Centauri Dreams

One reason I wanted to run yesterday’s article about the Opher et al. paper on the heliosphere, aside from its innate scientific interest (and it is a very solid, well crafted piece of work) is to illustrate how much we still have to learn about the balloon-like bubble carved out by the solar wind. The entire Solar System fits within it easily, but we observe only from inside and have little knowledge of its structure. None of the paper’s authors would argue that we have the definitive answer on the shape of the heliosphere. That will take a good deal more data, as the paper notes:

Future remote-sensing and in situ measurements will be able to test the reality of a rounder heliosphere. In Fig. 6, we show our prediction for the interstellar magnetic field ahead of the heliosphere at V2. In addition, future missions such as the Interstellar Mapping and Acceleration Probe will return ENA [energetic neutral atom] maps at higher energies than present missions and so will be able to explore ENAs coming from deep into the heliospheric tail. Thus, further exploration of the global structure of the heliosphere will be forthcoming and will put our model to the test.

We’ll learn more from the Voyagers, in other words, as well as from IMAP (more about this one in a later article), New Horizons, and whatever probe we next send out to system’s edge. Our two Voyager spacecraft may well last another seven years, which would give them 50 years of data return since their launch in 1977.

Image: And here’s something we’ve learned from New Horizons. The SWAP instrument aboard the spacecraft has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft. Credit: Southwest Research Institute; background artist rendering by NASA and Adler Planetarium.

The interstellar probe NASA has been contemplating, under study at various centers but most visibly at the Johns Hopkins Applied Physics Laboratory (APL) would, unlike Voyager, be built from the start with a 50 year goal in mind. Voyager 1 is now about 141 AU from Earth (21.2 billion kilometers). Interstellar Probe (APL capitalizes its design) would go for 1000 AU, but at much improved speeds, reaching the distance in 50 years.

How to do this? For one thing, achieve a boost from one of the huge rockets now coming onto the market, perhaps NASA’s own Space Launch System (SLS), or a commercial entry from a private company, perhaps SpaceX or Blue Origin. We’re not talking about launching until 2030, and that’s assuming the mission gets the green light in the upcoming heliophysics decadal survey, which will put in place missions related to the Sun over a ten year period.

A gravity assist at Jupiter added on to its kick from a massive booster would put us in familiar territory, given Jupiter’s history of flinging spacecraft like Voyager and New Horizons on their way, but a solar gravity assist is also contemplated, one that would take Interstellar Probe a good deal closer to the Sun than the Parker Solar Probe. You’d think closer is better, but at this stage in our technology, the perihelion numbers will be decided by factoring the weight of the required heat shielding. A balancing act ensues to get the most bang for the buck.

Exactly which instruments would fly on this modern era Voyager Plus would depend upon how instrument packages can be combined to save mass while maximizing power and data rates on the communications side. If you have a look at the APL page devoted to Interstellar Probe, you’ll see a notional payload, meaning this is what we’d like to cover with an ideal probe. The instrumentation includes:

A particle and fields suite for exploring the interstellar medium and its interaction with the heliosphere, with detectors such as:

• energetic neutral atom (ENA) camera
• energetic particles/cosmic rays
• solar /interstellar plasma and neutral wind
• vector helium magnetometer
• plasma wave

Beyond the particle and fields instrumentation, the probe should include:

• Optical cameras for flyby imaging and astrometry
• A suite to measure dust and its basic composition
• Infrared cameras for obtaining the 3D distribution of dust beyond our planetary neighborhood

We know that Voyager 1 and 2 have both left the heliosphere, Voyager 1 in August of 2012 and Voyager 2 in November of 2018, the two craft on widely divergent trajectories (recall Voyager 1’s dogleg at Saturn to get a look at Titan, whereas Voyager 2 moved on for close passes at Uranus and Neptune). Yesterday’s paper offered a new proposal for the shape of the heliosphere which is rather interesting in this regard. If the heliosphere really is more circular, lacking that presumed cometary ‘tail,’ then getting outside it won’t necessarily be determined by what would have been considered the shortest route, avoiding a tail that was estimated to trail thousands of AU. Here astrophysics and engineering work together in the choice of optimum trajectories.

