Ken Herkenhoff

Intro:

Ken Herkenhoff is a research geologist at the U. S. Geological Survey’s Astrogeology Science Center, since 1998. He previously worked at the Jet Propulsion Laboratory for nine years, working on the Mars Observer, Cassini and Mars Pathfinder missions and was the science lead for the Mars Exploration Rover MI cameras. In this interview we focus on the maps of the polar regions of Mars.

Hargitai:

How did you get involved or interested in Mars polar research and not something else on Mars?

Herkenhoff:

When I was a second-year graduate student, Larry Soderblom from here at the USGS was a Fairchild Scholar at Caltech, where I was a grad student. And at the time, this was the early 80s, I was interested in the Saturnian satellites. The data were new, we had seen them for the first time ever, and I was discussing that with him. Of course, he's an expert on those satellites as well as Mars. And he said, "No, Ken, I recommend you focus on Mars. Better projects for you to work on in Mars geology." My background was in geology. Initially, I was interested in astronomy and the planets and soon discovered that study of the planets was not considered real astronomy.

They were more interested in galaxies, and cosmology, and all that, which I was less interested in. So that's how I got into geology. And then, of course, Larry was right that Mars was a good topic to study. And in particular, Mariner 9 had obtained the highest resolution images of the Martian south polar region at that time. The problem was that the Mariner 9 cameras needed recalibrating. And so, I started on that project, which took years, and finally got them in the condition where I could use them for geological interpretation, photoclinometry, that kind of thing.

Hargitai:

It was too bright, the polar ice?

Herkenhoff:

No, even the summertime images needed better calibration, it was not a problem with too much brightness. The cameras just were not well-calibrated. These were old vidicon cameras, so the old television cameras. There was a fair amount of work to reduce the noise in the images. If you look back at them, they're still pretty noisy. But it was good enough to get some interesting information out of. So that's how I got interested in the south polar region, mainly because Larry suggested that I do that. And then, that led to the mapping eventually.

Hargitai:

What is the jump into extra unit mapping?

Herkenhoff:

In the summertime, the south polar residual cap, which is CO₂ frost, retreats to the point that the frost-covered area is very small, and the layered deposits are exposed without any frost on them. And so, that allowed mapping of those deposits, which appear dark. They don't appear as bright as the cap, the seasonal or the residual cap. And so, that made it possible to map them.

Hargitai:

Was it recognized immediately that it's dark, so that the polar ice was not just the bright thing that was previously thought, but also the dark?

Herkenhoff:

At the time, we didn't know the composition of the layered deposits. People suspected that they were ice-rich, but we didn't really know at the time. But they did appear dark, so they can't be pure ice. But it only takes a small amount of dust or dirt mixed into the ice to make it appear dark. The albedo is not a very good constraint.

Hargitai:

When you first started to make the geologic maps, how did you select the types of units that you would map?

Herkenhoff:

Those initial maps were very simple. I was not able to subdivide the layered deposits into individual layers. We just didn't have good enough resolution across the exposures of the layer deposits. So I mapped just one unit as the layer deposits. Another unit was the bright residual cap. And then, there were underlying units that were exposed below the layer deposits. So not that many geologic units to map in that area at the time.

Figure 1. Herkenhoff, K and Murray B 1994, Geologic map of the MTM-85080 quadrangle, Planum Australe Region of Mars. (revised)

Hargitai:

And did it develop further as–your first map is from '94.

Herkenhoff:

'92 actually.

Hargitai:

And then, the early 2000s. But you used Viking images.

Herkenhoff:

That's right. Viking and Mariner 9. That's all that were available at the time.

Hargitai:

Did computer technology enable you to map more precisely or better at the end of this?

Herkenhoff:

Yes. In fact, that's how Jim Skinner got involved in that later map, because by that time, we had Mars Global Surveyor MOC images of the south polar region, and there were THEMIS images. A lot more data to use for mapping. And therefore, we needed to use GIS. Previously, there weren't enough images–well, and GIS was rather primitive in those days. We didn't need to use GIS to put all the data together, merge it into a single view. And because Jim was an expert in GIS, that's how I got him involved, and he ended up taking the lead in the mapping, and combining multiple datasets, and using GIS methods to map the contacts rather than just doing it by hand. The original maps were inked by hand on Brownline or mylar.

Hargitai:

I don't know what that…

Herkenhoff:

I think I can show you. Here's an example. The base would be–this is a Viking orbiter base map. And I think you can see seams and that kind of thing. But then, the contacts were inked, using a pen, onto this and then used to reproduce the map.

Hargitai:

So you used the same resolution for mapping as the base map?

Herkenhoff:

That's right. And it's 1 to 500,000. Half-million. Yeah.

Hargitai:

And in your last north pole map, did the mapping scale increase?

