Space Sunday: Venus, Pluto, and a mini round-up

This cylindrical map of Venus reveals the planet’s hostile surface beneath the clouds, a place of volcanoes and vast volcanic plains with few impact craters. The latter demonstrates both how volcanism has played a roll in “smoothing over” the surface of Venus in the past, and how effectively the dense atmosphere acts as a shield in burning-up incoming space debris. Credit: NASA

Once regarded as a planet that may harbour life, Venus – as we know it today – is a hellish place. Cursed with a runaway greenhouse effect, the surface temperatures (averaging 735 Kelvin or 462°C / 863°F) are hot enough to melt lead and mark it was the hottest planetary body in the solar system. The atmosphere is both a toxic cauldron so dense that it exerts a surface pressure 92 times greater than our own – the equivalent of being 900 m (3,000 ft) under water on Earth.

Venus is also unusual in other ways: it has a retrograde rotation (it spins on its axis in the opposite direction to Earth and most of the other planets), and it takes 243 terrestrial days to complete one rotation but only takes 224.7 days to complete an orbit of the Sun, making a “day” on Venus longer than a year.

Despite its hostile conditions, it has long been believed that Venus was at one time in its ancient past a far more hospitable world, potentially warm a wet, and spinning a lot faster on its axis (quite possibly in the same direction as the Earth spins). However, at some point  – so the accepted theories go – Venus experienced a massive impact, one sufficient enough to slow – and even reverse – its rotation and which also left it the broiling, hostile world we know today.

An artist’s impression of how Venus might have appeared some 2.5 – 3 billion years ago, at a time when a globe-spanning ocean might have started to affect the planet’s rotation, slowing it and eventually giving rise to the planet’s runaway greenhouse effect. Credit: NASA

However, a new study involving the University of Bangor, Wales, the University of Washington and NASA, suggests not only did Venus once had a liquid water ocean, but that ocean may have actually been the catalyst that brought about the planet’s dramatic change.

To put it simply, tides act as a brake on a planet’s rotation because of the friction generated between tidal currents and the sea floor. On Earth, this results in the length of a day being shortened by about 20 seconds every million years. Given this. the team responsible for the  study investigated how such interactions might impact Venus. Using a numerical tidal model, the accepted belief that Venus once had a world-girdling ocean, and applying it to planetary rotational periods ranging from 243 to 64 sidereal Earth days, they calculated the tidal dissipation rates and associated tidal torque that would result from each variation in ocean depth and rotational period. Their work revealed that ocean tides on Venus would likely have been enough to slow the planet’s rotation it down by up to 72 terrestrial days every million years.

This might not sound a lot, but of the course of around 10-50 million years, it would have been enough to slow Venus’s rotation and bring it to how we see it today. In turn, this slowing of rotation would have accelerated the evaporation of an ocean waters on the sunward facing side of the planet, both increasing the atmospheric density and trapping more heat within the atmosphere, accelerating the planet’s greenhouse effect, in turn increasing the rate of ocean evaporation in what would have been a closed cycle. Add to that the planet’s known volcanism, and the team estimate that it would have taken around 100-120 million years to turn Venus into the planet we see today.

This work shows how important tides can be to remodel the rotation of a planet, even if that ocean only exists for a few 100 million years, and how key the tides are for making a planet habitable.

– study co-lead Dr. Mattias Green, University of Bangor

The study findings have potentially important implications for the study of extra solar planets, where many “Venus-like” worlds have already been found. From this work, astronomers have a model that could be applied to exoplanets located near the inner edge of their circumstellar habitable zones, helping to determine whether they might have at some point potentially have had liquid water oceans, and how those oceans may have affected their development.

Fly Your Name to Mars

Mid July through August 2020 will see NASA’s next rover mission launched to Mars, and as with a lot of their recent exploratory missions, NASA is giving members of the public the opportunity to have their names flown with the vehicle.

Between now and September 30th, 2019, NASA is inviting one million members of the public to submit their names and postal codes to Send Your Name (Mars 2020). These names will then be laser-etched onto a little chip roughly the size of a penny that will be mounted on the rover and carried to Mars. In return, successful applicants obtain a “boarding pass” similar to the one shown below, indicating their name will be flown on the mission.