Yesterday we looked at the need to get beyond the heliosphere so we could study its structure and gain insights into other planetary systems. But there are other reasons that take us much farther afield. It’s worth remembering that within the heliosphere, we have to contend with the foreground infrared radiation from dust within the Solar System, known as the zodiacal cloud. Going beyond the heliosphere opens up the possibility of studying diffuse infrared radiation from other stars and galaxies that has been effectively blocked for us by that cloud.

We also get a look at the nature of the dust disk, one that we can observe around other stars but are unable to measure in terms of large-scale structure from within our own. Learning how the Sun affects the structure of the heliosphere will help us understand the dynamics of other stellar systems, and the data a probe like this will take will be crucial at defining the local interstellar medium, through which our much longer-range probes will eventually move.

Needless to say, a great deal of science can be accomplished along the way. Interstellar Probe would reach the Kuiper belt in a scant four years, where flybys of KBOs and long-range observation of the environment there would complement and extend what we are learning from New Horizons. The APL trade study is designed to craft “a realistic mission architecture that includes available (or soon-to-be available) launch vehicles, kick stages, operations concepts and reliability standards.” All of this produces the reference materials that will be needed for the science and technology definition team that will turn aspirations into hard designs.

We should always be thinking about the kinds of mission that might one day fly, the long-range improvements that can enable them, and the audacious targets we someday want to reach. But as we draw up these conjectures and think about eventually engineering them, we also must be thinking about the kind of missions that can fly today. An interstellar probe of the kind now under study at APL and other NASA centers was a part of the discussion for the last decadal survey, but only now are we reaching a technological level to make 1000 AU in 50 years possible.

We need these early steps to make the broader strides that will occur later, on a path toward a Solar System infrastructure that will eventually support probes into the Oort Cloud and one day beyond. So tracking the fortunes of Interstellar Probe will be a priority for Centauri Dreams in coming months.

We have all too little information about the heliosphere, the only data from beyond it being what we have collected from the two Voyagers. Altogether, only five spacecraft — Pioneer 10 and 11, the Voyagers and New Horizons — have escaped the gravity of the Sun enroute to interstellar space. To understand how the heliosphere operates, and the interactions between the solar wind of charged particles and magnetic fields with what lies beyond, we’d really like to be able to look back at our system in its entirety. The Interstellar Probe concept being pondered at Johns Hopkins Applied Physics Laboratory and elsewhere is one possible way to do this.

I’ll have more to say about Interstellar Probe in coming days, though I do want to give a nod to its history, which can be traced as far back as 1958 and a report from the National Academy of Sciences. APL’s Ralph McNutt has been studying interstellar concepts for decades, and was a major source as I worked on my original Centauri Dreams book. It’s important to realize that interstellar studies around mission concepts to get beyond the heliosphere have been in play for a long time, and the next Heliophysics Decadal Survey could contain this one. You might remember that McNutt’s Innovative Interstellar Explorer was an earlier design.

Even now, though, we’re learning more about the heliosphere itself, and that boundary region known as the heliopause, where the solar wind bumps up against the deep currents of the interstellar medium and is shaped by them. Look at an older diagram of the heliosphere and you usually find it depicted with a shape something like a comet, a rounded nose up front and a tail stretching out well behind. Interstellar Probe would reach 1,000 AU and give us that view back that we need to learn more.

But to firm up a mission concept like this, we need more data on the target, and it turns out that the shape of the heliosphere is anything but established. It was in 2015 that Merav Opher (Boston University) and James Drake (University of Maryland) put forth a new computer model that, incorporating Voyager data, suggested a heliosphere shaped something like a crescent, so that instead of a single, comet-like tail, two jets would extend downstream from the nose.

