Why aren't there any robotic missions on Europa or Enceladus?

Question

As per Space.com's Methane in plume of Saturn's moon Enceladus could be sign of alien life, studies suggest that Enceladus and Europa are the two most promising solar-system bodies on or in which to search for extra-terrestrial life.

Why does Mars continue to been given so much focus when we already have several rovers there, instead of sending arial or sub-surface rovers to Enceladus and Europa? Budget wise I believe the missions would cost the same as that of Mars, while Mars is pretty much dry and the research focuses on finding signs of life that "may" have existed in the past, so how come it supersedes the mission to find signs of life that have the potential to "exist right now" in Enceladus and Europa?

It appears to me at least that billions of US dollars are being spent on a barren planet rather on these two moons that may support life. Though thin, Europa's atmosphere is composed of oxygen.

Answer 1

@GremlinWrangler's answer sums up several important points;

1. Getting a low mass rover from Earth all the way to landing on the surface of one of those Moons requires much much more rocketry (delta-v) than landing a much heavier and more capable/diverse probe on the surface of Mars.
2. Solar won't work well there and RTG's are quite scarce

Let's also consider that

3. Thanks to the evolving series of rovers and orbiters at Mars working in a highly coordinated way over decades, the effectiveness and scientific return (bang per buck) of missions there continue to increase, as well as experience in interpreting evidence of life rather than a search for actual living things looking back at you.
4. Both of those moons have 10+ km thick ice crusts. There may be cracks and holes in places and water from oceans may have been deposited on the surface and frozen and exposed to radiation and so anything in it will have likely died. Luckily, experience gained from all those Mars rovers of looking for signs of life rather than living stuff will be helpful here.
5. Missions going through the ice and into the ocean are currently hypothetical and problematic. You need a lot of energy to get through 10 km of ice no matter how you do it, and unless your spacecraft is absolutely completely 100% certain sterile you may contaminate a lifeless ocean or disastrously infect it with Earth organisms. To my knowledge there is not yet documented capability that spacecraft can be absolutely sterilized and yet still functional.¹

¹note added in proof: ...therefore if a mission is to be carried out, efforts have to be made to minimize this risk precisely because it can not be eliminated. Those might include further studies or calls for proposals for new ways to reduce biologically viable contamination. Alternatively this could become a sticking point and "maybe we shouldn't go just yet" thinking might kick this class of mission down the road a bit further.

In order to verify that others share my notion that complete sterilization is not currently possible I've just asked:

• Is there any demonstrated or even proposed technology that can sterilize a spacecraft with 100% certainty and yet leave it electronically functional?

Answer 2

There are probably many answers, but for guesstimating hypothetical missions a look at delta-V/subway maps like this are highly informative in terms of problem scale.

Working from that getting into LEO is 9.4 km/s, these are the rockets you see launching regularly, and can get about 5% of their launch mass into orbit. Earth escape needs another 3.41 km/s, from there getting to a mars intercept is another 0.39 km/s and because mars has an atmosphere we do not need rockets for most of the orbit insert/landing Delta-V, just a reasonably capable heat shield.

If we want a Jupiter intercept it is another 2.7km, and because Europa has very little atmosphere we probably need to use a rocket to descend deep into Jupiter's gravity well rather than aerobraking, meaning for a given booster we get far less mass (possible as little as 5%) to Europa orbit as we can get to Mars for the same mission cost, and need for more complex power, heating and radio systems for a further reduction in 'useful' payload.

The planned Europa Clipper mission uses one of the largest boosters currently available to get 6000kg en route to Jupiter and then burns around 4000kg of fuel getting into orbit (using gravity assists to get more payload than above), with a payload of instruments of 353 kg. Taking this as a template we could redesign the craft to land - using that 353 kg to add landing gear, more fuel and more structure to support the solar panels during landing but suspect we come up with a negative available payload mass. Or we could remove the instruments and make a 300kg lander, which would certainly be possible (similar D/V for a Lunar orbit to Lunar surface) but would be battery powered and very basic.

Getting to Saturn and Enceladus requires even more engine performance, and is beyond the point where solar power is useful adding political costs from launching nuclear RTGs into the mix.

So missions to these places are certainly possible, but the payloads are going to be far smaller than to mars and look far more like Huygens in terms of delivering a couple of pictures and some basic chemistry for a limited period than what we get on mars. Certainly would not be a meaningful search for life, and not much better than the orbiting probes have gathered.

So to date for a given amount of money there has been more return seen taking a complex craft to mars than a more basic craft to the outer moons. As Mars becomes better studied, this is changing hence the progress on Europa Clipper, which at one stage appears to have included a lander at similar cost to an entire mars mission.

Answer 3

The motive for the exploration of Enceladus and Europa is different from that of Mars. The primary motive for exploration of the two moons is the possibility of finding an independent instance of life, while the exploration of Mars also includes gathering information for likely future human colonization.

Of the three worlds, Enceladus is the most challenging because Saturn is really big (massive) and Enceladus orbits at just over four Saturn radii out, so is in an extremely deep gravitational well. We did a study of a possible flagship mission to Enceledus in 2009 (1). Although more has been discovered about Enceladus since then, I believe the Enceladus Report is still the most detailed study of the nuts and bolts of an actual mission there.

In order to save enough fuel to get down to Enceladus once we reached Saturn, we had to assume multiple gravity assists, so wouldn't reach Enceladus until about 10 years later (actual data in ref 1) and the MTBF (Mean Time Before Failure) for many flight items is not rated for more than 10 years. Section 3.1.1.1 of the report discuses some of the obstacles that must be overcome for such a mission.

So, if you ask "If searching for a second instance of life is so important, why are we planning multiple missions to Europa and none for Enceladus right now?". The answer is: it's a lot harder (and more expensive) to get to Enceladus.

(1) https://www.lpi.usra.edu/opag/Enceladus_Public_Report.pdf

Answer 4

This is a late answer, but ...

Budget wise I believe the missions would cost the same as that of Mars.

You have grossly underestimated the cost of a Europa lander. The increased delta V needed to get to and land on Europa alone vastly increase the cost of a lander mission to Europa compared to a lander mission to Mars. The enhanced radiation protection needed to make a Europa lander viable vastly increases the cost even more.

And then there's planetary protection. If the current placeholder value of one in ten thousand chance of infecting Europa with Earth life remains intact, this increases the costs even more. This would require the Jet Propulsion Laboratory and the launch facility to upgrade their class 10000 clean rooms by orders of magnitude. A class 10000 clean room means there are up to 10000 tiny particles, including bacterial spores, per cubic foot of air. While that represents a two order of magnitude reduction from the number of dust particles in room air, it means that the most recent Mars rover mission probably brought half a million bacterial spores to Mars.

Yet another factor is terrain. We do not yet know if Europa's terrain is lander friendly, let alone hopper friendly or rover friendly. There are some who speculate that Europa is covered with ice spikes. A three meter tall ice spike would not be visible in the currently available images of Europa. We need better images of Europa before we even think of sending a lander or hopper or rover to Europa.