EVE - European Venus Explorer
An in-situ mission proposal to Venus within the ESA Cosmic Vision 2015-2025 programme
Executive Summary
The Cosmic Vision plan sets out the next decade of Europe's priorities in space. Its first two questions focus on the formation of planets and life, and on the solar system itself. Central to both of these goals is the study of the terrestrial planets. The three such planets in our solar system - Earth, Mars, and Venus - show a wide range of evolutionary pathways, and so represent a “key” to our understanding of planets and their habitability (potentially including life development aspects) everywhere, including in other planetary systems. Earth research of course is well established, and Mars exploration has been recognised with its dedicated Aurora funding line; study of Venus similarly deserves to be prioritised highly.
Venus science is an important and well-developed field of research, building on the foundation of extensive missions from the US and USSR, in particular the latter’s series of Venera and Vega missions, the last of which successfully demonstrated deployment of a small balloon on Venus. However, these countries have not launched any Venus missions since 1989, and Europe plays a dominant role in Venus research in the 21st century with the Venus Express spacecraft. A large, vibrant, and world-leading Venus science community has been created in Europe around the Venus Express project. A follow-on international Venus mission will take advantage of Europe's lead role in this field, while incorporating the years of experience from our international partners (in particular Russia).
ESA's Venus Express mission is answering many questions about Venus; but there are many questions which cannot be addressed by orbital measurements alone, in particular relating to the isotopic ratio and cloud chemistry objectives, issues which are the keys to understanding climate evolution on Venus.
We propose an European in situ mission, called the European Venus Explorer (or EVE), consisting of one balloon probe, one descent probe and one orbiter, to address two of the four Cosmic Vision questions:
(1) What are the conditions for planet formation and emergence of life; and
(2) How does the solar system work.
From these questions are derived the following science goals for the EVE mission, which cannot be addressed from orbit only:
1) To derive a unified model of the formation and evolution of terrestrial planets, by studying for the first time the complete record preserved in the elemental and isotopic composition, especially that of noble gases, and characterizing the escape processes.
2) To study the stability of the current climate on Venus, by quantifying exchange of atmospheric constituents with the surface and interior of the planet, and at the interface with space. This will enable modelling of the evolution of the climate of Venus, and the history of water and other volatiles. Could the enormous greenhouse effect be a geologically recent phenomenon on this Earth-like planet?
3) To study the cycling of water and sulphur compounds in the complex cloud environment, by simultaneously studying chemical, radiative, and dynamic processes. Are the clouds maintained by a constant volcanic input of sulphur compounds? Do the clouds, which contain liquid water (albeit highly acidic), provide an environment where pre-biotic compounds or bacteria can survive?
4) To re-construct the geological history of Venus, by mapping the structural elements on the shallow subsurface to better understand volcanic episodes and formations, and anomalies in the ionosphere that can be correlated to present subduction activities.
5) To study the dynamics of the super-rotation of the lower and middle atmosphere and transition to the solar-antisolar regime in the upper atmosphere, by obtaining in situ measurements at a range of altitudes, longitudes, and latitudes as well as remote wind measurements. How is it that the atmospheric super-rotation of Venus is 50x (compared to ~10% in the jet streams on Earth?)
6) To study the role of electrical processes in the atmosphere, by studying their chemical, electrical, acoustic, optical and possible gamma-ray signatures.
Considered together, these science investigations will contribute to the central theme of the mission, which is to understand the evolution of Venus and its climate, with relevance to terrestrial planets everywhere.
The baseline EVE mission consists of one balloon platform floating at an altitude of 50-60 km, one descent probe provided by Russia, and an orbiter with a polar orbit which will perform science observations as well as relay data from the balloon and descent probe. The minimum lifetime of the balloon is 7 days, required for one full circle around the planet, much longer than the 48 hour data returned from Russia's VEGA balloons. Earth-based VLBI and Doppler measurements provide tracking information for the orbiter, allowing measurement of the variations in the planet's gravity field, and for the balloon and descent probe to yield wind measurements in the lower atmosphere. The descent probe’s fall through the atmosphere is expected to last 60 minutes, followed by a lifetime of 30 minutes on the surface. The Japanese space agency (JAXA) also proposes to include another independent platform, a small water vapour-inflated balloon which would be deployed at 35 km altitude and would communicate directly to Earth.
EVE is an M-size mission launched through GTO (capacity of 3000 kg) by a Soyuz Fregat 2-1b from Kourou. It consists of one spacecraft, delivering probes to Venus from transfer orbit, then directly inserted in a ~1 day period orbit and used both as data relay and science orbiter. The first phase is mainly devoted to probe operations and data relay to Earth, with limited orbital science. In a second phase, after the end of balloon operational period, the orbit will be lowered by aerobraking. The balloon data transmission rate to Earth through the orbiter relay is estimated to be ≈10 Mbit per day during balloon operation period.
The balloon payload is focussed on all aspects of cloud-level processes. The key instrument is a sophisticated GCMS system to analyse the cloud and gas composition; this is backed up by optical spectrometry of the cloud particle composition, and measurement of dynamics, radiative balance, and microphysical properties. In particular, the balloon offers a stable vehicle for isotopic mass spectrometry, which requires long integration times (~ hours). Note that the VEGA balloons of 1984, while demonstrating the technological feasibility of deploying balloons on Venus, carried only a very small payload of pressure, temperature, light flux, and backscatter sensors. The EVE balloon would carry a full chemistry lab and isotopic analysis.
The orbiter payload includes a range of instruments optimized for context of in situ measurements made from the probes, with major instruments including a subsurface sounding radar, a lidar, a sub-mm sounder to directly measure atmospheric winds for the first time, and a thermal IR spectrometer which will recover of the science goals from the unsuccessful PFS instrument on Venus Express.
The Russian descent probe payload will include many of the same instruments including a GCMS; but in this case the focus of the payload is the vertical profile of abundances, radiative fluxes and convective stability, with a particular science aim of characterizing near-surface chemistry. The probe will return descent and surface images, and will study surface composition after landing using techniques including gamma-ray spectroscopy. The planned landing site is in the highland tesserae regions, which are understood to be the oldest terrain on Venus, and have not yet been visited by spacecraft.
The payload of the JAXA balloon is optimised for study of dynamics below the clouds, with a few physical sensors and a transmitter for trajectory determination using VLBI. In particular, this balloon aims to resolve uncertainties of circulation in this region, which is critical for understanding the super-rotation but is inaccessible to remote sensing observations.
The science team for this proposed mission includes more than 150 scientists from across Europe as well as from Russia, USA, Canada, and Japan.
The large scope of this mission is particularly enhanced by a major collaboration with Russia. As currently envisaged, ESA is to provide a balloon platform, and an orbiter. Russia will participate at all levels in this mission, with a major science participation and payload contributions for every platform. Russia will also provide the descent probe, building on its extensive experience of Venera probes, will provide a Soyuz launcher (with ESA to meet the Kourou element of the launch costs), and will build the entry/descent systems for both balloon and descent probe. In addition, Japan is considering the provision of a small balloon for low altitude studies. US participation is also envisaged, with widespread science team involvement and at least one NASA-led instrument.
However, the EVE mission will still be a robust and useful mission in the case of non-participation from international partner; in this case we would propose an ESA-only mission consisting of a balloon platform without an orbiter, which would satisfy core science objectives with a reduced data rate.