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Venusian meteorites?
JRehling
post Jan 28 2019, 02:54 AM
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Today, reading of the discovery of an apparent Earth meteorite fragment contained within a sample from Apollo 14, a curious possibility crossed my mind, and it seems like someone else may have thought of this, but I haven't found any sign of it: Could we find Venusian meteorites on the Moon? Better yet, could we find samples that offer a snapshot of Venus' past conditions from previous eons as preserved on the Moon?

It seems like in principle the answer is yes, and the question is how much lunar regolith would be required. Additionally, would the process have destroyed information that we might hope to get from the sample? That seems like a more complex question.

There's a good bit of speculation that Venus may have had a radically different and more earth like climate in the past. It is noncontroversial that Venus has lost a lot of H2O over time, and if there was ever a large reservoir of H2O in the past, that seemingly would have meant an atmosphere with less CO2 partial pressure, which in turn would have made it easier for spallation of fragments from the surface to escape velocity. The fact that terrestrial material has made it to the Moon indicates that material can reach high orbit from Earth through spallation and that is quite close to the same velocity needed to go from the surface of Venus to escape.

Now, making the interplanetary journey is a somewhat bigger leap, but not much more so. A fraction of Venusian materials making it to solar orbit would accidentally get gravity assists from Venus to Earth/Moon.

Needless to say, a program of sampling lunar regolith for Venusian material would find a lot of other materials of interest in the process, including those sampling Mercury and the past environments of Earth.

Substantive appeal here would seem to include the following: There is no technical challenge beyond current technology, though the strategies needed to deal with the needed scale might not be straightforward. (A long-life rover could choose, via terrestrial control and/or onboard AI, which out of many potential samples to return.) There are some particularly high-interest questions here, and to put a rather dramatic point on this, with the Sun's output in the past being 70% of what it is now, the past of Venus is an outstanding candidate for an Earthlike planet, and it is unthinkable that we would ever get a sample return from any of the others in the next few centuries, while we could potentially have an Ancient-Venus sample before 2025. (It's possible that we already have some in the Apollo samples!) And the potential discoveries are… no need to get speculative here, but extremely striking, when we consider the idea of an earth-sized planet with earth like levels of heating and a known history of high water content.

I don't know if this idea has been mulled over before, but it seems like for the potential outcome, it should be of extremely high interest.
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HSchirmer
post Jan 28 2019, 05:10 AM
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QUOTE (JRehling @ Jan 28 2019, 03:54 AM) *
Today, reading of the discovery of an apparent Earth meteorite fragment contained within a sample from Apollo 14, a curious possibility crossed my mind, and it seems like someone else may have thought of this, but I haven't found any sign of it: Could we find Venusian meteorites on the Moon? Better yet, could we find samples that offer a snapshot of Venus' past conditions from previous eons as preserved on the Moon?

It seems like in principle the answer is yes, and the question is how much lunar regolith would be required. Additionally, would the process have destroyed information that we might hope to get from the sample? That seems like a more complex question.

There's a good bit of speculation that Venus may have had a radically different and more earth like climate in the past. It is noncontroversial that Venus has lost a lot of H2O over time, and if there was ever a large reservoir of H2O in the past, that seemingly would have meant an atmosphere with less CO2 partial pressure, which in turn would have made it easier for spallation of fragments from the surface to escape velocity. The fact that terrestrial material has made it to the Moon indicates that material can reach high orbit from Earth through spallation and that is quite close to the same velocity needed to go from the surface of Venus to escape.

Now, making the interplanetary journey is a somewhat bigger leap, but not much more so. A fraction of Venusian materials making it to solar orbit would accidentally get gravity assists from Venus to Earth/Moon.

Needless to say, a program of sampling lunar regolith for Venusian material would find a lot of other materials of interest in the process, including those sampling Mercury and the past environments of Earth.

Substantive appeal here would seem to include the following: There is no technical challenge beyond current technology, though the strategies needed to deal with the needed scale might not be straightforward. (A long-life rover could choose, via terrestrial control and/or onboard AI, which out of many potential samples to return.) There are some particularly high-interest questions here, and to put a rather dramatic point on this, with the Sun's output in the past being 70% of what it is now, the past of Venus is an outstanding candidate for an Earthlike planet, and it is unthinkable that we would ever get a sample return from any of the others in the next few centuries, while we could potentially have an Ancient-Venus sample before 2025. (It's possible that we already have some in the Apollo samples!) And the potential discoveries are… no need to get speculative here, but extremely striking, when we consider the idea of an earth-sized planet with earth like levels of heating and a known history of high water content.