Figure 2. Skinner, J.A., Jr., and Herkenhoff, K.E., 2012, Geologic map of the MTM 85200 quadrangle, Olympia Rupēs region of Mars: U.S. Geological Survey Scientific Investigations Map 3197, pamphlet 12 p., 1 sheet, scale 1:500,000.

Herkenhoff:

No, it was still 1 to 500,000. But that's the one where we used GIS for the mapping.

Hargitai:

Can it be said that the photomosaic maps were explicitly created to help geologic mappers?

Herkenhoff:

Yes. In fact, the proposals for geologic maps at that time were limited to quadrangle base maps that had been created using Viking orbiter mosaics. So other areas that we didn't have good enough coverage or for whatever reason the mosaic was not available could not be proposed for mapping.

Hargitai:

You inked it on this?

Herkenhoff:

I inked it directly on this. And then, that was used by the production people. They would, I think, scan it in and then use it to finalize the map by a draftsman. And so, the final map does not include my original line work.

Hargitai:

And how did you decide on the colors? Was it a standard?

Herkenhoff:

There was. It's been a long time, but as I recall, yes, there were standards for colors or at least limited choices for units of various ages. So I had to conform to those.

Hargitai:

Did you arrange the units according to their age from the beginning? Or the first maps were more focused on the albedo and morphology, not age?

Herkenhoff:

Both. We could tell that the layered deposits were younger than the underlying units and that the frost cap was younger than anything. Those were Amazonian. But that's a huge, three-billion-year span that it could've been. We know that they're much younger than that. But that was the finest distinction. Amazonian versus Hesperian was the finest distinction that we could map.

Hargitai:

And in your last north polar map that used GIS, how did it give you a richer view to either morphology or the age?

Herkenhoff:

Primarily, it was better coverage with higher resolution data. Viking orbiter, both the south and the north, the best resolution images were, I think, around 100 meters per pixel. Mariner 9 as well, maybe a little bit better, 60 to 80 meters per pixel for Mariner 9. But then, Mars Global Surveyor MOC has resolution as good as 1.5 meters, so much better. Less coverage, but then there was THEMIS data available as well. So yeah, basically, we just had more image coverage at higher resolution.

Hargitai:

Could you distinguish new kinds of features using that? I saw that the north polar map had individual craters.

Herkenhoff:

Yes, and we could resolve layers in the layer deposits much better. And in units below, these basal units ended up having layers as well that were not resolved in the earlier Viking data. So that was very helpful.

Hargitai:

There are eight possible quads around the north pole, but only four are mapped for the north pole. Do you consider it final for now, this set of maps? Or is there a chance that somebody will continue this 500,000 series?

Herkenhoff:

There's certainly a chance that someone could propose and complete more maps. I've moved onto other things, so I haven't proposed to do that. I guess I could've. But yeah, chose not to. And apparently, no one else has chosen to either. But the photomosaic bases are available if people want to map them. I think just nobody's proposed yet.

Hargitai:

And what was the basis of the choice of these particular quads and not the others?

Herkenhoff:

I think it was a combination–and I was not involved in this, but my impression that it was a combination of scientific interest and image coverage. So even if there's an area that may be very scientifically interesting, if we don't have enough coverage for that whole quadrangle to make a good mosaic base, then it wouldn’t have been created.

Hargitai:

And did the Mars polar landers landing site selection play any role in this?

Herkenhoff:

No, that was later. I was not involved in mapping the landing ellipse.

Hargitai:

You moved from JPL to USGS. Did it change anything related to your mapping methods?

Herkenhoff:

It certainly made it easier to collaborate with people here, and it enabled that north polar map to be completed with the GIS, including much more data. So yeah, it was quite helpful.

Hargitai:

How did you share the work with a long project with your coauthors when you had coauthors?

Herkenhoff:

Well, initially, Bruce Murray was my thesis advisor. And so, he gave broad instructions and guidance. But he didn't actually do much of the real work, that was me.

Hargitai:

Is it typical for geologic mapping that it's basically a one-man project?

Herkenhoff:

I think that's more often than not. But I have seen–particularly now that there are multiple datasets, not just imaging, but infrared reflectance, spectroscopy, thermal inertia. Much more information can be gained, and therefore there's room for more analysis in each quadrangle. And so, I'm seeing more and more people who will maybe have someone who–in addition to a photogeologist, someone who's an expert in, say, thermophysical properties or infrared spectroscopy. And they would collaborate to put together the geomorphology with the other remote sensing data to build a better map.

Hargitai:

But there still will be one person who does the same classical geologic mapping?

Herkenhoff:

Well, at least one. [Laugh] But I think I've seen proposals for collaborations between multiple photogeologists, so it doesn't have to be just one.

