1.1

Venus mission background

Several chemical components that could represent the first steps for further organic processes are found in the upper atmosphere of Venus (H₂S, OCS, SO₂, NH₄Cl, NH₂COOHN₄ and H₂O)^1,2. These chemical components could easily lead to consecutive creation of amino acids¹, which are the very first steps for further biological processes. As reported in³, UV Raman devices could be a key tool to detect the presence of such components. On the other hand, the high heterogeneous absorption between 320 and 400 nm, in the top cloud layer and mesosphere (~ 40-70 Km)⁴ and the atmosphere disequilibrium, are still questions that could be potentially solved by using a single optical payload. The two main reasons to explain these processes could be the concentration of FeCl₃ and SiO₂, or the presence of non-oxygenic photosynthesis lifeforms⁵.

An in-situ analysis tool, as proposed here, could be implemented by using a Raman spectroscopy device and a laser-induced florescence (LIF) device mounted on a landing probe. This study will help the further understanding of the composition and behaviour of the planet’s atmosphere. Missions proposing the need for performing spectroscopy experiments on Venus clouds have already been suggested⁵. Examples of the most recent ones are the Venera-D mission from Russia⁶, which is already planning a probe instrument including an imaging camera, a chromato-mass spectrometer, an alpha P-X-spectrometer, a gamma spectrometer, a laser spectrometer, a hazemeter and a seismic detector⁷. The spectrometer will use a multi-channel tuneable diode laser spectrometer. On the other hand, NASA started developing ideas for a CubeSat mission and descending probes to study Venus’ atmosphere that incorporate similar instruments^8,9.

The goal of our feasibility study will be to identify the specific science to be conducted, to propose an optical payload baseline design for a descending probe and to present a rough estimation of the budget for the mission realisation.

1.2

Brief history of scientific missions in the Venusian atmosphere

The history of Venus explorations starts in the early 60s with the first Russian missions performing flybys and landing on our nearest planet. Although these first missions were not as successful as planned, they paved the way for future Soviet missions which could finally analyse the planet’s atmosphere. The first successful atmospheric mission, the Venera 4 probe (383 Kg), was carried out by the Soviet bloc in the year 1967¹⁰. The mission already included a UV spectrometer that helped to analyse the atmosphere composition of Venus for the first time. In the year 1984, the VeGa missions were launched from Baikonur. They successfully tested the possibility of studying the alien atmosphere by sending two landing probes installed in independent balloons deployed at around 60 km altitude¹¹. The inclusion of a helium balloon to be deployed in the descending probe helped scientists to obtain a much longer analysis time and area of study, due to the dense atmosphere composition and presence of strong winds¹². Earlier, in 1978, NASA also sent the Pioneer Venus multiprobe (around 600 Kg) that performed atmosphere analysis thanks to a battery of different independent probes and instruments (amongst others, a mass spectrometer and gas chromatograph to measure atmosphere composition, a cloud particle size spectrometer and temperature and pressure sensors) ¹³.

From the 80s on, many more missions have been sent to Venus; mainly by American, European, and more recently Japanese contributions. However, since the earliest first missions during the 60’s to 80’s decades, no more landing probes were sent into the planet’s atmosphere. However, nowadays, the current advances in Raman spectrometers would be able to provide a much more accurate and extensive analysis than the missions four decades ago. Moreover, new technological developments would allow to do it on a much smaller and lighter platform, which would result in a much more cost-effective mission.

1.3

Space Raman Devices

Raman spectroscopy is an interesting technology for planetary science, since it allows for in situ analysis of samples thanks to the inelastic scattering of light. Fundamentally, the experiment just requires a narrow light excitation source, usually provided by a laser; and a spectrometer instrument to analyse the Raman light shift, which will provide accurate information of vibrational and rotational molecule levels of the illuminated sample. Raman spectroscopy can measure samples up to 200 m away and so can be useful for long-range atmospheric measurements³.