My Mars 2020 boarding pass

The Mars 2020 rover is based on the same chassis and power system as used by the Mars Science Laboratory Curiosity rover. It will also use the same type of landing system, featuring a rocket-powered “skycrane” that will hover a few metres above the surface of Mars and then winch the rover down to the surface. However – and for the first time in the history of planetary exploration – Mars 2020 will have the ability to accurately re-target its landing point prior to committing to lower the rover, thus allowing it to avoid last-minute obstructions that might otherwise damage the rover or put it at risk.

Core to this capability is a instrument called the Lander Vision System (LVS), which has been undergoing tests in California’s Death Valley attached to a helicopter. LVS is designed to gather data on the terrain the lander is descending towards, analyse it to identify potential hazards and then feed the information to a guidance system called Terrain-Relative Navigation (TRN), which can then steer the landing system away from hazards, allowing the skycrane to winch the rover to the ground in a (hopefully) a safe location.

The Mars 2020 rover’s LVS under test in Death Valley, California, mounted on the front of a helicopter. Credit: NASA/JPL

Mars 2020 is due to be launched between July 17th and August 5th 2020 to arrive on Mars at Jezero Crater on February 18th, 2021.

Insulating Gas Layer Could Give Pluto A Liquid Ocean

One of the most fascinating discoveries made by NASA’s New Horizons probe as it zipped past Pluto in July 2015,  was the remarkable ice plain that has been dubbed . From the initial receipt of the images of this vast expanse of ice, it has been theorised that, while not made of water itself, it carries with it numerous suggestions that it is being affected by convection currents which may be driving a liquid water ocean under the planet’s surface.

The existence of such an ocean, however, has been seen as inconsistent with other factors. For one thing, Pluto is thought to be around 4.5 billion years old; for another, it is so far from the Sun it can never receive much in the way of exterior warmth. These two facts alone mean that either any subsurface ocean should have frozen solid aeons ago or, should have frozen to the point where any water kept in a liquid form from heat escaping the planet’s core would be so far deep within the planet it would not to affect the surface nitrogen ice of Sputnik Planum.

A close-up of the “Sputnik Planum” ice field, showing the “cells” of ice between 16 and 38 km across, which had been thought to be in motion relative to one another as the result of convection currents rising from the planet’s core through a liquid ocean. A new study suggests that this hidden ocean is also kept liquid by a insulating later of gas sitting between it and the ice. Note that as further evidence of water on Pluto, the slug-like object in the centre of the image appears to be a large dirty iceberg of water ice, “floating” on the denser nitrogen ice (image: NASA / JHU/APL / SwRI)

Now, a new study led by researchers from the University of Hokkaido, Japan, offers a means to reconcile the inconsistencies. It proposes that Pluto’s liquid ocean is projected from freezing by an “insulating layer” of clathrate hydrates trapped in the slushy ice between the frozen surface and the liquid ocean. These types of molecules have low thermal conductivity, preventing the cold seeping into the water and freezing it at increasing depths. Thus, the water is kept liquid, and heat generated from deep inside Pluto’s core is sufficient (along with Charon’s influence) to create the kind of convection currents thought to be responsible for the surface features seen across the nitrogen ice of Sputnik Planum.

The study authors arrived at their conclusions after developing two models reaching back to Pluto’s believed formation. These models confirmed that without a gas hydrate layer, any ocean inside Pluto would have frozen out after a few hundred million years, but with even a relatively modest layer of insulating hydrates, the same ocean would remain predominantly liquid.

The study has implications elsewhere as well. Surface details on the moons Callisto, Mimas, Titan, Triton, and data gathered from NASA’ Dawn mission to Ceres, all suggest they have liquid oceans inside them. However, unlike Europa, Ganymede and Enceladus, they are not subject to the same forces that would generally be required to keep any interior oceans they might have in a liquid form. Thus, it might by that they also have clathrate hydrates acting as a insulating boundary between their frozen crusts and any water below.

In Brief for the Week

NASA Announce Project Artemis Timeline

Responding to criticism over their plans to send humans back to the Moon in 2024, NASA announced its timeline for crewed missions on Thursday, May 23rd.