The shape of the heliosphere has been under debate ever since, with Cassini data (based on correlations between particles echoing off the heliopause and ions measured by Voyager) being used to suggest a spherical heliosphere. Both crescent and sphere were controversial, says Voyager veteran and Cassini scientist Tom Krimigis. “You don’t accept that kind of change easily. The whole scientific community that works in this area had assumed for over 55 years that the heliosphere had a comet tail.”

Now Opher and Drake, working with Harvard’s Avi Loeb and Gabor Toth (University of Michigan), are back with an even more complex possibility. In a paper just published in Nature Astronomy, the scientists distinguish between charged particles from the solar wind and hot ‘pickup’ ions, particles that enter the Solar System in electrically neutral form, only to subsequently lose their electrons. We’ve learned from New Horizons that these particles become far hotter than ordinary solar wind ions as they are carried along within that wind.

Voyager 2’s crossing of the heliosphere boundary showed how pickup ions dominate in the region of the heliosheath, but this paper is the first to explore the impact of pickup ions on the structure of the heliosphere. The authors examine the temperature, density and speed of both groups of particles independently. Their model produces a shape far different from the comet view, a rounder, baggy-looking topology that only vaguely preserves a crescent and one that can be further manipulated by how we define the heliosphere’s edge. Have a look.

And let’s contrast this with the older view, at the left in this figure from the paper. In the caption, the reference to ‘case B’ notes one of two cases run in the scientists’ model, based on the fact that the interstellar magnetic field strength and direction are not well constrained (another reason for a properly instrumented probe in this region). For more on the distinction between the two cases, see the paper (citation below).

Image: This is Figure 4 from the paper. Caption: The new heliosphere. a, The HP [heliopause] is shown by the yellow surface (case B) defined by a solar wind density of 0.005 cm^?3. The white lines represent the solar magnetic field. The red lines represent the interstellar magnetic field. b, The standard view of a comet long tail extending thousands of astronomical units. V1 and V2 are shown in this artist rendition; V2 has now passed the HP. The yellow dot represents the Sun. The supersonic solar wind region is represented by the blue region around the Sun. The extended region beyond the blue region represents the HS [heliosheath]. Credit: NASA/JPL-Caltech / Opher et al.

The authors describe the heliosphere’s shape as:

…a more ‘squishable’ heliosphere that has a smaller and rounder shape (Fig. 4a). This global structure is drastically different from the standard picture of a long heliosphere with a comet-like tail that extends to thousands of astronomical units (Fig. 4b). The distance from the Sun to the HP in the new round heliosphere is nearly the same in all directions.

Building a spacecraft like the APL Interstellar Probe would allow us to put instruments specifically designed for operating in this environment into play, among other things detecting pickup ions near the heliopause. This will further refine the heliosphere’s shape and also allow us to better compare our own heliosphere with those around other stars. There are astrobiological implications here, for the heliosphere deflects charged particles that could disrupt DNA. How interstellar particles enter stellar systems could thus play a role in evolution.

The paper is Opher et al., “A small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions,” Nature Astronomy 16 March 2020 (abstract).

Centauri Dreams rarely looks at Mercury, the operative method being generally to focus on the outer Solar System and beyond. But a new paper out of the Planetary Science Institute in Tucson (AZ) raises the eyebrows in suggesting that parts of Mercury may once have been able to shelter prebiotic chemistry and perhaps, according to the authors, even primitive life forms. Such a finding might thus extend our ideas of ‘habitable zones’ much closer to parent stars than previously assumed.

It seems a long shot, given surface temperatures reaching 430? in the daytime and -180? at night, but the PSI work turns up interesting possibilities in some subsurface regions of Mercury. The heart of this research is found in the datasets returned by the MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) spacecraft. The Mercury orbiter identified numerous volatile-bearing surfaces on Mercury, with high abundances of sulfur, chlorine and potassium, and polar ice in permanently shadowed craters at the planet’s poles.

