Space Sunday: Curiosity’s 5th, Proxima b and WASP-121b

On August 6th 2016, NASA delivered the Mars Science Laboratory (MSL) to the surface of Mars in what was called the “seven minutes of terror” – the period when the mission slammed into the tenuous Martian atmosphere to begin deceleration and a descent to the surface of the planet which culminated in the Curiosity rover being winched down gently from a hovering “sky crane” and then lowered until its wheels made firm contact with the ground.

The “seven minutes of terror” actually had a double meaning. Not only did it represent the time MSL would smash into Mars’ atmosphere and attempt its seemingly crazy landing, at the time of the event, the distance between Earth and Mars meant it took seven minutes to be returned to mission control from the red planet. Thus, even as the initial telemetry indicating the craft was entering the upper reaches of Mars’ tenuous atmosphere was being received, mission controls knew that in reality, the landing had either succeeded or failed.

Obviously, the attempt succeeded. Everything worked flawlessly, and Curiosity was delivered to the surface of Mars at 05:17 GMT on August 6th, 2012 (01:17, August 6th EDT, 22:17 PDT, August 5th). In the five years since that time, it is helped revolutionise our understanding of that enigmatic world – as well as adding somewhat to its mystery.

To call the mission a success is not an exaggeration; within weeks of its arrival inside the 154 kilometre (96 mile) wide Gale Crater, Curiosity was examining an ancient riverbed en route to a region of the crater dubbed “Yellowknife Bay”. It was there the rover made its first bombshell discovery: analysis of the area showed that billions of years ago it was home for the ideal conditions to potentially kick-start microbial life. It was, in essence, the achievement of mission’s primary goal: to identify if Mars may have once harboured the kind of conditions which might have given rise to life.

This 360-degree view was acquired on August 6th, 2016, by Curiosity’s Mastcam as the rover neared the “Murray Buttes” on lower “Mount Sharp”. The dark, flat-topped mesa seen to the left of the rover’s arm is about 15 metres (50 ft) high and about 61.5 metres (200 ft) long. Credit: NASA/JPL / MSSS

For the first year following its arrival on Mars, Curiosity continued to survey the regions relatively close to its landing zone, finding more evidence of a benign ancient environment. Then it started out on the next phase of its mission: the long traverse towards the massive bulk of “Mount Sharp” – officially called Aeolis Mons. A huge mound of rock deposited against the crater’s central impact peak, “Mount Sharp” rises from the crater floor to an altitude of some 5.5 km (3 mi), and imagining from orbit strongly suggested its formation was due, at least in part, to the presence of water in the crater at some point in Mars’ past.

The 8 km (5 mi) trip took the rover a year to complete, in part due to its relatively slow speed, in part due to the fact is had to travel a good way along the base of “Mount Sharp” to reach a point where it could commence an ascent up the slope; but mostly because there were a number of points of interest along the way where the mission scientists  wanted to have a look around, investigate and sample.

Mount Sharp as seen from Curiosity, on January 24th, 2017. The light grey banding befpre the sandy coloured slopes is the clay unit the rover will reach in about 2 years. In front of it is the “Vera Rubin Ridge”, the next location for study by the rover. Credit: NASA/JPL / MSSS

For the last three years, the rover has been slowly making its way up “Mount Sharp”, climbing around 180 metres (600ft) vertically above the surrounding crater floor and visiting numerous points of interest – such as “Pahrump Hills”, the mixed terrain where “Mount Sharp” merges with the crater floor. Along the way, Curiosity has both confirmed that “Mount Sharp” was most likely the result of sedimentary deposits laid down during several periods of flooding in the crater before the water finally receded and wind action took over, sculpting the mound into its present shape down through the millennia.

The lakes within Gale crater may have actually been relatively short-lived, perhaps lasting just 1,000 years at a time, but Curiosity has shown that even during the dry inter-lake periods, water was very much a feature of Gale Crater, finding evidence of compressed water channels within the layers of rock which sit naturally exposed on “Mount Sharp’s” flanks.

