plutos upper atmosphere is similar to that of Titan and Triton. Methane is processed into heavier hydrocarbons by far-ultraviolet sunlight and also solar Lyman photons. The solar occultation results show that the upper atmosphere is much colder than previously survey. The observed N opacity at high altitudes was lower than expected. The absorption of sunlight in the 57-64nm wavelength range by N at high altitudes (850 to 1400km) constrains the tempe rature of the upper atmosphere to be approximately 70K. The mechanisms by which Plutos upper atmosphere is being cooled are not yet understood.The existence and complexity of Plutos hazes, as detected by LORRI and MVIC, was unexpected. Extensive, optically thin hazes extend to altitudes of 200km. Distinct layers are present which vary with altitude but are contiguous for over 1000km. In the highest resolution images from MVIC about 20 haze layers are resolved. The haze is unexpectedly blue, suggesting a composition of very small particles thought to be tholin-like in composition from the strewing properties observed. The layers in the haze are possibly the result of internal gravity waves driven by sublimation forcing orographic forcing.Pluto has a much lower than predicted escape rate. Prior to New Horizons the escape rate to space of N was calculated to be in the region of 2.8 x 10 molecules s based on estimates of Plutos surface pressure and radius, as well as CH and CO mixing r atios. However these calculations did not take into account the cooling of the upper atmosphere. Its now calculated that the escape rate for N is 1 x 10 molecules s. The escape rate calculated for CH is 5 x 10 molecules s which is much proximate to estimates prior to New Horizons and also 500 times faster than that of N. If these rates for N and CH are stable over a single Pluto orbit and over the age of the solar system, the equivalent thickness of N and CH surface ice lost to space would be approx. 6cm and 28m respectively. This relatively small amount of N loss is consistent with an undetected Charon atmosphere but appears to be inconsistent with the erosional features seen on Plutos surface. This suggests that N escapes in the past may have been now and then higher. The loss of methane is a suggested origin for Charons north polar red colour, involving varnishing of the winter poles over millions of years through cold-trapping and polymerisation of escaping hydrocarbons from Pl uto.CharonThe EH of Charon has two prominent features a tectonic belt of ridges and canyons in the equatorial region and a dark reddish cap to the North pole.The tectonic belt is more than 200km wide in places and consists of scarps, ridges and troughs which are almost parallel. There are two long, narrow, steep-sided depressions (chasmata). Serenity Chasma is 50km wide and 5km deep and Mandjet Chasma reaches 7km deep. Both chasmata are similar to extensional rifts visible on several mid-sized icy satellites such as Saturns Tethys. Its assumed that the tectonic belt is the result of substantial, aligned tectonic extension of Charons icy crust. The fact that several large craters are visible on the chasmata implies that the extension is geologically old.North of the tectonic belt there is rugged, cratered terrain. Mountains of 20km can be seen in the limb profiles. The crater density at large sizes on the northern terrain implies a surface age older than 4 billion years. The Northern hemisphere is capped by dark reddish region named Mordor Macula (MM), the extent of which does not correlate with any specific terrain boundary or geological feature. Layer This is an unusual feature because polar caps on other bodies tend to be bright, not dark, collectible to some kind of reflective ice or frost. Because the red-stained areas of Pluto look similar to MM it was originally thought that they might have similar origin. Its now known that Plutos red-staining is due to atmospheric tholins and since Charon has no atmosphere the origin could not be the same. Its now proposed that the tholins on Charon are made from methane escaping from near-by Pluto. The methane sticks to the winter pole where the temperature is lowest and the ultraviolet light received at night is sufficient to start to link the methane molecules together. As daytime comes, the molecules are heavy enough to remain on the surface and sunlight completes the process of polymerisation to form tholins.Sout h of the tectonic belt the surface is smoother, comprised of seemingly continuous plains named Vulcan Planum. Tectonic resurfacing is one possible origin of these plains. Areas of relatively low crater density and at least one pancake-shaped unit might imply cryovolcanic resurfacing.The spatial distribution of tectonic features across Charon is not consistent with the types of patterns predicted from tidal or de-spinning stresses. This may point to Charon having had an ancient subsurface ocean that subsequently froze producing the extensional features and possibly allowing the eruption of cryovolcanic magmas.The small moons of PlutoWhen the New Horizons mission was green-lighted only the dwarf planets Pluto and Charon were known. Then in 2005 the two small moons Nix and Hydra were discovered by the Hubble Space Telescope, followed by the even smaller moons, Kerberos and Styx, in 2011 and 2012 respectively. It had been expected that New Horizons would detect additional satellites but no other moons larger than approx. 1.7km in diameter are present at orbital radii between 5,000 and 80,000km.The customary hypothesis is that Pluto and its satellites were produced by the collision of Pluto with a similar Kuiper Belt object and it was hoped that New Horizons would provide information on whether this was the case. Several findings have helped to reinforce this hypothesis. First, the small moons are highly elongated, suggesting they formed and grew by the agglomeration of small objects, but, due to their size, their gravity was not sufficient to pull the material into a spherical shape. Indeed, from New Horizons images Kerberos appears to have a double-lobed shape suggesting the merger of two bodies. The shapes are consistent with the hypothesis that they all formed in the remnant disk produced by the collision that formed the Pluto-Charon binary.Second, it has been found that all four satellites have high geometric albedos, ranging from 0.56 0.05 to 0.83 0.08. I n contrast, the majority of small KBOs have geometric albedo of 0.1. This is further evidence that the moons were formed from the remnant disk rather than being captured gravitationally from the general Kuiper Belt population. Third, 11 craterlike features have been identified on Nix, and 3 craterlike features on Hydra. Crater densities have been calculated which exceed the values found on the older regions of Pluto and Charon and suggest that the surfaces of Nix and Hydra date back to at least 4 billion years ago. This fact again supports the formation hypothesis.From the high surface albedo of the moons, its strongly suggested that, like Charon, they are covered with water ice. Unlike Pluto and Charon, which rotate synchronously, the small moons are not synchronous and rotate much faster than expected with rotation periods ranging from 0.43 days to 5.31 0.10. In addition, the rotational poles of the small moons are almost at right angles to the common rotational poles of Pluto a nd Charon. These rotation speeds and axes have not been observed in other regular satellite systems and imply that tidal spinning has not played a major role in the moons rotational histories. A future study will determine whether chaos has played a part.