Hargitai:

Is there anything that you've discovered during the mapping that you haven't expected?

Herkenhoff:

That's a good question. I think at the time, people had noticed that there were very few craters on the north polar layer deposits and the residual cap. And through the mapping, we basically confirmed that and put limits on the crater density. And that helped us constrain the age of the north polar residual cap and layer deposits. And it was very young. And no crater's bigger than 300 meters. That was maybe not a surprise, but it was helpful to constrain the age.

Hargitai:

How much these geologic maps are long-lasting? And for the lunar geology, they remapped every decade the same region, so is it permanent?

Herkenhoff:

No, and that's true also for Mars, particularly because we've gotten so much more data in the last couple decades that it's enabled higher resolution and just better-informed mapping. So yeah, it continues to evolve, and those old maps are less useful these days. HARGITAI: Jumping to your current work with the Curiosity [?], is there something in your system that can be called a map for Curiosity?

Herkenhoff:

We do use orbital data to map the units that we're driving through. So yes, there are geologic maps of the landing sites, not just Curiosity but in Jezero for Perseverance as well that were created before landing to help us guide decisions on where we want to traverse. So yeah, those maps are very useful. Key.

Hargitai:

I've heard that there are quadrangles or quadrangle-like things, but I've never identified any files that would show those quadrangles.

Herkenhoff:

They're not published yet. They're internal use. And the quadrangles were used to divide up the area so that multiple mappers–each mapper would pick a quadrangle and map that one, then discuss with other mappers differences in where they're mapping contacts or interpretations of the geologic units. And then, eventually, they were all put together into one big geologic map. And I think for Jezero, that was published. Katie Stack I think was the lead author. But there's been more recent mapping that is still unpublished.

Hargitai:

On the traverse, you use the same principles as doing photogeologic maps from space?

Herkenhoff:

Oh, yes. For the rover traverses? Oh, yes. In addition, there are studies of roughness and traversability over rough or rocky areas.

Hargitai:

It's engineering.

Herkenhoff:

Yeah, it's more engineering-focused. Yeah, that's right.

Hargitai:

Any difference between the difficulty of mapping the different regions, or it's all the same?

Herkenhoff:

The north and south polar regions are fairly different. And what's interesting is that the south polar layer deposit's apparently been exposed much longer, like, 100 million years, than the north polar, which is maybe just tens of thousands of years. And so, yes, so different features are exposed. Exposures are better in the north than the south. And there are different units there. There're these darker basal units under the north polar layer deposits that don't appear in the south. I'm not sure if I've answered your question, but there are certainly differences.

Hargitai:

If you combined all your map regions, where would you like to be?

Herkenhoff:

Where would I like to go personally if I were an astronaut?

Hargitai:

Yes.

Herkenhoff:

I think the north polar layer deposits. Even though that was my original thesis interest, they are younger and therefore record more recent climate changes, like ice ages on Earth. And so, there's a record there to be interpreted in terms of those climate variations. So yeah, like ice ages on Earth and Antarctic cores, it would be very interesting to go to the north polar region and extract that history.

Hargitai:

Have you ever used actual Antarctic maps?

Herkenhoff:

I did not use Antarctic mapping to guide the polar mapping. Probably could have. I know I've seen radar data showing layering in Antarctic ice, but a major difference there is that the terrestrial ice sheets are flowing, whereas on Mars, as far as we can tell–this was debated years ago–they're static. Some people probably still believe that there's evidence for flow in the north polar layer deposits, but I think the consensus is that they're frozen to the base, not flowing. At least not currently or recently.

Hargitai:

Last question is, if I were a young audience, and you'd try to persuade me to go and become a planetary mapper, what is the good thing in planetary mapping as a career?

Herkenhoff:

Related to that, the polar regions, I think, are particularly interesting because of that record of climate changes. Obviously, we have a long climate history here on Earth, and comparing that with Mars I think is very interesting. Perhaps we could learn lessons from Mars, which is a much simpler system. No biology, no oceans. Much simpler system. And there's certainly evidence for astronomical variations causing these climate changes. Which happen on Earth as well, the Milankovitch cycles. But much stronger on Mars, so I think there are lessons there. But I think what's most interesting to me about planetary mapping is that you're exploring areas that nobody previously knew anything about. And it's wide-open in that sense for new, younger scientists to come in and help explore Mars and help understand a different planet in a new way. For me, it's curiosity-driven. I just want to know how Mars works, how it got that way, and the history. I would think that would be interesting to younger people as well.

Hargitai:

In the long run it may save the Earth, with the climate…

Herkenhoff:

Yeah, I hope so. If we can understand climate change on Mars, it may help us understand climate change on Earth better.

Hargitai:

Thank you very much for your time.

Herkenhoff:

Oh, you're welcome. Yeah. Happy to help.

[End]