Simone Marchi's Home Page

Simone Marchi
Research Fellow

Solar System Exploration Research
Virtual Institute
Southwest Research Institute
1050 Walnut St., Suite 300
Boulder, Colorado 80302 USA

Phone: +1 720 208 7220
Fax: +1 303 546 9687
Email: marchi [AT]
Simone Marchi

Research Interests

My research interests span from asteroids to terrestrial planets. In particular:

• Formation of terrestrial planets and the Moon;
• Geology of asteroids and terrestrial planets;
• Spectroscopy & dynamics of minor bodies;
• Meteorites.

I am most active in the field of terrestrial planets and asteroids collisional evolution. Their rocky surfaces are sort of "snapshots" of the bombardment history of the inner solar system. One may say that early processes in the solar system that are no longer observable are locked into cratered terrains. By studying these battered surfaces one may gain insights on the magnitude and frequency of early collisions in the inner solar system, including our own Earth.
I am also involved in several space missions. In particular, I am:

• Associate Scientist of OSIRIS and VIRTIS on board ESA Rosetta;
• Associate Scientist of VIR on board NASA Dawn;
• Chair of ESA MarcoPolo-R working group on crater properties;
• External collaborator of NASA MESSENGER Geology Discipline Group.

Stories: Asteroid Vesta, Vesta's Bombardment: Hot and Heavy, Mercury the Younger

Asteroid Vesta

An example of my research activities can be found in the recent work on the cratering history of the main belt asteroid Vesta in the framework of NASA Dawn mission. Images acquired during the early stages of the mission were used to construct a preliminary catalog of craters larger than about 4 km in diameter. The overall distribution of craters can be seen in the following "circles on Vesta" image:

Craters distribution of Vesta

Figure 1. Craters distribution on Vesta. Every each yellow circle corresponds to an impact crater. This image contains 1872 craters. Note the two large craters that wrap around the south pole: there are called Rheasilvia and Veneneia and they both measure about 500 km across (Figure after Marchi et al, Science, 2012). The spatial resolution of the map is ~300 meter per pixel. Click here for the full paper.

Since the publication of this work, Dawn has completed its operations at Vesta. In doing so, a better image resolution was achieved (down to 20 meter per pixel) and also better coverage of the northern hemisphere (in shadow in Figure 1).

Beside the (remarkable!) fact that Vesta is one of the few asteroids ever reached by a spacecraft, Vesta is a really unique body for many other reasons. In particular, I find it absolutely mind-blowing that we do have chips of Vesta in our meteorite collections. This is truly a striking feature. Among the ~500,000 kg of extraterrestrial material in our labs, we have only established genetic links for three objects so far: Vesta, the Moon and Mars. Among them, Vesta is the farthest from us: ~1.1 astronomical units (~164,557,657 km) at the closest. Here is picture of a chip from Vesta's deeper crust (a rock type called diogenite, also common on the Earth) sitting on my desk:


Figure 2. Diogenite "Tatahouine" found in Tunisia.
The parent meteorite fell on June 27, 1931 near the town Foam Tatahouine. Several fragments (for a total of ~13 kg) were collected afterward. The meteorite is also known as "Green Meteorite" given its greenish color. The fragment shown in this picture is about 1 cm wide. In addition to diogenites, eucrite and howardite (the latter is a mixture of the first two) meteorites are also thought to come from Vesta.

Thanks to Dawn, we now have a better understating of Vesta and its link to the howardite, eucrite and diogenite meteorites. To me, one the most intriguing outcome of Dawn observations is that they clearly disclosed a body with two distinct "faces". There is the "young" face shown by the hilly and rugged southern hemisphere, while the northern hemisphere appears to be much "older" given its smooth and gently sloped surface as a result of eons of impact cratering. There is the "dark" face expressed by localized spot of low albedo (< 20%) opposed to the "bright" face where the albedo can be as high as 40%. Also, the composition is rather heterogeneous, from diogenitic-rich to eucritic-rich material.

The two faces of Vesta

Figure 3. Animation showing the two faces of Vesta.
Well, this is not exactly what Dawn saw at Vesta, but it helps visualizing the two faces of Vesta (the faces are a representation of the roman god Janus Bifrons on a coin, ~200 BCE).

Vesta's Bombardment: Hot and Heavy

Fragments from asteroids provide an unique opportunity to study the processes that shaped the early solar system. Some of these rocks, found on the Earth as meteorites, reveal signs of impact processes on their parent bodies, recorded as tiny variations in the amount of radiogenic elements, like the noble gas Argon (see Figure 4). Particularly intriguing are the signatures found in many chondritic meteorites (e.g. the ordinary chondrites enriched in iron), and in a major clan of achondrites (e.g. the howardite, eucrite and diogenites; originated from asteroid Vesta).

Argon and Potassium

Figure 4. Potassium and Argon. The noble gas Argon (Ar) entrapped in meteorites as a result of radioactive decay of Potassium (K) is widely used to constrain the time when major impact processes took place on the meteorites' parent bodies. Impacts produce heating that trigger Ar release from the lattice. Thus, the relative abundance of Ar (highly volatile) to K (less volatile) tell us the elapsed time since the last major impact. These ages are commonly called impact-reset ages.