In December 2014, NASA issued a report on how “Mount Sharp” was likely formed. On the left, the repeated depositing of alluvial and wind-blown matter (light brown) around a series of central lakes which formed in Gale Crater, where material was deposited by water and more heavily compressed due the weight of successive lakes (dark brown). On the right, once the water had fully receded / vanished from the crater, wind action took hold, eroding the original alluvial / windblown deposits around the “dry” perimeter of the crater more rapidly than the densely compacted mudstone layers of the successive lake beds, thus forming “Mount Sharp”

Alongside the sedimentary layering of the mudstone comprising “Mount Sharp” and the compressed and long-dry water channels, a further sign that the region was once water rich comes in the form of the mineral hematite, which Curiosity has found on numerous occasions. Right now, the rover is making its way towards a feature dubbed “Vera Rubin Ridge” which orbital analysis shows to be rich in this iron-bearing mineral which requires liquid water to form. Beyond that is a clay-rich unit separating the hematite rich ridge from an area which show strong evidence for sulphates. These are also indicative of water having once been present, albeit less abundantly than along “Vera Rubin Ridge”, and thus hinting at a change in the local environment. Currently, Curiosity is expected to reach this area in about two years’ time, after studying “Vera Rubin Ridge” and the clay unit along the way.

Selfies from Mars: how Curiosity has weathered the dust on Mars over five years – the dates are given as Sols – Martian days, top left and the locations where the pictures were taken. Credit: NASA/JPL

Throughout the last five years, Curiosity has remained relatively healthy. There has been the odd unexpected glitch with the on-board computers, all of which have been successfully overcome. There has been some damage to the rover’s aluminium wheels. This did give rise to concern at the time it was noted, resulting in a traverse across rough terrain being abandoned in favour of a more circuitous and less demanding route up onto “Mount Sharp”. But overall, the wheels remain in reasonably sound condition.

The one major cause for concern at present lies with Curiosity’s drill mechanism. Trouble with this first began when vibrations from the drill percussive mechanism was noted to be having a negative impact on the rover’s robot arm.

More recently – since December 2016, in fact – all use of the drill has ceased, limiting Curiosity’s sample gathering capabilities. This has been due to an issue with the drill feed motor, which extends the drill head away from the robot arm during normal drilling operations, preventing the arm physically coming into contact with targets. Attempt to rectify the problem have so far been unsuccessful, so engineers are loot at ways to manoeuvre the rover’s arm and place the drill bit in contact with sample targets, avoiding the need to use the feed motor.

So with five years on Mars under its belt, and barring no major unforeseen incidents, Curiosity will continue its mission through the next five years, further enhancing our knowledge of Mars.

Proxima B Unlikely to Have Earth-Like Atmosphere

Proxima b, the planet discovered orbiting the closest star to our own, Proxima Centauri (see here for more). Located 4.25 light years away, Proxima Centauri is a M-type red dwarf star. Such stars are highly variable and unstable compared to other types of stars, and this might weigh heavily against Proxima b having the right conditions for life to arise.

However, since its discovery, the planet has been the subject of numerous studies to determine what, if  any, atmosphere it may have might be like, and whether it could be Earth-like. This has been name harder by the fact that the planet hasn’t been seen crossing in front of its host star, so these studies have relied upon simulations rather than direct observations. One such study, carried out by NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, suggests Proxima b would not be able to retain an Earth-like atmosphere for very long.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO

Event though it sets within the so-called “habitable zone” of its parent star – the distance at which a planet might support liquid water and an atmosphere – it is still encounters bouts of extreme ultraviolet radiation hundreds of times greater than Earth does from the sun. That radiation generates enough energy to strip away not just the lightest molecules—hydrogen—but also, over time, heavier elements such as oxygen and nitrogen, doing so as much as much as 10,000 times faster than occurs with Earth’s atmosphere.

