Astrobiology Top 10: Lessons from Venus

As 2014 comes to a close, Astrobiology Magazine is counting down our ‘Top 10’ stories from the past year. Venus fills out the number 9 spot with two stories. The first story explains how our knowledge of Venus has been used to define a ‘Venus Zone’ around distant stars. The study could help scientists distinguish Venus-like planets from Earth-like planets in the Universe.

The second story marks the end of the European Space Agency’s Venus Express mission after eight years of exploration. Venus Express was an incredible success and provided researchers with a wealth of data about our neighbor in the Solar System.

In the Zone. The Venus Zone: Seeking a twin of our twin among the stars was originally published on September 10, 2014.

Venus Express Goes Silently into the Night was originally published on December 17, 2014.


In the Zone. The Venus Zone: Seeking the Twin of our Twin Among the Stars

By Sheyna E. Gifford

Artist's concept of lightning on Venus. Image credit: ESA

Artist’s concept of lightning on Venus. Image credit: ESA

What if, in our quest to find another Earth, we happen upon another Venus?

We should celebrate, of course. Venus is often called Earth’s “twin” because it shares of lot of our home planet’s physical characteristics: surface area, composition and density. Also, roughly speaking, both planets inhabit the area around the Sun’s habitable zone – though Venus is near the inner edge, while we on Earth occupy the relative center. Bearing the similarities and differences in mind, scientists Ravi Kumar Kopparapu, Stephen Kane and Shawn Domagal-Goldman explored how distant analogs to Venus might be detected and differentiated from Earth-like planets occupying the same relative space. The paper pinpointing their finding was published in Astrophysical Journal Letters on 9/10/2014.

Successfully detecting analogs of our inner planets out in the Universe, as Kopparapu and colleagues describe, means that at least two important events have taken place.

One says something about us. At present, our ability to identify the existence of other planetary systems is increasing by the day. When it comes to divining which exoplanet is a mini-Neptune and which is a mega-Earth, we still have a ways to go. Gaining the ability to pick out Venus-like planets will imply that we have gotten really good at sorting exoplanets.

The other is a larger statement about the Universe: If Venus-like planets are found in abundance, then Solar Systems like ours may be the rule rather than the exception. Discovering a twin to Venus around another star might well spark our interest in focusing our observations there, both for signs of another Earth and for clues about the dynamics of exoplanetary systems that harbor conditions similar to our own.

In their paper, Kopparapu, Domagal-Goldman and Kane explain how we can determine the distance between a planet and a star from calculations we can make today. They project that in the near future, when the James Webb Space Telescope takes to the skies, measurements of exoplanetary atmospheres will distinguish Venus-like from Earth-like from Mars-like. In the meantime, Kane and colleagues made some important calculations that will assist astronomers in the search for distant Venuses.

Not every planet in or near a habitable zone is habitable. Inhospitable Venus is an excellent example. Credit: NASA/JPL/Caltech

Not every planet in or near a habitable zone is habitable. Inhospitable Venus is an excellent example. Credit: NASA/JPL/Caltech

First, they estimated how close a planet can be to a star and still retain its atmosphere. Those figures pertain to planets like Mercury, with close-in orbits where the Solar Wind strips away nearly all atmospheric particles. Then, they approximated the furthest distance from a star likely to sustain a planet-wide runaway greenhouse effect. Taken together, these parameters describe the Venus zone: a place where we can start looking for planets with characteristics of own second planet from the Sun.

Before we move on to finding planets in other solar systems, we should talk a bit about our own Solar System. Here to do that and then discuss his findings is Dr. Shawn Domagal-Goldman, one of the paper’s authors and the mind behind The Pale Blue Blog at astrobio.net.

Astrobiology Magazine (AM): Shawn, the discovery of just how much Earth and Venus differ is relatively recent. What did it take for us to figure out that our own twin in the Solar System was not just uninhabited, but utterly uninhabitable?

Domagal-Goldman: This is one of the things we’ve learned through telescope and spacecraft observations of Venus over the last century or so. And this highlights two of the things I love about this paper – it leverages what we’ve learned about planets from observations of the ones in our own solar system to inform exoplanet data; and it also reinforces the notion that two planets with fairly Earth-like “astrophysical” properties such as mass/radius can be dramatically different in terms of their habitability.

AM: Where is our own “Venus zone”?

Domagal-Goldman: The outer edge of the “Venus zone” is, by the way we’ve defined it here, the same as the inner edge of the habitable zone. This is roughly the “border” between where we think a planet is more likely to be Venus (closer to the Sun than the border) or more like Earth (further from the Sun than the border). This border therefore is between Earth and Venus. The inner edge of the “Venus zone” is the distance at which Venus would lose it’s atmosphere from all high energy input from the Sun. In our system, this is VERY close to the Sun – about twice as close to the Sun as Mercury is.

AM: We’re just beginning to find rocky planets in so-called habitable zones around other stars. Why is now a good time to start breaking up these habitable zones into discrete bands that reflect Earth and Venus? How will we be able to tell a Venus-zone from an Earth-zone at such tremendous distances? Will there be a Mars-zone as well, when all is said and done?