I don't know if this idea has been mulled over before, but it seems like for the potential outcome, it should be of extremely high interest.



Interesting question, it helps to remember that everything heading away from the sun is "uphill", so the most likely place to find Venusian meteorites is Mercury. Just like rocks and sediment work their way "downhill" under the influence of Earth's gravity, things tend to spiral inward due to the Sun's gravity.
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nprev
post Jan 28 2019, 06:38 AM
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There are doubtless Venusian, Mecurian, and pick-any-rocky-body meteorites on the Moon, and even more on Earth. It's a statistical inevitability. The problem is identifying them as such absent detailed mineralogical data on the surface materials of the bodies of origin, which apparently requires in situ analysis.

Pretty sure that we only have data that meet that standard for the Moon and Mars, though admittedly at least two asteroids seem to have been positively identified as likely points of origin for many meteorites (Vesta and Pallas). I don't think that any of the Venera missions were capable of conducting the required level of compositional analysis (nor did they last long enough on the surface to do so), but perhaps the Messenger data set soon to be augmented by Bepi-Colombo will be enough to eventually identify Mercurian meteorites wherever and whenever we may stumble upon them.


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tanjent
post Jan 28 2019, 07:09 AM
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Remember that there is no atmospheric drag to contend with once a rock has left Venus behind. An object chipped off the surface of Venus would already begin its journey with Venus' angular momentum relative to the sun. The issue of "in" or "out" would then depend on whether the impact and subsequent encounters with Venus itself added to this initial momentum or subtracted from it, and the sign of that increment seems equally likely to be positive as negative. (Someone with better knowledge of orbital mechanics might set me straight if I am wrong about this.)

This is why it took approximately the same energy differential to set the Parker Solar Probe on a course inward towards the sun, as to set New Horizons on a course outwards towards the Kuiper belt.


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JRehling
post Jan 28 2019, 09:20 AM
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According to this analysis

http://adsabs.harvard.edu/abs/1993Metic..28Q.398M

about 30% of Venus ejecta would strike Earth (which is roughly the same as the fraction of Mars ejecta expected to reach Earth). Although the abstract doesn't mention the possibility, this implies that probably not too much less than 3% would strike the Moon, as the interplanetary trajectory is ~identical, and the Moon has about 10% the Earth-Moon system's surface area.

Indeed, many meteorites are all but known to have originated from Vesta, and many other meteorite types have a very short list of suspected parent bodies. (One of the quirkiest misnomers in astronomy, the pallasite meteorites are not suspected to be from the asteroid Pallas, but were first discussed by a scientist himself named Peter Simon Pallas!)

All meteorites offer clues as to the size of the parent body and Venus meteorites could in many cases give us a very short list of candidates because the solar system has no great abundance of bodies larger than Mars, and the only one that's not Venus, we know extremely well.

Any scheme like I've suggested (including the hunt for meteorites on Earth) depends crucially on the quick and efficient identification of candidates, so as to exclude the vast and overwhelming majority of non-meteorites. It's for this reason that no martian meteorites have ever been found outside of a desert on Earth, and Antarctic ice is a wonderfully easy place to distinguish meteorites from the local solid surface material (ice and meteorites being quite obviously different). To suggest a candidate architecture for the lunar hunt for meteorites, I might suggest a rover pulling a large rake behind it, then performing a multispectral analysis of all the pebbles that are turned up. Anything looking non-lunar would be a candidate to pick up and inspect further. Would a Venusian rock (or some fraction of them) be immediately distinguishable from lunar regolith on the basis of vis/IR spectra? Sounds like a research question; obviously, since we don't know the nature of ancient Venus, there must be some uncertainty there.

Venera has given us some information on the composition of venusian rocks. However, if we're interested in ancient Venus, that information is not authoritative. However, we have a lot of information about the composition of lunar and terrestrial rocks, so it ought to be possible to sift through candidates to test for non-lunar/non-terrestrial isotopic ratios. Out of those, we may keep the martian candidates or also reject those.

Certainly, the main difficulty here is in examining enough lunar regolith to find a nonzero number of venusian candidates. The question is if that ratio and the means for performing the sorting make a mission like this practical. And, furthermore, the willingness to fund such a search for the results it may return (which would be sure to add a lot of non-Venus science).
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HSchirmer
post Jan 28 2019, 04:08 PM
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QUOTE (JRehling @ Jan 28 2019, 09:20 AM) *
I might suggest a rover pulling a large rake behind it, then performing a multispectral analysis of all the pebbles that are turned up.