Atmospheric loss occurs in the highest amount over the polar regions, where planets are the most sensitive to interaction with the solar wind of their parent star along the line of the planet’s magnetic field. When these magnetic field lines at the poles are closed, charged particles remain trapped near the planet, helping to retain the atmosphere. But if the magnetic field lines are open, they provide a one-way route for atmospheric elements to escape.

The NASA study suggests that with a very warm thermosphere temperatures, as Proxima b would likely have, give its proximity to its star, and an open magnetic field, the planet would probably lose a volume of atmosphere equivalent to Earth’s entire atmosphere in just 100 million years. Even if very low thermosphere temperatures and a closed magnetic field is assumed, the same volume of atmosphere would be lost in around 2 billion years – half the total time the planet has been in existence.  Thus, it would seem unlikely the planet has a dense, rich atmosphere like we have here on Earth. While this doesn’t entirely rule out Proxima b having some form of tenuous atmosphere, it does mean that it’s unlikely to be of a kind that would make the planet in any way habitable for life.

Hubble Observes Stratosphere of a Jupiter-sized exoplanet

As of August 1st, 2017, the Kepler Observatory has been responsible for the discovery of 5,017 candidate exoplanets, with 2,494 confirmed. Many of these world have been the subject of follow-up studies using both ground-based telescopes and the Hubble Space Telescope (HST), and it is Hubble which is responsible for observing a stratosphere  – a layer of atmosphere where temperature increases with higher altitudes – in the atmosphere of one exoplanet.

The planet in question is WASP-121b, a Jupiter-sized planet orbiting a yellow-white star slightly larger than the Sun, 880 light-years from Earth. Roughly 1.2 times the mass of Jupiter, the planet has a radius that is about 1.9 times that of the largest planet in the solar system, and orbits its parent star once every 1.3 days. So close is it to its parent, it is close to the Roche limit, the point at which it is so near its parent, the star’s gravity could literally rip the planet apart.

The study utilised Hubble’s Wide Field Camera 3 to gather spectrographic data on the planet. Analysis the different wavelengths of the atmosphere’s light-curve revealed some wavelengths were glowing rather brightly in the infra-red band – most likely as a result of presence of water vapour at the top of the planet’s atmosphere. The emission of light from water means the temperature is increasing with height – strongly indicative of the planet having a stratosphere.

The discovery is important because it shows that a common trait of most of the atmospheres in our solar system — a warm stratosphere — also exist in exoplanet atmospheres. This means scientists can now compare processes in exoplanet atmospheres with the same processes that happen under different sets of conditions in our own solar system.

A digital representation of Wasp-121 and it’s “hot Jupiter” planet, Wasp-121b. Credit: Engine House VFX, At-Bristol Science Centre, University of Exeter

However, at this point in time scientists are unaware as to what is driving the heating of Wasp-121b’s stratosphere. On Earth, the temperature of the stratosphere does not exceed 270 K (-3°C; 26.6°F).  On Titan, where the heating is due to the interaction solar radiation, energetic particles and methane – temperatures don’t change by more than 56 °C (100 °F). However, on WASP-121b, temperatures in the stratosphere increase by about 560 °C (1,000 °F).

One possibility is that compounds such as vanadium oxide and titanium oxide are active within the planet’s atmosphere. Not only are these compounds believed to be common to brown dwarfs (aka. “failed stars”, which have much in common with gas giants), they also require the hottest temperatures possible in order to keep them in a gaseous state.

The study is one of the first results to come out of a new observing programme being carried out by an international team of scientists, who have been awarded 800 hours of time to use Hubble to study and compare 20 different exoplanets. It represents one of the largest time allocations for a single programme Hubble’s 27-year history.

“This new research is the smoking gun evidence scientists have been searching for when studying hot exoplanets. We have discovered this hot Jupiter has a stratosphere, a common feature seen in most of our solar system planets.” says Professor David Sing, the study’s co-author and Associate Professor of Astrophysics at the University of Exeter.

“It’s a truly exciting find as we’re seeing dramatic differences planet-to-planet which is giving valuable clues in figuring out how planets behave under different conditions, and we’re only just scratching the surface of all the new Hubble data.”

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