Several research groups have been working in the last decades to provide compact Raman systems that are capable of operating in extreme environments, including the possibility of operating in the different bodies of our Solar system. Already in 2006, the University of Hawaii implemented several changes to standard Raman spectroscopy devices to be able to create a small portable system to be operated outdoors¹⁴. The group, already planning for space exploration, designed a device using a 532 nm laser with an energy of 35 mJ and with 8 ns pulses. The team reported that they were successful in their goals, thanks to the use of volume holographic transmission gratings, which provided a better resolution (70-2470 cm^-1 line spacing with a resolution of 9 cm^-1). The device had a laser source using a Nd:YAG laser with second harmonic generation (SHG) to obtain the 532 nm emission. The incoming light from the sample was later collected thanks to a Makusutov Cassegrain 127 mm telescope¹⁴.

Furthermore, several planetary mission are currently using compact Raman devices to perform research. The best known example is the NASA 2020 mission to Mars. The mission will use the SuperCam instrument, to perform 7 m distance Raman analysis. The SuperCam includes, in addition to Raman, a suite of measurement techniques, such as LIBS, Time-Resolved Fluorescence (TRF) spectroscopy, Visible and InfraRed (VISIR) reflectance spectroscopy and high-resolution imaging. The mission also uses a robotic arm that integrates the Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) Raman device, to perform proximity measurements. In this last case, the Raman device operates with a 248.6 nm laser emission¹⁵. Similarly, the European Space Agency (ESA) is planning to launch an instrument in the frame of the ExoMars mission in 2022, which will also have Raman capabilities^16,17.

1.4

Venus atmosphere conditions

One constraint of doing Raman spectroscopy in Venus’s atmosphere, is the presence of extreme and challenging conditions under which the devices will have to perform. The investigation of existing chemicals and possible amino acids will have to be performed in very different pressure and temperature regimes, depending on the atmospheric altitude^18,19.

The atmospheric region of interest for the present study (although the probe could be analysing data down to the planet’s surface) is the one between 70 to 40 km altitudes. At these altitudes, the atmosphere has a sulphuric acid clouded region, with temperatures approximately between 200 to 400 K, and pressures approximately between 0.1 to 10 atm¹⁹.

In contrast, the presence of strong winds, up to 400 Km/h, could make balloon exploration very favourable, since the probe would be able to travel horizontally, covering vast atmospheric areas²⁰. This has already been used by the Russian VeGA mission in 1984, giving successful results, and was proposed also to ESA in 2013 in response to the Cosmic Vision Call for Ideas²¹. Once, and if the probes reach the planet’s surface and are still analysing data, the instruments will encounter high pressures (around 90 atm) and high temperatures (around 700 K).

Figure 1.

Venus atmospheric profile. Source²².

2.1

Science

Several scientific articles have theorized about the possibility of Venusian life in the planet’s atmosphere^5,23. Venus’s atmosphere is mainly composed of CO2, but also contains other relevant elements related to Earthly biochemical processes. Moreover, the already mentioned UV absorption processes could result from a possible anoxygenic photosynthetic life form²³.

Raman spectroscopy is a powerful analysis technique by which a precise identification of both mineral and organic phases can be carried out, without disturbing the analysed sample. This analysis can be done in any state in which the sample is found (solid, liquid or gas) and with micrometre resolution. In addition, Raman spectroscopy is a very fast technique that allows measurement in a few seconds. Along with this, the data products are very simple, so the memory consumption and processing are minimal. This technique can be applied both in contact mode, bringing the signal through optics and fibers to the spectrometer, as well as remotely, from a few centimetres to tens of meters. Contact measurements can be carried out using a continuous laser and in low light conditions, while remote acquisition is carried out using a pulsed laser, which allows samples to be analysed without interference from outside light and other phenomena which could interfere. Therefore, a properly designed instrument based on this technique can perform fast and detailed analysis of samples in-situ.

These characteristics and this design would make it very suitable for the detection of amino acids and their precursors in the atmosphere of Venus. There is already extensive experience in the use of this technology in the determination and differentiation of amino acids in various environments, whether deposited on inorganic surfaces, in solution or in aerosols. In addition, the Raman spectroscopy technique makes it possible to uniquely distinguish each type of amino acid by having a unique fingerprint based on its molecular composition. Moreover, in recently published articles²⁴, it is stated that the discovery of phosphine in Venus’s atmosphere could be produced by certain types of bacteria. This could also be easily studied by a Raman technique. In particular, researchers believe that performing distance Raman using a pulsed UV excitation source could help to determine the presence of organics, amino acids, or other more complex biomarker molecules. The Raman shift range between 500 and 1700 cm^-1 gives a sufficient signal for the precise determination of the compounds. Which will further simplify the detection design.

