The atmospheres around planets contain clues on the conditions in which those planets formed and on how they have evolved. As a result, missions to place probes into planets have remained a high priority since planetary exploration began. To date, the atmospheres of Venus, Mars, and Jupiter have been explored in situ by probes. (In the case of Mars and to a lesser extent Venus, landed missions have conducted composition and structure measurements.) The Mars Science Laboratory due to land next year will make new and more precise measurements of that world's atmosphere. If selected in the current New Frontiers competition, the Venus SAGE lander would act as an atmospheric probe during its descent. The Neptune and Uranus mission concepts include atmospheric probe options to extend measurements to the ice giants.
The Saturn probe mission concept proposes to return to the gas giants with a probe that would complement the Galileo probe's measurements at Jupiter. In some key respects, the Galileo probe was unable to fulfill its goals because it had the bad luck to descend through a dry atmospheric "desert" that was atypical of the planet. A mission to deliver a new probe (actually two) to Jupiter was prioritized in the last Decadal Survey. Unfortunately, the facilities to test probes for the harrowing entry into Jupiter's atmosphere no longer exist. (A portion of the probe mission's goals now will be performed through remote sensing by the Juno mission.)
Saturn in a way substitutes as a next best option to a new Jupiter probe -- it is the only other gas giant in the solar system. However, a probe mission to Saturn also would be a compliment. Saturn is believed to have formed in an adjacent region to Jupiter, and a probe's measurements would help scientists better model conditions in the early solar system. Measurements could help determine the mechanisms by which the gas giants formed. In particular, measurements of the abundances of heavier elements would help determine the mechanisms of gas giant formation.
The mission concept prepared by the Decadal Survey is probably the simplest of all the concept missions. A carrier/relay craft with the probe would arrive at Saturn approximately seven years after launch. Thirty days or more before arrival, the probe separates from the carrier/relay craft. The probe would enter the atmosphere and begin measurements at 0.1 bars (a bar is the atmospheric pressure at sea level on Earth). At 1 bar, the probe would detach from its parachute for a more rapid descent to 5 bars and the end of the nominal mission after 55 minutes of data collection. The probe would be designed to survive to 10 bars, and the carrier/relay would continue to listen for as long as the entry site remains visible.
The Decadal Survey requested that the study team consider only the simplest mission with a single atmospheric probe and just two instruments: a mass spectrometer and an atmospheric structure instrument. With just this core mission, the estimated cost would fit into the FY15 New Frontiers mission cost.
The study team notes that an actual mission team might well suggest additional instruments. From previous studies, additional instruments such as nephelometer to study clouds and haze structure or a net flux radiometer to study energy balance would be among several candidates. One addition might be a module attached to the probe with a microwave radiometer to conduct deep remote sensing measurements of the atmosphere similar to Juno prior. (The module would be jettisoned just before entry.) If funds permit, a second atmospheric probe likely would be a high priority.
Editorial Thoughts: The arguments for atmospheric probes as high priority missions have long been convincing to me. I hope that the Decadal Survey will propose at least one probe mission. For me, a Venus probe to better understand the formation of terrestrial worlds and the unique evolution of that world would be highest priority. (The 1978 Venus Pioneer probes left many unanswered questions that modern instruments could address.) I would then prioritize a probe to Uranus or Neptune second to explore a different class of worlds. However, if budgets or the Decadal Survey members better understanding of research priorities results in a Saturn probe, this would be a mission I would whole heartedly support.
There is just one addition that I really would like to see added. Cameras are now relatively cheap and lightweight. A camera on the back of the aeroshell and on the probe to take images before and after entry could produce some of the most beautiful images in planetary exploration. Think of the rings seen from just above or within the atmosphere. A 2006 Saturn probe summary listed an imager as a possible instrument, so this may not be an impossible wish.
Appendix: Science Objectives
The following objectives, copied from the concept study report, are similar to the goals for most atmospheric probe missions. With some minor changes, they could be applied to probe missions to Uranus and Neptune. A Venus probe mission would have goals that echo these but also would be focused on chemical interactions with the surface and evolution of the atmosphere's composition.
- Determine the noble gas abundances and isotopic ratios of H, C, N, and O
- Determine the atmospheric structure at the probe descent location*
- Determine the vertical profile of zonal winds as a function of depth at the probe descent location**
- Determine the location, density, and composition of clouds as a function of depth in the atmosphere**
- Determine the variability of atmospheric structure and presence of clouds in two locations**
- Determine the vertical water abundance profile at the probe descent location**
- Determine precision isotope measurements for light elements such as S, N, and O found in simple atmospheric constituents
*Results would be enhanced with microwave radiometer measures of atmospheric structure prior to probe entry to obtain measurements at depths greater than probe will reach while operating (less useful at Uranus and Neptune because of depth of key structures)
**Measurements from multiple probes would enhance results
The Decadal Survey mission concept studies can be found here.