I remember reading about something similar, but with a magnetic rake to collect iron meteorites and iron dust.

Vague recollection is that this was part of a 1960s lunar habitat proposal. Collect iron-rich regolith to use as aggregate for moon concrete, simple way to make structures that would be well shielded against solar storms & cosmic rays.
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JRehling
post Jan 28 2019, 06:36 PM
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QUOTE (HSchirmer @ Jan 28 2019, 09:08 AM) *
I remember reading about something similar, but with a magnetic rake to collect iron meteorites and iron dust.


When magnetism is the property you're looking for, there are wonderfully useful procedures. It is quite easy to create an unlimited amount of iron-bearing materials from the beach sand near me.

It is also easy to identify iron meteorites visually, which is why they have been spotted easily in photos from Opportunity, et al, and why meteorite hunters on Earth find them at a much higher frequency than their share of the total that fall.

What would distinguish ancient Venus meteorites from lunar rock (and would make it resemble, in most cases, some subset of Earth meteorites that made it to the Moon) could include:
• Formation in contact with water
• Metamorphic rock that formed under greater pressure than is typical of lunar regolith (Venus has ~5x lunar surface gravity)
• An absence of rare earth elements


Venusian basalts could resemble native lunar rocks quite a bit and be harder to locate.

Venusian sedimentary rocks, if any, would be very interesting and could be easy to distinguish from lunar rocks, but may also have a poor chance of surviving the ejection procedure.

Some interesting discussion of lunar rock composition vs. HED (Vesta), Martian, terrestrial rocks, and other meteorites is here:
http://meteorites.wustl.edu/lunar/howdoweknow.htm

Finally, there is likely a way to match isotope ratios that would distinguish nonlunar rocks from other large parent bodies, but I don't know that the Venera data suffices for this. It should be possible in principle but may require better in situ analyses on Venus, and time consuming evaluation of candidates found on the Moon.
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tanjent
post Jan 28 2019, 10:17 PM
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JRehling, it sounds as if you have read something interesting pointing to an expected difference in rare earth abundance between Venus and the Moon. I did not know that any of the Soviet landers were able to make actual chemical analyses of the Venusian soil, so what is this prediction based on? Lack of groundwater and plate tectonics? It's the first time I have seen this topic mentioned, so if you can point me to a good article, please do.
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JRehling
post Jan 29 2019, 01:48 AM
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tangentm, good question, and I should have been more cautious on that detail. We should expect the differentiation of early Venus to be similar to that of Earth, but the rare earths are not as good of an indicator of that as I thought, now that I look into it.

The Venera landers did analyze the composition of the Venusian surface. This may, however, not represent the ancient Venusian surface. What would be useful information would be isotopic analyses of elements that are not represented in the Venusian atmosphere. So, most of the major crustal elements besides oxygen. (Though there is an outstanding possibility that iron of all things is present as a trace component of the Venusian atmosphere in the form of ferric chloride as a candidate for the UV absorber.)
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HSchirmer
post Jan 29 2019, 02:17 AM
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QUOTE (JRehling @ Jan 29 2019, 01:48 AM) *
tangentm, good question, and I should have been more cautious on that detail. We should expect the differentiation of early Venus to be similar to that of Earth, but the rare earths are not as good of an indicator of that as I thought, now that I look into it.

The Venera landers did analyze the composition of the Venusian surface. This may, however, not represent the ancient Venusian surface. What would be useful information would be isotopic analyses of elements that are not represented in the Venusian atmosphere. So, most of the major crustal elements besides oxygen. (Though there is an outstanding possibility that iron of all things is present as a trace component of the Venusian atmosphere in the form of ferric chloride as a candidate for the UV absorber.)


Remember, an orbiter looking at Earth's (inorganic) surface areas would report back mostly- H2O (oceans and polar ice) silicate (sandy deserts) and phylosillicates (clays).
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JRehling
post Jan 29 2019, 03:43 AM
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The Venera landers got more detailed information about surface composition than an orbital observation might hope to make, as Table 2 here shows:

http://www.planetary.brown.edu/pdfs/3437.pdf

However, the mineralogy won't necessarily help us differentiate venusian rocks from lunar ones, particularly when we're interested in venusian eras very much before the present. Isotopic breakdowns would help, and we could get those with future landed missions. That might bear some signature that would differentiate them from native lunar rocks.
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