Figure 2.

Raman shift range vs laser excitation.

Figure 3.

Raman and Fluorescence emission ranges for a 250 nm excitation.

2.2

The instrument-Optical Payload

As the Raman shift provides a weak signal compared to fluorescence effects, the instrument could have two different channels to assure success. The mission could include a Raman spectroscopy device but could also study atmosphere fluorescence. Both processes could be stimulated by using a single laser and the response signals could be analysed by a single spectrometer. Time resolved measurements should be included to differentiate the raised signal coming from the Raman shift or from the scattered fluorescence effect. In the case of the fluorescence signal, being of higher intensity, it could be used to acquire information in the tens of meters distance range across the atmosphere³. A similar approach is planned to be used in the OrganiCam instrument from Los Alamos National Laboratory, in a mission to Jupiter’s moon Europa. The mission plans to use a combination of fluorescence/Raman instruments for a study of life²⁵. Thus, authors suggest an instrument base line architecture as follows (see Figure 4):

1) The UV laser source will be separated using beam-splitters to transport the emitted light in free space into the different regions of study:

2) Both fluorescence and Raman signals, could be later joined by using a beam combiner element. The incoming light would then enter the spectrometer device that will independently analyse both signals. The Raman signal will be then used to study the molecules chemical bonds and possible functional groups, while the florescence signal will be focused on electronic structure to analyse aromatics, keynotes and aldehydes²⁶.

The laser source and spectrometer instruments will have to be inserted into the protected core of the probe; guaranteeing that the sensitive instruments are kept away from the extreme environments.

Figure 4.

Schematic of the basic instrument base line. The laser could be obliquely placed near the receiving telescope.

2.3

The laser excitation source

Due to the Venus atmosphere thermal ranges and the interest in studying the presence of amino acids and their response to certain wavelengths, the proposed excitation source will be in the UV region, where these molecules show a great absorption and later fluorescence. UV lasers have recently been market accessible²⁷. Even if nowadays reliable UV lasers can perform Raman spectroscopy in a lab environment, the laser qualification for being used in space missions is not yet a straight forward and standardized process²⁹. However, examples of UV spectrometer laser devices, being used in space missions already exist, such as the SHERLOC instrument of the 2020 NASA mission¹⁵ or the UV-DPSS at the MOMA (Mars Organic Molecule Analyser) for the ExoMars 2022 mission²⁸.

Different options could be available for the mission’s purpose. However, the best solution will always be a trade-off between science capabilities (different sample analysis and used methods) and the mission constraints (budgets and environmental conditions). As a base-line, our preferred option due to the principal objective of studying amino acids, plus budget constraints and space heritage, we decided to propose a hollow cathode ion laser by a neon copper emitting at 248 nm wavelength³⁰; similar as the one used for the SHERLOC instrument³¹.

Other UV configurations could be a slightly different UV laser emitter, changing the hollow cathode ion laser to a frequency doubled solid state laser, and even tuneable lasers with a wider emission wavelength³².

2.4

Optics and analyzing setup

The telescope implemented on the device acquiring the reflected signal from the Venusian atmosphere could have a baseline aperture around 100 mm. UV light has the inconvenience of having a strong absorption when transmitted or reflected with optical components. Luckily, in recent years many research groups worked on MgF2 and LiF coatings, which can provide UV and also far-UV (FUV) reflectance up to 80-90% (range around 270 nm)³³, making UV science feasible and efficient for such purposes. Similarly, the spectrometer gratings could be implemented using reflection holographic gratings, which are already available from companies such as Horiba.

Finally, we propose on doing a time-resolved Raman analysis³ by using an intensified charge-coupled device (ICCD) time gated measurement detector, to better separate the Raman and fluorescence signal.