Strofio: Exospheric Sampling of Mercury's Surface Composition

1. Strofio: Exospheric Sampling of Mercury's Surface Composition US involvement in BepiColombo and SERENA-STROFIO prospects Stefano A. Livi Southwest Research Institute Stefano.Livi@swri.edu 210-522-3310

2. Mercury’s Exosphere • Difficult to study as Mercury is very near the Sun • Only elements with strong doublet lines (Na, Ca, K) can be identified from Earth

3. Existing Measurements • Mariner 10 Ultraviolet Spectrometer: • H, He: cold and hot components • O: much less abundant then expected • Ground optical measurements • Na, K: daily variations • Ca: higher energy Together, these six elements account for only 20% of the observed total pressure inferred from radio occultation measurements.

4. Why Chemical Composition Studies? • Elemental and isotopic composition is a powerful tool to study planet geological history • Neutrals come directly from the surface, hence reflect the chemical composition of the regolith • Neutrals circulation and outflow modify regolith composition • Neutral spatial and temporal variability strongly constrain models of surface evolution • Neutrals are the source of planetary ions

5. Processes Generating the Exosphere Thermal evaporation (day/night asymmetry) H, He, Na (subsolar), K (subsolar), S, Ar, H2O Photo-desorption: (day/night asymmetry) Important for volatiles H, He, Na, K, S Ion-sputter: (Solar wind activity related) Important for refractories Ca and others Impact Vaporization (episodic) Everything? (more volatiles than refractories) Important for creation of glass Important for loss of water deposits and volatiles Venting, volcanism (spatially locked) everything? (more volatiles than refractories)

6. What is Strofio? Strofio is a scientific investigation of the Hermean exosphere based on a novel mass spectrograph that determines particle mass-per-charge (m/q) by a time-of-flight (TOF) technique. The name comes from the Greek word Strofi, which means “to rotate”: the phase of a rotating electric field “stamps” a start time on the particles’ trajectory. The detector records the exit position (hence the start time) and the stop time. Strofio accomplishes its high-sensitivity (0.14 counts/s when the density is 1 particle/cm3) within very limited resources (1.9 kg). The mass resolution (m/m≥ 60) is achieved by fast electronics and does not require tight mechanical tolerances.

7. The Hermean exosphere is a fascinating environment, very difficult to study from Earth • The six measured elements and an extensive body of computer simulations are guiding our understanding. Direct measurements of neutrals and ions are needed to advance our understanding of Mercury and its exosphere • The suite SERENA onboard BepiColombo MPO has been tailored to study in-depth the neutral and ionic environment of Mercury

9. Strofio as NASA Contribution • Strofio activities in the US must be financially supported by NASA • Since 2004 NASA has been regularly briefed about Strofio and the exospheric community intentions • As of today (May 2008) there has been no commitments of NASA towards funding the experiment

10. Past Events • April 2006: Proposal submitted to NASA in response to the Discovery XII AO • October 2006: NASA notifies that Strofio was not selected • December 2006: debriefing at NASA HQ: • Category III: “Excellent science with appropriate methodology, but requires further development” • Instrument-to-spacecraft interfaces insufficiently defined and constrained • No funding available to support further development

11. The Saga Continues … • December 2006: ESA requests clarification • January - April 2007: further requests from ESA, ASI, JHUAPL, and SwRI • May 2007: Serena ISRR review • June 4 and 5: bilateral meeting ESA-NASA • June 14 and 15: bilateral meeting ASI-NASA

12. Light at the End of the Tunnel? • June 21: NASA calls SwRI and requests “mini-proposal” to carry on activities until end of September 2007 • Further plans call for an ad hoc review to be held on or around September 24th • Successful review would open up possibility for approving Strofio without the need for a new proposal in response to a future AO

13. NASA: the Saga continues (1) September 27th: Based On a review of NASA's requirements, the Science Mission Directorate has decided to further evaluate the technology readiness of Strofio for flight on the European Space Agency's Beppi-Colombo mission to Mercury. The evaluation will be based on a Technology Readiness Report and a site visit. The review will be performed by a team of scientists and engineers external to NASA. The results of the evaluation will be communicated to the Directorate Program Management Council who will decide if Strofio will be selected to proceed into Phase A. …. NASA's final decision regarding Strofio will be communicated to you and ESA on October 31,2007.

14. NASA: the Saga continues (2) October 1st: Technology Readiness Report issued October 19th: Questions from the review team received October 22th: Technology Readiness Review takes place

15. NASA: the Saga continues (3) October 31st: Report from the evaluation committee (distributed on 3/17/2008 !!): The Strofio instrument risk is rated as MEDIUM. There are 3 major weaknesses driving the medium risk rating. The proposed schedule is aggressive and lags the ESA schedule, which could negatively impact Strofio. The mass margin is inadequate at this stage of development and not under the control of Strofio. Also, the Phase C/D cost reserves are only 20% as calculated by the TMC panel, compared to the 25% required by the AO. However, there are two major strengths that are mitigating factors. The instrument concept is well suited to obtain the required measurements and the proposing team is highly qualified to do the implementation. The TMC Panel feels that with careful coordination with ESA/SERENA and with dedicated mass margin allocation, the instrument can be successfully implemented for BepiColombo.

16. NASA HQ: the Saga continues (4) October 31st: Nothing happens November 14th : The late autumn and early winter months are a time of great budget uncertainty at NASA. The President's 2009 Budget Proposal is not presented to Congress until early February. Before that, NASA must negotiate its portion of the 2009 Budget with the Office of Management and Budget. Those negotiations are currently incomplete. Due to this uncertainty, NASA cannot, in all prudence, undertake any new commitments at this time. December 26th: Budget is signed by pres. Bush

17. NASA: the Saga continues (5) January 30th: Nothing happens January 31st: ASI contact with Jim Green: • Strofio still has some problem with mass …. • and with development of critical items (HVPS) • NASA is willing to support the necessary further development • Final decision on financing of Strofio delayed

18. NASA: the Saga continues (6) My personal reading of the tea leaves: • NASA is indeed willing and wishing to finance Strofio • They cannot find an appropriate way • They may be cautious of creating a precedent

19. The SALMON Opportunity • On March 14th, NASA announced the “Stand Alone Missions of Opportunity Notice” (SALMON), an umbrella AO to offer timely opportunities for participating to mission of other agencies • PEA H2: Lunar and planetary science partner missions of opportunity fits with Strofio science and goals and has the financial volume to support the Strofio flight program BUT ……….

20. The SALMON Opportunity • Salmon has 6 elements, to be released one after the other starting May 15 • Nobody seems to know the sequence • Even in the most optimistic scenario, money will not be available before mid-2009