What has been called Exploration Mission 1 (EM-1) the first uncrewed flight of the Space Launch System, NASA’s new super-booster, has been re-christened Artemis 1. Targeted for a June 2020 launch, it will remain pretty much as EM-1: flying an uncrewed Orion Multi-Purpose Crew Vehicle (MPCV) around the Moon and back on a 6-day mission, marking the second flight for Orion.

Then it 2022, somewhat earlier that the original schedule, Artemis-2 (formerly Exploration Mission 2), will fly a crewed Orion vehicle around the Moon. Then in 2024, Artemis 3 will send a crew of four to the nascent Lunar Orbital Platform – Gateway (LOP-G) and thence to the lunar surface in 2024.

An artist’s impression of the Lockheed Martin lunar lander vehicle concept. Credit: Lockheed Martin

LOP-G itself will undergo initial assembly with three flights targeting 2022-2024. The first of these will deliver the Power and Propulsion Element (PPE), to be developed by Maxar Technologies to lunar orbit via a commercial launch vehicle. This will be followed in 2023 by a pressurised module with docking adapters, then ahead of the Artemis-3 mission in 2024, a third mission will deliver elements for the first landing (including the lander vehicle itself?). Further construction / expansion of LOP-G will then continue through three more flights between 2025 and 2028.

Outside of flying the later elements of the LOP-G, this is – as I’ve mentioned before – a massively ambitious schedule, with just five year to develop test and prove the lunar lander alone.

UFOs Over The Netherlands? No, It’s Just SpaceX

A Dutch website set up to record UFO sightings was flooded with reports in the early hours of Friday, May 24th, European time after a “train of stars” was spotted crossing the Netherlands’ skies, sparking fears of an alien invasion. The website was inundated with more than 150 sighting reports, some quite panicked in nature.

But the “train of stars” wasn’t the start of an intergalactic invasion, but rather the results of the first launch of SpaceX’s planned Internet constellation called Starlink that will eventually comprise 12,000 mini-satellites in orbit around the Earth to provide Internet access around the globe.

For the first launch, a single Falcon 9 rocket placed 60 Starlink satellite into orbit, and the nature of their release, some 440 km above the surface of the Earth, left them strung out like beads on an invisible string, visible to the naked eye – and thus causing the called to the Dutch UFO website.

Amateur astronomer Dr. Marco Langbroek filmed the Starlink satellites passing over the Netherlands on Friday, May 24th, 2019

Over the next several weeks, the satellites will collectively increase their altitude to 550 km, moving further apart as they do so, whilst also become a lot fainter to the naked eye.

Nuclear Thermal Propulsion for Deep Space?

In the 1960s / early 1970s, the United States investigated the potential of nuclear thermal propulsion for use in space (see Project NERVA). However, for a variety of reasons – including loss of political backing and concerns over what might happen if a rocket carrying the nuclear propulsion system exploded within the Earth’s atmosphere, the programme was eventually cancelled.

Now it might be back on the cards again. For the last several years there have been a number of studies into the use of nuclear thermal engines to overcome some of the issues in getting crews to and from Mars. As part of their federal budget proposal for 2020, the House Appropriations Committee approved US $125 million in NASA budget  for nuclear thermal propulsion development. This brings the total awarded to NASA in respect of nuclear thermal propulsion over the period 2019/2020 to US $225 million , with the requirement for the space agency to have a flight demonstration ready by 2024.

An artist’s impression of a Mars exploratory vehicle using a nuclear thermal propulsion in Low Earth Orbit, n Orion vehicle docked against he habitat module. Credit: NASA

This is again an ambitious time frame given there are many regulatory requirements around the use of nuclear thermal systems that must be considered. It’s unclear, though, how nuclear thermal propulsion fits into NASA’s long-term exploration plans; all road maps put forward thus far, including those put forward by potential commercial partners, all rely on more conventional propulsion technologies, including chemical and solar electric propulsion. Such systems don’t offer the shorter travel times presented by nuclear thermal propulsion, but they also aren’t hamstrung by its technical, political and environmental (again, what happens if the rocket carrying a nuclear propulsion system blow up whilst ascending through the atmosphere?) and regulatory hurdles.

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