In the present-day Solar System with the Sun in its current state, Venus is outside of the habitable zone (HZ) – or rather, it is closer to the Sun than the inner boundary of where water will be found in liquid form. The stability of water as a liquid sets the lines for the traditional habitable zone, outlined here in blue. However, for exoplantary systems with characteristics distinct from ours – where the stars exhibit different luminosity, the planets rotate differently, or the atmosphere conditions vary on those planets vary – the HZ will vary in size and distance from the star. Image Credit: NASA

In the present-day Solar System with the Sun in its current state, Venus is outside of the habitable zone (HZ) – or rather, it is closer to the Sun than the inner boundary of where water will be found in liquid form. The stability of water as a liquid sets the lines for the traditional habitable zone, outlined here in blue. However, for exoplantary systems with characteristics distinct from ours – where the stars exhibit different luminosity, the planets rotate differently, or the atmosphere conditions vary on those planets vary – the HZ will vary in size and distance from the star. Image Credit: NASA

Domagal-Goldman:: There could be a Mars-zone, as well! But getting at that will require us to understand the degree to which Mars was habitable, for how long, and what caused the demise of the red planet’s habitability. These are all questions currently being explored by Curiosity, and we look forward to answers on all those topics.

Ultimately, these sorts of categorizations are going to be done better when we can analyze exoplanets in more detail with bigger future telescopes. The reason we’re doing all this now is for two reasons. First, this gives the community scientific hypotheses for us to test with that sort of mission. Second, it helps us design those missions, and prioritize which objects we would look at first when those missions happen.

AM: The size and location of the Venus zone in each system is going to be dependent on a lot of factors: for example, the luminosity and size of the primary star. A white dwarf star will have a Venus zone much smaller and closer in than our Sun’s. What else will we need to consider in trying to size up Venus zones?

Domagal-Goldman: The other thing that’s really needed now is more simulations of Venus-like atmospheres. This is something that’s very difficult to do, as Venus has been one of the planets that is most difficult to simulate in our computer models. Making advances in that will help us determine the boundaries of both the Venus zone and the habitable zone. Ultimately, we want to define these boundaries with observations from telescopes, but until that happens the best thing we can do will be to improve our simulations and use those results to refine the concept of the Venus zone.

AM: The Kepler Space Telescope has been the workhorse of our planet-hunting mission thus far. When the James Webb takes to the skies, what will change, in terms of finding the Venus Zones, the Venus-analogs, the Earth-analogs and places where we should focus the search for life?

Domagal-Goldman: If we’re lucky, we’ll get a couple Venus-like candidates to study, as the first tests of the hypotheses in this paper. And if we’re extremely lucky, we may get a potentially habitable world or two for us to study, as well. That will be the first mission to move us from studying the “physics” of these planets to being able to study the “chemistry” for a large number of them. Eventually, with a future mission, the goal is to study the biology of such worlds.


Venus Express Goes Gently Into the Night

Source: ESA

Visualisation of the Venus Express aerobraking manoeuvre. Copyright ESA–C. Carreau

Visualisation of the Venus Express aerobraking manoeuvre. Copyright ESA–C. Carreau

ESA’s Venus Express has ended its eight-year mission after far exceeding its planned life. The spacecraft exhausted its propellant during a series of thruster burns to raise its orbit following the low-altitude aerobraking earlier this year.

Since its arrival at Venus in 2006, Venus Express had been on an elliptical 24‑hour orbit, traveling 66 000 km above the south pole at its furthest point and to within 200 km over the north pole on its closest approach, conducting a detailed study of the planet and its atmosphere.

However, after eight years in orbit and with propellant for its propulsion system running low, Venus Express was tasked in mid-2014 with a daring aerobraking campaign, during which it dipped progressively lower into the atmosphere on its closest approaches to the planet.

Normally, the spacecraft would perform routine thruster burns to ensure that it did not come too close to Venus and risk being lost in the atmosphere. But this unique adventure was aimed at achieving the opposite, namely reducing the altitude and allowing an exploration of previously uncharted regions of the atmosphere.

The campaign also provided important experience for future missions – aerobraking can be used to enter orbit around planets with atmospheres without having to carry quite so much propellant.

Venus Express aerobraking. Copyright ESA–C. Carreau

Venus Express aerobraking. Copyright ESA–C. Carreau

Between May and June 2014, the lowest point of the orbit was gradually reduced to about 130–135 km, with the core part of the aerobraking campaign lasting from 18 June to 11 July.

After this month of ‘surfing’ in and out of the atmosphere at low altitudes, the lowest point of the orbit was raised again through a series of 15 small thruster burns, such that by 26 July it was back up to about 460 km, yielding an orbital period of just over 22 hours.

The mission then continued in a reduced science phase, as the closest approach of the spacecraft to Venus steadily decreased again naturally under gravity.