These rocks show signs of multiple impacts that took place about 4 billion years ago, during the so called "late heavy bombardment" or "lunar cataclysm", a characteristic also shared by many lunar rocks. Clearly, the Moon, Vesta and other meteorite parent bodies do not share similar collisional histories. In fact, objects like Vesta are surrounded by a large reservoir of asteroids, the Main Belt, confined between the orbits of Mars and Jupiter. These objects have orbits that often intersect one with each other, implying an high collisional rate.

On the other hand, the Moon resides in a relatively quieter region of the solar system, where collisions with interplanetary rocky bodies are much more uncommon. Thus, how come that lunar rocks and several asteroidal meteorites share strikingly similar collisional patterns? Is that a mere coincidence or rather is telling us something profound? While this puzzling coincidence was long been recognized by several researchers, a satisfactory answer to this conundrum has only been suggested in a recent multidisciplinary work.

Researchers have linked the lunar ad asteroidal datasets and found that the same population of projectiles responsible for making craters and basins on the Moon around 4 billion years ago were also hitting Vesta at very high velocities, enough to leave behind a number of telltale, impact-related ages (see Figure 5).

Lunar and Vestan samples

Figure 5. Lunar and Vestan samples.
The new research demonstrates how to use howardite, eucrite and diogenite (HED) meteorites, that originated from Vesta, to study the lunar cataclysm. Interestingly, the total mass of lunar rocks stored in our laboratories is approximately 448 kg. For comparison, HEDs sum up to 1332 kg. Thus, thanks to the new interpretation of HEDs impact-reset ages, this new work expanded by about three times the total mass available to study the lunar cataclysm.

This novel interpretation of the howardites and eucrites was augmented by recent close-in observations of Vesta's surface by NASA's Dawn spacecraft. In addition, the team used the latest dynamical models of early main belt evolution to discover the likely source of these high velocity impactors, finding that the population of projectiles that hit Vesta had orbits that also enabled some objects to strike the Moon at high speeds.

The findings support the theory that the repositioning of gas giant planets like Jupiter and Saturn from their original orbits to their current location destabilized portions of the asteroid belt and triggered a solar system-wide bombardment of asteroids billions of years ago, the lunar cataclysm. The research also provides new constraints on the start and duration of the lunar cataclysm, and demonstrates that the cataclysm was an event that affected not only the inner solar system planets, but the asteroid belt as well.

The paper, published on April 2013 in Nature Geoscience, can be found here.

I can tell you that doing this work was a lot of fun!

Mercury the Younger

Mercury is a fascinating world. Among the terrestrial planets it is the closest to the Sun and, for this reason, its study has always been a challenge: both for ground-based and spacecraft observations. Mercury was observed at close range for the first time by the spacecraft Mariner 10 in the 70s. After more than 30 years of loneliness, in 2011 the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission has inserted into orbit around Mercury enabling planetary scientists to perform unprecedented analyses of its surface, tenuous atmosphere, and interior.

My interests in Mercury reflect some of my latest work about the Moon and Vesta. Thanks to these studies, I was able to provide news evidence in favor of the so-called "late heavy bombardment" (a period of intense bombardment in the inner solar system occurred about 4 billion years ago). Therefore, I was eager to study the effects -if any- of this heavy bombardment on Mercury's surface. In collaboration with the Geology Discipline Group of the MESSENGER team, I performed an analysis of the oldest visible terrains on Mercury (namely, those with the highest crater density). Then, I applied a new chronology that I helped to develop, which allowed us to convert the crater density measured on a given terrain into its absolute age. The results were puzzling. With my great surprise, the oldest terrains on Mercury date around 4.0-4.1 billion years ago (see Figure 6):

Mercury crater density

Figure 6.
The animation compares the actual surface of Mercury, as seen by MESSENGER (left-hand panel: obtained using a publicly available mosaic of Mercury found here), to the crater areal density (right-hand panel: number of craters larger than 25 km averaged over neighbor regions of 300 km in radius). The crater density is shown in color, ranging from dark blue (minimum) to white (maximum).

One may ask: so, what's the big deal? Well, first of all, this finding implies that the first 400-500 million years of Mercury's history (which is thought to have formed about 4.5 billion years ago, as supposedly for the Earth and Moon) are lost. In other words, something tremendous must have happened early in Mercury's history to erase its surface. What could that have been? Hard to say, given that the most ancient surface is lost (!), and with it also the traces of what may have caused the resurfacing. However, additional indirect information indicated that voluminous volcanism took place on Mercury, and perhaps it was strong enough to wipe out its surface. Our work also suggests that this erasure could have been aided by the heavy bombardment itself.

This research was published in the July 4th 2013 issue of Nature magazine (here).

>> More stories to come soon. Check out this page regularly.

Last update: 2 April 2014

© 2013 Simone Marchi