Key to the research are Mercury’s ‘chaotic’ terrains, hilly and fractured areas first seen in 1974 in the flybys of Mariner 10. The planet’s spectacular Caloris Basin is a crater about 1525 kilometers across ringed by mile-high mountains. Interestingly, numerous chaotic terrains are found directly on the other side of the planet from the Caloris Basin, leading to the theory that the impact produced them. The new paper finds that idea unconvincing. Instead, the chaotic terrains seem to have formed through gradual developments of a non-catastrophic nature. From the paper:

We attribute the immense volume losses, which we infer to have occurred during chaotic terrain formation… to widespread collapse associated with the devolatilization of hundreds of meters to a few kilometers of upper crustal materials. In the context of this hypothesis, we define “collapse” as encompassing elevation losses due to (1) mass wasting associated with the sublimation of surface/near-surface volatiles and (2) gravity-driven terrain disintegration over zones of deep volatile evacuation.

Image: Extent of a vast chaotic terrain (white outline) at the antipode of the Caloris basin (~5 x 10⁵ km²). Credit: Rodriguez et al.

In other words, no impact needed. The authors note that the chaotic terrain is not geographically limited to the Caloris antipode, suggesting that the volatile-rich crust may have been global. They also find that the antipodal area experienced an active phase about 1.8 billion years ago for reasons unknown. The paper identifies large areas of surface elevation losses within the antipodal chaotic terrains, a finding the authors interpret as the result of crustal volatiles turning into gas and escaping from Mercury’s upper crust over an area of about 500,000 square kilometers.

Daniel Berman (Planetary Science Institute) is a co-author of the paper:

“The deep valleys and enormous mountains that now characterize the chaotic terrains were once part of volatile-rich geologic deposits a few kilometers deep, and do not consist of ancient cratered surfaces that were seismically disturbed due to the formation of Mercury’s Caloris impact basin on the opposite side of the planet, as some scientists had speculated. A key to the discovery was the finding that the development of the chaotic terrains persisted until approximately 1.8 billion years ago, 2 billion years after the Caloris basin formed.”

Image: Zoom in showing variable magnitudes of collapse, which includes a relatively unmodified rim section that is smooth but not broken into knobs (arrow 1). This area adjoins another part of the rim that has been almost entirely removed (arrow 2). The adjacent intercrater regions also exhibit deep and abrupt relief losses (arrows 3 & 4). Credit: Rodriguez et al.

Where did this volatile-rich crust come from? One possibility is impacts from outer Solar System objects or perhaps main belt asteroids. Another is outgassing of volatiles from the interior. PSI’s Jeff Kargel, likewise a co-author, makes this interesting point:

“We also observe evidence of surficial devolatilization, probably caused by solar heating. If so, we have an opportunity to infer the range of Mercury’s volatile properties and compositions… While not all volatiles make for habitability, water ice can if temperatures are right. Some of Mercury’s other volatiles may have added to the characteristics of a former aqueous niche. Even if habitable conditions existed only briefly, relics of prebiotic chemistry or rudimentary life still might exist in the chaotic terrains.”

Image: Context view showing the location and extent of the chaotic terrain antipodal to the Caloris basin (outlined in white) relative to the ray systems of the Kuiper and Debussy impact craters. (B) Close-up view of panel A that provides the context and locations for panels C and D. The numbers 1-9 identify individual rays within the region’s view. (C, D) Close-up view showing crater rays that extend over the chaotic terrain (green lines 6, 8, 9) and other crater rays that appear truncated over the chaotic terrain (red lines 1-5, 7). We provide the location of the hollow hosting crater Dario in panel C. Credit: Rodriguez et al.

That volatile losses may not all be ancient is inferred from the fact that crater ejecta rays are not found in extensive areas of the chaotic terrain, which the authors interpret as indicating more recent activity. Mercury’s ‘hollows’ — small depressions resembling melt pits in terrestrial permafrost — have likewise been investigated as signs of near-surface volatile losses, although the matter is still under investigation. Whatever the case, it’s apparent that MESSENGER has returned enough information to offer up a first round of potential landing sites to investigate the planet’s volatile-rich crust and conceivably study its unexplored potential for astrobiology.

The paper is Rodriguez et al., “The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System,” Scientific Reports 10, Article number: 4737 (2020). Full text.