Under the assumption that there was some propellant still remaining, a decision was taken to correct this natural decay with a new series of raising manoeuvres during 23–30 November, in an attempt to prolong the mission into 2015.

However, full contact with Venus Express was lost on 28 November. Since then the telemetry and telecommand links had been partially re-established, but they were very unstable and only limited information could be retrieved.

“The available information provides evidence of the spacecraft losing attitude control most likely due to thrust problems during the raising manoeuvres,” says Patrick Martin, ESA’s Venus Express mission manager.

“It seems likely, therefore, that Venus Express exhausted its remaining propellant about half way through the planned manoeuvres last month.”

Venus. Copyright ESA/MPS/DLR/IDA, M. Pérez-Ayúcar & C. Wilson

Venus. Copyright ESA/MPS/DLR/IDA, M. Pérez-Ayúcar & C. Wilson

Unlike cars and aircraft, spacecraft are not equipped with fuel gauges, so the time of propellant exhaustion for any satellite – especially after such a long time in space – is difficult to predict. The end could not be predicted but was not completely unexpected either.

Without propellant, however, it is no longer possible to control the attitude and orient Venus Express towards Earth to maintain communications. It is also impossible to raise the altitude further, meaning that the spacecraft will naturally sink deeper into the atmosphere over the coming weeks.

“After over eight years in orbit around Venus, we knew that our spacecraft was running on fumes,” says Adam Williams, ESA’s acting Venus Express spacecraft operations manager.

“It was to be expected that the remaining propellant would be exhausted during this period, but we are pleased to have been pushing the boundaries right down to the last drop.”

“During its mission at Venus, the spacecraft provided a comprehensive study of the planet’s ionosphere and atmosphere, and has enabled us to draw important conclusions about its surface,” says Håkan Svedhem, ESA’s Venus Express project scientist.

Venus has a surface temperature of over 450°C, far hotter than a normal kitchen oven, and its atmosphere is an extremely dense, choking mixture of noxious gases.

Is Venus volcanically active? Copyright ESA/AOES

Is Venus volcanically active? Copyright ESA/AOES

One highlight from the mission is the tantalising hint that the planet may well be still geologically active today. One study found numerous lava flows that must have been created no more than 2.5 million years ago – just yesterday on geological timescales – and possibly even much less than that.

Indeed, measurements of sulphur dioxide in the upper atmosphere have shown large variations over the course of the mission. Although peculiarities in the atmospheric circulation may produce a similar result, it is the most convincing argument to date of active volcanism.

Even though the conditions on the surface of Venus are extremely inhospitable today, a survey of the amount of hydrogen and deuterium in the atmosphere suggests that Venus once had a lot of water in the atmosphere, which is now mostly gone, and possibly even oceans of water like Earth’s.

Also just like Earth, the planet continues losing parts of its upper atmosphere to space: Venus Express measured twice as many hydrogen atoms escaping out of the atmosphere as oxygen atoms. Because water is made of two hydrogen atoms and one oxygen atom, the observed escape indicates that water is being broken up in the atmosphere.

Studies of the planet’s ‘super-rotating’ atmosphere – it whips around the planet in only four Earth-days, much faster than the 243 days the planet takes to complete one rotation about its axis – also turned up some intriguing surprises. When studying the winds, by tracking clouds in images, average wind speeds were found to have increased from roughly 300 km/h to 400 km/h over a period of six Earth years.

Venus, southern hemisphere. Copyright ESA/VIRTIS-VenusX IASF-INAF, Observatoire de Paris (R.Hueso, Univ. Bilbao)

Venus, southern hemisphere. Copyright ESA/VIRTIS-VenusX IASF-INAF, Observatoire de Paris (R.Hueso, Univ. Bilbao)

At the same time, a separate study found that the rotation of the planet had slowed by 6.5 minutes since NASA’s Magellan measured it before completing its five-year mission at Venus 20 years ago. However, it remains unknown if there is a direct relationship between the increasing wind speeds and the slowing rotation.

“While the science collection phase of the mission is now complete, the data will keep the scientific community busy for many years to come,” adds Håkan.

“Venus Express has been part of our family of spacecraft in orbit since it was launched in 2005,” says Paolo Ferri, Head of ESA Mission Operations.

“It has been an exciting experience to operate this marvellous spacecraft in the Venus environment. The scientific success of the mission is a great reward for the work done by the operations teams and makes us more proud than sad in this moment of farewell.”

“While we are sad that this mission is ended, we are nevertheless happy to reflect on the great success of Venus Express as part of ESA’s planetary science programme and are confident that its data will remain important legacy for quite some time to come,” says Martin Kessler, Head of ESA Science Operations.

“The mission has continued for much longer than its planned lifetime and it will now soon go out in a blaze of glory.”

“Venus Express was an important element of the scientific programme of ESA and, even though mission operations are ending, the planetary science community worldwide will continue to benefit from more than eight years of Venus Express data and major discoveries which foster the knowledge of terrestrial planets and their evolution,” says Alvaro Giménez, ESA’s Director of Science and Robotic Exploration.