In his emails, Murchie explained the advantages that he believes landing provides. "The minerals most diagnostic of origin are carbonates, olivine, and organics. These are all minor phases and could only be seen as individual grains. MERLIN does carry an imaging spectrometer. It is a microscopic imaging spectrometer on the arm that will distinguish individual grains, and its [spectral] wavelength range covers absorptions due to all these phases."

"Very closely similar meteorite analogs are also distinguished by small differences in abundances of volatile elements, especially carbon, sulfur, zinc, manganese, and hydrogen. MERLIN's APXS on the arm measures sulfur, zinc and manganese, and maybe carbon. The landed gamma-ray spectrometer gets a very strong signal because it's not limited by time within one [Phobos] body radius, the usual limitation on the signal to noise ratio for gamma rays. It sits on the surface for months and gets very sensitive measurements of carbon, hydrogen, and sulfur. Between the two investigations we'll measure 20 elements which is far more powerful than the three to eight measurable from orbit to distinguish proposed compositions."

"The imaging spectrometer will map hydrogen at the grain scale so we can see if it is linked to specific minerals and intrinsic to the moon, or a surface coating painted onto everything by the solar wind. The microscopic imaging spectrometer and landed stereo imager will together do a phenomenal job of measuring particle size and texture."

"Most important is the arm. We'll use it to scrape away surface soil, measuring regolith strength, and see how the surface has been altered and what fresher soil looks like. Finally the APXS measures volatile elements at 100's of microns depth; the gamma-ray spectrometer measures them at ~10 cm depth. Comparing the two measurements will show how space weathering has depleted volatile elements." (Space weathering, the change in surface composition by exposure to the space environment, is a particularly nasty problem for studying asteroids. It alters the top few microns of the surface, potentially changing its color and spectra. Since this topmost surface is what we can most easily see with remote sensing instruments, this can lead to incorrect composition assumptions and misidentification of analog meteorite types.)

Another way the MERLIN mission would differ from its competitors is in how it would study the interior of Phobos. All three missions will use the tracking of the spacecraft’s radio signal to measure the gravity of these two moons and hence how their mass is distributed internally. These studies can more precisely be done from orbit than from flybys. PADME would do flybys of both moons, Pandora would orbit both, and MERLIN would flyby Deimos and orbit Phobos.

MERLIN’s landings on Phobos would enable an additional investigation into that moon’s interior. As bodies rotate about their axes, they also wobble slightly, which is technically referred to as libration. These tiny oscillations can be sensitive measures of the distribution of mass within a body. Murchie writes, "By landing, we are able to do a very, very robust measurement of Phobos' libration that will do a superb job of measuring internal density heterogeneity. Another important discriminator of origin." (You can see the effects of libration by spinning raw, soft boiled, and hardboiled eggs. Their different interiors cause them to spin and wobble quite differently.)

In mission design, there rarely are free lunches, and the MERLIN mission takes on additional complexity that its competitors do not. It must be able to function both as a highly capable spacecraft and a fully functional lander. The latter requires, among other things, that the spacecraft be able to track its descent, have batteries that will keep the spacecraft functional during nights on the surface, and an antenna that can track Earth from on the surface.

One of the team’s approach to managing the overall mission complexity is to use existing instrument designs for most of the scientific payload. The spacecraft’s main camera that will image Deimos and Phobos during flybys and orbit is a simplified version of the camera used on the Mercury MESSENGER orbiter. The cameras used on the surface will be a copy of the 2020 Mars rover’s color Hazcam cameras. Murchie told me, "two of the instruments are flight spares, and three are really cheap. Their power comes from getting close."

As for the landing itself, "Landing on Phobos is the easiest landing of any planetary body. There is no Mars-like entry, landing, and descent. We land at a few centimeters per second, taking so little fuel that we do a practice down to 100 m, then a few weeks later land for real, and if needed there is plenty of fuel for a third landing. It's not 7 minutes of terror, it's 70 minutes of boredom. But there is enough gravity that the spacecraft weighs enough for "normal" (albeit snail-like) operation. No need for harpoons. We call it landing in the Goldilocks zone (of gravity)."

After the bounce the Philae lander did when it tried to land on a comet, another small body, I asked Murchie about this. "Gravity is about 2 orders of magnitude higher on Phobos [than on the comet]. Philae landed in a free fall, and bounced because its harpoons (required for the low gravity) failed to deploy. MERLIN on the other hand has a controlled descent to negate the terminal velocity, landing at less than 10 cm/s. The landing gear absorb most of what little shock there is. We've analyzed the bejeepers out of the landed configuration. With regard to bounce, we've looked at that and on account of Phobos' much higher gravity, spacecraft design, and controlled descent, bounce is expected to be less than 7 cm. Yes centimeters. Further, we target one of several dozen areas with low regional slope. We've modeled landing on a surface in one of these areas that has a block abundance like those measured on Phobos, Eros, or Itokawa, using a NASA-developed autonomous landing and hazard avoidance technology—the probability of landing with a stable orientation is much greater than 99 percent."

In the end, it appears that NASA has three good proposals to explore the Martian moons. All three would address the key questions of composition and internal structure that should reveal the origin of these moons and much about the nature of small rocky worlds. While this post has emphasized the unique traits of the MERLIN proposal, each of the other two have their unique strengths, too.

There is no single or right way to explore the Martian moons. PADME offers simplicity and low cost. Pandora offers a thorough reconnaissance from orbits of both worlds. MERLIN offers in-depth analysis of two and possibly three landing sites on Phobos but only flybys of Deimos. These three very different proposals emphasize the creative aspect of designing planetary missions.

Within the tight budget of a Discovery mission, no mission can do it all. Each of these three teams had to make choices about which measurements to emphasize and which to forgo. Behind almost every description of a real or proposed Discovery planetary mission, you will see the creative act of deciding how to meet tough scientific goals within a tight budget.

NASA’s managers expect to pick finalists in September for further study from among the 28 Discovery proposals that were submitted to explore worlds ranging from Venus to Saturn’s moon Enceladus. These (typically three) semifinalists will be those that combine both the absolutely best science with the most solid technical proposals. We will learn then if the choices made by the MERLIN team, or the other Martian moon teams, provided the right combination of both.

Selection of the final mission should come late in 2016 with the mission to launch by the end of 2021.